Transistors 2SD1062/B826 are Q431, 433 in the left channel at least. They are TO220 size power transistors and are shown on the schematic immediately above and below the output transistors - assuming of course, that the 7240PE schematic and your amplifier match. Physically, they are located away from output stage, almost in the middle of the board.
I guess you already realise there is a important procedure for replacing the output transistors which involves those solder bridges. You'll need to re-solder them, after fitting replacements.
I assumed that since you removed the output transistors, you could operate the amplifier as long as no load was connected, since it was being tested with dead transistors previously and they would have been shorted C-E, I imagine. Any excessive current from the output stage should now be limited by the light bulb.
I guess you already realise there is a important procedure for replacing the output transistors which involves those solder bridges. You'll need to re-solder them, after fitting replacements.
I assumed that since you removed the output transistors, you could operate the amplifier as long as no load was connected, since it was being tested with dead transistors previously and they would have been shorted C-E, I imagine. Any excessive current from the output stage should now be limited by the light bulb.
Thank you for providing those Q positions, I found them right away. The service manual I am referencing is (sorry to admit) the one on HiFi Engine. It says a B912 is in position Q431 and that is what's on the board. Q433 lists a B911 that too is what's present. I looked them up and unless I misinterpreted, both are PNP.
I also looked up the two transistor numbers you provided for Q431 and Q433 - 2SD1062 and 2SB826 respectfully. I was puzzled because (again unless I misinterpreted) 2SD1062 is NPN and B826 is PNP.
To my limited understanding, Your "arrangement" makes sense if they are supposed to be a complimentary pair. Can the manual I was referencing be wrong in this case?
I thought it best to understand this discrepancy before measuring their terminal voltages.
With regards to what you said about output transistor bridges. It did look like they had an abundance of solder and I took this to be evidence of the sloppy workmanship some attribute to NAD products. It was near the end or just after I removed those devices, that I considered some leads need to be connected to two places - bridged as you pointed out.
Since there is no foil side diagram in the manual, I am hoping the board will guide me and jog my memory when it comes time to install the new ones.
I also looked up the two transistor numbers you provided for Q431 and Q433 - 2SD1062 and 2SB826 respectfully. I was puzzled because (again unless I misinterpreted) 2SD1062 is NPN and B826 is PNP.
To my limited understanding, Your "arrangement" makes sense if they are supposed to be a complimentary pair. Can the manual I was referencing be wrong in this case?
I thought it best to understand this discrepancy before measuring their terminal voltages.
With regards to what you said about output transistor bridges. It did look like they had an abundance of solder and I took this to be evidence of the sloppy workmanship some attribute to NAD products. It was near the end or just after I removed those devices, that I considered some leads need to be connected to two places - bridged as you pointed out.
Since there is no foil side diagram in the manual, I am hoping the board will guide me and jog my memory when it comes time to install the new ones.
Wha? 😕? I used the same HiFi Engine source for the 7240PE manual, see my links. The schematic, pages 13&14, was the reference. Now that you've drawn my attention to it, the parts list is where the difference lies and I didn't refer to it. I think the parts you have are European complementary types BD911 and BD912 as shown, probably from STmicro or Mospec. They have higher maximum ratings which might be better suited there. http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00001277.pdf
'Funny that NAD would change from Japanese to Euro types but it's probably down to the available Japanese TO220 types.
re the solder bridges -, I'm talking about when you have all the parts installed and ready for test, as described in the manual, Page 5. Don't do that until all else is in order.
I checked terminal voltages at Q431 and compared them to what is on the schematic:
B +55.3 volts measured B +71 volts schematic
C +43.3 volts measured C +43 volts schematic
E +55.3 volts measured E +71 volts schematic
and Q433
E -55.3 volts measured E -71 volts schematic
C -43.5 volts measured C -71 volts schematic
B -55.1 volts measured B -43 volts schematic
For comparison, I measured and compared terminal voltages for Q432:
B +55.7 volts measured B +71 volts schematic
C +55.7 volts measured C +43 volts schematic
E +55.7 volts measured E +71 volts schematic
and Q434
E -55.7 volts measured E -43 volts schematic
C -55.7 volts measured C -71 volts schematic
B -55.7 volts measured B -71 volts schematic
In case it's incorrect, let me mention that for a ground point I am using the common negatives of C505 & C506.
During the time it took to take these measurements, I noticed a mild burning odor. No smoke, just the scent.
Perhaps this is due to still using a 100W bulb in the DBT? Although I have visited three second hand stores today, I found no incandescent light bulbs. Tomorrow morning, I am making the drive for the output transistors which will give me the opportunity to look around thrift stores in different outlying communities. Maybe I'll have better luck. If not, I'll re-read the earlier post re Halogen lamps. Those things get darn hot though.
B +55.3 volts measured B +71 volts schematic
C +43.3 volts measured C +43 volts schematic
E +55.3 volts measured E +71 volts schematic
and Q433
E -55.3 volts measured E -71 volts schematic
C -43.5 volts measured C -71 volts schematic
B -55.1 volts measured B -43 volts schematic
For comparison, I measured and compared terminal voltages for Q432:
B +55.7 volts measured B +71 volts schematic
C +55.7 volts measured C +43 volts schematic
E +55.7 volts measured E +71 volts schematic
and Q434
E -55.7 volts measured E -43 volts schematic
C -55.7 volts measured C -71 volts schematic
B -55.7 volts measured B -71 volts schematic
In case it's incorrect, let me mention that for a ground point I am using the common negatives of C505 & C506.
During the time it took to take these measurements, I noticed a mild burning odor. No smoke, just the scent.
Perhaps this is due to still using a 100W bulb in the DBT? Although I have visited three second hand stores today, I found no incandescent light bulbs. Tomorrow morning, I am making the drive for the output transistors which will give me the opportunity to look around thrift stores in different outlying communities. Maybe I'll have better luck. If not, I'll re-read the earlier post re Halogen lamps. Those things get darn hot though.
Just so we are on the same page, here's what I am referring to from Home Depot Canada, in this example. The halogen bulb itself is inside a standard A-Line envelope and you don't get fried so much if you touch it 
: https://www.homedepot.ca/en/home/ca...rical/l3-lightbulbs-2016/l4-halogenbulbs-2016
Both rail switch transistors should have same E and B voltages until they turn on at high levels. The 71V rails are pretty low at only 57V or so. Check this voltage at the power supply bridge rectifier BD502 or caps C509,510. If these are low too, the +/-71V power supply is overloaded and you need to turn off and find the problem - probably the L channel transistors or other failures there. If you remove the supply to the L channel, the R channel supply should rise correctly because voltages there look consistent and probably normal. If the power supply voltages are much higher than say, 58V, the PTC protection resistors PTC1 and PTC2 are doing their job and limiting current to the PE circuit. Measure across these to see what they are dissipating.
Anyways, I think I need to read up on that design as much as you now 😱.
: https://www.homedepot.ca/en/home/ca...rical/l3-lightbulbs-2016/l4-halogenbulbs-2016Both rail switch transistors should have same E and B voltages until they turn on at high levels. The 71V rails are pretty low at only 57V or so. Check this voltage at the power supply bridge rectifier BD502 or caps C509,510. If these are low too, the +/-71V power supply is overloaded and you need to turn off and find the problem - probably the L channel transistors or other failures there. If you remove the supply to the L channel, the R channel supply should rise correctly because voltages there look consistent and probably normal. If the power supply voltages are much higher than say, 58V, the PTC protection resistors PTC1 and PTC2 are doing their job and limiting current to the PE circuit. Measure across these to see what they are dissipating.
Anyways, I think I need to read up on that design as much as you now 😱.
decorative lamps, like candle shaped, are usually exempt from these "ECO" rules.
They are usually very low wattage typically 25W and 40W for our UK's 240Vac.
As said earlier, the halogen replacements are tungsten filament inside a double envelope. These make a good PTC resistor with a 12:1 upto 15:1, hot:cold resistance ratio.
They are usually very low wattage typically 25W and 40W for our UK's 240Vac.
As said earlier, the halogen replacements are tungsten filament inside a double envelope. These make a good PTC resistor with a 12:1 upto 15:1, hot:cold resistance ratio.
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I managed to locate incandescent bulbs today. Two dollars for a bag of 12 tested 40W and 60W - the DBT is now equipped with a 60 watt.
The voltages at BD502 measured +/-46 volts not +/-71 volts.
For comparison I checked voltage at BD503. +/-20 volts not +/-28 volts.
In order to see if BD503 rises to 28v, do I just deprive BD502 of power by removing fuse F505?
TPC1 and TPC2 look like disc capacitors but I see the parts list identifies them as thermistors. The voltage across each is less than 1mv.
It sounds like there will soon be checking for failed transistors. This would be all that are on the left channel?
The "other failures" you mention can certainly apply to fusible resistors. Presumably it would be prudent to compile a list as they will need to ordered. Along with, by the sound of things, more transistors.
The voltages at BD502 measured +/-46 volts not +/-71 volts.
For comparison I checked voltage at BD503. +/-20 volts not +/-28 volts.
In order to see if BD503 rises to 28v, do I just deprive BD502 of power by removing fuse F505?
TPC1 and TPC2 look like disc capacitors but I see the parts list identifies them as thermistors. The voltage across each is less than 1mv.
It sounds like there will soon be checking for failed transistors. This would be all that are on the left channel?
The "other failures" you mention can certainly apply to fusible resistors. Presumably it would be prudent to compile a list as they will need to ordered. Along with, by the sound of things, more transistors.
12 bulbs for $2? That's a deal you can't afford to miss!
I don't quite understand those rectifier voltages though. You verified +/-43V at Q431/433 collectors and +/-55V at the emitters so something has changed there if you are measuring DC at the same appropriate points. The common ground point you used earlier is correct anyway. Measuring across the terminals of each of those smoothing caps is the best place to check each DC supply voltage, if you have access.
However, if the DBT is now glowing, the mains voltage to the transformer primary winding could have dropped to around 70VAC now and all secondary winding AC voltages would have dropped proportionally. Similarly, the rectified and smoothed DC supplies. That seems likely and is what a DBT normally does in the event of a load approaching the bulb rating but perhaps you haven't checked that yet.
You could remove fuses to both power amplifier supplies by pulling F501-504 to check the load there. I'm not certain what would happen if you removed the 43V supplies with the 71V supplies still in place but I don't think it would be good. BTW, you mentioned that all fuses were intact when you first looked at the amp but perhaps that's because the owner already replaced any that failed, in the vain hope that would fix the fault. After all, when a safety device cuts the power, there must be fault with the safety device, right?
That's how many consumers react, anyway.
Still, you might expect the 28V supply for the preamp to then rise some. There is a second power transformer supplying the tuner, tuner microprocessor and displays and this is also fused (F801,802) and possible to isolate. It's not used for any other role like DC or overload protection relays but I'm not sure how much current that section draws normally. It could be significant with its large LED display. I would pull the fuses anyway, knowing you may have to re-enter the tuner presets in any event.
I suspect the Q431/433 transistors are OK but there is still a problem involving them. Pull them and recheck their DC supply voltages again, after first checking that you can restore the figures in #44 by refitting the DBT bulb you used during those measurements.
We could work with the lower voltages you now measure but need to know how much has changed due to substituting the 60W bulb. I think there is quite a load there somewhere, perhaps 25W even with no audio output. It would be useful to know if the extra load is due to the tuner, which can be simply eliminated.
Finally, yes a PTC is a positive temperature coefficient resistor a.k.a. thermistor. Once common in tube era TVs and later as soft-start resistor devices. Not seen much now. You may notice this design is like the old NAD3020 and similar models in that there is no output protection other than small, thermally actuated (bimetal switch) circuit breakers. They take seconds to trip and won't help much in a shorted leads or DC fault situation but I guess NAD feel that their customers won't be needing much protection at their low average power levels.
I don't quite understand those rectifier voltages though. You verified +/-43V at Q431/433 collectors and +/-55V at the emitters so something has changed there if you are measuring DC at the same appropriate points. The common ground point you used earlier is correct anyway. Measuring across the terminals of each of those smoothing caps is the best place to check each DC supply voltage, if you have access.
However, if the DBT is now glowing, the mains voltage to the transformer primary winding could have dropped to around 70VAC now and all secondary winding AC voltages would have dropped proportionally. Similarly, the rectified and smoothed DC supplies. That seems likely and is what a DBT normally does in the event of a load approaching the bulb rating but perhaps you haven't checked that yet.
You could remove fuses to both power amplifier supplies by pulling F501-504 to check the load there. I'm not certain what would happen if you removed the 43V supplies with the 71V supplies still in place but I don't think it would be good. BTW, you mentioned that all fuses were intact when you first looked at the amp but perhaps that's because the owner already replaced any that failed, in the vain hope that would fix the fault. After all, when a safety device cuts the power, there must be fault with the safety device, right?
That's how many consumers react, anyway. Still, you might expect the 28V supply for the preamp to then rise some. There is a second power transformer supplying the tuner, tuner microprocessor and displays and this is also fused (F801,802) and possible to isolate. It's not used for any other role like DC or overload protection relays but I'm not sure how much current that section draws normally. It could be significant with its large LED display. I would pull the fuses anyway, knowing you may have to re-enter the tuner presets in any event.
I suspect the Q431/433 transistors are OK but there is still a problem involving them. Pull them and recheck their DC supply voltages again, after first checking that you can restore the figures in #44 by refitting the DBT bulb you used during those measurements.
We could work with the lower voltages you now measure but need to know how much has changed due to substituting the 60W bulb. I think there is quite a load there somewhere, perhaps 25W even with no audio output. It would be useful to know if the extra load is due to the tuner, which can be simply eliminated.
Finally, yes a PTC is a positive temperature coefficient resistor a.k.a. thermistor. Once common in tube era TVs and later as soft-start resistor devices. Not seen much now. You may notice this design is like the old NAD3020 and similar models in that there is no output protection other than small, thermally actuated (bimetal switch) circuit breakers. They take seconds to trip and won't help much in a shorted leads or DC fault situation but I guess NAD feel that their customers won't be needing much protection at their low average power levels.
Although the parts list included fuses F801 and F802, they are not present on the board. There are silkscreen outlines for fuses, but in there are place are jumpers designated J62 and J63. If isolating the tuner section from power is desireable, I could disconnect a leg each from both jumpers
I removed Q431 and Q433 and swapped the 60W DBT bulb for the 100W I used before.
The DC supply voltage to Q431 E & B is +55 volts and C is +43 volts. Supply voltage to Q433 B & E is -55 volts with -43 volts at C.
While removed, I checked both transistors and they tested good.
I removed Q431 and Q433 and swapped the 60W DBT bulb for the 100W I used before.
The DC supply voltage to Q431 E & B is +55 volts and C is +43 volts. Supply voltage to Q433 B & E is -55 volts with -43 volts at C.
While removed, I checked both transistors and they tested good.
Nice going. So it was the bulb rating but I think you should use the lowest rating that enables rail voltages near spec. That means removing other loads and it would not hurt to lose the tuner whilst testing. Perhaps there is another way to disconnect power to it, like disconnecting mains to its transformer but the wiring diagram doesn't look hopeful.
Very very stupid question: is the input volt selector set to right voltage? Did not read the first two pages.
The OP has the specified rail voltages for the main +/- 43V supply rails, either connected normally or with the 100W lightbulb limited supply current, so no problem with the mains voltage. The limiter will glow and reduce the mains voltage if there is a significant load or short though, as intended.
I disconnected the jumpers that are in lieu of fuses F801 and F802. The tuner display is no longer functioning so presumably there is no power going to the tuner section.
I'm somewhat unclear as to which bulb should be in the DBT. As things are now, with the 60W voltages are 50V/40V and 55V/43V with the 100W.
EDIT: I have a 40W if that would be more appropriare.
I'm somewhat unclear as to which bulb should be in the DBT. As things are now, with the 60W voltages are 50V/40V and 55V/43V with the 100W.
EDIT: I have a 40W if that would be more appropriare.
OK, you pulled the tuner and the rail voltages rose from what they were at +/-28V and +/- 20V on the L channel - is that right? If so, that's good now and shows mostly just the different loads of either channel power amplifier now, I think.
Now you have the R channel voltages about right, it still shows there is more to look at in the L channel but at lower test current - still good so far. Now you can leave the 60W bulb in place and go ahead with looking at C-E shorts and Vbe voltages of Q435,7,9,441 if not already done so. Then test the forward voltage drops of D431,3,5,7 and the reverse drops of zeners D431,433. Inspect and check the small resistors around there too. If in doubt, use the voltages in the R channel for comparison. I would assume it's fine and if node voltages including those marked on the schematic are similar, there won't be much of a problem there. Post any obvious voltage discrepancies but others that are similar should be unnecessary.
Now you have the R channel voltages about right, it still shows there is more to look at in the L channel but at lower test current - still good so far. Now you can leave the 60W bulb in place and go ahead with looking at C-E shorts and Vbe voltages of Q435,7,9,441 if not already done so. Then test the forward voltage drops of D431,3,5,7 and the reverse drops of zeners D431,433. Inspect and check the small resistors around there too. If in doubt, use the voltages in the R channel for comparison. I would assume it's fine and if node voltages including those marked on the schematic are similar, there won't be much of a problem there. Post any obvious voltage discrepancies but others that are similar should be unnecessary.
I feel like I'm late with an instalment. Lately there has only been 6 hours between getting home from work and when I must get out of bed to do it again.
I have yet to check the indicated transistors for C-E shorts. I will do that today if in my neighbourhood with a gap between assignment. If not, tonight then. I will give removing and checking them priority.
Regarding the diodes. I was able to locate and check voltages on D431, D433 and D435. But I cannot find D437 anywhere on the board, on the schematic, on the locator diagram or in the parts list.
Not being 100% clear on "forward voltage" I measured across the three I could find in both directions. comparing it to the schematic.
D431 was +\- 8 volts. The single figure on the schematic is 8.2 volts.
D433 was +\- 8 volts. The single figure on the schematic is 8.2 volts.
D435 was +\- .70 volts. This diode is in series with D415. The only figure on the schematic (that is not after a resistor) is on the side of D415 farthest away from D435 and that figure is +(I think) 1.7 volts.
D437 as mentioned previously is not present.
Also as previously mentioned, the voltages of D431 and D433 was + 8 volts on each end of both diodes.
Other than the fusible resistor an initial visual inspection of the board did not reveal damage to any others.
I have yet to check the indicated transistors for C-E shorts. I will do that today if in my neighbourhood with a gap between assignment. If not, tonight then. I will give removing and checking them priority.
Regarding the diodes. I was able to locate and check voltages on D431, D433 and D435. But I cannot find D437 anywhere on the board, on the schematic, on the locator diagram or in the parts list.
Not being 100% clear on "forward voltage" I measured across the three I could find in both directions. comparing it to the schematic.
D431 was +\- 8 volts. The single figure on the schematic is 8.2 volts.
D433 was +\- 8 volts. The single figure on the schematic is 8.2 volts.
D435 was +\- .70 volts. This diode is in series with D415. The only figure on the schematic (that is not after a resistor) is on the side of D415 farthest away from D435 and that figure is +(I think) 1.7 volts.
D437 as mentioned previously is not present.
Also as previously mentioned, the voltages of D431 and D433 was + 8 volts on each end of both diodes.
Other than the fusible resistor an initial visual inspection of the board did not reveal damage to any others.
Update
I managed an opportunity to come home for an hour about lunchtime and removed & tested transistors Q435/37/39/41. The small signal transistors Q439 (2SC2240) and Q441 (2SA970) were OK. Not so for the larger two which I now know are Darlington.
There was voltage present where none should be plus OL where there should have been voltage.
Q435 is a BD680 whereas the service manual says 2SB649 and Q437 is a BD679 not a 2SD699. I wonder if the switch was by design intention or just what was available. In any case, I will use what was on the board to define a replacement.
Just checked now on alltransistors.com and there are 45 substitutes listed for each of these. Any chance you can recommend a couple of pairs which would be contemporary and theoretically easy to get?
And no further visual indication of damage to resistors other than the fusible ones. Mouser have 68 ohm 1/4 watt fusibles but none of the others on the board. Digikey had none at all. Not sure yet what to do about that. Although they don't sell such things, a company in California, Quest Electronics, did offer to make inquiries of their suppliers if I gave them a list.
I managed an opportunity to come home for an hour about lunchtime and removed & tested transistors Q435/37/39/41. The small signal transistors Q439 (2SC2240) and Q441 (2SA970) were OK. Not so for the larger two which I now know are Darlington.
There was voltage present where none should be plus OL where there should have been voltage.
Q435 is a BD680 whereas the service manual says 2SB649 and Q437 is a BD679 not a 2SD699. I wonder if the switch was by design intention or just what was available. In any case, I will use what was on the board to define a replacement.
Just checked now on alltransistors.com and there are 45 substitutes listed for each of these. Any chance you can recommend a couple of pairs which would be contemporary and theoretically easy to get?
And no further visual indication of damage to resistors other than the fusible ones. Mouser have 68 ohm 1/4 watt fusibles but none of the others on the board. Digikey had none at all. Not sure yet what to do about that. Although they don't sell such things, a company in California, Quest Electronics, did offer to make inquiries of their suppliers if I gave them a list.
Hi. Only 6 hours sleep? Something has to give there 
Anyway, you made some more progress with a little light at the end of the tunnel, it seems. I probably confused anyone reading this by numbering the diodes wrongly - I don't know how because I rechecked them before posting. They should have read D421,3,5,7 which would have made more some sense.
As for diode forward voltage drop, a standard silicon diode conducts current only one way, called forward but when it does, there's a small voltage drop across the junction of about 0.7V. Here's the whole story: https://learn.sparkfun.com/tutorials/diodes/real-diode-characteristics
A zener is a little different. It's still a diode with a forward voltage drop but it also has a an induced reverse voltage drop, called the zener voltage, which is adjustable within the manufacturing process to be in a standard range from about 2.5 - 33V or even higher, though precision and thermal stability are poor at higher voltages. Needless to say, whether the diode current is forward or reverse, the voltage measured across its terminals will only be correct in one direction, according with the meter probe polarity.
I'm beginning to wonder if the schematic I'm reading has any semis at all that are the same as in your amplifier, Perhaps someone has been in here before and tried to repair it. Substituting Darlingtons for 2SD669A/B649A is not necessarily wrong but I wonder what the motivation to use them was. It would depend on whether the problem was just a lack of gain since the 4A current rating should plenty for the driver transistors. I would have thought BD139-16, BD140-16 could have done that job quite well. There are Chinese clone 2SD669A/649A available but whilst they work acceptably as low-medium power driver transistors, I have no idea how they compare in ruggedness to the obsolete Hitachi originals. I'd be inclined to avoid them but any comments folks?
Alltransitors tend to give a pretty strange list of substitutes, often missing the most suitable ones completely. Don't trust NTE's substitutes either. The problem here is to decide which parts are original, revised or correct and best suited in the original amplifier - no point match something that failed.
Unless you are convinced that all those semis are original, I would follow the schematic you found which had the BD911/12 part correct at least. If it had 2SD669/BD649 drivers for those, Use high Hfe BD139/40 or those with a 16 suffix. If there is room, TO220 types could be used but I haven't considered what's available yet.
On fusible resistors for DIY repairs, I have to say I find it impractical to refit them. You need big orders to get a range of suitable values and I admit that in my own gear, I have to just jacked standard MO (metal oxide) types up off the PCB by a few mm. So far, I haven't had to deal with too many problems involving those unobtanium fusible resistors anyway.
Anyway, you made some more progress with a little light at the end of the tunnel, it seems. I probably confused anyone reading this by numbering the diodes wrongly - I don't know how because I rechecked them before posting. They should have read D421,3,5,7 which would have made more some sense.
As for diode forward voltage drop, a standard silicon diode conducts current only one way, called forward but when it does, there's a small voltage drop across the junction of about 0.7V. Here's the whole story: https://learn.sparkfun.com/tutorials/diodes/real-diode-characteristics
A zener is a little different. It's still a diode with a forward voltage drop but it also has a an induced reverse voltage drop, called the zener voltage, which is adjustable within the manufacturing process to be in a standard range from about 2.5 - 33V or even higher, though precision and thermal stability are poor at higher voltages. Needless to say, whether the diode current is forward or reverse, the voltage measured across its terminals will only be correct in one direction, according with the meter probe polarity.
I'm beginning to wonder if the schematic I'm reading has any semis at all that are the same as in your amplifier, Perhaps someone has been in here before and tried to repair it. Substituting Darlingtons for 2SD669A/B649A is not necessarily wrong but I wonder what the motivation to use them was. It would depend on whether the problem was just a lack of gain since the 4A current rating should plenty for the driver transistors. I would have thought BD139-16, BD140-16 could have done that job quite well. There are Chinese clone 2SD669A/649A available but whilst they work acceptably as low-medium power driver transistors, I have no idea how they compare in ruggedness to the obsolete Hitachi originals. I'd be inclined to avoid them but any comments folks?
Alltransitors tend to give a pretty strange list of substitutes, often missing the most suitable ones completely. Don't trust NTE's substitutes either. The problem here is to decide which parts are original, revised or correct and best suited in the original amplifier - no point match something that failed.
Unless you are convinced that all those semis are original, I would follow the schematic you found which had the BD911/12 part correct at least. If it had 2SD669/BD649 drivers for those, Use high Hfe BD139/40 or those with a 16 suffix. If there is room, TO220 types could be used but I haven't considered what's available yet.
On fusible resistors for DIY repairs, I have to say I find it impractical to refit them. You need big orders to get a range of suitable values and I admit that in my own gear, I have to just jacked standard MO (metal oxide) types up off the PCB by a few mm. So far, I haven't had to deal with too many problems involving those unobtanium fusible resistors anyway.
I should have searched for the manuals a different way - found the schematic which is annotated with the BD679/680 substitution. It was only after 5,500 units (seems like the other substitution of BD911/912 didn't work out in production testing). The stupid part is that one substitution only appears on parts list, the other is on the side notes of the schematic. This is not a good way to document anything, regardless of the manual's issue date and control system. I'm assuming that the anonymous upload is of only one manual.
Short story is that you're right to replace the fitted parts, as you say. BD681/2 may be easier to find and just as good though. Watch the brands, some available parts (often fakes and generic copies) with those numbers are not up to spec. Use STmicro, On Semi or old SGS, Philips, Siemens, TFK parts from the bin, if you have any. I wonder why 4A max. rated parts failed but the BD911/912 they were driving didn't. Hmm....it suggests they have failed too but any suggestions, anyone?
Short story is that you're right to replace the fitted parts, as you say. BD681/2 may be easier to find and just as good though. Watch the brands, some available parts (often fakes and generic copies) with those numbers are not up to spec. Use STmicro, On Semi or old SGS, Philips, Siemens, TFK parts from the bin, if you have any. I wonder why 4A max. rated parts failed but the BD911/912 they were driving didn't. Hmm....it suggests they have failed too but any suggestions, anyone?
Actually that 6 hours is time at home. Time asleep is included in that 6 hours but does not consume all of it.
And that probably contributes to my having difficulty wrapping my head around measuring the diode voltage drop. Things are much clearer when I've been up for a while and sitting in my truck cab with the notes I've taken and my phone to go online but at home, with the actual hardware in front of me, I sometimes just stare at it and wonder "what was I supposed to be doing again?"
I followed the link you sent and unless I am mistaken (which I frequently am), voltage drop is measured out of circuit? If so, I need to do that later and will do if that step is necessary.
In the meantime, I measured as I had before, in circuit with power on and swapping probe leads on each end:
D421 +/- 8.3 volts
D423 +/- 8.6 volts
D425 +/- .663 volts
D427 +/- .641 volts
I'm glad for your follow up post as I anticipated being able to sub for either 2SB649 / D699 (as per manual) or BD680 / BD679 (as are on the board).
I stopped in at the two parts stores I frequent and thought I had a bit of luck at one. The BD pair was not abvailable but they can order genuine original Hitachi 2SB649 / D699 pairs. The landed cost would have been $30~$32 for 4 and an ETA of 2 to 6 weeks. I am glad i held off.
Your revised recommendation of BD681 and BD682 (neither with a letter/number suffux) are obtainable from Mouser (ST Micro) and I should have them in about a week.
They also have 1/4 watt 68 ohm fusible resistors. I shall get a quantity of those and use where needed. They also have two other values of fusible resistors that are on the board but they are not 4 watt, they're 5 watt. I think I shall get some of those too as the shipping cost will not be greatly affected if at all. If other values of fusible resistors are needed, I will just use the blue metal film ones as you suggested.
I actually may not even need to replace the 4 watt resistors but I'll at least have something on hand if need be.
I did not realize that the Darlington transistors were instead of non-Darlingtons. I was going to say that the solder joints to me looked original but you referenced material saying the change was intentional. One thing I noticed in the Darlington's specs was what I thought a very high hfe of 750. Might that be the reason they went that route?
And that probably contributes to my having difficulty wrapping my head around measuring the diode voltage drop. Things are much clearer when I've been up for a while and sitting in my truck cab with the notes I've taken and my phone to go online but at home, with the actual hardware in front of me, I sometimes just stare at it and wonder "what was I supposed to be doing again?"
I followed the link you sent and unless I am mistaken (which I frequently am), voltage drop is measured out of circuit? If so, I need to do that later and will do if that step is necessary.
In the meantime, I measured as I had before, in circuit with power on and swapping probe leads on each end:
D421 +/- 8.3 volts
D423 +/- 8.6 volts
D425 +/- .663 volts
D427 +/- .641 volts
I'm glad for your follow up post as I anticipated being able to sub for either 2SB649 / D699 (as per manual) or BD680 / BD679 (as are on the board).
I stopped in at the two parts stores I frequent and thought I had a bit of luck at one. The BD pair was not abvailable but they can order genuine original Hitachi 2SB649 / D699 pairs. The landed cost would have been $30~$32 for 4 and an ETA of 2 to 6 weeks. I am glad i held off.
Your revised recommendation of BD681 and BD682 (neither with a letter/number suffux) are obtainable from Mouser (ST Micro) and I should have them in about a week.
They also have 1/4 watt 68 ohm fusible resistors. I shall get a quantity of those and use where needed. They also have two other values of fusible resistors that are on the board but they are not 4 watt, they're 5 watt. I think I shall get some of those too as the shipping cost will not be greatly affected if at all. If other values of fusible resistors are needed, I will just use the blue metal film ones as you suggested.
I actually may not even need to replace the 4 watt resistors but I'll at least have something on hand if need be.
I did not realize that the Darlington transistors were instead of non-Darlingtons. I was going to say that the solder joints to me looked original but you referenced material saying the change was intentional. One thing I noticed in the Darlington's specs was what I thought a very high hfe of 750. Might that be the reason they went that route?
The diode measurements check out there, you don't need to remove any that show an appropriate voltage drop in the forward or reverse (zener) direction. When diodes are fitted across a transistor junction, you do have to lift one leg to check because they may then be confused, being inverse parallel or even parallel connected with another junction.
re the Darlingtons: There seems to have been a change to Euro semis in the design mods after going to production around 1985. Perhaps that reflects on a change of design engineers or where it was made and local parts sources. I think that most available Euro types then would have been standard types such as BD139/40 which could not match the design Hfe (gain) of the specified type without sorting. Now, the BD139-16/140-16 pre-sorted option might do quite well.
I've been reading other forum comments on these receivers too. It seems they were full of dry joints and needed resoldering wherever you saw the faint, dull ring around the leads on the solder side. Sometimes its more distinct: How to spot a dry joint, (fixing relay units or ecu) You could always solder everything in sight but it is kind of
boring and unimformative.
re the Darlingtons: There seems to have been a change to Euro semis in the design mods after going to production around 1985. Perhaps that reflects on a change of design engineers or where it was made and local parts sources. I think that most available Euro types then would have been standard types such as BD139/40 which could not match the design Hfe (gain) of the specified type without sorting. Now, the BD139-16/140-16 pre-sorted option might do quite well.
I've been reading other forum comments on these receivers too. It seems they were full of dry joints and needed resoldering wherever you saw the faint, dull ring around the leads on the solder side. Sometimes its more distinct: How to spot a dry joint, (fixing relay units or ecu) You could always solder everything in sight but it is kind of
boring and unimformative.
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