Get a clip lead, a wire with an allegator clip on each end. Put a cable in the inputs, and short the tip to ring of the other end. Does the amp still hum? Problem is Inside the amp.
How old is this amp? if relatively new (under 10 years) then possible problem is a dirty connector or a screw on a metal cover not making contact. Reseat the connectors, make sure all metal cover screws have star washers or at least a clean unoxidized lockwasher.
If over 10 years old, definitely over twenty years old, the electrolytic caps can have leaked out the water past the cracked rubber seals and gone low capacity. The ones in the power supply near the transformer are the most likely ones. A switcher supply (little toroid transformers) runs caps even harder and can wear them out quicker. What brand electrolytic caps in the power supply? Panasonic, nichicon, rubicon, sprague, make some long life quality caps. They also make some short life (cheap seal?) ones. Other brands, ???? The dead 3 year old PCAT power supplies I have contain Other brands. The TB Woods motor drives that I have that ran 12 years inside a hot oven motor compartment before anything failed, had nichicon e-caps. The SEMI motor drives that I have that all failed on cold mornings at about 3-5 years of age with a puddle of cap ooze in the motor box of the conveyor (not real hot environment), contained CDE e-cap's. Oh, BTW, Semi is out of business, the conveyor manufacturer is now substituting Yakima motor drives.
How old is this amp? if relatively new (under 10 years) then possible problem is a dirty connector or a screw on a metal cover not making contact. Reseat the connectors, make sure all metal cover screws have star washers or at least a clean unoxidized lockwasher.
If over 10 years old, definitely over twenty years old, the electrolytic caps can have leaked out the water past the cracked rubber seals and gone low capacity. The ones in the power supply near the transformer are the most likely ones. A switcher supply (little toroid transformers) runs caps even harder and can wear them out quicker. What brand electrolytic caps in the power supply? Panasonic, nichicon, rubicon, sprague, make some long life quality caps. They also make some short life (cheap seal?) ones. Other brands, ???? The dead 3 year old PCAT power supplies I have contain Other brands. The TB Woods motor drives that I have that ran 12 years inside a hot oven motor compartment before anything failed, had nichicon e-caps. The SEMI motor drives that I have that all failed on cold mornings at about 3-5 years of age with a puddle of cap ooze in the motor box of the conveyor (not real hot environment), contained CDE e-cap's. Oh, BTW, Semi is out of business, the conveyor manufacturer is now substituting Yakima motor drives.
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Do you have any instruments or tools or enough safety awareness and education to dig inside and measure voltages and replace components? Even a DC measurement across the output terminals (no signal or speakers connected) is a good start to finding out what ails your amp. It should read somewhere below 50 mV.
This is a fairly big amp, near 35 years old - way past the use-by date for many of the parts, mostly electrolytic capacitors, as Indianajo suggests. Those big electrolytics are what keeps the hum in the transformer, not struggling to power your amplifier, so if they are dried out or blown out, you get lotsa hum and sometimes the whole amp. can go down with them so don't try to use the amp with a load or signal longer than absolutely necessary whilst you routinely replace the main and in fact all the electrolytic capacitors.
It seems half the newb threads on the forum are about replacing the caps in old amplifiers so there will be no shortage of threads to refer to for guidance in choosing caps. I'd say the fault is a major reason they come up for sale in the first place. You can download schematics for parts lists if you need to be sure, yet none of the old stuff will be available new now (which it needs to be) so you will substituting newer, much smaller parts anyway. Likely, it wont hurt to over size on capacitance values up to 100% since the manufacturing tolerance may be that much. Same with voltage ratings which are not important so long as the marked working voltage is reached.
Here's a comprehensive listing from this forum of most Threshold models and references to information you may want.
http://www.diyaudio.com/forums/pass...overview-schematic-collection-all-models.html
This is a fairly big amp, near 35 years old - way past the use-by date for many of the parts, mostly electrolytic capacitors, as Indianajo suggests. Those big electrolytics are what keeps the hum in the transformer, not struggling to power your amplifier, so if they are dried out or blown out, you get lotsa hum and sometimes the whole amp. can go down with them so don't try to use the amp with a load or signal longer than absolutely necessary whilst you routinely replace the main and in fact all the electrolytic capacitors.
It seems half the newb threads on the forum are about replacing the caps in old amplifiers so there will be no shortage of threads to refer to for guidance in choosing caps. I'd say the fault is a major reason they come up for sale in the first place. You can download schematics for parts lists if you need to be sure, yet none of the old stuff will be available new now (which it needs to be) so you will substituting newer, much smaller parts anyway. Likely, it wont hurt to over size on capacitance values up to 100% since the manufacturing tolerance may be that much. Same with voltage ratings which are not important so long as the marked working voltage is reached.
Here's a comprehensive listing from this forum of most Threshold models and references to information you may want.
http://www.diyaudio.com/forums/pass...overview-schematic-collection-all-models.html
That being said, I set my Volt Meter to DC & connect no speakers or interconnects to the amp, thereby measuring the DC voltage across the + & - terminals of each channel. Is that correct ?
I had read in one of the posts is that the speakers need to be connected..
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Yes, correct and no, not with speakers or any signal connected (short the inputs even) because you are trying to see what the static, idle or quiescent condition of the amplifier is. If you load it that way you have a abitrary condition, even allowing background noise into the amp, dependent on your speaker.
The replies made on forums can be wrong because some of us are not versed in SS amplifier design or understand operation at all. Others parrot stuff they read but get the story backwards (hearsay). It happens but sometimes the actual test being described does indeed call for a fixed load but that will be for a dynamic test such as power output, stability, bandwidth etc.
So, what do you read?
The replies made on forums can be wrong because some of us are not versed in SS amplifier design or understand operation at all. Others parrot stuff they read but get the story backwards (hearsay). It happens but sometimes the actual test being described does indeed call for a fixed load but that will be for a dynamic test such as power output, stability, bandwidth etc.
So, what do you read?
So to summarise:-
1. I disconnect the speaker cables from the amp
2. I use Shorted RCA pins on both inputs of the Amp
3. I set the meter on DC & check for voltage across the speaker terminals after switching the amp on..!!
Correct ?
Yes.
Note though - first up, don't worry about shorting the inputs. Check that the readings are low with the meter range at 200 mV or lowest DC range if it's not an autoranging meter. Then, when you have, turn off the amp and fit the input shorts
Power on again - same readings again, power off and fit speakers and how is the hum now? same, less, more?
Note though - first up, don't worry about shorting the inputs. Check that the readings are low with the meter range at 200 mV or lowest DC range if it's not an autoranging meter. Then, when you have, turn off the amp and fit the input shorts
Power on again - same readings again, power off and fit speakers and how is the hum now? same, less, more?
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So u first want me to check the DC at the speaker terminals without speakers connected but without shorting the inputs.
And thereafter connect the speakers & listen to the hum with the inputs shorted..??
Don't try to summarise, it's taking us in circles. Just follow single steps.
1. Set the DMM as above post #7
2. Remove all signal and output connections.
3. Power up
4. Measure DC voltage across output connections. Verify that it is below 50mV or so. If ...not, just record it but don't proceed with further testing.
5. Power down.
6. Short inputs to amplifiers.
7.Power up.
8.Measure DC voltage across output connections.
9.Power down.
10. Connect speakers.
11. Set volume low and power up. Is hum less, same, more?
12. Power down
13. Reconnect amplifier as normal.
Reply when you can
1. Set the DMM as above post #7
2. Remove all signal and output connections.
3. Power up
4. Measure DC voltage across output connections. Verify that it is below 50mV or so. If ...not, just record it but don't proceed with further testing.
5. Power down.
6. Short inputs to amplifiers.
7.Power up.
8.Measure DC voltage across output connections.
9.Power down.
10. Connect speakers.
11. Set volume low and power up. Is hum less, same, more?
12. Power down
13. Reconnect amplifier as normal.
Reply when you can
Speakers & Inputs disconnected :-
Right channel : 10.5mv
Left channel : 22.5 mv
Inputs shorted :-
Right channel : 10.5mv
Left channel : 18.5mv
Speakers connected with inputs shorted:-
Hum remains the same. No increase nor decrease..
mv = AC voltage?
Did you measure the DC voltage as well?
We need to see both AC mV and DC mV in both test conditions (inputs open and inputs shorted) to assess what might be happening. That's eight test readings.
Inputs open:
R x.y mVac, x.ymVdc
L x.y mVac, x.ymVdc
inputs shorted to signal ground:
R x.y mVac, x.ymVdc
L x.y mVac, x.ymVdc
Did you measure the DC voltage as well?
We need to see both AC mV and DC mV in both test conditions (inputs open and inputs shorted) to assess what might be happening. That's eight test readings.
Inputs open:
R x.y mVac, x.ymVdc
L x.y mVac, x.ymVdc
inputs shorted to signal ground:
R x.y mVac, x.ymVdc
L x.y mVac, x.ymVdc
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I measured the DC. Also I had opened the amp a while ago to check for any damaged parts. I found that a Green wire that's connected to the chassis is not connected to the Earth pin of the IEC socket. In fact, nothing is connected to the Earth pin of the socket. Also I have found a pair of Broken resistors in each channel with the colour coding as Orange>Black> Gold >silver which if I'm correct is 3 ohms. These connect to the two legs of all the transistors in each channel. I have taken pictures which I will post from the computer, as I'm writing this from my iPhone now...
I'm concerned too about the disconnected earth (safety earth). However, many modern amplifiers and particularly the source units like media players, CD player, preamplifier etc. don't have earth by design. They should, however, have a "square in square" symbol on the case rear to warn anyone that the device is double insulated and shouldn't be opened as there is no safety earth connection since the circuit earth is floating.
Do you see evidence that there was a connection to the IEC socket earth originally? Don't be surprised if the previous owner tried to remove the earth connection in the hope of solving the hum issue too. This is tricky trying to avoid hum loops in the earth wiring of amplifiers connected to other earthed devices. He probably got it badly wrong and gave up. 'Problem is, it's likely none too safe now, so it should be restored.
Then you can proceed to isolate the real cause which might have something to do with those broken emitter resistors. That's a worry too, since the large output transistors can't work without their emitter leads connected to something like the output terminal , via one of those resistors which should be lower, at about O.3 ohms or just 0R3. Measure a good one with your meter. The associated transistors are probably toast as a result of the damage but at least the remaining transistors seem OK, as the DC offset you measured seems healthy.
At this point, you should decide who is responsible for this fault and whether you should get professional help because the pressure is on now. It's not just a quick-fix tweak that's required and you need schematics and some expertise identifying the faulty parts, selecting and refitting new ones and testing your work. I can't judge your competence to do this but I can tell that you are uncertain of what is what in there.
Do you see evidence that there was a connection to the IEC socket earth originally? Don't be surprised if the previous owner tried to remove the earth connection in the hope of solving the hum issue too. This is tricky trying to avoid hum loops in the earth wiring of amplifiers connected to other earthed devices. He probably got it badly wrong and gave up. 'Problem is, it's likely none too safe now, so it should be restored.
Then you can proceed to isolate the real cause which might have something to do with those broken emitter resistors. That's a worry too, since the large output transistors can't work without their emitter leads connected to something like the output terminal , via one of those resistors which should be lower, at about O.3 ohms or just 0R3. Measure a good one with your meter. The associated transistors are probably toast as a result of the damage but at least the remaining transistors seem OK, as the DC offset you measured seems healthy.
At this point, you should decide who is responsible for this fault and whether you should get professional help because the pressure is on now. It's not just a quick-fix tweak that's required and you need schematics and some expertise identifying the faulty parts, selecting and refitting new ones and testing your work. I can't judge your competence to do this but I can tell that you are uncertain of what is what in there.
I just re-connected the Chassis to the Earth pin on the IEC socket. Trust me it sounds sooo much better. I am uploading some pictures i have taken when i opened the Amp. Yes im sure the previous owner had tried all sorts of weird things in this amp. Such an a**hole i swear. Im sure you will agree once you take a look at the pics..
https://www.dropbox.com/sh/yal9w15nw9544yl/ZMNbM32a0C
Great that you have improvement! Yes, it will likely work fine at low power, even with some of the paralleled output transistors toasted. Set your DMM to lowest resistance range or even to continuity buzzer (if it has one) and read the value in ohms across the resistors (power off) to verify how many need replacement of 5 + 6 in series in the +ve rail (2N5878) and 5+6 in series in the -ve rail (2N5676)that that I count from the schematic.
The big composition emitter resistors still look small for the job but that may be a safety feature. Nelson Pass would likely know whether you should substitute another type as these carbon composition ones are now rare and way too expensive, if that's what they actually are or even need to be.
Looking closer, the type specified on the schematic is 1 ohm, not 0.3 or 3 ohm. If the solder work there looks new, I'd say you have more a**hole work to deal with. You never know, the transistors associated with the damaged resistors may still be OK. Replacing the transistors, which may require matching for a proper restoration, would not be cheap for these obsolete parts and modern, drop-in equivalents may just not be available.
Check out those resistors and carefully measure the base-emitter voltage drop across each power transistor. These are the 2 pins and the collector connection is the case, as you can see by the connections on the PCB to the mounting bolts. NPN will read reverse polarity to PNP types but both should measure around 0.6Volts DC (power on and take care not to slip or short anything as it's curtains if you do.
). If your meter has a diode check feature, you could measure the diode voltage approximately this way, without power, for safety. Obviously, check either way round if you aren't sure whether you are measuring a PN or NP junction.
Note that the transistors and resistors sets are in parallel, so don't be confused by the fact that you could be reading 5 or 6 at once - the resistors need to be the broken ones or unsoldered at one end to get individual transistor measurements. After you check B-E voltages, remove the resistors in one bank at a time of paralleled ones and replace all with equal value ones, rated about 2 Watt as I estimate, depending on what you see as the correct type and correct resistance.
Sorry if it begins to seem vague now, but hacked amps are not easy to sort out without good info. as well as the schematic. Check the schematic posted for your own assurance.
The big composition emitter resistors still look small for the job but that may be a safety feature. Nelson Pass would likely know whether you should substitute another type as these carbon composition ones are now rare and way too expensive, if that's what they actually are or even need to be.
Looking closer, the type specified on the schematic is 1 ohm, not 0.3 or 3 ohm. If the solder work there looks new, I'd say you have more a**hole work to deal with. You never know, the transistors associated with the damaged resistors may still be OK. Replacing the transistors, which may require matching for a proper restoration, would not be cheap for these obsolete parts and modern, drop-in equivalents may just not be available.
Check out those resistors and carefully measure the base-emitter voltage drop across each power transistor. These are the 2 pins and the collector connection is the case, as you can see by the connections on the PCB to the mounting bolts. NPN will read reverse polarity to PNP types but both should measure around 0.6Volts DC (power on and take care not to slip or short anything as it's curtains if you do.
). If your meter has a diode check feature, you could measure the diode voltage approximately this way, without power, for safety. Obviously, check either way round if you aren't sure whether you are measuring a PN or NP junction. Note that the transistors and resistors sets are in parallel, so don't be confused by the fact that you could be reading 5 or 6 at once - the resistors need to be the broken ones or unsoldered at one end to get individual transistor measurements. After you check B-E voltages, remove the resistors in one bank at a time of paralleled ones and replace all with equal value ones, rated about 2 Watt as I estimate, depending on what you see as the correct type and correct resistance.
Sorry if it begins to seem vague now, but hacked amps are not easy to sort out without good info. as well as the schematic. Check the schematic posted for your own assurance.
Great that you have improvement! Yes, it will likely work fine at low power, even with some of the paralleled output transistors toasted. Set your DMM to lowest resistance range or even to continuity buzzer (if it has one) and read the value in ohms across the resistors (power off) to verify how many need replacement of 5 + 6 in series in the +ve rail (2N5878) and 5+6 in series in the -ve rail (2N5676)that that I count from the schematic.
The big composition emitter resistors still look small for the job but that may be a safety feature. Nelson Pass would likely know whether you should substitute another type as these carbon composition ones are now rare and way too expensive, if that's what they actually are or even need to be.
Looking closer, the type specified on the schematic is 1 ohm, not 0.3 or 3 ohm. If the solder work there looks new, I'd say you have more a**hole work to deal with. You never know, the transistors associated with the damaged resistors may still be OK. Replacing the transistors, which may require matching for a proper restoration, would not be cheap for these obsolete parts and modern, drop-in equivalents may just not be available.
Check out those resistors and carefully measure the base-emitter voltage drop across each power transistor. These are the 2 pins and the collector connection is the case, as you can see by the connections on the PCB to the mounting bolts. NPN will read reverse polarity to PNP types but both should measure around 0.6Volts DC (power on and take care not to slip or short anything as it's curtains if you do.
Note that the transistors and resistors sets are in parallel, so don't be confused by the fact that you could be reading 5 or 6 at once - the resistors need to be the broken ones or unsoldered at one end to get individual transistor measurements. After you check B-E voltages, remove the resistors in one bank at a time of paralleled ones and replace all with equal value ones, rated about 2 Watt as I estimate, depending on what you see as the correct type and correct resistance.
Sorry if it begins to seem vague now, but hacked amps are not easy to sort out without good info. as well as the schematic. Check the schematic posted for your own assurance.
I tried measuring the resistors last evening, but since it were in the circuit, it showed Zero. I wil try removing one leg & measure again. I have never measured transistors ever, so I guess I need to read a bit on it. Also, shall I change all the resistors to 1 ohm 2 watt ones?
The previous owner has tampered the traces as well so he has looped it with pieces of wire. What an a**hole again. I think he has changed a lot in the amp, including the hot lead going from the IEC inlet to the fuse to the switch with an ordinary 18 gauge wire. The Neutral lead is around 14 gauge. Earth was 20 gauge & disconnected. I need to re-do this area as well I guess...
Take real care with mains wiring. Even if your local power is only 120V, the potential and current capacity are still quite enough to deep-fry anyone. I guess you know what your regs are about wiring and usually the Earth wire cross-sectional area is at least equal to the largest conductor. That is the wire that should never burn even when the A and N have melted to copper pools.
I'm unsure whether the substitution of 3R resistors was a widely accepted modification or not. It seems utterly stupid to throttle your amp that way unless you only wanted to drive high impedance loads or nurse a sick amplifier.
What you have, looking at the schematic, is a row of parallel NPN transistors attached at the collectors to the positive rail and their emitters are commoned to a second row with 1R resistors. The top 5 and their driver transistor, are thus in series with the second 6 and their driver. Then a parallel combination of a power diode and another 1R resistor connects the the second row of commoned emitter resistors to the output node. There is then a mirrored, PNP assembly, same as this one, from the negative rail, also to the output node. I assume you have found the schematic in that listing and looked at it, despite the weak and scratchy image. Otherwise it won't make sense explaining anything.
This is one fancy and heavy-duty way of getting high power out of low voltage transistors without risking stability or poor load-sharing. Just look at all those redundant power transistors (by todays standards)!
I see no problem with fitting 1R0 emitter resistors provided you replace any damaged transistors to be certain the load will be safely shared. I suggest Metal Oxide (MO) type, 2-3 Watt, not carbon composition or metal film (MF). Failing that, use low inductance, wire wound types if available. Parallel transistor applications require bullet-proof sharing precautions or risk heavy damage. That means same parts, all working and adequately matched. You could suspect some to be damaged, which might explain the 3R resistors to reduce power but who knows?
Measuring transistors is no different to any other part, as long as you just connect the meter safely to the 2 pins, without shorting so you can measure the voltage, Vbe. IC clips fitted to the meter leads are ideal if you can't be careful enough holding contact. The pins are always as per previous post, Base and Emitter. Download a datasheet from a supplier's on-line catalogue to verify any connections. Expect about 0.6V with a functional transistor but this may be lower due to the arrangement here. Zero, high or very low voltage and different to the other transistors likely means a toasted part. Obviously, use the lowest DC range again.
I'm unsure whether the substitution of 3R resistors was a widely accepted modification or not. It seems utterly stupid to throttle your amp that way unless you only wanted to drive high impedance loads or nurse a sick amplifier.
What you have, looking at the schematic, is a row of parallel NPN transistors attached at the collectors to the positive rail and their emitters are commoned to a second row with 1R resistors. The top 5 and their driver transistor, are thus in series with the second 6 and their driver. Then a parallel combination of a power diode and another 1R resistor connects the the second row of commoned emitter resistors to the output node. There is then a mirrored, PNP assembly, same as this one, from the negative rail, also to the output node. I assume you have found the schematic in that listing and looked at it, despite the weak and scratchy image. Otherwise it won't make sense explaining anything.
This is one fancy and heavy-duty way of getting high power out of low voltage transistors without risking stability or poor load-sharing. Just look at all those redundant power transistors (by todays standards)!
I see no problem with fitting 1R0 emitter resistors provided you replace any damaged transistors to be certain the load will be safely shared. I suggest Metal Oxide (MO) type, 2-3 Watt, not carbon composition or metal film (MF). Failing that, use low inductance, wire wound types if available. Parallel transistor applications require bullet-proof sharing precautions or risk heavy damage. That means same parts, all working and adequately matched. You could suspect some to be damaged, which might explain the 3R resistors to reduce power but who knows?
Measuring transistors is no different to any other part, as long as you just connect the meter safely to the 2 pins, without shorting so you can measure the voltage, Vbe. IC clips fitted to the meter leads are ideal if you can't be careful enough holding contact. The pins are always as per previous post, Base and Emitter. Download a datasheet from a supplier's on-line catalogue to verify any connections. Expect about 0.6V with a functional transistor but this may be lower due to the arrangement here. Zero, high or very low voltage and different to the other transistors likely means a toasted part. Obviously, use the lowest DC range again.
Take real care with mains wiring. Even if your local power is only 120V, the potential and current capacity are still quite enough to deep-fry anyone. I guess you know what your regs are about wiring and usually the Earth wire cross-sectional area is at least equal to the largest conductor. That is the wire that should never burn even when the A and N have melted to copper pools.
I'm unsure whether the substitution of 3R resistors was a widely accepted modification or not. It seems utterly stupid to throttle your amp that way unless you only wanted to drive high impedance loads or nurse a sick amplifier.
What you have, looking at the schematic, is a row of parallel NPN transistors attached at the collectors to the positive rail and their emitters are commoned to a second row with 1R resistors. The top 5 and their driver transistor, are thus in series with the second 6 and their driver. Then a parallel combination of a power diode and another 1R resistor connects the the second row of commoned emitter resistors to the output node. There is then a mirrored, PNP assembly, same as this one, from the negative rail, also to the output node. I assume you have found the schematic in that listing and looked at it, despite the weak and scratchy image. Otherwise it won't make sense explaining anything.
This is one fancy and heavy-duty way of getting high power out of low voltage transistors without risking stability or poor load-sharing. Just look at all those redundant power transistors (by todays standards)!
I see no problem with fitting 1R0 emitter resistors provided you replace any damaged transistors to be certain the load will be safely shared. I suggest Metal Oxide (MO) type, 2-3 Watt, not carbon composition or metal film (MF). Failing that, use low inductance, wire wound types if available. Parallel transistor applications require bullet-proof sharing precautions or risk heavy damage. That means same parts, all working and adequately matched. You could suspect some to be damaged, which might explain the 3R resistors to reduce power but who knows?
Measuring transistors is no different to any other part, as long as you just connect the meter safely to the 2 pins, without shorting so you can measure the voltage, Vbe. IC clips fitted to the meter leads are ideal if you can't be careful enough holding contact. The pins are always as per previous post, Base and Emitter. Download a datasheet from a supplier's on-line catalogue to verify any connections. Expect about 0.6V with a functional transistor but this may be lower due to the arrangement here. Zero, high or very low voltage and different to the other transistors likely means a toasted part. Obviously, use the lowest DC range again.
Oh so the use of 3R resistors instead of the factory-rated 1R will reduce output power??- I will order MO 1R resistors & change the entire range. I live in India & the voltage is 230v. There is no Earth connection going to the transformer. Neither did I see one in the Transformer schematic. The Earth wire from the IEC inlet was connected to a screw in the chassis, which I guess was later removed by the previous owner. Refitting it back improved things quite a bit.
Another things is that the front LED was working when I bought this amp last week. As the amp was playing, it suddenly went away & did not work since then.
Take real care with mains wiring. Even if your local power is only 120V, the potential and current capacity are still quite enough to deep-fry anyone. I guess you know what your regs are about wiring and usually the Earth wire cross-sectional area is at least equal to the largest conductor. That is the wire that should never burn even when the A and N have melted to copper pools.
I'm unsure whether the substitution of 3R resistors was a widely accepted modification or not. It seems utterly stupid to throttle your amp that way unless you only wanted to drive high impedance loads or nurse a sick amplifier.
What you have, looking at the schematic, is a row of parallel NPN transistors attached at the collectors to the positive rail and their emitters are commoned to a second row with 1R resistors. The top 5 and their driver transistor, are thus in series with the second 6 and their driver. Then a parallel combination of a power diode and another 1R resistor connects the the second row of commoned emitter resistors to the output node. There is then a mirrored, PNP assembly, same as this one, from the negative rail, also to the output node. I assume you have found the schematic in that listing and looked at it, despite the weak and scratchy image. Otherwise it won't make sense explaining anything.
This is one fancy and heavy-duty way of getting high power out of low voltage transistors without risking stability or poor load-sharing. Just look at all those redundant power transistors (by todays standards)!
I see no problem with fitting 1R0 emitter resistors provided you replace any damaged transistors to be certain the load will be safely shared. I suggest Metal Oxide (MO) type, 2-3 Watt, not carbon composition or metal film (MF). Failing that, use low inductance, wire wound types if available. Parallel transistor applications require bullet-proof sharing precautions or risk heavy damage. That means same parts, all working and adequately matched. You could suspect some to be damaged, which might explain the 3R resistors to reduce power but who knows?
Measuring transistors is no different to any other part, as long as you just connect the meter safely to the 2 pins, without shorting so you can measure the voltage, Vbe. IC clips fitted to the meter leads are ideal if you can't be careful enough holding contact. The pins are always as per previous post, Base and Emitter. Download a datasheet from a supplier's on-line catalogue to verify any connections. Expect about 0.6V with a functional transistor but this may be lower due to the arrangement here. Zero, high or very low voltage and different to the other transistors likely means a toasted part. Obviously, use the lowest DC range again.
I removed one leg of the Resistors & measured them. They measure 1 ohm as per the schematic. I guess the first colour band has turned from Brown to almost orange & that's why I thought it must have been changed to 3R.
Measured the Transistors as well. Almost all measure 0.56 to 0.6v DC, except one in each channel that measure 6.8vdc. Is this normal ? I also replaced the one broken resistor on the left channel with a 1R 2 watt. The Hum has reduced quite a bit, but I can see another broken resistor on the right channel as well (though not as bad as the one I replaced). There seem to be a total of 48 resistors altogether, one for each transistor. Is it worthwhile changing all of them to 1R Mills MRA-5 ? The original resistors seem to be made of some wood-kind & are easily broken. They have a thin wire wound inside..
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