Read preceding posts for comments on in-circuit testing, bias setting and rechecking your work, Mikedrz.
DC coupled amplifiers can collapse like dominoes so you must check both outputs and the driver transistors. That may not be the end of it either. You have to remove them eventually as it takes a bit more skill and knowledge to be certain of transistors in-circuit.
For safety, current on first power-up needs to be limited to mere tens of mA to protect as a bulb would whilst the bias is adjusted. IMHO, this will be simpler with a bulb than regulated supplies. If the bias does not set simply within the midrange of adjustment, something is wrong. Fix it by checking circuit and critical parts, colour codes, the lot. Then you can consider applying full power and readjusting the bias.
DC coupled amplifiers can collapse like dominoes so you must check both outputs and the driver transistors. That may not be the end of it either. You have to remove them eventually as it takes a bit more skill and knowledge to be certain of transistors in-circuit.
For safety, current on first power-up needs to be limited to mere tens of mA to protect as a bulb would whilst the bias is adjusted. IMHO, this will be simpler with a bulb than regulated supplies. If the bias does not set simply within the midrange of adjustment, something is wrong. Fix it by checking circuit and critical parts, colour codes, the lot. Then you can consider applying full power and readjusting the bias.
Ian, thank you. I will research further. I guess there is no better way to learn than to break something and have to fix it. The amp was working fine with pot in the middle earlier, I am pretty sure turning it to one extreme is caused this disaster. The book didn't mention any precautions; and simply said "adjust the trimmer pot until the voltage is at a steady 47 mV.
IMO, it's very likely that Q16 and Q17 died together. Possibly Q14 and/or Q15 as well. I would at least check Q17.
That bias circuit looks horribly sensitive. As you turn the trimmer, I would expect nothing to happen until it's at about 70%, then then suddenly the current shoots up fast and... well, you already found out what happens next.
The trick is to go sloooowly. The low-current safe position is with the wiper near the top in the circuit diagram, so Q9's base is connected to it's collector. The high-current "burn-out" setting is with the wiper rotated "down" so Q9's base is connected to it's emitter.
Good luck
Attachments
That is not the way to wire up the pot to a Vbe multiplier.
That is asking for trouble.
Fit a fixed resistor (R1) from base to collector to replace the upper half of the pot.
Fit a series pair of pot (VR2) + fixed resistor (R3) from base to emitter. That replaces the lower half of the pot.
The sum of R1+R3 should be ~ = Old pot value. R1~4*R3
New pot value = Old pot value.
if Old pot = 2k2 then R1 ~ 2k, VR2 = 2k2, V3 ~ 560r
This will give a range of Vbe voltage of ({2k / [2k2+560] } + 1) * Vbe ~ 1.7*Vbe when pot is adjusted to max.
Upto ({2k / 560} +1 ) * Vbe ~ 4.5 * Vbe.
Note, that the range 1.7Vbe to 4.5Vbe suits driving a two stage Driver + Output double EF stage.
If you are driving a CFP or outputs direct then the 2k needs to be ~ half that value, i.e. 1k0
If your are driving a three stage Pre-driver + Driver + Output EF stage then the 560r needs to be lower. 360r gives <=6.5Vbe
That is asking for trouble.
Fit a fixed resistor (R1) from base to collector to replace the upper half of the pot.
Fit a series pair of pot (VR2) + fixed resistor (R3) from base to emitter. That replaces the lower half of the pot.
The sum of R1+R3 should be ~ = Old pot value. R1~4*R3
New pot value = Old pot value.
if Old pot = 2k2 then R1 ~ 2k, VR2 = 2k2, V3 ~ 560r
This will give a range of Vbe voltage of ({2k / [2k2+560] } + 1) * Vbe ~ 1.7*Vbe when pot is adjusted to max.
Upto ({2k / 560} +1 ) * Vbe ~ 4.5 * Vbe.
Note, that the range 1.7Vbe to 4.5Vbe suits driving a two stage Driver + Output double EF stage.
If you are driving a CFP or outputs direct then the 2k needs to be ~ half that value, i.e. 1k0
If your are driving a three stage Pre-driver + Driver + Output EF stage then the 560r needs to be lower. 360r gives <=6.5Vbe
Godfrey, you have quite the talent for visualizing the problem from a simple explanation and a quick look at the schematic.
After giving my solder pump quite the workout, I found that all of the power transistors tested fine. Aside from Q16 as previously mentioned. I've tested Q17 the PNP out of the pair, and it seems to show a reading from emitter to collector for one second and then goes to zero. I am going to replace it as a precaution.
I have visited several local suppliers and none of them have these transistors in stock. So I'm once again waiting on the mail, just put an order in with Digikey.
AndrewT at this point I no longer have the patience to hack at the PCB board any longer. haha. I will have to tread very lightly this time and hope I don't have to go through this again. At least with this project.
I will not give up just yet, and will keep you all up to date on my progress, many thanks to you all.
After giving my solder pump quite the workout, I found that all of the power transistors tested fine. Aside from Q16 as previously mentioned. I've tested Q17 the PNP out of the pair, and it seems to show a reading from emitter to collector for one second and then goes to zero. I am going to replace it as a precaution.
I have visited several local suppliers and none of them have these transistors in stock. So I'm once again waiting on the mail, just put an order in with Digikey.
AndrewT at this point I no longer have the patience to hack at the PCB board any longer. haha. I will have to tread very lightly this time and hope I don't have to go through this again. At least with this project.
I will not give up just yet, and will keep you all up to date on my progress, many thanks to you all.
Mikedrz, you can use a number of transistor types in Randy's designs. He picked 6 types to use in all his BJT designs to keep inventory down but you can use BC546/56 pairs or MPSA06/56 pairs or many others to replace 2N5401/5551 here without problems. Since the RENESAS drivers are almost unobtanium or fakes now, they will need to be substituted soon too.
The plan was to make it as cheap as possible so chasing his specific parts at higher cost is not what he intended, I'm sure.
The plan was to make it as cheap as possible so chasing his specific parts at higher cost is not what he intended, I'm sure.
Thanks Ian, I was going to ask about substitutes as the parts in his designs are out dated. Digikey has lightening fast shipping. I got my parts overnight. I've replaced the parts and have been trying to set the current bias. It doesn't seem to want to stay steady and keeps creeping up on its own.
I set it as best I could, and hooked it up to a speaker. It was working alright except for an annoying buzzing in the background. After about 5 minutes there was another melt down. I looks like the amp is hitting thermal runaway. It got hot enough to melt the mica spacer. I turned it off as fast as I could and disconnected the speaker. I let everything cool down and turned the current bias to min.
When I turned it back on it was still working, but the output transistors started to warm up again and I decided to disconnect it from the power supply.
There is obviously something not right here. It would work well with the current limiting supply for an hour straight, most likely because it will limit the current to 1.5 amps.
I set it as best I could, and hooked it up to a speaker. It was working alright except for an annoying buzzing in the background. After about 5 minutes there was another melt down. I looks like the amp is hitting thermal runaway. It got hot enough to melt the mica spacer. I turned it off as fast as I could and disconnected the speaker. I let everything cool down and turned the current bias to min.
When I turned it back on it was still working, but the output transistors started to warm up again and I decided to disconnect it from the power supply.
There is obviously something not right here. It would work well with the current limiting supply for an hour straight, most likely because it will limit the current to 1.5 amps.
Mikedrz,welcome to the show. it would seem you once had this amp working, but you've now got a little problem of overheating.. one question, did you check the current draw when the amp was first working before all started to go wrong?...
some times, we can use use limiters in each supply rail to aid fault finding.. simple high watt resistors are best for this job, say 100 ohms in each supply and feel if they get hot at power up. In a working 'amp' they should stay cold and if there's a heavy short they'll heat fast.... and we've all used 60w/100w bulb within the primary side of the power transformer to check for shorts etc...
some times, we can use use limiters in each supply rail to aid fault finding.. simple high watt resistors are best for this job, say 100 ohms in each supply and feel if they get hot at power up. In a working 'amp' they should stay cold and if there's a heavy short they'll heat fast.... and we've all used 60w/100w bulb within the primary side of the power transformer to check for shorts etc...
I am almost ready to throw this thing out of the window. The only thing I can think of is that the biasing transistor is not on the same heat sink as the output transistors; they all have their own heatsinks. I also did not use a 10 ohm resistor to connect signal common. I've connected it directly to common of the power supply.
it worked before, just retrace things...sometime we've all spent many a long hour working on peace's of equipment that's just refused to work via changing this and that..but, in the end it pay's off to keep at it and get it back to working order..it's all part of service/fault/repairs.
just go back over things and check to see if you've not put any part in the wrong way round.
just go back over things and check to see if you've not put any part in the wrong way round.
At last...the big mistake!
!!To have thermal regulation you have to sense something with that bias generator transistor. It should ideally be clamped on top of one of the output transistors but it will work like most Emitter Follower designs like this by clamping to the heatsink between the outpuit transistors. That is what it's for - to prevent thermal runaway by pegging back bias voltage as the output stage heats up.
I suspect you may not have seen or understood the full text of this design - it won't survive without this feature or a very close physical approximation.
So get the output transistors onto a single heatsink and place the bias transistor as close as poss. but still insulated like the rest with a pad. or mica washer and silicone grease (not the conductive sort used on computers).
Lead length may be a problem but a short couple of extra cm. to its leads won't hurt if it now has a chance to do its job. Then see if drifts - not likely!
There is always a mistake. haha I read the text several times but it seemed to not emphasize these important parts. It stated the transistor should ideally be on the same heat sink for thermal tracking. Thats it, no mention that the design was dependant on it.
Thank you, I will make the necessary changes. What about the 10 ohm resistor, is that a must?
Thank you, I will make the necessary changes. What about the 10 ohm resistor, is that a must?
Hi again
The 10 ohm "earth lift" resistor is a good idea in many amplifiers, to reduce hum arising from small earth currents flowing into the input from the source at very low impedance. This helps keep amps quieter but is not essential in an ideal system or even with a floating source, like a battery powered type, where you don't need it.
You also don't need to place the drivers on the main heatsink - they will only need small sinks anyway, as you likely have in use now.
If you are having difficulty with 2SD 669/B 649 sources, BD139/140 will work there OK with this design and BD139 will also work as the bias transistor. I don't know what brands you you can obtain easiest but so far, I've quoted you types that On-semi produce as I believe they are widely available in Canada.
The 10 ohm "earth lift" resistor is a good idea in many amplifiers, to reduce hum arising from small earth currents flowing into the input from the source at very low impedance. This helps keep amps quieter but is not essential in an ideal system or even with a floating source, like a battery powered type, where you don't need it.
You also don't need to place the drivers on the main heatsink - they will only need small sinks anyway, as you likely have in use now.
If you are having difficulty with 2SD 669/B 649 sources, BD139/140 will work there OK with this design and BD139 will also work as the bias transistor. I don't know what brands you you can obtain easiest but so far, I've quoted you types that On-semi produce as I believe they are widely available in Canada.
A note on my post above. I suggested BD139/140 in lieu of the drivers but didn't see the rail voltage. At +/- 42V, it's just too high for them, sorry. There are plenty of other drivers but availability is not so good on many. I find MJE 243/253 good and tough but there are newer, faster Japanese types if you wish to improve on it.
What supply voltage are you using here Mikedrz?
What supply voltage are you using here Mikedrz?
Hello sir, I am using 2 24 V @ 2 A transformers with their secondaries in series. For the caps I've got 2 x 6800 uF @ 50 V. The rectifier is a 200 v @ 4 amp bridge. The rail voltage comes out to about 36 volts with a load on it.
I have fastened the bias transistor to one of the outputs and the current is finally stabilizing somewhat. However I think its too far gone and one of the transistors may be bad again. It just doesn't sound right. The buzzing sound just grows louder and comes and goes, also only one of the output transistors gets hot, while the other is almost cold to the touch.
I have fastened the bias transistor to one of the outputs and the current is finally stabilizing somewhat. However I think its too far gone and one of the transistors may be bad again. It just doesn't sound right. The buzzing sound just grows louder and comes and goes, also only one of the output transistors gets hot, while the other is almost cold to the touch.
Hi
Thanks for info.on the power supply. At 36V rails, there will be no problems with the alternatives I gave you but above 40V rails, things change and higher ratings than the common types are usually necessary. The small transitors would best be MPSA42/92 for instance.
I have only seen the effect you describe on a couple of repairs to commercial amplifiers on which I routinely replaced both transistors so I can't be precise in saying what is wrong and what may be about to fail also.
When semis fail, it can be in with different effects and junctions behaving erratically so it is not always clear what is about to get worse or is just a problem elsewhere. Without testing them at a working current like half an amp, the few milliamps of tests with your DVM means very little with power transistors. Usually, we wind up having to replace the lot anyway or get serious and build/buy a tester just for power transistors.
I can only suggest that's likely the case but test the drivers as best you can too. As explained, these things collapse like dominoes, burning or at least stressing preceding and series connected semis. There is also a chance of oscillation with the bias transistor in its new location, so try moving the leads about by skewing the mounting and see if there is a change in temp.
Really, the correct bias is quite small and niether transistor should get hot on a proper size heatsink. Here, a rating of 2 degrees C/Watt would be minimal and the pairs of transistors should heat up the same, if much at all with the right bias.
Keep at it but if you want to save burning up transistor after transistor, you must have current limiting like the methods suggested, whilst ever the problems remain.
Thanks for info.on the power supply. At 36V rails, there will be no problems with the alternatives I gave you but above 40V rails, things change and higher ratings than the common types are usually necessary. The small transitors would best be MPSA42/92 for instance.
I have only seen the effect you describe on a couple of repairs to commercial amplifiers on which I routinely replaced both transistors so I can't be precise in saying what is wrong and what may be about to fail also.
When semis fail, it can be in with different effects and junctions behaving erratically so it is not always clear what is about to get worse or is just a problem elsewhere. Without testing them at a working current like half an amp, the few milliamps of tests with your DVM means very little with power transistors. Usually, we wind up having to replace the lot anyway or get serious and build/buy a tester just for power transistors.
I can only suggest that's likely the case but test the drivers as best you can too. As explained, these things collapse like dominoes, burning or at least stressing preceding and series connected semis. There is also a chance of oscillation with the bias transistor in its new location, so try moving the leads about by skewing the mounting and see if there is a change in temp.
Really, the correct bias is quite small and niether transistor should get hot on a proper size heatsink. Here, a rating of 2 degrees C/Watt would be minimal and the pairs of transistors should heat up the same, if much at all with the right bias.
Keep at it but if you want to save burning up transistor after transistor, you must have current limiting like the methods suggested, whilst ever the problems remain.
Mikedrz, driver transistors can also be tip41's and tip42's these are 100v at about 6amp..yes you can use mj15030/31 and for small signal long tail i/p can be general type 2n5401 2n5551 or mpsa 92/42's these are 300v @500ma there pin out are e b c flat side facing upwards.
Beef up your bridge diode unit as 4amp type is bare min on that supply rail current of 2 amp aside...buy or salvage one from audio equipment, you can get blot down types with spade connections.
when building a power supply for audio work it's best to up grade things as that way it will not sag so much when the amplifier is being pushed hard.
regarding biasing the o/p transistors, what voltage drops was you reading across the emitter resistors? did you follow the set in the Randy's book? there's also a quick way of checking current draw via placing your meter in series with the + rail and adjust bias trim pot. do you have a scope/ generator? have you checked the remaining parts on the pcb for damage such as o/c resistors. anyway as stated keep the limiter's on till the circuit is working or your burn more parts up!
Beef up your bridge diode unit as 4amp type is bare min on that supply rail current of 2 amp aside...buy or salvage one from audio equipment, you can get blot down types with spade connections.
when building a power supply for audio work it's best to up grade things as that way it will not sag so much when the amplifier is being pushed hard.
regarding biasing the o/p transistors, what voltage drops was you reading across the emitter resistors? did you follow the set in the Randy's book? there's also a quick way of checking current draw via placing your meter in series with the + rail and adjust bias trim pot. do you have a scope/ generator? have you checked the remaining parts on the pcb for damage such as o/c resistors. anyway as stated keep the limiter's on till the circuit is working or your burn more parts up!
I have one of those bigger bridges in my current limiting power supply. I have a scope out on loan from my cousin. I am going to take a look at the tip41/42's as they're probably easier to mount to a heat sink.
For now I think I am going to give this a rest before I lose my patience haha. I am over budget for parts and the wife is starting to get upset when another package arrives every other day.
For now I think I am going to give this a rest before I lose my patience haha. I am over budget for parts and the wife is starting to get upset when another package arrives every other day.
Fair enough reason, Mikedrz
Here's hoping Christmas season finds you with some spare time and some cheap spare parts.
Here are some options to cut costs if you have to continue the hard way:
Outputs: 2N3055/MJ2955 or their plastic equivalents
Drivers: BD139/BD140 You can use others if cost is not the problem but these will be cheaper and better with their much higher Ft (gain-bandwidth product)
This will save a bundle on spares whilst you sort out the problems. You then fit the recommended parts when it's working properly. good luck and
Here's hoping Christmas season finds you with some spare time and some cheap spare parts.
Here are some options to cut costs if you have to continue the hard way:
Outputs: 2N3055/MJ2955 or their plastic equivalents
Drivers: BD139/BD140 You can use others if cost is not the problem but these will be cheaper and better with their much higher Ft (gain-bandwidth product)
This will save a bundle on spares whilst you sort out the problems. You then fit the recommended parts when it's working properly. good luck and
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.