Sorry, was almost asleep
Yes, I meant to write "the to Power Amp signal" , which is present at the "Preamp Out" jack.
Good eye.
Unfortunately badraven is so desperate by now (this said with a good meaning, I would too in the same situation
) that he does whatever we write here, even if it was meant as an example or a comparison and not an actual repair tip.My "Plan B" is to replace the full power amp with something new and proven.
I suggested in that case removing al Power Amp parts and cleaning the PCB because unfortunately this is an "everything in a huge PCB".
In another amp, as in, say, a Laney one with separate PCBs it's easy to remove the Power one and fit a new one.
In my own amps, which I have been making for 43 years now, for the last 30 years I have been using different Power schematics, from 2N3055 to TIP142/147 to Mos Fets but *all* of them are mountable on the same back panel/heatsink with the same 2" separation screws, and have the same connection wires (transformer/speaker out/ground/audio in) so you can easily pull a dead Power unit, junk it, and install a new one in the same place, same wiring.
In fact one of mine will work there ; I'm not offering you one simply because I'm 6000 Miles away.
I have used them to repair hopelessly burnt Marshalls, Peaveys, Laneys, Gallien Krueger, Acoustic, Crate, etc. , typically after they went through a couple ham fisted "Techs" who think a jackhammer, a chainsaw and a welding torch are proper tools for amp repair.
Oh well.
By the way, Sessionnette 75 amps use a similar idea.
For many years they concentrated on improving the Preamp sound , where the real work is, and used a generic ILP 100W OEM Power Module .
There are both MosFet and Bipolar versions.
Since ILP disappeared some years ago, they became unobtainable, so they designed and made a new module, different technology but which can be mounted instead of the original one, with the same footprint and wiring.
End result? : those wonderful 35 Y.O. SS amps, preferred by Pete Townshend and Jan Akkerman (among others) are still alive and kicking, how's that?
NOTE: *if* you decide to mount a new power amp to the underside of that chassis, I suggest this one:
Velleman, Inc.
Velleman, Inc.
http://www.vellemanusa.com/downloads/0/illustrated/illustrated_assembly_manual_k8060_rev1.pdf
The way it was designed, it's a physically "flat" amp, ideal to build a "Powered cabinet" Plate Amp or what I suggest, on the down side of the chassis.
Although, of course, maybe you can find a used complete working amp for less than that.
R95 R96
I changed R95 R96 from 2.2k to 3k and didn't get much of a difference. Still overheated and it didn't make much of a change time wise.
I changed them to 4.6k and it took about 5 minutes longer for the outputs to overheat. Now if I continue raising this value there would come a point where the transistor would shut off and kill all current in the C-E terminals, thereby shutting the entire circuit off by virtue of no power flow. Since the overheating is a byproduct of too much current flow at the output transistors, the trick here is to figure out why there is too much current flow (I know, that's obvious huh? I'm thinking out loud)
Taking Q15 into consideration: Between E-B we have R96 2.2k Between B-C we have R97 2.2k and R98 10 ohm and D31 BAT85. A diode drops voltage not current, is that correct?
I don't understand what D31 is doing in this circuit. I am attaching the Datasheet for it because I can't find a value for it. Can someone help me with this please.
On a different note: Since Q19 and 21 do not appear to have bias circuits, Q15 appears to set the bias for the entire circuit on the -42V side. Since the overheating is a result of too much current flow, this really shouldn't be this difficult to diagnose should it? On the other hand, this amp worked just fine (for the most part) for 5 + years. So it seems strange that there would be a need to change the bias as much as I did last night to get the change in the time it took to overheat with the new bias resistors.
BR
I changed R95 R96 from 2.2k to 3k and didn't get much of a difference. Still overheated and it didn't make much of a change time wise.
I changed them to 4.6k and it took about 5 minutes longer for the outputs to overheat. Now if I continue raising this value there would come a point where the transistor would shut off and kill all current in the C-E terminals, thereby shutting the entire circuit off by virtue of no power flow. Since the overheating is a byproduct of too much current flow at the output transistors, the trick here is to figure out why there is too much current flow (I know, that's obvious huh? I'm thinking out loud)
Taking Q15 into consideration: Between E-B we have R96 2.2k Between B-C we have R97 2.2k and R98 10 ohm and D31 BAT85. A diode drops voltage not current, is that correct?
I don't understand what D31 is doing in this circuit. I am attaching the Datasheet for it because I can't find a value for it. Can someone help me with this please.
On a different note: Since Q19 and 21 do not appear to have bias circuits, Q15 appears to set the bias for the entire circuit on the -42V side. Since the overheating is a result of too much current flow, this really shouldn't be this difficult to diagnose should it? On the other hand, this amp worked just fine (for the most part) for 5 + years. So it seems strange that there would be a need to change the bias as much as I did last night to get the change in the time it took to overheat with the new bias resistors.
BR
Looks like the amp is finally cured!!!!!
Gajanan Phadte
I don't have a way of testing caps other than to ohm them for a short. I just happened to have two .47uf 50v caps on hand so I replaced C40 and instant WONDERFUL!!!
No more buzzing, the outputs are running about 87 degrees after about 15 minutes of playing on all three channels. And it sounds wonderful! Did I mention that yet? lol Of course I still have the 4.6k resistors at R95 and 96. Is that going to be a problem? I ask because I really like the sound of the amp like this. It seems like it has a fuller sound. Although that could very well be because this amp has always sounds WAY better than my other amp. Maybe I should put the 2.2k resistors back in and see what it sounds like. Hmmm
Anyway I ohmed the old cap and its shorted. So I guess its no wonder the outputs were running hot.
Hopefully this is the end of problems with this amp for a while.
Of course I wish to thank Gajanan for his advice here because it led me to the immediate solution. But I wish to thank all of you for your tireless help. Sorry this went so long. I didn't give up though and I have learned so much from you guys and all the research I have had to do. Thank you thank you thank you! You guys are wonderful!
BR
Time for a beer!
Gajanan Phadte
I don't have a way of testing caps other than to ohm them for a short. I just happened to have two .47uf 50v caps on hand so I replaced C40 and instant WONDERFUL!!!
No more buzzing, the outputs are running about 87 degrees after about 15 minutes of playing on all three channels. And it sounds wonderful! Did I mention that yet? lol Of course I still have the 4.6k resistors at R95 and 96. Is that going to be a problem? I ask because I really like the sound of the amp like this. It seems like it has a fuller sound. Although that could very well be because this amp has always sounds WAY better than my other amp. Maybe I should put the 2.2k resistors back in and see what it sounds like. Hmmm
Anyway I ohmed the old cap and its shorted. So I guess its no wonder the outputs were running hot.
Hopefully this is the end of problems with this amp for a while.
Of course I wish to thank Gajanan for his advice here because it led me to the immediate solution. But I wish to thank all of you for your tireless help. Sorry this went so long. I didn't give up though and I have learned so much from you guys and all the research I have had to do. Thank you thank you thank you! You guys are wonderful!
BR
Time for a beer!
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I changed R95 and 96 back to 2.2k and the sound definitely changed. It was noticeably thinner and there was some low level buzzing coming from the speakers. Also the output transistors got a lot hotter, like 145 degs in a short time.
Anyway, I put the 4.6k resistors back. I guess this means its not really repaired, it basically has a bandaid on it. It still gets up in the 140 deg range when I am cranking the volume but I think that should be expected shouldn't it? I think I am going to install a cooling fan inside like the kind used in a computer power supply. I have red plenty of articles where people say a fan is an excellent idea in any amp. I am putting the amp back together for now. I'm glad to be getting it off my bench.
BR
Anyway, I put the 4.6k resistors back. I guess this means its not really repaired, it basically has a bandaid on it. It still gets up in the 140 deg range when I am cranking the volume but I think that should be expected shouldn't it? I think I am going to install a cooling fan inside like the kind used in a computer power supply. I have red plenty of articles where people say a fan is an excellent idea in any amp. I am putting the amp back together for now. I'm glad to be getting it off my bench.
BR
So the fault was a s/c electrolytic ?
Had you followed my suggested plan then you would have found that pretty quickly.
Take out all SS and test them and while they are out check all passives.
Never mind I guess you were taking instruction from numerous sources.
If one electrolytic is gone then its likely the rest (due to age) are fading away.
I would replace all electrolytics including smoothing caps.
Had you followed my suggested plan then you would have found that pretty quickly.
Take out all SS and test them and while they are out check all passives.
Never mind I guess you were taking instruction from numerous sources.
If one electrolytic is gone then its likely the rest (due to age) are fading away.
I would replace all electrolytics including smoothing caps.
Nigel
Don't get your feelings hurt man, when I began I had no knowledge and it took until recently before things began making some sense. In the beginning a lot of what you guys were saying was more jargon that help for me. I don't remember who it was but one of you said not to worry about the caps because that is rarely the problem.
Didn't make sense to me but what did I know? Sorry if I took the wrong path, I did the best I could with what I had to work with.
I will get the caps ordered so I can replace them all. I still don't know what "s/c" electrolytic is.
I take it putting 4.6k resistors in R95 and 96 isn't a problem.
Thanks!
BR
Don't get your feelings hurt man, when I began I had no knowledge and it took until recently before things began making some sense. In the beginning a lot of what you guys were saying was more jargon that help for me. I don't remember who it was but one of you said not to worry about the caps because that is rarely the problem.
Didn't make sense to me but what did I know? Sorry if I took the wrong path, I did the best I could with what I had to work with.
I will get the caps ordered so I can replace them all. I still don't know what "s/c" electrolytic is.
I take it putting 4.6k resistors in R95 and 96 isn't a problem.
Thanks!
BR
Unless the amp case has lots of large vents then for a power amp you definitely need a fan or two.
I use pc cases for my amps and have 2 fans at opposite corners, one sucks air in while the other blows it out the opposite corner.
It sounds like your bias is at fault for the overheating problem. You need to find the setup procedure for the amp.
A simple approach would be to play a sine wave through the amp and look for crossover distortion on a scope. I was surprised just how little bias you can get away with.
However quite a few people prefer the sound if the bias is run a bit hot.
I use pc cases for my amps and have 2 fans at opposite corners, one sucks air in while the other blows it out the opposite corner.
It sounds like your bias is at fault for the overheating problem. You need to find the setup procedure for the amp.
A simple approach would be to play a sine wave through the amp and look for crossover distortion on a scope. I was surprised just how little bias you can get away with.
However quite a few people prefer the sound if the bias is run a bit hot.
Well I will change the caps as soon as they come in.
When you say it sounds like the bias is at fault. I assume you mean because I had to double the resistance of R95 and 96? That cuts the current doesn't it? Or do I have that backwards?
As for the fans, mine are marked 12v I don't have any experience with venting amps. Will these run off the +16v rail or so I have to mock up a circuit to cut the voltage? Or better yes, is it better to create such a circuit? And will wiring two in series cause enough of a V drop to accomplish this?
When you say it sounds like the bias is at fault. I assume you mean because I had to double the resistance of R95 and 96? That cuts the current doesn't it? Or do I have that backwards?
As for the fans, mine are marked 12v I don't have any experience with venting amps. Will these run off the +16v rail or so I have to mock up a circuit to cut the voltage? Or better yes, is it better to create such a circuit? And will wiring two in series cause enough of a V drop to accomplish this?
I should check C39 as well - C40 on it's own shouldn't cause any ill effects, as it's referenced to ground on both sides.
Nigel Goodwin
Nigel
I'm ordering caps for the amp. C40 may not be the whole problem but things sure changed dramatically when I changed it.
Nigel
I'm ordering caps for the amp. C40 may not be the whole problem but things sure changed dramatically when I changed it.
Nigel
It is great playing my amp again!!!
How do I get the Setup Procedure for my amp? From Fender? If so, they will give this info to non-dealers?
Do you have a suggestion for creating the sine wave you are talking about? Of course I don't have access to a scope for maybe it doesn't matter at this point.
It is great playing my amp again!!!
When you say PC Cases are you talking about a PC power supply to power the fans?
How do I get the Setup Procedure for my amp? From Fender? If so, they will give this info to non-dealers?
Do you have a suggestion for creating the sine wave you are talking about? Of course I don't have access to a scope for maybe it doesn't matter at this point.
There is no "setup procedure", after all there are no adjustments. Fender publishes the schematic, and that is all there is. it is expected any service tech can do what is necessary to check for bias being appropriate and take whatever measures might be required. Ther is no procedural manual. Sorry.
It will be very hard to look for a crossover notch in the waveform without a scope.
Is the overheating happening while the heat sink is bolted to the chassis? Or is the board off the chassis at the time? The board must be mounted to chassis to provide proper cooling.
Q14,15 are thye bias transistors, I forget, are they stock types? And have any of the diodes been substituted? D30,D31 are schottky types, having a low forward voltage drop. Putting a "regular" diode in their place would warm the amp up.
It will be very hard to look for a crossover notch in the waveform without a scope.
Is the overheating happening while the heat sink is bolted to the chassis? Or is the board off the chassis at the time? The board must be mounted to chassis to provide proper cooling.
Q14,15 are thye bias transistors, I forget, are they stock types? And have any of the diodes been substituted? D30,D31 are schottky types, having a low forward voltage drop. Putting a "regular" diode in their place would warm the amp up.
Enzo
That's what I was thinking but I didn't want to contradict.
Last night and the night before I sat here and played the amp for several hours with no problems that I know of.
I have not changed D30 or 31. Not sure how to test them yet.
That's what I was thinking but I didn't want to contradict.
The "primary" heatsink where the outputs are mounted and the thick aluminum block/spacer that goes between the chassis and the mounting for the transistors is what I had for a heat ballast until I mounted everything back in the case the other night. I had clamped a pair of locking pliers on the heatsink to add to the ballast capacity of the sink. The heating problem pretty much went away when I replaced C40.
Last night and the night before I sat here and played the amp for several hours with no problems that I know of.
Q15 was replaced with a KSA1013 I installed a TIP31C at Q14 because that's the only NPN I had with the correct pinout. When my order arrives I will swap in the correct transistor.
I have not changed D30 or 31. Not sure how to test them yet.
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Its easy enough to hear if pure tones are being played.
I actually rather liked the sound of my amp (when it was working) with a little crossover distortion going on.
Badraven, if I counter someone else's stratement, it is not done out of disrespect, it is a point of information, hopefully useful information. We all have things we are used to having, but not all situations comply with those expectations. I sure as heck have made faulty assumptions in trying to post things in forums like this, and unless someone points out my error, it won't be corrected. SO if something seems wrong or you don't understand, say so.