Peavey Bass 400 Series A: Short with Load Attached

[Neutronarmy]

Hello!

I've been working through a Peavey Bass 400 Series A that came to me from a friend with a few issues. The original owner had attempted to repair the amp and did so pretty shoddily. A friend of mine picked it up and replaced the power cord and filter caps. However, one filter cap - that on the -40V line - was replaced backwards and exploded. Prior to the cap replacement the amplifier worked.

I replaced the blown filter cap with a properly oriented one and replaced the rectifier diodes as one tested bad in-circuit and I wasn't receiving -40V. The output transistors were also replaced as one was shorted (though that transistor may be unrelated and my fault during the testing process). Currently when running the amplifier through a light bulb limiter it registers as fine without a load attached, but once a load is attached it registers as a short. Currently I read roughly +40V, -40V, and +24V where expected; I say roughly as I have done the readings with the limiter in place.

Any ideas on what might be the culprit? Thanks in advance for any help!
Chris

[Jazz P Bass]

Is there any Vdc on the output without a load?

[Enzo]

Adding: whenever you are blowing fuses, work WITHOUT a speaker load on a solid state amp. Only once the thing is stable and holds a fuse that way AND there is no DC voltage on the output, only then will we connect a load.

If you replaced an output transistor, you must always check all resistors associtated with that transistor. And any time an output fails, it is not at all unlikley for a driver transistor to fail too.

SInce your thing only blows fuses with a load, I also suspect you have DC on the output.

[Neutronarmy]

Adding: whenever you are blowing fuses, work WITHOUT a speaker load on a solid state amp. Only once the thing is stable and holds a fuse that way AND there is no DC voltage on the output, only then will we connect a load.

If you replaced an output transistor, you must always check all resistors associtated with that transistor. And any time an output fails, it is not at all unlikley for a driver transistor to fail too.

SInce your thing only blows fuses with a load, I also suspect you have DC on the output.

Tomorrow I will double check the resistors around the output transistors and see if there are any problems. I replaced the whole gang of them when the problem arose, so I'm not 100% which was shorted.

Last time I measured (and I will once again tomorrow) I didn't have DC on the output without a load. I have only attached a load with a light-bulb current limiter inline when I thought that all was okay. Upon adding the load - with limiter still inline - the bulb lights as if a short was present. Thus I have never blown a fuse as I didn't remove the limiter once the short seemed present. But again, I will double check and report most current and accurate reports tomorrow.

Thanks again everyone for the help!

[J M Fahey]

Do you turn your amp on first and after that (at least 15 seconds ) you add the load?
Or add the load first and then turn the amp on?
Not the same.
In both cases, *through* the lamp limiter.

[Neutronarmy]

Do you turn your amp on first and after that (at least 15 seconds ) you add the load?
Or add the load first and then turn the amp on?
Not the same.
In both cases, *through* the lamp limiter.

I've always added the load and then turned the amp on. I had never realized there would be a difference doing it either way. I'll give it a shot while doing testing this afternoon. What is the advantage of turning the amp on first when testing? Thanks for the insight.

[Jazz P Bass]

I believe what Juan is getting at is this.
With the amp in an unknown state (ie: just repaired) you need to be careful how the load is applied.
Obviously when the amp is working correctly you do not need to hook up the speakers after you turn the amp on (although some 'protect' circuits do just that)
I would suggest monitoring the output for Volts dc when the amp is turned on.
With a load & without a load.
If the amp is not blowing fuses, ditch the lamp limiter.
If you are getting a brief voltage that may imply that the amp is not working correct as of yet.
If you are seeing a voltage I would monitor the base voltages of the top & bottom output sections of the output transistors (that is what turns them on).
The amp has to be stable before a load is applied.

[Neutronarmy]

Well, I have some additional information but no solution. I am still noticing a lack of any DC on the output; I read about 10ma, but that may be from my less-than-perfect multimeter. I still read +40, -40V, and +24V (well, more accurately a little less than that because of the limiter).

I checked all of the flameproof resistors and they measured proper. I also checked a good chunk of the transistors/diodes throughout and most tested as I'd expect. I received a strange result from testing the drivers (430C/431C; they read a voltage when diode testing them forward and reversed) in-circuit, so I am going to pull them tomorrow; I have replacements if they still test bad out of circuit.

As everyone advised against a load I avoided one entirely for these tests. I ran everything listed above with the limiter in place and no short was shown (a quick burst of light for the filter caps and then a very low ambient light). However, when I attempted to run without a load and without the limiter the amp very quickly began to make the sort of noise I have come to associate with blown components - a gradually increasing hum accompanied with a whine. When I noticed that I shut it off completely and reattached the limiter which still showed the same level of current consumption. I did notice when checking the fuse after that incident that the previous owner put an eight amp slow blow fuse in place of the six amp suggested, so I am going to remedy that tomorrow.

Anything I should add to the troubleshooting agenda? Any texts that I should be consulting? I have done some minor solid state repair in the past, but this is definitely the most serious project thus far. I'm enjoying the challenge, but I still feel like I could be much better educated.

[Jazz P Bass]

Are you in possesion of the correct schematic?
Is this a MarkIII?
If so, post it here.
There are voltages marked on most all Peavey schematics.
They all have to be met.
Usually if a component is off enough to make the power supply squeal, it will show up as an abnormal voltage.
As to the 8A or 6A fuse.
The 6A rating is for full, flat out loading of the output.
At idle both are way over what this amp should be pulling.
A working amp will pull about 50 watts at idle, so those fuses are not going to help.
What is strange is the fact that the amp is pulling current with a load but there is not any Vdc on the output.

[nashvillebill]

I'm not as advanced as the previous posters, so I would approach this particular situation with the "divide and conquer" approach. I'd start by disconnecting the PS rails from both the pre and and power amp--now on my Series 400B the diodes and smoothing caps are on the power amp board, so the PITA factor may be high here. But once the entire power supply is isolated, then does it power up fine on the dim bulb tester? And does the PS then pwoer up without the dim bulb tester and measure per the schematic?

Then I'd enable the LV rail only, doing only the power amp board side first, and using the DBT then without if it passes. Then plug in the preamp board to the LV rail and try again on the DBT then without if it passes. Then all that is left is the power amp; pull the TO3's off and re-connect the main rails. Pass the DBT? then try wthout...

The drivers do sound suspect, but you were testing them in-circuit.

[Neutronarmy]

Are you in possesion of the correct schematic?
Is this a MarkIII?
If so, post it here.
There are voltages marked on most all Peavey schematics.
They all have to be met.
Usually if a component is off enough to make the power supply squeal, it will show up as an abnormal voltage.
As to the 8A or 6A fuse.
The 6A rating is for full, flat out loading of the output.
At idle both are way over what this amp should be pulling.
A working amp will pull about 50 watts at idle, so those fuses are not going to help.
What is strange is the fact that the amp is pulling current with a load but there is not any Vdc on the output.

BASS 400 A SERIES 1972 schematic.pdf

It is the original Peavey "The Bass" 400 Series A (without the series/parallel inputs). The schematic really only shows +40V, -40V, and +24V and I can match all of those (within approximation because of the limiter). I was only mentioning the fuse as perhaps it was rated too high and thus blowing components before fuses, making it a more dangerous situation for the currently functioning parts. I didn't assume that the fuse swap would cause the issue at hand. I agree that the lack of DC voltage is really strange. I double checked it today, but I'll still check it again tomorrow. Does seem awfully strange.

@nashvillebill - I am going to run through the amp in its current state again, replacing the drivers and double checking any associated components. If that doesn't work, dividing it up into chunks does seem like a very logical approach.

Thanks again everyone for the help. It is very much appreciated.

[nashvillebill]

Well, hopefully that may fix it, but I've found that merely replacing components, without having a good idea if they are bad or not, is seldom an efficient way to repair amps....it only takes a few seconds to unplug the 24 volt plug that goes to the preamp and a few more seconds to pull the TO-3's. That will at least isolate the problem, to the power amp board (if it still persists) or to the other components if it has gone away. And don't rule out a shorted 24 volt plug/wire to the preamp.

[Jazz P Bass]

I don't know if this will help or not.
Here is the 400R schematic.
It has the voltage markings that I mentioned.
The basic topology is there.
There are some 'minor' differences if you can work around them.

[nashvillebill]

I just wish Peavey had assigned numbers (C1, R1, Q1, etc) to their components on those old schematics....

[Neutronarmy]

Today's update: replaced the drivers and replaced a suspect diode (the diode was the one on the right of the schematic connecting the -40V line to the 22k/inductor by the output). Without a limiter I am measuring ~80mv DC on the output. The voltages read roughly +41.5, -41.5 and +22V.

I managed to get what I assumed was a stable amplifier even without the limiter and was thus excited. I then added the limiter back in and attempted a load and same result: bright 100W lightbulb and a terrible noise (I was using an old 8ohm speaker as my dummy load project isn't yet finished). The amp was quickly turned off and the load removed. With the load removed the amp returned to how it was acting prior.

I did at one point today have a significant negative voltage on the output, but quickly realized I had replaced the diode backwards. When it was reversed the negative voltage went away. It looks like I need to do some further troubleshooting; I am still befuddled. Thanks again for all of the help!