80s Peavey Amp Repair Attempt

[tmoneygetpaid]

A friend gave me a Peavey Mark III Centurion that had been sitting in his basement in non-working state for quite a while. I figured this would be a good learner project (I would like to learn amp design and repair), opened it up, found a bad power transistor and rectifier diode, replaced all the power transistors and rectifier diodes, and the filter (reservoir) caps for good measure, since they were the originals. It still blew a fuse on power up.

So I ordered and installed new power resistors and filter caps for the remainder of the power section, replaced the TL074 and procured a schematic from Peavey. After replacing these, it's still blowing a fuse on power up. I noticed that the +-28V / +-16V section of the power supply (see the schematic) has no fuses, so it's entirely disconnected, but I also don't see where these are used. Am I missing where these end up? And could this be causing the issue?

If not, what else I should check? There's nothing glaring (obvious shorts/ solder bridges, blown up resistors or caps), but given, I haven't looked at the Preamp section.

[Alex R]

You need to trace the fault, not make assumptions about what it might be. Get hold of a multimeter with a diode tester and check the semiconductors for shorts, especially drivers for the power transistors. It's a fair bet your power transistors have shorted again so test them too. And when you turn it on use a light bulb limiter or preferably a variac and ammeter in the power supply as well, bring it up slow watching for excessive current draw.

[tmoneygetpaid]

I have a good multimeter, and I'll start looking around systematically. But before I do so, my main question still stands-- do I need to put fuses in the power supply section that supplies +-28 and +-16, and if not, why is that there? I ask this not from laziness, but because I want to understand the design. I've also seen some receivers that have PSU sections lacking fuses, which also confused me.

Thanks for your reply!

[Jazz P Bass]

Check the power supply rectifer diodes for a short.
And all that Alex R said.

[tmoneygetpaid]

Forgot to mention, I replaced those as well. I'll check them to make sure they didn't go in these last couple rounds of power ups.

[52 Bill]

If you have a light bulb limiter, use it.

First thing I would check, is to pull off both red and orange transformer secondary plugs from the pc board. Now power up the amp and see if the main fuse holds. If it does, then the transformer and primary wiring are probably okay.

There are two separate power supplies one for the power amp board and one for the preamp board. Each one has it's own set of rectifier diodes. All of them need to be tested.

While transistors and diodes are usually the problem, other things like filter caps can short as well.

[Enzo]

Hi, welcome.

As the others suggested, don;t just guess, is it this? is it this? is it this? You need to systematically isolate the problem. Once we know WHERE the problem is, we can determine WHAT it is.

If one of your output transistors was bad, that is a clue. We must also then check all resistors associated with it to see if any are now open. Any power transistor is driven by a smaller one, and when the larger one is damaged, it often takes out the smaller.


Aside from not just throwing ports at the amp, we also don't need to redesign the amp. More fuses might be nice, but the amp and all the ones like it have not needed those extra fuses for the last 35 years, I suspect they won;t be needed now. There are no fuses on the 260C power amp. Is that the power amp you have? Or are you trying to tell us there are empty fuse holders on your amp?

Look up "light bulb limiter" and make one and use it.

Let us get systematic. Your amp blows fuses. Quick checks would be for shorted output transistors. Next check all the rectifier diodes for shorted. There is a cable from the power amp board up to the front panel board. Disconnect it. If that stops the blowing fuses, then we know the preamp has the problem. That is unlikely, but good to know. Assuming the issues are on the power amp. DO NOT connect a speaker or load to the power amp until we know it is stable and not making DC. We check the power transistors to make sure none are shorted to the heat sink or chassis.

And it is last on the list, but, the transformer. The red and orange wires from the transformer (high and low voltages) plug onto the power amp. Disconnect them and power up the amp. If the fuse still blows, then the transformer has a problem. If it sits there happy, the tranny is OK.

I noticed that the +-28V / +-16V section of the power supply (see the schematic) has no fuses, so it's entirely disconnected, but I also don't see where these are used. Am I missing where these end up?

I don't understand how the lack of fuses means anything is disconnected. The red wires are rectified and filtered to make the +42 and -42 power supply voltages. And the orange set of wires makes 28v supplies which then form the +16 and -16. Where are those used? Look in the schematic. The pair of 42v supplies connect to the output transistors. The orange wires make the lower voltages. The 28v rails are not used anywhere, they are just the raw voltage feeding the 15v regulators. ANywhere you see +/-15v supplies, expect to see op amps and other ICs. The +/-15 goes to the preamp, which we have unplugged. The 15v rails also feed the ICs on the power amp.

[tmoneygetpaid]

Hi, welcome.

If one of your output transistors was bad, that is a clue. We must also then check all resistors associated with it to see if any are now open. Any power transistor is driven by a smaller one, and when the larger one is damaged, it often takes out the smaller.

Thanks for your reply. Yes, that's what I'll check next- those transistors feeding the power transistors.

]
Aside from not just throwing ports at the amp, we also don't need to redesign the amp. More fuses might be nice, but the amp and all the ones like it have not needed those extra fuses for the last 35 years, I suspect they won;t be needed now. There are no fuses on the 260C power amp. Is that the power amp you have? Or are you trying to tell us there are empty fuse holders on your amp?

Ok, so I am clearly misinterpreting or misunderstanding something about my amp or the schematic. I do have the 260C power amp. I haven't added any fuses to the design. On the schematic I've attached, and the layout, and on my board, there are two fuse holders (on the board itself) that immediately follow the orange rail from the power transformer. In my amp, they are empty, which based on the schematic would mean that that whole +-28V/ +-16V supply is disconnected. Right?

I was initially unclear as to where this was even used, since the schematic shows the +-42 V being divided and run across some Zener diodes to produce a +-15V supply, but I see on the layout that the connector to the Preamp board is labeled for +-16V, so I understand that's how the Preamp is powered. Still, since it's been disconnected this whole time by not having the fuses, can I just limit my search and destroy to the power amp section and its associated half of the power supply?

Look up "light bulb limiter" and make one and use it.

Let us get systematic. Your amp blows fuses. Quick checks would be for shorted output transistors. Next check all the rectifier diodes for shorted. There is a cable from the power amp board up to the front panel board. Disconnect it. If that stops the blowing fuses, then we know the preamp has the problem. That is unlikely, but good to know. Assuming the issues are on the power amp. DO NOT connect a speaker or load to the power amp until we know it is stable and not making DC. We check the power transistors to make sure none are shorted to the heat sink or chassis.

Yes, I've done everything here except disconnecting the preamp, which I'll do.

And it is last on the list, but, the transformer. The red and orange wires from the transformer (high and low voltages) plug onto the power amp. Disconnect them and power up the amp. If the fuse still blows, then the transformer has a problem. If it sits there happy, the tranny is OK.

I checked the transformer-- with the red and orange connectors disconnected the fuses stays intact. So the trafo is ok.

THANK YOU!!!

[Audiotexan]

I noticed that the +-28V / +-16V section of the power supply (see the schematic) has no fuses, so it's entirely disconnected, but I also don't see where these are used. Am I missing where these end up? And could this be causing the issue?

I only see two fuses (both 1amp, and both in the +/-16v supply rail). I see no 28v rail as yet in the schem you posted. 0o As mentioned before.. are you sure you have the right schem for the amp you have? I see pos./neg. rails for 42v, 15v 16v.

At any rate: No chance of missing fuses causing the issue.

Being that there are fuses missing from the circuit would say to me that there was a person trying to isolate the fault before, and removed those fuses to remove power from the secondary supply (16v) to make sure that something on that section wasn't what was causing the short. (Something that's not in circuit cannot blow fuses!).

Put the 'fuse issue' in the above regard on the back burner. As something else that IS still receiving voltage is the source of the current set of issues.

[tmoneygetpaid]

I only see two fuses (both 1amp, and both in the +/-16v supply rail). I see no 28v rail as yet in the schem you posted. 0o As mentioned before.. are you sure you have the right schem for the amp you have? I see pos./neg. rails for 42v, 15v 16v.

No, you are right. They aren't rails, the schematic just shows a point on the +-16v supply where voltages are +-28, I guess for checking the voltage regulators? I hadn't noticed there weren't any arrows there...

[J M Fahey]

No, you are right. They aren't rails, the schematic just shows a point on the +-16v supply where voltages are +-28, I guess for checking the voltage regulators? I hadn't noticed there weren't any arrows there...

Since voltage regulators drop voltage, you have to start with higher "raw" unregulated voltage first.
The +/-15V rails are guaranteed, within 1% or so; while the "28V" can drop to as low as 20 or 21V with no ill effect.

[Enzo]

My humble apology. I opened my own 260C file instead of yours. Now I see you have a 260C '81, I was looking at the original 260C.


yes indeed they added fuses to the low volt rails on that later version, sorry. And if the clips are empty, then those supplies are unpowered and are not involved with the main fuse blowing. And indeed there are two sets of +/-15v rails, one set for the power amp ICs and the other for the preamp.

PK, on same page now.

There are only so many things that can blow the main fuse. With the red wires off, we check the main rectifiers for shorts, but also check the two pins where the red wires go, and the red stripe wire in the center. Are the two end pins shorted together? (Extremely rare C28 short). Are either shorted to the center pin? Is either big blue cap shorted? Remember, the can tabs on these are not active pins, only the two in the center count.

The power amp itself can be blown. Shorted output transistors, bad driver transistors, open resistors.

And the parts might work, but both sides are turning on at once, as if the dual bias diode were open.

Somneone been in this before you? Make sure the parts in it are right. All four outputs are the same type, and all NPN. Any smaller parts been subbed?

Simple DC on the output won't blow fuses, UNLESS we put a load on it.

SO: power supply shorted to ground or shorted between polarities. Outputs shorted, shorted to ground, shorted side to side. Both sides conducting.