Old Peavey bass combo capacitor question

[mrmcsick]

I have a late 70's/early 80's Peavey Mark iii 260c bass combo with a 15" black widow I picked up for $40 the other day. It functioned but poorly. I noticed the speaker was locked up but instantly found the reason with some internet research. Fixed that and was good to go. Only thing is the amp had alot of hum all the time. The other night I started going over the amp and cleaning it up. When I looked at the poweramp board I noticed a cap was disconnected on the positive side. Doesn't look like it burned off or anything. Also noticed that there were 2 diodes soldered in parallel with 2 caps, did someone add these or is it stock? It had acouple other things played with inside as well like both the external speaker 1/4" jacks were disconnected and what I assume were the wires going to them halfway down the board cut to about 3" of length. Someone also wired in a mono 1/4" preamp out jack inline with the stock 1/4" one and just left the stock one dangle inside. The stock one is stereo and operates with sound going out of the amp and the preamp out in the when inserted halfway in and shuts off the amp speaker out when inserted fully so Im told. I never used it.

Just wondering what the 1 larger cap is for that isn't connected and if the diodes ontop of the 2 other caps are stock.

[mrmcsick]

[mrmcsick]

here is the factory schematics.

[Jazz P Bass]

Looks factory to me.
Other than the cut speaker leads & the unsoldered capacitor.
Resolder the capacitor.
In fact, while you are at it, resolder the whole board.
Reattach the speaker leads to the jack.
I am sorry but I cannot see where the "diodes across the capacitors" are.

[mrmcsick]

Thanks for the reply. The diodes are on caps 31 and 32. They are left and right of the 2 pin connector with the red and black cut wires.

What is the function of that disconnected cap? Noise filter? Why did the amp still function with it not connected?

[Jazz P Bass]

The red & black cut connector is the speaker output leads.
Red +
Black Ground.
The diodes across C31 & C32 are a factory revision for spike protection.
The cap that came loose is C30. 1000uf/35V.
It is the main filter capacitor for the minus 25 volt rail.
Yeah the amp will "work" but not correctly.

[mrmcsick]

Thanks. I got 2 new 1000uf/35v axials from radioshack and am about to put them in.

What exactly does this cap filter on the minus 25v rail? Noise? How would it affect the amp with it connected/not connected?

Sorry for the noob questions.

[Jazz P Bass]

Power Supply Primer.
The + & - 15 volt supplies.
The Org, Org Wh, & Org are transformer secondaries. Volts AC.
CR 20 & Cr 21 are diodes which take the top half of the ac waveform & rectify it to +25 Vdc.
C 28 is the "smoothing" capacitor. It helps to make the rectified voltage a more consistant +25Vdc.
This voltage then enters LM340 which is a +15 volt regulator.
It's capacitor is C31, which is used more as a reservoir.

CR22 & CR 23 take the bottom half of the Vac waveform & rectify it to -25Vdc.
C30 (which you will observe has the polarity reversed compared to C28, plus goes to ground) smoothes the voltage for LM 320. This is the -15Vdc regulator. C32 is it's reservoir capacitor. (reversed polarity again)

So when C30 came loose on your amp. the voltage entering the LM320 was ragged & ripply.
The Lm 320 most likely did not handle this very well.
The consequense was more than likely a ripply (as measured in Vac) -15 volt rail.
The ic's in the preamp run on the + & - 15 volts.
They must have a ripple free power supply for proper operation.

[mrmcsick]

I wired in the external speaker output jacks to the red/black in parallel. Seems to work fine. I soldered in the new c28 and c30 caps. Also desoldered the mono preamp out jack that was wired in with the stock stereo.

Amp no longer gets signal through it!

When power is turned on the speaker leads get about a -20v surge sucking the speaker into the full backwards excursion position. Lasts for less than a second. you can hear the amp is on and the very faint his/hum coming out of the speaker to verify it. My pre gain pot is dirty so when I move it around I can hear the static coming out the speaker. Figured this must be a problem with the preamp output jack/signal. Inserted a cable halfway in and ran it into the input of another amp and it gets signal that is at a constant volume with no eq/vol knobs affecting it. Push it all the way in and it no longer works. Resoldered the mono jack they had wired inline with the stock jack. When cable is inserted into this there is no output due to it being the full in position of the other jack. Tried the internal speaker leads and both external leads with no change. When shut off it doesn't have a voltage spike like when turned on, just a little blip. Tried other cables and basses and that did nothing either.

Seems as if the signal isn't going into or out of the power section for some reason I guess. Im now lost.

You said that 1 of the larger caps has reversed polarity? the old ones went on the board facing the same direction and I soldered them in the same way. IDK? Oh nevermind you are talking about the ones with the diodes ontop of them, c31, c32.


Also wondering if I have the internal speaker leads reversed. I have the 1 pin connector right by c15 as the positive and the 1 pin connector that is by cr20 (it doesn't show this pin on the board layout pic above) as the negative.

[mrmcsick]

Disregard last post. Had a connector from the preamp to poweramp backwards. so dumb it fits both ways with no indication.

Amp sounds much healthier now. Barely any buzz/hum and it seems louder too lol.

Only thing I can't figure out now is the preamp out.
When I use the stock one the amp and the signal out both work when in the halfway in position. When all the way in the re is no sound or signal. There should be a signal coming out of the pre out still. oh well.
When I use the mono jack it functions like the stereo one at the halfway in position.

Thanks for all your help. Really appreciate it.