Ampeg BA600 repair

[angelothewolf]

Hi there!

So, it is an Ampeg BA600-115 it came with a massive electrolyte leakage from a filter capacitor (C16) that exoplode after some PSU issue.
PSU was shorted, one diode bridge (D8).
I replace Mosfets Q1 and Q2 PWM controler U1, both rectifiers D7 & D8 and four large filter capacitors D14/15/16/17

This is like a dual PSU as far as I can tell, One for the preamp rails with the HT for the preamp tube and another one for the AMP.
Now everything works but I have an issue with the amp rails (+/-30VDC) the positive rail is about +25V but the negative is -12V, the Amp actually works but when it reaches a volume threshold the tension drops too muchs and it cuts out.

any clue will greatly apreciated ´couse this is driving me nuts.

ampeg_ba300_ba600_sch.pdf

[J M Fahey]

Just thinking aloud, no experience with SMPS.
1) you practically replaced all main components of the large power supply.
2) +30 and -30 are 2 identical supplies in all aspects, just one has negative grounded, other has positive.
They *should* provide exact same voltage.
3) that supply is regulated, so you should at least have one +30V rail, yet best is +25, let alone the "officially bad" -12V one
4) "just to do something" I´d scope U1 pin 6 to check whether it´s putting out a symmetrical square wave (it should) so as to drive Q1 and Q2 the same.
5) then would scope secondary T1b , then T1c, and find if there are any differences
6) then cathode of D7 and of D8
"just thinking it as if it were a conventional supply and looking for differences or abnormalities"

Certainly somebody SMPS savvy will be able to offer more approppriate analysis or suggestions, I´m just mentioning what I´d be doing for starters if I had it on my bench today.

[nickb]

Please note you will need to use an isolating transformer if you are going to 'scope anywhere on live side of the power supply.

[Mick Bailey]

The two secondaries T1-B and T1-C are off the same primary T1-A. I would expect the voltages to be symmetrical even if the primary voltage is too low. The main cap failure C16 is on the negative rail, and you still have a problem with that rail (ignoring for the time being that the +25v isn't correct). Re-check the rectifier solder joints and make sure you don't have any AC leakage superimposed on the rectifier outputs (maybe from a faulty 1nf snubber cap) that could pull down the filtered voltage.

[nickb]

I see there are +24V and -15V rails too which are very close to what what you measured. So, I have to ask and don't be mad with me, are you sure you measured the 30V rails in the correct place?