Vox Cambridge 30R Oscillations

[DrGonz78]

The amp suffered a failure that took out the fuse and the TDA2050A chip. Replaced those components and hooked up a speaker only to find the amp was motor boating. Looked over the circuit board and came across some nasty soldering somebody did to replace the bridge rectifier. Re-soldered the bridge connections and it appeared to fix the motor boating as the amp passed signal with no oscillations. The amp sounded pretty good except for a bit of a hum, no DC on the output measured. Tried the amp out again and it is back to motor boating. I measured some intense leakage off of C16 up to 5-10vdc and replaced it. C16 is not leaking like it was but monitoring it I did see it read -1vdc for moment. C20 has no leakage at all. If you pull out the preamp tube it cuts the oscillations but when the 12ax7 is removed there is 27vdc on the output of the amp. Why would pulling out the preamp tube put 27vdc on the output of the amp?

Edit: Just had to post the pic of the nasty solder job as it was just bad. So somebody had been in the amp and it is hard to tell what is a Vox last minute add on or mod gone bad. The bridge rectifier definitely was replaced.

[52 Bill]

Is the tube filament connected to the dc power supply?

What happens to the dc voltages when you pull the tube out?

Have you tested the basic filter caps?

[DrGonz78]

When I pull out the tube the -/+28vdc supply goes up to about -/+29.9vdc, the -/+16vdc goes to about -/+17vdc and -/+9.1vdc goes to about -/+9.4.(I hope that is what you meant in your question) Yes the Filament supply is connected to the main DC supply. The filter caps test fine and the AC ripple is in the very normal range.

[DrGonz78]

I think the overall problem here might be the TDA2050 chip has shorted out again or something. I measure DC on the output now regardless of the tube in or out of circuit. About 2-3vdc on the output and I measure DC voltage after C16 that jumps around 12v, 5v, 2v, 1v, -12v, -5v so on. The amp did work one time with an underlying hum. When I first installed the TDA2050 I had 15mv on the output even when it was motor boating. Should I remove the output chip to troubleshoot? I am really scratching my head on this one trying to figure this one out.

[52 Bill]

So pulling the tube had nothing to do with the voltage on the output of the power amp chip.

If you inject a signal into the input of the power amp chip do you get a signal at the output? When you are taking dc readings, where are you grounding the meter? If you pull the tube and measure dc voltage at C16 what do you get? If C16 is leaking dc the tube will not change that.

Have you checked for bad grounds everywhere in the amp? I haven't seen one of these in a while, but if I remember correctly some of these recent Vox amps have really odd ways of connecting grounds through the amp.

[DrGonz78]

I will try injecting signal into the chip and check that out later tonight. All voltage measurements are taken in respect to chassis ground. With the tube installed the voltage at C16 will actually start at 10vdc and then drops down to 0v after power up, within a few seconds. With the tube removed C16 jumps around 12v, 5v, 2v, 1v, -12v, -5v fluctuating voltages around. I will check a bunch of grounding on the amp later tonight too.

On a side, I had ordered some tda2050's a few months ago to replace on an old Epiphone EP1000 amp. Of course the chip was ordered through eBay and that might have something to do with this issue too. I ordered two of them and that Epiphone is still sitting in the corner of shop, head scratcher. Such an easy repair but it had strange problems where it passed signal but my rail voltages were off balance. I guess I half suspected something odd with this repair due to the other amp having a strange repair saga. Perhaps I got a couple fake or defective TDA2050's? Unfortunately 2050's are hard to come by these days and it's hard to find a good source. I ordered some more from another source to see if I have better luck and to have some on hand.

[Jazz P Bass]

LM1875 should work as a replacement.

It's pretty hard to 'kill' a real TDA2050 other than trying to pull too much through it.

[DrGonz78]

LM1875 should work as a replacement.

It's pretty hard to 'kill' a real TDA2050 other than trying to pull too much through it.

Oh yeah the LM1875 is perfect for this amp! Rail voltages up to -/+30 at 8 ohms for 30 watts. Seems like that should have been the chip in the amp in the first place really. Definitely going to order some of those for this particular repair. Thanks Jazz!

[DrGonz78]

Well I got the LM1875 chip installed and there is no VDC on the output now and oscillations have stopped. I am pretty sure C48 must have been leaky or something as I saw a brown heat stain on it. C48 tested good but probably only when under low voltage meter probing, so I replaced that guy too. C48 could have been the cause of the oscillation stability issues on the amp as the originally replaced 2050 chip showed burn fault between pins 4 & 5. So now the amp is passing signal, no hum, but it is very quiet. My gut says to replace the vactrol in the tremolo circuit as I know the stock one is complete crap.

[DrGonz78]

Another reason I believe it is the vactrol is that the tremolo is not working. So it must be stuck in a state at it's lowest tremolo level which brings both ch1 & ch2 to a very low volume. Also, it says shorting out the resistor side of the vactrol will test if that is the issue and it will restore the volume. Does that sound about right? I will have to test that out later tonight after work.

[g1]

Yes. It looks like the resistive side of opto is in series with the output of the NJM2120 (IC3) between R41 and R42.
Shorting it should tell you if it is bad.

[DrGonz78]

Well I definitely pulled a rookie mistake. Was about to order another vactrol and then it just hit me that I forgot to tweek the variable pots for the tremolo. I had put them to a state of off while trouble shooting before replacing the new output chip. Went right in there and boom!! Loudness!! I guess the rule here is don't fix what aint broken. However, it seems very much like a recommended part to replace. I will monitor the amp as it will be used by my uncle and if it goes out I will just replace it for him. Thanks again for the help everyone here!