crate amp help g-212

[boog]

Crate G-212
This comes on, power amp seems ok, had a thump when powered up, some controls made static with reverb, it won't switch off of channel A.
It has been extremely hot around the zeners just after the drop resistors on the preamp board, I figured out one was backwards, reversed it, and now the negative rail has -17 at the drop resistor, but the positive rail is only 3v, but goes up to 10v if the input jacks are not in the chassis.
I'm checking on the 1w resistors before the zeners, those along with all other diodes tested good, I also changed all 4 4558's, and in the pics there are 2 resistors lifted from me trying to narrow down where the problem is.
Where the board is burnt seems to be ok.

[DrGonz78]

First thought is that board where it's burnt might be conductive. Scrape a bit into it to see if it is more cosmetic on the top surface. If it's charged deep in the board then I would try to remove some of the board to see if that is the problem. Might be something else going on but I would look at the board closely.

[vintagekiki]

Crate G-212
... negative rail has -17 ... positive rail is only 3v, but goes up to 10v...

- Desolder one end R46 and R48 (47 Ohm) and measure the voltages at TP13 (+ 16V) and TP14 (-16V).
- Check that on the PCB has not burned in the area where the negative and positive rail connects to the ground.

[Enzo]

Let me get this straight, the amp was basically working fine, you just couldn't change channels? Why would we then decide one of the power supply diodes was backwards?

Please check your zener diodes They are wired opposite of each other. In other words, one has its cathode grounded, the other has its anode grounded. HArd to tell from your photo, but it appears to me that you have both cathodes grounded.


Get power supply restored and then we can attack the channel switching.

[DrGonz78]

Let me get this straight, the amp was basically working fine, you just couldn't change channels? Why would we then decide one of the power supply diodes was backwards?

Yeah that 2nd picture shows both cathodes ground as Enzo said... Is that picture before or after you reversed it??

[vintagekiki]

Check on the PCB has not burned in the area where the R47 / R49 (330 Ohm) is connected to the zener diodes, and properly polarize the zener diodes.
On the picture, both cathodes are currently grounded.


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[boog]

sorry the pic was before I reversed the one on the positive rail, and the amp wasn't passing guitar signal other than scratches from dirty pots and reverb could be heard.
The problem ended up being one of the 47uf caps had gone shorted, some of the problem during tests was me not grounding the jacks, oops.
Anyway it's up and working fine now, thanks for the replies.