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MOSFET 100 3210 woes

Started by buffalocomputer, April 04, 2023, 12:00:40 PM

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buffalocomputer


The clean channel on the amp works nicely, but on the lead channel, the signal is barely discernible out of the speakers. I see R46 and 47 look burnt on the board, and R67 and 68 look like the wrap on the resistor has cracked due to old age.

On the schematic c38 and 39 are marked to be 10 uf, but I have 100s in there, and c40 and 41 on the schematic are 47 uf, but 68s are installed. I cannot read the resistor values for R44 45 46 47...

I do have a good meter, and scope. Any suggestions would be appreciated.

When checking v+ ground to pin 1 on IC1, the lead channel will kick on as long as I have the positive lead touching the pin from my meter. Next thing to check?Plz keep a bit layman, was years ago that I took digital repair 😂

Also I'm reading 40 volts DC across resistors 46 and 47 which are getting incredibly hot, 6 volts across resistors 44 and 45. The zeners looked to test okay, and I see no problems with the caps there. When I do click on the overdrive channel, the LED on the front comes on very very dimly.

g1

Attaching schematic, please post yours if it is different.
Do those zeners each have around 15V across them?
The 4 resistors R44-R47 should also each have around 10 to 15V across them.
The excess voltage across some seems to indicate a faulty or shorted part.
Do any of the IC's feel hot?

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buffalocomputer

I tested a couple of them quickly last time I turned it on, and they seemed normal temperature. The two hot resistors I could feel a few inches away from the board. I will check it out again.

Tassieviking

Here is the 3210 issue 7 schematic if it helps.
It is a lot clearer then the issue 2 schematic but there are most likely differences somewhere.
There are no stupid questions.
There are only stupid mistakes.

g1

Thanks TV, that schem. is so much easier to read.

So R44-R47 are not all the same value and I was wrong to say they should drop around 15V each.
The zeners should have about 15V each across them, R44 and R45 each with approx. 20V across them, and R46,R47 each with about 15V across.

As it was reported that R46 and R47 were overheating with about 40V across each, it seems something on the +/-34V lines is over-conducting.
The only  thing I see running off those lines is the little power amp driving the reverb tank (TR4,TR5).  Those transistors (and associated circuit) would be a good place to look.

buffalocomputer

Quote from: Tassieviking on April 05, 2023, 10:20:12 AMHere is the 3210 issue 7 schematic if it helps.
It is a lot clearer then the issue 2 schematic but there are most likely differences somewhere.

That is a far better schematic, but the component values are definitely different.

Tassieviking

I also think that the 5275 amp schematic has some strong similarity to the 3210, I think it could be of some use to work out what the component values are that are unreadable.
There are no stupid questions.
There are only stupid mistakes.