Help finding fault with old bass amp that blows fuses

[duke37]

EDIT - Sorry - I just adjusted the bias pot (it was way off) and the voltage between C-E changed to 2.437...if that matters.

What do you mean 'it was way off'? by twiddling you may have introduced another fault. The voltage is there to compensate for the base drop voltage of the driver transistors, these would be about 0.7V each. The current is limited by the resistors in the emitters of the driver transistors and the bias should be set for the output trnsistors to run with little crossover distortion.and little heat dissipation.

 

[permanentrecordstudios]

What do you mean 'it was way off'? by twiddling you may have introduced another fault. The voltage is there to compensate for the base drop voltage of the driver transistors, these would be about 0.7V each. The current is limited by the resistors in the emitters of the driver transistors and the bias should be set for the output trnsistors to run with little crossover distortion.and little heat dissipation.

Per the service manual, this bias pot needs to be turned so that test connector (cn101) reads 5mV ± 2mV. cn101 was reading about 15mV before I adjusted it. It's now reading 5.4mV post adjustment.

This was just something I'd been wanting to check & adjust if necessary. It seemed a little silly taking current measurements without adjusting this first.

 

[permanentrecordstudios]

If all is well and no sign of things over heating etc. Have you tried it with out blub limiter and no speaker hooked up?

Now if all is well use a different speaker instead of it's own bass speaker and power up with with the limiter in is it ok?

The big test is no limiter and power up, guitar
Amps can be tested with signal generators set at 1k of around 300mv applied to the input and watch the clean sign wave on a scope.

Now let's say an amplifier is 100w at 4 ohms
It's out ac volts will be v divided by load = current times volts out..

20v ÷4 = 5amp times 20v power 100w

At 8 ohms will be 50w..v÷r=I times v=p.

"Have you tried it with out blub limiter and no speaker hooked up?"
I haven't, because I haven't fixed anything yet. I want to test what I can, while I have power (I don't have my new fuses yet, I'm using 2a's I found in another amp...and I don't want to risk blowing those too).

As soon as I get the new fuses (got 10 on the way) I can test it under normal circumstances. But yeah, I had planned on taking that same course of action (one step at a time, adding additional pieces as the previous test ok).

"Now if all is well use a different speaker instead of it's own bass speaker and power up with with the limiter in is it ok?"
Is this safe to place a speaker in place with the limiter in place too? I accidentally kept my speaker plugged in when I first got the limiter in place, when I powered up the amp, and there was a hum coming out of the speaker. I was afraid I may damage a speaker with the limiter in place.

 

[Wayne Phillips]

If there was a hum in the sound you can remove the speaker load and check the offset volts at the out put..it should be low say 24mv or up to 100mv ..

 

[permanentrecordstudios]

If there was a hum in the sound you can remove the speaker load and check the offset volts at the out put..it should be low say 24mv or up to 100mv ..

2.3mV at speaker output.

 

[Wayne Phillips]

That is ok..

 

[permanentrecordstudios]

That is ok..

Hey just an update...
So I plugged the speaker back in (I got rid of the hum, just didn't have the ground connected properly before) and everything was fine. The light bulb lights brightly first, then dims quickly now (it didn't light before).

Then I decided to plug my bass into the amp input to see what happens. The 1/4" cable wasn't plugged into the bass yet, but as the cable tip touched the input connector (didn't even make it in, it just touched the outside of the input connector) as I was trying to insert it into the input, the lightbulb light up brightly, and stayed lit brightly. I quickly switched off the supply voltage.

Now the input connector is on a separate pcb. That pcb is also out of the amp casing (so that I can test points underneath). I don't know if it being unattached to the casing is interrupting grounding (I assume this board is normally grounded to the case via the pot knobs that screw thru the front...and since they're not screwed into the front panel right now is the grounding disrupted?)) that caused this...or if this may in fact be my very problem.

Is the input somehow shorting when I'm inserting a cable, blowing these fuses?
Sorry, that may seem like an obvious question, but I guess there is the possibility something else shorted as I slightly moved the board when I grabbed it to plug in the input. I didn't short anything or any part of the board to be clear, but if some component is loose, got slightly jolted and shorted as I moved the board.

 

[permanentrecordstudios]

Now the input connector is on a separate pcb.

I don't know why I said that...I'm getting confused....this input is on the same pcb....which is why it's not currently attached to the front panel/case.

 

[permanentrecordstudios]

Another update:
Regarding "as I was trying to insert it into the input, the lightbulb light up brightly, and stayed lit brightly. I quickly switched off the supply voltage."
The input was just coincidental here......it was more about the timing.

I removed the speaker and powered up again, the light bulb dimmed.
I put a 1/4" phone plug (open) into the output, another into the input (open). No problems.
I removed everything, plugged the speaker back in, powered up (bright glow then dimmed), waited about 20 seconds, the light bulb suddenly started glowing brightly (speaker started making a low hum at the exact moment too).
I removed the speaker, powered up again, waited over a minute, no problems.

Seems to be down to when the speaker is plugged in.
Resistance at the speaker 1/4" phone still shows 4 Ohm.

 

[permanentrecordstudios]

one more update: I placed a dummy load (I think) of 9 Ohm on the output, powered everything up, waited, and all was good.

The "dummy load" I used was just another light bulb, which measures 9 Ohm resistance...so I don't know if
that qualifies as a suitable dummy load or not. I don't have another speaker to check with.