Ampeg B100R again...

[mtlbasslad]

Greetings - here we go again with this amp blowing fuses...



Tested for shorts from rails to ground & to each other, all OK.
Fired it up with light bulb limiter, OK & without limiter - fuse holds. When I did the same thing with Tip's in, with limiter was OK but without blew fuse??

Any ideas?

[mtlbasslad]

Okay, reading that post I seem to have missed something, let's try again.
Had a short, replaced output TIP142 & 147's with what I thought were good ones. Hooked up light bulb limiter, fuse held but no signal, hmm? Removed limiter, instant blown fuse.
Removed TIPs checked anything I could think of for shorts, including rails to ground & to each other. Now with no TIPs in fuse holds. New TIPs on the way.
Was I safe to assume no shorts without TIPs means board is OK? I know, that word assume...
Oh and this is the same amp from my post of years ago.

[g1]

Not quite sure what you mean about board being ok, but there can still be other problems on the board that make the Tip's turn on hard when they are installed. Then the fuse would not blow with them removed.

[Enzo]

DO NOT CONNECT a speaker or load to the amp until it is certain not to make DC. An amp can sit there making 40vDC on the output, but with no load, no current flows and fuses do not blow. As g1 said, all the amp needs to do is make the output swing to DC and it will blow fuses when loaded.

[mtlbasslad]

Well, I'm happy to see the usual suspects chiming in here - hello g1 & Enzo.

I've pretty much checked the bigger passive components (R's & C's, bridge) maybe it's time to look at Q3 & Q4? The fact that the fuse holds with the bulb limiter confuses me. I'm easily confused 'cuz I'm over my head here.

I've had fair success lately with more evident problems - a 70's Fender bassman with obvious leaking caps or an intermittent pedal that needed a new 3PDT, but this one has me almost ready to take it to a pro (how humbling)
Oh well, it's my own amp - nobody breathing down my neck & I do have other amps...

[g1]

The bulb limiters job is to save fuses (and other components). It limits current to the primary as it lights up. So if you were measuring the AC volts on the primary you would see it drop when the bulb lights. Reduced voltage at the secondary means there won't be enough current to blow the fuse.

If you still have the Tip's out, what are the DC voltages at TP5 & 6 ?

[mtlbasslad]

Yes the TIP's are still out, new ones coming today...

With .2V 1000hz input, TP5 reads +18.7 VDC TP6 +18.7 VDC if I read the pinout on IC2 right. Pin 4 is -17.5 VDC & pin 8 +22.8 VDC.
At R39 -16 VDC at R38 +16 VDC.
TP4 on IC1 reads 9.4 VDC.
I didn't expect anything at TP7 but got .003 VDC -fluctuating...

Does that tell you anything?

[g1]

Can you double check those DC readings with no signal applied.

[mtlbasslad]

Double checking now - stand by...

[mtlbasslad]

OK put on boots & coat run out to the shop and... not much change - TP5 now -21 TP6 +19.5 everything else the same.
New TIP's arrived but sure not putting them in until...

[g1]

Are IC1 supply pins + & - 15V ?

[mtlbasslad]

IC1 TL074 Pin 4 +15 Pin11 -15
Outside -14C Brrr

[g1]

Those 15V supplies are good. Not sure if the 9VDC at IC1 pin14 is due to it's inputs or a bad chip. What are DC volts at IC1 pins 12,13, and double check 14 (TP4) ?

[mtlbasslad]

IC1 Pin 12 0 VDC Pin13 -.002 VDC
TP4 9.6 VDC

[g1]

How about other outputs of the quad IC1, pins 1, 7, & 8. Any DC ?