current limiter and trouble-shooting

[J Rindt]

A neighbor kid brought me a PV Series3 Bass Amp.
He said it was humming loudly with no output.
I plugged in a guitar and powered up. I DID hear guitar, but also the VERY Loud hum. Turning the Pre Volume on either channel made the hum go away, and so did disconnecting the  -/+15V supply.
Wiht the amp still powered on i reconnected the 15 Volt Supply and that snapped the mains fuse. It is supposed to be 8 amps, there was a 7.5 in there, the closest i have is 7.0, so i installed that and plugged the amp into a Light Bulb Limiter.
The 100 watt bulb is bright. So my question is, how do i proceed to check the amp when it is on this current limiter. With a tube amp, i could pull all the tubes and go from there.
But this amp is SS and i know almost nothing about Solid State. How can i find what is causing this short.?
I would really like to help this kid if i can.
Thank You
 
http://el34world.com/charts/Schematics/files/peavey/peavey_mark_3_bass_with_pa.pdf

[NewYorker]

J:

This sounds similar to a problem that I fixed in my nephew's Acoustic bass amp.  Short version of the story - one of the filter cap's in the B+ supply was leaking DC - not quite shorted but leaking enough to blow the mains fuse.  It also hummed before blowing the fuse.

Try removing or disconnecting the filter cap's, and then bring the amp up on the current-limiter.  You probably should substitute a dummy load for the speaker, in case what I'm suggesting causes the amp to REALLY hum.  That or, if possible, disconnect the +/-15Vand +/-52V supplies from the rest of the amp.

You probably ought to check the rectifier diodes as well, to make sure they aren't shorted.  (i.e. check to confirm they conduct in only one direction.)

It looks like this amps has a +/-15V supply and a +/-52V supply, so you have two places to check.

Start there.  Good luck.

Ed

[PRR]

> this amp has a +/-15V supply and a...

Very unlikely the 15V minor supply can be blowing 7A fuse or lighting lamps.

Agree with the process: big caps, big rectifier, back to the PT.

[NewYorker]

"Very unlikely the 15V minor supply can be blowing 7A fuse or lighting lamps."

PRR / J:

Good point.  However, if the (big) filter cap's end up being the problem, might it be a good idea to replace them all while you're in there?

Ed

[J Rindt]

OK...to check this stuff in circuit (is that possible) can or should i lift R70 and check to see that the big 55Mic caps are not shorted, and i can probably measure those big rectifier diodes in circuit, or do i need to get them off of those big caps also?
The diodes would most likely read as a short to themselves if they are bad.?
Thank You

[J Rindt]

It would seem PV sent me the "wrong" schem. They are very close, but i need the BH version. However, it does not show the layout or schem for the preamp...just the power supply and output, but maybe the preamp is the same for both amps.?
Instead of One Resistor i needed to lift two, and i lifted the rectifier diodes. The diodes all test about a voltage drop of 0.510.
The two resistors are good.
The Caps test about 0.15 ESR with my Blue Meter.
When i set my DMM to capacitance i get OL. Looking at the book, my meter has a max reading of 3999. So i guess the 5500 Mic Caps are beyond the limits of my meter.?
It would seem the Power Supply is good. I must have a problem in the preamp.?
Thanks for the help.....
the correct schem is below

[PRR]

The preamp power is NOT why it blows a 8Amp main fuse.

The small supplies are fed through either 1K resistors or 1A regulators. They CAN'T suck much power.

Your cap measurements do not prove the caps are good. They may break-down at a few Volts.

A rude test: while on the Lamp-Limiter, probe for Volts at R65 R68 1K 5W. + and - 50V is nominal. I would suspect you have much lower V on one or both sides.

The amp is from the 1980s. Also they put nominal 52V on 55V caps. I would assume the 5000uFd caps C35 C36 are shot. Actual uFd is not critical, 4,700u is right, 3,300u will work, 6,800u if it fits and doesn't spoil the budget. But get at-least 60V rating for a short-term repair, 75V if you want it to work a long time. "Snap in" caps are available in these values and will probably fit height/diameter. The legs won't be right spacing/diameter, drill the PCB to suit.

[J Rindt]

Right...i understand what you are saying regards the Mains Fuse. I forgot to mention, when the fuse blew it also burned up R58 on the Collector of Q12. That is on the 52V supply you are talking about.
It could all just be coincidence, but like i say, i plugged a guitar in, it passed guitar signal but was Real Noisy. I unplugged a Molex Jumper to try and isolate the noise, and when i plug it back in.....the fuse and R58 smoked. If you know what i mean..... the first time i work on a SS Amp, do a little trouble-shooting, and the amp burns up. Just making sure i did not cause this, make a stupid mistake, etc etc.  I was wondering if i did something "weird" ...as far as the Transistor Bias was concerned, that wanted all that current.
Anyway.....on my way out the door, will apply your suggestions later on.
Thank You So Much

[J Rindt]

OK...i had this amp all apart...got it back together now.
Those two resistors, i measured on the 52 Volt and the 15 Volt side of each one.
They are:
+ 7.6 and 2.5
- 7.8 and 1.9
With the amp on the limiter, i do not how to interpret that.
Thank You

[PRR]

> R58 smoked

Then Q3-Q6 and Q13-Q16 are blown.

This becomes a major repair with dubious economics.

[J Rindt]

> R58 smoked

Then Q3-Q6 and Q13-Q16 are blown.

This becomes a major repair with dubious economics.

OK...Thanks. So the power transistors.
Can i (reliably) test 13 and 16 in circuit, or do i need to get these off the chassis.?
Thank You

[PRR]

I would test out of circuit.

[J Rindt]

OK...that is what i thought.
Thanks