Peavey Special 112 No Output

[misterc57]

This amp had no output on initial testing and no excessive current draw.

15 V rails are good. I started stepping through the signal path. I got to the preamp out jack and noticed scratchy noise on the volume pot. I shut down and gave all the pots a good spray and many turns.

Next startup had excessive current draw. So a new problem arose. Gotta love this hobby.

Help please on steps and where to start!

Thank you, MC

Schematic attached.

peavey_special_112.pdf

[misterc57]

Just to be clear. My primary issue now is excessive current draw.

Thank you

[glebert]

I recently had an issue with a solid state amp where the output was weak. I went to jump the effects loop and as I was putting the plug into the jack it got noisy and was overheating. I am pretty sure that the dirty jack (and pots in my case since there are effects level pots) caused it to start oscillating. Cleaning the jacks and pots fixed it for me. I would give those preamp out/power amp in jacks another good cleaning and see if it changes anything.

[g1]

Are you on a limiter lamp? If so, try with no load connected.

[misterc57]

Are you on a limiter lamp? If so, try with no load connected.

I am using a variac.

With no load there is no excessive current!

[misterc57]

I recently had an issue with a solid state amp where the output was weak. I went to jump the effects loop and as I was putting the plug into the jack it got noisy and was overheating. I am pretty sure that the dirty jack (and pots in my case since there are effects level pots) caused it to start oscillating. Cleaning the jacks and pots fixed it for me. I would give those preamp out/power amp in jacks another good cleaning and see if it changes anything.

Gave them a good cleaning. Connected the load. Seems to be good now. More testing tomorrow.

Yay!

[misterc57]

Edited....


High current still happening. This is getting more interesting.

With signal inserted into either input jack...

If volume is set to zero. I get no high current draw using either channel.

If volume is turned up. I get high current draw using the gain channel. High current increases as the VOL is turned up.

If volume is turned up. I get high current draw using the clean channel. High current increases as the VOL is turned up.

I am injecting a 1K signal using my signal generator at around 250 mV. If I shut off the signal generator there is no high current.

[Helmholtz]

Looks like oscillation. Do you have a scope?

Are input jack switches working?

[glebert]

Just to clarify, what are you considering "excessive current draw" and what impedance load are you using? This is a high powered amp, it will draw some good current when it is cooking.

[misterc57]

When we say "oscillation", what does that mean in the case? I am thinking the amplified signal keeps being regenerated?

Input jack switches are working.

I have a 4 ohm dummy load.

Vol at "5" is drawing 1 A current., and climbs more rapidly after Vol "5"

Vol at about "4" is drawing about 0.55 A. Pics below of Normal channel Vol at 4 and the 1K input signal. Scoped at the amp output...

[Helmholtz]

When we say "oscillation", what does that mean in the case? I am thinking the amplified signal keeps being regenerated?

A self-oscillating amp has turned into a signal generator due to some fault causing positive feedback.
Often the frequency is ultrasonic, so inaudible.
Nevertheless the oscillation produces output power and heats up the load (or voice coil).
Consequently there may be considerable mains current draw.

Edit: I misread and thought you were getting high current without feeding a signal.

[g1]

Vol at "5" is drawing 1 A current., and climbs more rapidly after Vol "5"

It's a 160W@4 ohm amp. So at full power you should see at least 1.3A more line current than with no signal (with 120VAC line voltage).
Once you hit clipping it will ramp up much faster.

[glebert]

When we say "oscillation", what does that mean in the case? I am thinking the amplified signal keeps being regenerated?

Input jack switches are working.

I have a 4 ohm dummy load.

Vol at "5" is drawing 1 A current., and climbs more rapidly after Vol "5"

Vol at about "4" is drawing about 0.55 A. Pics below of Normal channel Vol at 4 and the 1K input signal. Scoped at the amp output...

For some reason I'm not seeing pictures with this, even though the HTML looks like there should be some embedded. Is it just me?

[misterc57]

I see the pics in my browser.

Anyway, all seems good now. Another lessons learned here.

The speaker that was in the amp is a 8 Ohm Celestion G12 Modern Lead 70. I just discovered that it is blown, hence no output.

I am guessing that the original speaker was a 4 ohm Scorpion 200 watts or higher. Anyone know?

After all the tune-up work and connecting to a temporary working 8 ohm speaker, all seems good.

Thank you to all for your valued input.

MC

[nosaj]

Only pics showing are in Post 1.
nosaj