Ampeg SVT 5 PRO Anyone had one on the bench?

[selloutrr]

I just put an Ampeg SVT 5 Pro on the bench.
one of the channels (clean) is super low level and distorting. The rest of the amp sounds great.
The preamp tube tests 116% side A 100% side B no gas no leaks.

here is the schematic if anyone else needs it.

[Andy Le Blanc]

the scheme has test point figures, start at the power side of the section that's misbehaving.
then input a wave form and see what the test points tell you.

[selloutrr]

sounds like a plan.

I was thinking of injecting a signal and working thru that ch. monday.

reading up on these amps they have a voltage starving problem.
IC4's heat sinks not big enough.
47k and 1.5K resistors drifting or burning out causing transistors to blow.
lots of power section problems.

[selloutrr]

anybody have a schematic for the SLM preamp overdrive ch?

here are the power amp schematics for LOUD

[MarkusBass]

Has anyone schematic of the preamp? You posted twice the schematic of the power amp and the power supply. I need to solve a problem in preamp.

Mark

[selloutrr]

What is the production year and version?

The sticker inside on the board should say.

[MarkusBass]

I haven't got the amp yet. I may have it in a few days. But I know that it has a problem with channels switching. Were there more than one version?

Mark

[selloutrr]

Almost a dozen versions they switched manufacturing during production.

I've had several on the bench. You are in for a special treat. You will have to most likely build a test harness to operate the cards outside the housing.

First test the tube then chopstick the relay I had one that broke free of the solder - easy fix. The Jack has also gone bad before. The shunt wasn't shunting.

Make notes some models aren't well labeled and a bitch to put back together. It takes me almost an hour to pull one apart and 90 min to rebuild and double check before power up.

Good luck.

When you get the version and year I probably have the schematic.

[ampdoc1]

You'll earn your money on St. Louis Music Ampegs. Had a new one in my shop one time,..turned out two of the wiring connector harnesses had been installed backwards. NO WAY it ever passed any kind of inspection.

[jelle]

Wow.

[tubenoob]

I'm having a look at one of these for a friend and I really have no idea what I'm doing.
I could use some tips on how to approach troubleshooting the amp.
The symptom is it makes a loud horrible buzz/hum when you power it up. Problem occurs in both channels with all controls at zero and preamp tube pulled.

Doesn't blow fuse or trip breaker but it sounds so loud and awful you want to turn it off immediately.

I guess it's a problem in the power amp?
What should I do to troubleshoot?
Any help will be immensely appreciated!

[eddie25]

Is there a 12AU7 in those amps? Those rack mount SVT's drive tend to drive those hard, but I don't think that's normally the symptom.

I would check for DC voltage on the output while there is NO load. If it's there (more than a few mV), then I think you can expect one or more of the output transistors to be bad.

[tubenoob]

I'll check that, by no load do you mean no signal going into it?
I can't leave this thing on for more than a few seconds without panicking, it's so loud and awful sounding and it definitely had a loa do it at idle pulling over 1.5A with no signal going in and controls at zero and the preamp tube removed.

[eddie25]

No load is no speaker connected.

I would hook it up to a lightbulb limiter though, if it's pulling a lot of current.

You can measure DC voltage across the .47ohm 5 watt resistors as well and see if one or more gives an odd reading, those correspond to the power transistors. Just be careful.

[gui_tarzan]

I'm having a look at one of these for a friend and I really have no idea what I'm doing.

You worry me by making that statement. Are you at least aware of the safety rules?

I just put an Ampeg SVT 5 Pro on the bench.
one of the channels (clean) is super low level and distorting. The rest of the amp sounds great. The preamp tube tests 116% side A 100% side B no gas no leaks.

I just finished a XXX (yep, another one) that was distorting on the clean channel at very low volume. The high gain channels were distorted anyway but didn't seem to be very loud. After checking all of the normal points (burned parts, solder joints, etc.) I found the connections on the ribbon cable going to the power amp board were hanging by just a few strands in some locations. I trimmed the cable and resoldered everything.

Still had distortion and low volume. I checked the tube voltages and found the PI to have very unusual readings so I swapped it with a new one and it came back to life and was clean up to around 70% volume.

Just a thought.