Ampeg SVT VR protect circuit

[xtian]

Ampeg SVT VR on the bench. Powers up and warms up, drawing ~100w; after about 20 seconds, relay clicks and amp draws ~200w. Green bias LEDs are lit. After only a minute or two, relay clicks and power draw drops back to 100w. So, I want to examine the protect circuit.

In the schematic, page 3, area C1, it shows "RELAY CONTROL", but I can't find the relay in the schematic. And I don't know what the circuit is sensing. Help appreciated.

Ampeg_SVT-VR.pdf

[Jon Snell]

There, unfortunately, is no power input schematic.
I think it is the Sort Start circuit relay control ... if it has one, most of the high powere Ampegs do.
Look on the board for J9 and check out where it goes.
200Watts is a little high for idle current, even for this model!

[nevetslab]

The Relay is on the AC Mains PCB on the left rear of the chassis, has the IEC Mains connector & Fuse holder on it. I've attached those drawings below, along with some other files from my SVT-VR folder.

AC Terminal PCB Schematics and Parts List (07P319-04_2).pdf

Ampeg SVT-CL Power & Heater Xfmrs.pdf

Ampeg SVT-CL, -AV, -VR Output Transformer.pdf

Bass Amplifier PCB Layout (598PWA_A).pdf

Tech Note -- Transformer connections drawing.pdf

PWA 300 Watt Power Supply PCB Layout (792PWA_A).pdf
:

[xtian]

Thanks for that AC Termnal schem, which shows the relay. The Relay Control that I mentioned above is just a 20 second time delay...it's not monitoring anything in the amp, as far as I see.

I guess I'll check all the power tubes' support resistors, and try to find if one or more power tubes is malfunctioning.

[Jon Snell]

Don't miss the obvious!
Check the ECC82/12AU7 driver valve on the cathodes.

[nevetslab]

You're right. J9 is just the turn-on voltage to feed that on the Relay board via J35. Q3 in the time-delay turn-on circuit on the power supply PCB isn't monitoring anything, unlike that found in the SVT-CL, where there is control circuits that will shut down the power transformer and send the front panel LED into flashing GRN/RED.

Power draw is usually more like 230W at idle with the rear grille panel Fan connected, but that's a function of the power tube's bias settings.

For many years, I had been using a cardboard panel punched out with test points to access the plate resistors and screen resistors of the power tube PCB while powered up. Finally ordered some hi temp acrylic material from McMaster Carr and fabricated a transparent Service Plate to replace the metal cover plate, which let me see all of the power tube PCB, with openings to access the plate voltage points of the primary O/T, the connector terminals of the Grids, heater, cathodes and GND, and get at everything with the PCB mounted to the plate and plate to the chassis.



Prior to these two iterations, it felt reasonably dangerous without that tube PCB being anchored in place without the cover plate to secure it. The brown fiber cover plate helped, but still a PITA not being able to see the PCB below. I never did cobble an alternate fixture to stand up at a right angle so you had access below the power tube PCB while energized. Do-able, but would need some additional metal work to stabilize it. Another task for laterman.

[xtian]

UPS lost my package for two days, so now I've finally replaced the burned plate and screen grid resistors on the power tube board. But now the amp won't come out of time-delay startup. I may have lost my +/-15v supply. But I'm lost without a layout diagram for the SVT VR.

The filament transformer has six wires at its secondary: pairs of white, orange, red. Unloaded, the red pair is 13.4vAC, and the orange pair is 13.1vAC. These windings must feed the 15v bipolar supply:



But I don't know where to probe the 15v rails. Do I have to remove this PCB in order to see the parts numbers?

[g1]

But I don't know where to probe the 15v rails. Do I have to remove this PCB in order to see the parts numbers?

You should be able to probe the 8 or 14 pin op-amp from the underside to check for +/-15V supplies.