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.

[xtian]

Whelp, I removed the main PCB, tested good LV values (+15, -15, +12, +5 all good), reassembled, and started up successfully. However, just like in post #1, after 20 second warmup, relay switched, amp was drawing ~200w, I heard OK white noise from speaker, then about ten seconds later relay clicks off, and no more HV.

Why would the relay control switch drop its control voltage? Bad C19?

[Tom Phillips]

Can you monitor the voltage at Q3 base during startup until the fault? If that voltage rises to around 15V and does not change when the relay clicks off then I'd be thinking about a bad Q3 more than a bad C9. Could also be something going on between Q3 emitter and the relay coil. You could monitor the emitter voltage during start up too. Based on what you find, someone may have ideas of what to do next.

[xtian]

I'm considering bypassing this timer/relay circuit, as it duplicates the function of the standby switch. Opinions?

[g1]

You could monitor the emitter voltage during start up too.

Yes, make sure it is getting that proper 9V to begin with. It's possible it is weak already at start-up but just enough to engage the relay for a while.

[Tom Phillips]

I'm considering bypassing this timer/relay circuit, as it duplicates the function of the standby switch. Opinions?

Although I have not looked at the complete circuit, I will say that Turn On timer circuits are usually there to suppress loud sounds that would otherwise he sent to the speaker when the amp is first turned on. Given that, I would not bypass the circuit without some further input.

I hate the scavenger hunt to trace the circuitry over many separate pages!

Edit: OK. I found the relay on the "AC-TERM BRD." schematic. I see that the relay contacts appear to be in series with the standby switch. Therefore, my guess is that the amp needs time for something to stabilize before the B+ is applied. My first thought was the bias circuitry but I just got called away so ran out of time to look further at all the connections across the multiple pages. I'll just say that there must be a reason the relay circuit is there given the way cost control is usually enforced by the bean counters at the company.

[xtian]

You'll note the standby switch is in series with the timer circuit. In other words, operator can replicate the function of the timer circuit by using the standby switch. I found another tech who bypassed the timer circuit:

http://johnkvintageguitars.homestead.com/SVT-VR.html

I came to the same conclusion, and implemented the bypass of the timer circuit. Amps works perfectly.

[Tom Phillips]

... I found another tech who bypassed the timer circuit: http://johnkvintageguitars.homestead.com/SVT-VR.html I came to the same conclusion, and implemented the bypass of the timer circuit. Amps works perfectly.

Thanks xtian.
That seems to sum up the situation. Just leaves a mystery of why the amp was designed with the timer circuit in the first place. Maybe someone had strong feelings about importance of the standby but I won't start down that path. There are already plenty of long standby function discussions.