Ampeg SVT VR - repair

[Feindrum]

Hello everyone
I have already repaired much SVT VR but here, I have a new problem.
It's one SVT VR "Made in China"

Problem : No green ON light, No Led BIAS and No sound

My observations.

- No sound

- No Led BIAS

- No heather in preamp

- I have heather on power tube and tubes driver

- All tubes is ok

- Fuses is ok

- I have a modification on supply PCB : J28 is connected with yellow cable, normal ?
I don't have schematic for that.
Image : https://ibb.co/xfxxyx9

Anyone have any ideas?
Thanks so much !

[Jon Snell]

Check the filament transformer and the +-15 and +5volt outputs. Check power supply in general for good voltages.

[nevetslab]

Here's some of the schematics and drawings you'll need:

AC Terminal PCB Schematics and Parts List (07P319-04_2).pdf
Bass Amplifier PCB Layout (598PWA_A).pdf
Bass Amplifier PCB Schematics (598SCH_B).pdf
Power Supply PCB Schematics (792SCH_A).pdf
PWA 300 Watt Power Supply PCB Layout (792PWA_A).pdf
PWA Tube PCB Layout (365PWA_0).pdf
Tube PCB Schematics (365SCH_0).pdf
Tech Note -- Transformer connections drawing.pdf

The AC Mains PCB contains a relay which, when it closes, turns on the HV Xfmr. If that relay doesn't close, you'll have to track down the circuits to find why you're not getting the voltages to that. They originate on the main PCB, then up to / from the preamp PCB, and finally down to the AC Mains PCB. As Jon indicated, you'll have to verify you have the heater voltage from the small power xfmr, which also supplies the bipolar voltages. These amps are known for having solder joint fractures on the main PCB, the AC mains PCB, the preamp PCB and the output PCB. The sheer weight of the power and output xfmrs all too often have loose mounting hardware, which in transit, turn the amp chassis into a shaker table to vibrate everything, and break down solder joint connections.

I've written a number of technical posts on the care and service of the Ampeg SVT-CL and SVT-VR, including the circuit descriptions and troubleshooting guides, so spend some time using the forum's Ampeg SVT-VR listings. When I get a chance, I'll post some of them to lead you to the details, but this amp (and the SVT-CL) is well documented.

[The Dude]

Step 1- Turn the thing upside down and give it a good shake. I rarely find an Ampeg of this era that doesn't have screws and/or nuts rolling around inside. Ampeg has not yet discovered lock washers.

On that note, while you have it open, go around and tighten up everything. There are bound to be loose screws and nuts.

[nevetslab]

Compiling a list of SVT-VR Service threads from this forum:

https://music-electronics-forum.com/...-repair/46429-
https://music-electronics-forum.com/...-repair/49663-
https://music-electronics-forum.com/...-repair/49660-
https://music-electronics-forum.com/...otection-relay
(this one deals mostly with the SVT-CL, but there's good info on power tube selection and general info on the SVT tube amp series)
https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/48292-ampeg-svt

https://music-electronics-forum.com/...lown-tube-fuse
https://music-electronics-forum.com/...-service-plate
https://music-electronics-forum.com/...-repair/49830-
https://music-electronics-forum.com/...-repair/50093-
https://music-electronics-forum.com/...-repair/47152-

[Feindrum]

Hello everyone,

Thanks so much for help !

So my new observations on J4 connector (filament XFMR) :
Pin 1 : 15V
Pin 2 : 7V
Pin 3 : 7V
Pin 4 : 0V (normal ?)

So I dont have DC voltage for +12V, 5V ...

I have just -18V DC on C12

[Jon Snell]

All supplies, low voltage, must be present before it will show any signs of life.
BEWARE! the main HT may be charged so either wait for a couple of hours or discharge C23 & C29!
Work from the output ends of the power supply, backwards until you find the missing link.

[nevetslab]

What appears to be missing in my records are the details on both the SVT-VR and the SVT-CL/AV/Pro2 Filament xfmr. In the parts list of the amplifiers, those two filament xfmrs carry a different part number. On the SVT-VR Filament xfmr, which brings in two windings by way of J4 (on the main power amp PCB), Pins 1 & 4 is one winding, while Pins 2 & 3 is a second winding. Following the two discrete rectifier bridge circuit, there should be +/- 20VDC at the main unregulated filter caps C4, C5 & C12. Following dropping resistors R7 & R36 are 15V Zener diodes D2 & D3.

Also on the Filament xfmr is the high current heater winding that connects directly to the main power tube PCB, and that 6.3VAC winding leaves that board to feed both the heaters in the preamp, as well as on the main power amp PCB to run the heaters of the two driver tubes and the input tube of that power amp circuit. The AC Voltages between Pins 1 & 4, and Pins 2 & 3 at J4 should be the same voltage, or close to it.

[Feindrum]

Hi again,

Yes, so I have on J4 :
Pin 1/4 : 20VAC
Pin 2/3 : 15VAC

It's correct. After diode bridge, I don't have voltage in DC.
All diodes 1N4007 is ok, strange...
I make new soldering

I have just -18V DC on C12 (negative pole)

C7 was faulty, but nothing again with new capacitor.

[Jon Snell]

Crack on the PCB or poor/no ground.

[Feindrum]

Yes, I'am sur also for ground problem.

Just a question for the ground on main board.
The Ground is here :
https://ibb.co/gdXbfLt
If yes, I don't have shunt with star ground and screw, normal ?

[nevetslab]

Yes, I'am sur also for ground problem.

Just a question for the ground on main board.
The Ground is here :
https://ibb.co/gdXbfLt
If yes, I don't have shunt with star ground and screw, normal ?

You DO NOT have the grounding screw thru that portion of the PCB, into the metal standoff? YOU NEED THAT.