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SVT-4 Pro problem

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davebungo

Oct 13, 2007
13
0
#1
Hi everyone.

Hopefully an Ampeg expert can help me with this...

I'm attempting to debug/fix a problem with my Bass player's SVT-4 Pro. The fault first reared its ugly head at a gig last night. Basically, the amp cuts out after a short time (measured in seconds but not always the same.) The line output is still there - just no power amp output by the sound of it.

Coincidentally this occured after he tried out a new Ampeg cab for the first time - I assumed that it was a faulty cab, but we got his old one and the fault persists. We checked cables etc. also.

After a bit of physical jiggling about, we managed to get it going and completed most of the gig with it but eventually the same happened. Again, a bit of physical movement got it back.

Oh, also, the fault reset's after power off and back on.

I opened the thing up to take a look. One thing I noticed was that the black wire from the mains transformer is not connected to the main PCB along with the other wires. Is this wrong or is it simply a variation of a standard design. Any idea which connector it should go to assuming it should be connected? (J?? on PCB).

I'm sorry this is a bit rushed but if you have any questions please ask. ANY help greatly appreciated.

BTW I have contacted SLM for a service manual - I'm stabbing in the dark without one really.

Dave.
 
#2
it depends a little on which transformer you have. That SHOULD, if a recent one, be a 94-161-40.

If so, the black wire is a mains power (primary) wire. The secondary wires should be orange and yellow separate wires, and also red and green on a connector.

If you are here in the US, 120V, you should have the black wire and a brown wire going to J19 and J20 , and a white wire and violet wire going to J21 and J22.

If that looks right to you, go with it. If it does not, or you aren't sure, best not to do any more, but get it looked at.

if you are elsewhere, tell what your mains voltage is.

An old enough one may have only the black and white wires (94-161-01 transformer).

I don't know why that would affect the output, since at 120V the other half of the primary winding would still be supplying power, just not as efficiently. At any other voltage except 100V, the unit should be dead with that wire off.
 
#3
Hi Jerrold,
thanks for your reply.

The transformer is 94-161-41 and I'm in the UK so 220V.

The Tx wires are as follows:

Primary:
WHITE (J25)
BLUE (J23)
BLUE/WHITE (J19)
BLACK ------------------------------- (J??)
PURPLE (J22)
GREY (J26)
BROWN (J24)

In addition, the mains live input goes to the switch out via J18, back from the switch to the fuse and then from the fuse to J15.

Secondary:
RED
GREEN
RED
ORANGE
YELLOW
ORANGE

The RGR wires from the secondary are conected by a latching connector to a header on the 15/0/15 PCB, but the header is not the proper receptacle i.e. doesn't retain the socket as it should - it's just a 3 pin header. This is a bit of a shame because when I opened it up the socket wasn't fully home and straight - looks like it may have moved under vibration. SLM ought to know about this - perhaps I should drop them a line (to be honest this looks like a case of lack of proper stock so something else was fitted). Would the loss of the 15/0/15 have explained the symptom i.e. loss of power amp only or would all audio have disappeared i.e. does the 15/0/15 feed the pre-amp stages (other than the valve stuff of course).

BTW are the valves (sorry tubes) run off a proper HT - if so where is this getting to the pre-amp board?

<edited> just had a closer look and yes, the 450V caps are a dead give-away, so it looks like the loss of the RGR wires from the secondary could certainly spoil things. I have measured 320V across one of them so it looks like the HT is OK at least while it's working <end of edit>


Thanks,
Dave.
 
#4
For that voltage, the black wire isn't used, so that's OK.

The red and green wires being loose would probably explain it. The tube filament voltage comes from them, so if they were bumped so that they made contact, the tubes would get heater voltage, and even if contact was lost again, they would work for a while until they cooled down.

The tube plate voltage is supplied off a "voltage multiplier" circuit that is operated by the orange& white wires. The tubes get about 260V plate voltage.
 
#5
Hi,
I just noticed that the large relays which click on at power up are clicking (presumably off) whenever I flex the chasis. So there must be a dry joint somewhere - time to track it down I think. I'll keep you posted. I presume the relays connect the outputs - is that correct?

Also, is it OK to power up without a speaker load with the master volume down?

The more I look at this design from an electro-mechanical viewpoint, the more I don't like it. Having such a weighty transformer at one end with the main PCB anchored all around just seems to be a recipe for long term problems...
 
#6
Jerrold Tiers said:
For that voltage, the black wire isn't used, so that's OK.

The red and green wires being loose would probably explain it. The tube filament voltage comes from them, so if they were bumped so that they made contact, the tubes would get heater voltage, and even if contact was lost again, they would work for a while until they cooled down.

The tube plate voltage is supplied off a "voltage multiplier" circuit that is operated by the orange& white wires. The tubes get about 260V plate voltage.
Click to expand...

I was just typing my second reply as you posted your response - thanks Jerrold.

I now believe that this is simply a case of a cracked joint or track somewhere. The HT was probably OK all along as the line output was OK when the fault was evident.

Dave.
 
#7
I think the fault is now repaired but only prolonged testing will prove this to be the case. I have to say that although the inside of an SVT-4 Pro is nice and neat, this doesn't seem to equate to long term reliability. As far as I'm concerned, the way the PCB is mounted to the chassis coupled with the large weight of the transformer at one side is asking for trouble. If you lift one front panel handle up, the whole chassis flexes across the diagonal of the unit and the main PCB simply follows. The main bridge rectifier is clamped to a heatsink in one corner so the soldered PCB joints take some strain.

The fault seemed to clear up once these joints were re-worked.

It reminds me of a Trace Elliot valve combo I had (100W Trident) which was a fantastic amp and all, and equally neat and tidy inside but it suffered a lot of reliability problems due to the use of PCB connectors in critical places where a hard wired solution would have been better (although not from an assembly viewpoint).

Don't get me wrong, I'm not out to slag off Ampeg, just reporting what I see.
 

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