Ampeg SVT 4Pro Repair

[andy3055]

Posting this for anyone who may be able to use my observations.

I had this Ampeg SVT4 Pro for repair that stumped me quite a bit. It had two separate issues that confused me.

1. There was some repetitive noise which sounded like something was "oscillating."
2. Both channels were distorting with no real level of volume. Sounded like as if there was not enough signal from the pre-amp!

We were able to rule out the pre-amp by using another power amp.

My friend who owns the amp had only one cab and troubleshooting was time consuming as it took a while to switch the cab between channels in between. It also threw us off since the noise signal was prominent on pin 14 of IC 101 which did not clear up after replacing same. Added to this, the scope died on us while being left powered on overnight. It is an old Tektronix which I am still trying to repair!

Back to basic troubleshooting using a DMM. With no power we checked all the transistors on that area of the schematic and found no difference between channels. Even though checking the FETs would not give any clear indication, just for the kicks we did and found that the display was all over the place with Q 101 as against Q 201 which was not so.

As checking/removing any component involved pulling out the whole power amp board with the heat sinks out of the chassis, we took the opportunity to check all the critical resistors/caps while it was out. We were thinking it has to be a common component as both channels were distorting which usually meant one of the common ICs or a common part in the power supply. But we were wrong! What we found was that R 126 and R 226 were both open and that explained why both channels were distorting!

Replaced Q 101 and the resistors 126/226. Also replaced R 119 and R 219 as they were all the same 47 ohm 1/2 watt resistors. I felt the original flameproof resistors were so tiny to be 1/2 watt though. On close observation they were very slightly discolored and was easy to not suspect unlike regular carbon film resistors.

It is a solid amp but very poor construction with regards to servicing.

[Jazz P Bass]

It's a good thing when you win one.

[nevetslab]

Welcome to the forum! Glad to hear you were able to get thru the maze of the SVT4-Pro's construction and track down the critical culprits to the problem. And, happy to hear you didn't have to replace any of the MosFET's in the output stage. I heartily agree with you on the non-servicable design of this series. As I maintain a sizable backline gear rental inventory, where we have numerous SVT3-Pro thru SVT6-Pro amps, I've built simple Power Xfmr extension cables so you can run this amp outside the chassis, though still tedious. Pulling the cover off of one, and finding open power resistors in the output stage, down in the 'valley' between the two heat sinks, and finding a bank of shorted MosFET's..........it gives me pause. These are one amp I need to be in the right mind-set to dig in!

[andy3055]

Agree with all comments and thanks for same. Those flameproof resistors are very deceiving to say the least! I also find that the fiberglass material they have used for the board is of poor quality as the traces that are too thin in some places get lifted off the board very easily. For anyone who attempts a repair on one of these, extreme care is the only word I can say. At 67+ years, I have repaired all sorts of tube/solid state amps and I pride myself of pulling out components and soldering stuff in with no trace of the repairs I do. But this is a nasty material. The only advantage is that one can shine a light from one side and follow the trace on the opposite side on these boards. I was surprised that I was able to find the failed culprits just using the meter as the scope while it worked, threw me off and sent me in circles!
Thanks and regards.