Setton bs 5500

[bigdrums925]

Hi once again:
I am working on a Setton BS 5500 stereo power amp. The owner brought it to me because it blows the main fuse on one side when turned on. (this unit has 2 completely separate power supply's and power switches)
I found what I thought was a leaky filter cap and replaced it, which then seemed to be leaky itself. So I swapped out for one of a lesser value to see if the problem continued. My variac has a sensitive amp meter so I can see if the unit under test draws excessive current.
So here's the problem: When I ramp up the variac to about 40% the power draw slowly increases even after I stop the ramp up.
At about 1 amp draw I shut down the variac, but nothing seems to be getting hot measured with a infrared thermometer. I have disconnected the amp section from the power supply board and the problem persists. I do know when I disconnect the 2 yellow wires from the power supply board, the problem stops. They supply about 48 vac each direct from the power transformer.
That the power draw continues after I stop ramping up makes me think that some transistor is beginning to turn on into a shorted circuit. But I can find nothing shorted.
What are the thoughts here for me to chase down?
Apparently the RS 660 is similar which I have a service manual for but is too large to upload

[nickb]

Have you checked the rectifiers? Do you get a healthy DC output from the power supply with it disconnected from amp section?

[bigdrums925]

There are 4 large diodes on each power supply board and they check good. Tested all the transistors there too. I get +/- 24 volts even connected to the amp board.

[Jazz P Bass]

Here is the RS 660 schematic: Setton RS 660.pdf

And a mug shot of the BS 5500:

[nickb]

Looking at Jazz's pic and the RS660 tells me the RS660 differs in the power supply. The pic shows two identical transformers whereas the RS660 schematic has a high voltage and a low voltage transformer. Therefore, I don't have any confidence in using the RS660 schematic right now.

The BS5500 is a dual monoblock design so if one channel is working you can use that to measure against.

[nickb]

I've found some stuff that will help:

Power amp



PSU+

[bigdrums925]

Its been some time but I'm back working on this Setton. I have a feeling the problem could be leaky filter caps. When I disconnect 1 of the caps + leads the excessive current draw goes away. Comments?

[J M Fahey]

Either the cap is very old/was overheated/etc. and is VERY leaky or , quite possibly, is connected the wrong way.
Or by mistake a much lower voltage one could have been misplaced there.

Rise variac slowly and check voltage across all 4 main caps , meaning +/- rails in 2 separate supplies.
Also check whatīs printed on them and post everything here.

EDIT:

When I disconnect 1 of the caps + leads the excessive current draw goes away. Comments?

Yes, WHICH of them?

[bigdrums925]

I can only raise the variac to about 70% on the right side resulting in -44.x volts on 1 cap and +44.x volts on the other, drawing 1 amp and rising

The left side draws lots of current (over 1 amp) at only 5% variac. The voltage is very low on the caps of course

The problem seems to lie in the negative supply filter caps because when they are disconnected the excessive current draw goes away even on the left side

They are 15,000 uf 63 VDC

[Enzo]

So sub something. For this test even just 1000uf would work. All we want to see is if the cap is shorting the supply, we don;t care what performance would be like cranked. If a sub cap makes it stop drawing current, then you know to get a real replacement for the cap. Answers the question in a matter of seconds.

[J M Fahey]

Again, I suspect the negative cap is upside down (negative to ground) and/or IF itīs held by a metal clamp, it cut through the insulation sleeve or .... hope you get what Iīm hinting at.

A substitution cap, if connected wrong, will *also* cause premature high draw at very low Variac "mains" voltage.