Sunn SL260 output section short

[Tone Meister]

Fellas, I have some time on my hands so I'm digging into a few solid state projects. I'm a complete novice so I will need my hand held at times, especially in certain circuit sections (voltage regulation, voltage dividers, etc). That said, here is one I dredged up thinking I could use the power transformer on the Fender FM65R I'm working on in another thread.
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Sunn SL260 chassis lights the bulb on my LBL and amp's pilot light won't light because of the current draw. Pulled both output transistors and light does not light and amp's pilot lights normally. Power transformer secondaries measure about ~30 VAC with outputs pulled. Replaced outputs with 15015 NPN and 15016 PNP but bulb still lights up although not quite as bright.

Seems like my +/-15v rails are screwed up but I can't tell where the voltage regulation happens.

[52 Bill]

Pulled both output transistors and light does not light and amp's pilot lights normally. Power transformer secondaries measure about ~30 VAC with outputs pulled. Replaced outputs with 15015 NPN and 15016 PNP but bulb still lights up although not quite as bright.

What else did you test in the power amp section? Is there any dc voltage on the speaker output?

[Tone Meister]

Drats, can't believe I didn't think to check that. Will report findings tomorrow morning

[J M Fahey]

Test amp with NO speaker or any load connected.
Aim at no DC on the output, and rails at least 20 or 30V each.
IF so, you can proceed with other tests.

[Tone Meister]

Test amp with NO speaker or any load connected.
Aim at no DC on the output, and rails at least 20 or 30V each.
IF so, you can proceed with other tests.

Thanks Juan, I learned early on from you fellas not to connect a speaker until it is confirmed that no DC on is on the output.

On this amp, there is no DC (~2mV) on the output with or without the output transistors installed.

[Tone Meister]

Here is what I found. Q209 small signal NPN BC107 was open base to emitter and shorted. It had been replaced previously with a 2N5089 and I used a 2N4401
IC202 opamp 4558 also had DC on the outputs so I socketed that when installing the new chip

OK, tested the amp with the power PCB unscrewed from the chassis and isolated and have the amp working somewhat. I can plug a guitar into the input and get decent sound.
However, I have a massive current draw (>1.2A) from the chassis ground to the heatsink, which screws directly to the rear of the chassis. If I attempt to ground the heatsink to the chassis the bulb lights bright. Measure in excess of 1.2A of current with my Fluke.

There is no DC on the speaker outputs. I have pulled and tested every transistor in the amp, pulled and tested both 3000uF/50V filters and reciifiers. Pulled all the TIP29/30 and outputs to be sure they are insulated properly and all test good. Not sure how to check the IC203 CMOS chip (MC140118).

Any idea what could be pulling that much current?

[g1]

Something mounted on the heatsink is shorted to it. You should be able to locate with resistance measurements.

[DrGonz78]

Something mounted on the heatsink is shorted to it. You should be able to locate with resistance measurements.

For example, Q206 tip30b pin 2 is in electrical contact with mounting base tab, hence the mica insulator. However, looking at the mounting screw it's rather difficult to tell if they have insulators for the screws. Ohm out the tabs of the driver transistors to the heat sink to see if there is continuity. Just something that jumped out at me when I looked at the picture and it is an easy thing to check.

Edit: Thinking on it more it looks a Fender type Sunn product, not sure. But the board reminds more of Fender boards that were loaded in a Sunn type amp. So actually the screw does not actually mount through screw holes on the heat sink. Rather it has very large holes that clear it and then there will be nuts on the board holding it in place providing insulation. Still I would ohm out those tabs to the heat sink to see if the micas are damaged.

[Tone Meister]

For example, Q206 tip30b pin 2 is in electrical contact with mounting base tab, hence the mica insulator. However, looking at the mounting screw it's rather difficult to tell if they have insulators for the screws. Ohm out the tabs of the driver transistors to the heat sink to see if there is continuity. Just something that jumped out at me when I looked at the picture and it is an easy thing to check.

Edit: Thinking on it more it looks a Fender type Sunn product, not sure. But the board reminds more of Fender boards that were loaded in a Sunn type amp. So actually the screw does not actually mount through screw holes on the heat sink. Rather it has very large holes that clear it and then there will be nuts on the board holding it in place providing insulation. Still I would ohm out those tabs to the heat sink to see if the micas are damaged.

Well you missed that one pretty badly Doc, it was actually Q203 TIP29. You need to step up your game a little bit.

Actually, I need to pay closer attention to the minutiae because this was completely preventable. I don't know what I was thinking, but I replaced the old mica with a carefully cut piece of Q-pad, which of course is conductive. Details ...

[J M Fahey]

OUCH !!!!!!!!!!!

Q-Pad II is a composite of aluminum foil coated on both sides with thermally / electrically conductive Sil-Pad rubber. The material is designed for those applications in which maximum heat transfer is needed and electrical isolation is not required.

You might have just *soldered* the case to heatsink

[Tone Meister]

OUCH !!!!!!!!!!!

You might have just *soldered* the case to heatsink

Oh no, I'm pretty slow sometimes but I'm not retarded. All work to this point has been done on the LBL Deluxe plugged into my Staco 1520 with ammeter and volt meter. I brought it up slowly with the heatsink unattached, then I checked only momentarily to SEE if there was any current draw, which there was, BEFORE attempting to reattach the heat sink. So I now have all the output section shorts sorted out and am now moving on to the humming reverb circuit.

I was gifted this chassis as a donor amp, but it is so simple and so easy to work on that I figured I'd take advantage of the opportunity to learn a bit more in the SS world. I think I have almost $5.00 US invested in parts right now, plus a little of my time. What's the amp chassis worth, maybe $25?

Here is what keeps me from burning down the place Juan.