500w stageline amp repair help

hi guys

would be greatfull if anyone could help with this repair project, i have been at it for a while and am almost there,
i have been getting help on another forum but it seems to have ran dry,

https://www.electronicspoint.com/threads/stageline-500w-amp-repair-project.270979/

please take a look any help finishing this off would be greate!

I really don't want to get involved with a 500 W Amp as someone's first repair. Let's just say I worked on 13 kV regulators at 1.5 Amps, and I built the leach Amp from scratch and I used to do only the problematic amplifier repairs for a repair outfit in my spare time.

For the benefit of every one else what's the deal now? One channel doesn't work and that channel is ? and the protect light is on?

I'm not sure which channel doesn't work, but for the benefit of everyone that might get involved:
Define the bad channel: Left/Right
Don't bridge it.

Measure from ground: T1, T2, T3, T4, T5 and
The differential voltage between T32 and T31

and the equivalent points for the other channel.

everything needed to know is on that link, any how i am back where i started with the fault LEFT LED lit and the protect light on,
here is a schematic for the unit if its of help,

I'm with KeepItSimpleStupid, trying to repair 500W amplifiers isn't something you attempt with no experience - and there's no such thing as 'almost there' - it's either repaired or it's still faulty, and if not repaired correctly it will self-destruct again.

Sorry to say but I think you killed it.
I stopped reading the other posts when you said you reversed the power supply polarities to the output section and all the fuses blew.
It looks like the original problem started when the cooling fan stopped working, witch means all the components inside have been heat stressed.
Reversing the polarity to the output stages could have blown all the semiconductors and electrolytic caps.
As I have found in revers polarity cases unless you replace everything at once the new parts well just keep blowing.
Due to the fact the amp is nothing special and is only worth about $100 if it worked, I would not wast any time one it.
Even if you get it working one of the other stressed components will eventually fail.
Its refered to as a boat anchor in the repair industry.

I asked for voltages and the others for comparison.

Protect, usually means there is some amount of DC on the speaker terminals. Nominally -0.75 < x < _0.75 V and usually in the mV range. Protect usually just opens the speaker leads protecting the speakers. I have yet to stare at the protection circuitry, but sometimes (usually not though) you have to disable it to fix the amp. In my home built one, I have to.

Startup is usually done by using a Variac (or variable AC supply) while monitoring the current. With a signal and monitoring the output. You don't have that.

Some of your output devices are paralleled which means they need to be matched by current gain if replaced. You might have to buy 10 to get 2 sets of matched pairs, but you only needed two,

The "bias adjustment" can be temperamental. Two high and the amp overheats.

Amps are funny. When they die a lot of parts fail. Miss one of them and the amp will fail again, possibly immediately.

A load isn't necessary for solid state amps until final testing. It's mandatory for tube amps.

Reverse polarity could mean significant damage. I got an amp to fix that five shops refused. The components had "puddled" on the PCB.

KeepItSimpleStupid said:

The components had "puddled" on the PCB

Click to expand...

Sounds like a fun project. not

It wasn't too bad. I just had to use multiple vendors to get all of the parts. Bill was something like $300. The amp was borrowed and used in a disco and money was "no object" because of the "borrowed" aspect.

The toughest one was the Leach Amp. I made a mirror image of the board and did not realize it, I was able to use the board by moving the NPN's to the PNP spots and cutting and moving only two traces for the bias regulator. That was amazing. It went through a few iterations. I used a CVT with like a 20 A secondary that hummed to much and I used 100 uF 50 V caps on the board that EXPLODED. The supplies were 50. Won't do that agian.

So, I moved it to a 2 RU home built case and ordered a custom 35 VAC x4 at 3A toroidal transformer. In hindsight, it should have been higher current. The construction article said 3A total and I doubled it. 40,000 uF of capacitance total on 4 supplies. Thus I needed a slow turn-on circuit and protection. Nothing really fancy, but I switched the NPN and PNP outputs by mistake and only had to replace one resistor. Nothing but a 3A AGX fuse for speaker protection. But, if any of the amp rail fuses pop, a metal oxide resistor will pop.

Now,, it runs with a 500 W Sorenson AC sine voltage regulator as front end. I bought that for $100 in NY. Worth about $2000 when new. It gets improved bass with the regulator.

I added input and output thump protection. The input protection was necessary, because the delay was based on all of the filter caps getting to 2/3 final voltage. That's when the metal oxide resistor got shorted out. It's very unusual to have rail fuses, but a "power OP amp" on one rail isn't nice.

The input has a FET-Optocoupler in series and the audio ramps up logarithmically which is really cool.

The amp has an unheard of un-rolled off frequency response of 0 to 800 kHz. Not a typo. it's rolled off to be 0.5 Hz to 40 KHz. Built on a ground plane with 99% metal film resistors and a 10 turn pot for the bias adjust. The heatsink runs really really cool.

I never got around to a "clipping indicator" or a high temp sensor or improving the turn-on circuit. I made a clipping indicator or a work project that was really cool using a bi-color LED and therefore it detected both positive and negative clips and extended the duration. Turn-on improvement would have been something to prevent the resistor from popping. i.e. If all of the supplies weren't up to 2/3 max, then shut down the amp. The bigger problem is room in the enclosure.