Ampeg BA210 overheating after mofset repair

[Ligsy1]

Hello world,

I plead for some help with this absolute nightmare of a repair. I have been handed a a ampeg BA210, it was reported to have a buzzing and "farting" sound at intermittent volume stages. So I opened this can of worms up and began testing components to ensure all are as should be. I discovered that the mofset were rubbish and replaced them all. On doing some research I noted the bais pot on these or shocking bad, so I replaced it for a nice 1k multi-turn trim pot.

I turned the amp on to bais but it immediately blow the DC fuse, I tested the power resistors and found that R61 was dead, I replaced it and amp came on for a short time before a small amount of smoke came from the board around the big capacitors, I've tested the caps and all seem healthy.

I have no idea what to check for next, maybe I'm being silly and missing something out but this thing has got my head in a pickle. I don't want to turn it on incase I do damage to more sensitive components.

Any advice or experience would be greatly appreciated.

[Randall]

I would suggest this is a prime candidate for a Light Bulb Limiter, if you don't already have one.

[Jazz P Bass]

BA210.
Switch mode power supply with a Class D output?

[Jon Snell]

Check the output Mos Fets are not damaged and check Q1 & Q4.
Ensure AP1 is at miniimum resistance, if the MosFets are correctly installed, of the exact type and have survived, try again setting the 15-20mA quiescent setting as per the service manual

[Jon Snell]

BA210.
Switch mode power supply with a Class D output?

Class B output stage not D.

[g1]

What do you mean the outputs were all rubbish? What exactly did you test/measure to determine this? I doubt it would have worked if they were all bad.
Where did you source your replacements from? Counterfeit transistors are not uncommon these days.

[The Dude]

^^^^^^^^ THAT! My guess is that perfectly good outputs were replaced with fakes.

[Enzo]

Plain old linear power supply, Basic MOSFET power amp.

[Jazz P Bass]

Yeah.

The BA 210 V2 is the one that I was thinking of.

Oops.

[Ligsy1]

Hi all,

I am going to construct a lightbulb limiter and hopefully begin setting the bais. The replacement mofsets are genuine from a trusted UK supplier. Unfortunately because this is an older BA210 alot of the exact replacement components are really hard to find or source in the UK for a reasonable price.

Since my first post I have discovered that the thermistor was also broken, it literally crumbled away in my fingers. I have now replaced the thermistor for an all most exact match. As the original type was none existent in the UK.

From reading other similar forums, the advice has been to replace or resolder the large .33 and 3.30ohm resistors but I can't read or see any problems with them on my board.

I apologise for my late responses, I do appreciate you all taking time to answer.