BA283 Blown/Troubleshooting

I was using two BA283 type amps for makeup gain for a stereo Pultec.  While troubleshooting noise, I think I shorted something out and I'm pretty sure I blew up the amps and fried the output transformers.  I want to re-build the amps, but and replace any part that could have been damaged. 

Today I tested the amps by themselves, and the output was low and distorted.  I used a pot for gain adjustment (the 2k RV2 on the attached schematic), and this no longer affects the output.

Any help would be appreciated.

Craig

craigmorris74 said:

I think I shorted something out and I'm pretty sure I blew up the amps and fried the output transformers. 

Click to expand...

Why do you think you fried the output transformers?

craigmorris74 said:

Today I tested the amps by themselves, and the output was low and distorted.  I used a pot for gain adjustment (the 2k RV2 on the attached schematic), and this no longer affects the output.

Click to expand...

I would suspect the transistors first.
Easy way to test them is to get yourself a cheap, but great, tester.
De solder the transistors and test them one by one with the tester.

buy the MK328, great tester, really cheap

Hey Whoops,

I didn't know you could fry output transformers, but I sold them, and they didn't work in a different circuit.

The MK328 looks great and is cheap.  I'm going to order one now.

Thanks for the help,
Craig

You can generally troubleshoot simple semiconductors with a VOM...  Back before we had fancy digital VOMs with diode ranges, we tested diode drops with classic Simpson 260 using ohms scale.

You can often find faults without removing the parts if shorted or really trashed.

JR

I tried measuring the transistors in the circuit and some are giving me values I don't think are right.  When  I inject signal into each amp,  the distortion appears after the first transistor, and gets worse after the other two. 

I'm just going to replace all the transistors.  Are any other components likely to be bad?

craigmorris74 said:

I'm just going to replace all the transistors.  Are any other components likely to be bad?

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De solder the transistors and test them one by one with the tester.

It's not so easy to test transistors in circuit here because some of them are shunted by other transistors which gives you a false reading.  Better to pull them and test out of circuit.

Rob Flinn said:

It's not so easy to test transistors in circuit here because some of them are shunted by other transistors which gives you a false reading.  Better to pull them and test out of circuit.

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You can generally find bad ones that way a little easier, it is always possible for measurements of good ones to be conflicted by components in parallel.

JR

This needs an output transformer or choke load to work properly. Check transformer for continuity. Coil could burn o.c. In fault, 3055 would get hot and cook pcb and likely fail.

JohnRoberts said:

You can generally find bad ones that way a little easier, it is always possible for measurements of good ones to be conflicted by components in parallel.

JR

Click to expand...

There's only 6 transistors on a BA283 card, and they're all easily accessible.
It's pretty easy and quick to desolder them and test them outside the circuit

Whoops said:

There's only 6 transistors on a BA283 card, and they're all easily accessible.
It's pretty easy and quick to desolder them and test them outside the circuit

Click to expand...

first do no harm....  I used to operate a kit business and saw lots of horrible soldering, and some circuits boards lift traces from too much rework.

Use your own judgement, I prefer to try the easy non invasive test first.

JR

I'm about to replace all the transistors on my boards.  If the electrolytic are bad, could this damage other parts of the circuit of I power it up?

I'm not a fan of blindly replacing parts. If you have a scope you should be able to reason about what the problem is.

Is it possible that you reversed supply polarity? If you did for longer than a few seconds, it's very possible that the electrolytics are destroyed. Are any of them bulging or deformed? Did you smell anything funny around the time? If it is the electrolytics, then the transistors are likely OK.

craigmorris74 said:

I'm about to replace all the transistors on my boards.

Click to expand...

Why?
did you test them as suggested?

Whoops said:

Why?
did you test them as suggested?

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I've got them pulled out of the circuit right now.  If they test bad I'm replacing them with new ones.

Just testing the transistors with a multimter, none of them indicate that they are bad.

squarewave said:

it's very possible that the electrolytics are destroyed. Are any of them bulging or deformed?

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Check what squarewave said, any bulging or deformed electrolytics?

Also check the tantalum capacitors for possible shorts, I've replaced old tantalum capacitors in gear that failed shorted.

Check all the resistors also

Whoops said:

Check what squarewave said, any bulging or deformed electrolytics?

Also check the tantalum capacitors for possible shorts, I've replaced old tantalum capacitors in gear that failed shorted.

Check all the resistors also

Click to expand...

None of the electrolytics look damaged.

I forgot there were tantalums in there too.  Lots of things to go wrong.

craigmorris74 said:

Lots of things to go wrong.

Click to expand...

Actually it's a pretty simple circuit with a lower component count.
There's just a few things that could have gone wrong

It is a simple circuit.  Not complaining, but I've been told to check all the transistors, almost all the capacitors, and the resistors.  That means desoldering  almost every part. 

My question now is are there parts on BA283's that of bad, damage other parts in the cicuit?