Help with 33609 build (smoking)

[dudewithabeer]

Hello everyone, I’m having a very hard time pinning down the problem to R44 going up in smoke on my 33609 build.

This problem occurs when the op amp is plugged in and turned on while testing the circuit. This problem only occurs from one of the op amps because I tried switching them out on both main boards and the problem only follows the one. The weird thing is once r44 is totally burnt on one board, on the other board the like resistor will go. However on initial power up the problem with r44 burning follows only one of the op amp instantaneously.

I have tried looking for shorts, replacing the some of the transistors (one in particular was bad), diodes, trim pot (replacing it and turning it down, up) and even doubled checked the psu which is perfectly set to 24v on both + and - sides. I’m trying things just to try and I cannot for the life of me get anywhere. I feel like I’m loosing my mind..

Any insight would be greatly appreciated.

Ps. I am new member, I have a good basic understanding of electronic and electrical, very good at soldering (I’m a welder by trade). Completed aml 1073, recapped amps. And fixed amplifier issues in the.

 

[Rob Flinn]

If the problem only occurs when the op amp is plugged in then I would be looking at the op amp itself. When I was building these I made a test jig up so I could test the op amps out of circuit. I also used sockets for the op amps so that they were easier to repair.

 

[beatnik]

Definitely use sockets for the opamps and if you are testing them separately make sure to add some form of current limiting (a bench psu with current limit function would be ideal)

Meticulously check for solder bridges and wrong resistor values on the opamp PCB. Use visual inspection with a magnifying glass and double check with continuity function on the DMM. You can also compare resistance measurements with the working opamp to spot any differences.

Next I would test the diodes and transistors with a DMM while in circuit, any suspects can be removed and further inspected with a semiconductor analyzer (you can get these from China quite cheap these days)
If you have spare set of semiconductors it might be easier to replace the whole lot (double check the new ones before soldering them just in case)

 

[dudewithabeer]

If the problem only occurs when the op amp is plugged in then I would be looking at the op amp itself. When I was building these I made a test jig up so I could test the op amps out of circuit. I also used sockets for the op amps so that they were easier to repair.

For the sake of the rest of my 10k resistors! Lol

Finn, That a really good idea and thank you for taking time to respond. I did use sockets thankfully. Once it’s mounted to the test board just run the 24v into the 24v term. And out into the ground to test?

 

[dudewithabeer]

Definitely use sockets for the opamps and if you are testing them separately make sure to add some form of current limiting (a bench psu with current limit function would be ideal)

Meticulously check for solder bridges and wrong resistor values on the opamp PCB. Use visual inspection with a magnifying glass and double check with continuity function on the DMM. You can also compare resistance measurements with the working opamp to spot any differences.

Next I would test the diodes and transistors with a DMM while in circuit, any suspects can be removed and further inspected with a semiconductor analyzer (you can get these from China quite cheap these days)
If you have spare set of semiconductors it might be easier to replace the whole lot (double check the new ones before soldering them just in case)

Beatnik, thank you for your reply, that’s good idea as well, I’ll have to check the old eBay for a semi conductor analyzer, and also need a bench psu, this is really helpful as well, as it helps me reset my to a starting point after chasing this problem for a few nights.

 

[Rob Flinn]

For the sake of the rest of my 10k resistors! Lol

Finn, That a really good idea and thank you for taking time to respond. I did use sockets thankfully. Once it’s mounted to the test board just run the 24v into the 24v term. And out into the ground to test?

Yes, I have found those amps to be a bit temperamental until you get them working right. Therefore it's good to test them out of circuit.

 

[dudewithabeer]

Yes, I have found those amps to be a bit temperamental until you get them working right. Therefore it's good to test them out of circuit.

Makes total sense, and something I will keep in mind for future build as well, I acquired a variable psu from my boss an hour ago so that’s cool. I’ll try to rig something up tonight.

As a side note, is it just these neve style op amps that are so temperamental that are a pain or all of them in general, I wanted to build a couple capi op amps to replace the stock ones in wa tb12 that I have.

 

[inkster]

I use a 489 card as a 440 test jig with bench supply that has adjustable current, it's good practice to current limit circuits of unknown condition, new builds, overhauls etc.