PM700 Channel Cuts After Clipping

[JesseDismania]

Hi all,

This is my first time posting so I apologize for any formatting errors.

I have a PM700 I've been working with, it's been serving me well but I'm stuck on a problem that's popped up:

Channel 4 is cutting out completely after peaking. If I power down the console and bring it back on then the input works as expected again, but will cut out after clipping and will only send a signal to the cue until power cycled again.

I thought it might be a capacitor issue but it doesn't appear to be and I am stumped. If anyone has any insight I'd really appreciate it!

 

[JesseDismania]

I think it might be an op-amp issue, I bridged 2 of the pins on the second IC and it appears to stay on.
Anyone have a good site to order vintage parts from?

 

[gyraf]

What opamp?

 

[JesseDismania]

What opamp?

TA7136P

 

[drtechno]

TA7136P

That one is pretty well extinct by now. A lot of people move on and get or make an adapter board and use a TL071

You'll find someone out there that probably junked a PM4000 for those op amps and are still stuck with them.or the electronics hoarders that bought lots of them and trying to sell them 10 times their price to people that don't want to use adapter boards.

 

[JohnRoberts]

That symptom sounds like a circuit losing it's nominal DC bias, perhaps from a bad solder connection (common) or open resistor (uncommon) on a biasing component.

If you can probe around with VOM to look for a circuit that not at correct nominal operating DC voltage.

JR

 

[ohm]

Those opamps are nothing special and you'd probably do better replacing them like suggested with some little adaptors, though I'm not sure TL071s would be ideal as I dont think they are rated for the +/-22v that the TA7136P are probably running on.

If you're super desperate to replace them as exact, then you could look at swiping this PM180 Rack Mixer

https://www.ebay.com/itm/334845143819
It uses the same opamps PLUS has 5x Tamura input transformers and 2 large output transformers AND 2 NE80100 discrete opamps. I think it basically near the same circuit as the PM700 in a 3U rack.

I picked up one of these last year and I'll be looking for some replacement opamps and recapping with better caps when I get a chance