Pioneer SX-550 phono input cutting out

[jcsul]

I've got an SX-550 that has some phono stage issues. After 5-10 minutes (some times a little more or less) one of or both of the channels will drop down to almost nothing. Switching to a different channel (Radio, Tape monitor, Aux) seems to resolve the problem as whatever channel is out will immediately kick back in. A visual inspection of the phono stage looks good. No burt resistors, no leaking or bulging/vented caps. I've gone ahead an ordered replacements for all he PSU, driver, and output transistors to be safe, and also replacements for a general recap.

the reason I'm posting is I'm wondering if anyone has had a similar problem and the culprit have ben the TA7136 that is the IC that does all the Phone pre amplifying. While also looking around online I came across a forum post that lead me to this (http://cimarrontechnology.com/ta7136adapter.aspx) which is a drop in adapter that'll take any jellybean Dip-8 opamp that can handle the voltage the 550 uses in the phono stage (or I could whip up a little regulator for ones that don't) I'm curious if any one has used these adapters in a 550 before and what changes need to be made to the circuitry. If anyone has general advice on what the issue could be outside of a bad cap/transistor, the insight would be greatly appreciated. If Any one needs any testing done to help resolve the issue, I have a DMM but don't have a scope yet. I can take voltage measurements if it will help though.

 

[zebulon1]

Deox and Fader lube all the controls first.
My experience with the op amps is they start to pop and hum and generally make racket with the main amp.

 

[Watthour]

Verify the voltage at the OP amps, pins 4 and 7. The fact that one or the other or both channels drop might indicate that the problem is not one of the SIPs but something else.

Understand that the output signal from the phono equalization amp routes through a deck on S8, and selecting another source with that switch moves the contacts. Those contacts must be clean and reliable, just as the input jacks to the phono section must be clean. Verifying the grounds at pins 36/37 would be a good idea as well.

If a complete reacp will be performed, include those in the phono amp section.

 

[jcsul]

Deox and Fader lube all the controls first.

Done and done. All the switches and pots have been cleaned pretty throughly with contact cleaner. However, I know some times some pretty tough grime can build up on those depending on where they are stored, and I don't know where this guy has been for the past ~40 years. I know switches and pots can be disassembled and the contacts resurfaced, but that'd be breaking new ground for me. So I'm gonna try to stay away from that. Good to know that OpAmp failure presents differently than what I'm experiencing.

Verify the voltage at the OP amps, pins 4 and 7. The fact that one or the other or both channels drop might indicate that the problem is not one of the SIPs but something else.

Understand that the output signal from the phono equalization amp routes through a deck on S8, and selecting another source with that switch moves the contacts. Those contacts must be clean and reliable, just as the input jacks to the phono section must be clean. Verifying the grounds at pins 36/37 would be a good idea as well.

If a complete reacp will be performed, include those in the phono amp section.

I had already planned on replacing all the caps in the phono section as well since I noticed the issue, but testing voltages and knowing the pins to test now will be helpful, and will be simple to do. I'll make sure to check the grounds as well. The jacks seem clean, but telling whether or not they're clean on the inside is hard to tell. Will have to put the DMM in continuity and wiggle them around and see if I can't break the connection. I could try the same with the gangs on the switches. Thanks for the info.