Audio distortion from Phono Stage

[El Rubio]

Maybe you could use your scope to trace the signal path. Maybe it is the switch. I would poke with a chopstick the components and connections along the signal path as well. It may show a poor solder joint or flakey component. Also, since you mentioned it seems quiet at initial turn on, you could try freeze spray on those suspect transistors. I think tracing it with the scope may be the best bet.

 

[gadget73]

This is a long shot, but may be worth investigating. I saw a video a while back by someone who fixed noise in the phono stage of a Sony receiver from that era (STR-7065, if I recall correctly.)

The negative contact of the RCA jack is just a press fit into the ring that includes the connector to the internal wiring. He suggested that enough oxidation forms over time to disrupt the very weak signal from a phono cartridge. His solution was to make a little solder bridge on the back of the jack, between the outer ring and the back of the negative contact (similar to where the red dots are in this photo.)
View attachment 3094200
From the pictures I've seen, the STR-2800 appears to use the same jacks.

tube Sherwood gear has similar jacks, except they press through an aluminum plate. Can't solder it. I "fixed" it using a spring loaded center punch to tighten it up.

 

[HotRodStereo]

UPDATE

It turns out i did have some 1845s on hand so i threw them in until the 1815s come from mouser. The sound got a little better in volume, but a slight flutter remained. Since I didn't have new replacements for Q552 i swapped them side to side and sure enough, the noise followed the Q552 transistor to the left side. Looks like both transistors on that right side went bad the same way, at the same time. I believe in coincidence, but not that much coincidence. With what you guys pointed out about the RCA inputs and the noisiness i can hear in them, I'm going to take a look at firming them up as recommended, though i think my jacks are a bit different from the photos.

I have a bunch of other projects and some job offers on my plate right now so i will probably wait for the transistors to come this week before an update. Since this is a warranty job i want to get this guy his unit back asap and for good. I will grab photos as I go into the RCAs and post them here for posterity, great tips all around.