Strat 500 weak distorted channel B

[stasis190]

Hi guys, semi noob here (well maybe super noob whatever)

Have a strategy 500 power amp. Channel B is weak / distorted / almost no bass. Here is what I checked so far:
-Track 2 or track 1 functions the same and they switch ok
-Switched all power and preamp tubes from CH1 to Ch2, didnt change any symptom at all.
-Checked and double checked all speaker connections and tried the 4 ohm and 8 ohm taps with diff speakers/cables
Preamp:
-Checked preamp Plate voltages (all high 200's VDC)
-Checked preamp cathode voltages (all ~1-3VDC)
-Checked coupling caps (grids on all preamp tubes 0VDC while pre-cap it is high 200's from plate)
-visually looked for cold solder etc.. didn't see anything
Power amp:
-checked bias supply for push/pull/cha/chb all around -54VDC
-all plate volts high DC

And finally last night i busted out the oscilloscope and was comparing waveforms on the preamp grids. I found that pin 2 of tube 1 on the channel B has a super distorted / wanky waveform whereas channel A on that preamp tube pin has a sine wave (200HZ input.)

I am leaning towards removing the tubes and checking this again to see if the measurements are affected. Also It appears that the feedback loop is connected to this pin so I am slightly skeptical of that area of the circuit. All I got is a strategy 400 schematic which seems to be accurate in that area around the preamp tubes at least.

Any troubleshooting tips from some pros?
Strategy 400 Prelim Schematic.pdf

[Leo_Gnardo]

That strange looking waveform you point out is inside the feedback loop, and represents the loops attempt to correct the distortion. I see there's a funny lookin' thing on the cathodes of the pre and fb triodes, expect that's a constant current source between ground & pins 8&3. If everything else checks out, I'm suspecting that as the source of your problem. Maybe swap CCS's from one chan to the other & see if the problem follows.

[stasis190]

Leo,

That thing turned out to be a J508 current regulating diode. I switched them as you so aptly suggested. Same problem stayed on channel B.

Then I tracked down the feedback from the output transformer and disconnected it (on CHB.) Within 5 minutes of doing this and leaving it disconnected, CHB started to work great again after a few pops and sputters. hmm.

So I reconnected the feedback loop, reassembled, and am playing music through it now. I think I hear a *very slight* amount of hiss on channel A that was not there before i switched the CCS. So faint it is a hunch really. Anyways this is very strange to me but nevertheless I ordered some NOS J508's on ebay and am going to switch these out when they arrive. I wouldn't be surprised if that's all she wrote, and I also wouldn't be surprised if it starts acting funny again. Thanks again buddy for the sharp shooting! here's some pics for the curious!

[g1]

For the curious, the J508 current regulating diode is like a FET with gate shorted to source:
J508 | Vishay J508 Constant Current Diode, | Vishay