HP339A need help, distortion section not functioning correctly

[ausman1000]

I didn't get a chance to delve back into it today but hope to tomorrow. I quickly pulled the pcb assembly and checked C25, it is good. The parts have arrived so I will probably just change out both U3 and U5 and run over the same tests again.
Thank you!

 

[jan.didden]

I may be late to the party, but the tune LEDs seem to work correctly. They should be lit at over- or under-range which seems the case here.

Do you guys work from a schematic or groping around in the dark?

Jan

 

[BSST]

Hi Jan,

Working from schematic. I think arrows are probably working properly given inputs from phase control loop, but something's "broken" so plenty of uncertainty.

Hi Ausman,

I'm relatively confident of diagnosis re U3, but lots of uncertainty about U5. We can conduct more tests re U5 if you like. I'm loathe to have you replace it when I've made a lot of guessing about its health.

If you can get the amplitude loop working, it will be a good guide to signals expected in the phase loop. There's a remote chance all begin to work properly.

 

[ausman1000]

Ok I will just replace U3 first then test. I'm on the fence about using a socket, what do you think?

 

[BSST]

Sockets seem good to me, and this area is not in direct distortion signal path anyway.

 

[ausman1000]

Just got done changing out U3 and the same exact problems persist. If I take the external signal source down to approximately 800 Hz the CW led extinguishes (which I would expect?) but if I bring the frequency way up past 1 khz both led's light up (I would expect the CCW led to extinguish at some point?).

Should I go ahead and just change out U5?

I went ahead and changed out U5 and something has definitely changed in operation. I will report back after I have run a few tests.

 

[BSST]

Sorry for my unreliable presence, but it sounds like you're making great progress.

 

[ausman1000]

Are you kidding? I wouldn't mind if you only stopped by once a week! lol

Well....
I do believe we have a winner!
All Operational Verification Checks per the manual pass with flying colors and the led's are behaving.
Internal oscillator distortion is reading -98 dB (if I'm reading the scale correctly) at 1 kHz, 1 V RMS.

 

[Dilbert]

Good to hear you’re up and running, a low cost fix as well!

Too bad about those knobs, you might be able to get a parts unit cheap.

 

[BSST]

Congratulations, Ausman! Well done!

 

[ausman1000]

Yea, those knobs are virtually unobtainable. I may have a better chance of finding some HP winged knobs and swapping the clear section onto them.

Quite the contrary, congratulations to you BSST!
My hat's off to you dear sir. Thank you for sharing your knowledge and devoting your time and patience to my issue. I couldn't have done it without you!