PL700 Rebuild

[Overundr1]

Verified the buss bar was grounded to chassis. Added the star ground just to be sure with no noted change.

 

[George S.]

B+ and B- rail voltages are good?
If so, than see what Joe thinks.

 

[Overundr1]

B+ 109.9vdc

B- -109.7 vdc

On mains

 

[Overundr1]

Wondering if I need to install the other two rca pre drivers since this is an original 700 back plane, late tonight will try that tomorrow.

 

[Sunnbobb]

Signal from source is clean?

 

[Overundr1]

Verified clean scope trace yes

 

[mlucitt]

I would put my ungrounded scope on the outputs to see the distortion, then work backwards along the signal path until I found no distortion. If this issue is present on both channels, it is likely a power problem. Ensure the rail fuses are all good by verifying the voltage on the load side of each rail fuse.
Good luck hunting.

 

[Overundr1]

Will get back to this, family health issues.

 

[Overundr1]

Wife out of hospital and on the mend so back at it. New triac board installed, trimmer for led’s installed, new turret board installed. Still have output distortion. Bias still perfect about 1mv of offset. All fuse holders cleaned and correct agx fuses installed. VU meters respond correctly. Hooked up my scope and what in flouroscanland is this? No input and look at the double waveform (one probe). Duplicated on my old Techtronix as well. Have my suspicions but rather than guess thought I would ask for suggestions. Also have rca predrivers in all four bottom row locations.

 

[Bob Boyer]

Glad your wife is out of the hospital. Hope she's on the mend and can enjoy Thanksgiving. Been there, done this with one or the other of us. It's always tough, even as there is much for which to be thankful.

Good luck tracing this down, too. I'm watching from afar.

 

[Overundr1]

I found out just how fast a sepsis attack can affect us retired folks. In the space of two hours she went from I don’t feel so good to collapse and ambulance call. Thanks for the well wishes

 

[Gepetto]

Wife out of hospital and on the mend so back at it. New triac board installed, trimmer for led’s installed, new turret board installed. Still have output distortion. Bias still perfect about 1mv of offset. All fuse holders cleaned and correct agx fuses installed. VU meters respond correctly. Hooked up my scope and what in flouroscanland is this? No input and look at the double waveform (one probe). Duplicated on my old Techtronix as well. Have my suspicions but rather than guess thought I would ask for suggestions. Also have rca predrivers in all four bottom row locations. View attachment 84378 View attachment 84379 View attachment 84380 View attachment 84381 View attachment 84382 View attachment 84383

From your picture, it appears that you may have the ground channel to channel jumper in place on the board backside between pad 5L1 and 5R1. Cannot tell for sure but it appears that way in your photo. When you added the wires to the single point ground bus bar to terminal block 5R and 5L, that jumper was to be removed to prevent inadvertent ground looping.

 

[Overundr1]

Jumpers installed 11L 11R, 12L 12R,
No jumpers 5L 5R, 16L 16R, and 17L 17R
Original backplane all NPN outputs.

 

[Overundr1]

Almost like any real input level increase triggers the distortion. At ultra low levels amounts perfect. Out of time for scope work today will dig in again tomorrow. Did test on known good speakers just for my own sanity.
May have to inject signal directly into the board input point and start chasing the trace

 

[Overundr1]

Got to be something simple, both channels doing the same thing, need to go over the board component by component.

 

[Overundr1]

Also cannot figure out the crazy frequency 3.01 mhz?

 

[Bob Boyer]

I found out just how fast a sepsis attack can affect us retired folks. In the space of two hours she went from I don’t feel so good to collapse and ambulance call. Thanks for the well wishes

Oh my. Been down that path. About lost Pat to sepsis last year when they postponed surgery for two hours to wait until the morning. Two hours is all it takes. My thoughts are with you guys.

 

[Overundr1]

Looks like I need to probe pins 4 and 7 for voltage and pin 6 for output waveform on those op amps as a start tomorrow.

 

[Overundr1]

And looks like pin 9 on the control board is the output to the output array, correct?

 

[Overundr1]

First off the fact that Joe includes the schematic for his proprietary board makes trouble shooting so much easier, thank you sir. Verified clean signal generator input to rca jacks, input to control board clean, input both inverting and non inverting pins 2 and 3 on the op amp clean, pin 7 15.1v pin 4 -15.1v. However output on pin 6 was a disaster on either channel, wish I could hold phone oscope probe and take a picture all at the same time. basically a lot of hash and minimal waveform. anyways need to order some op134's.
Can only blame myself, even with checking followed the wrong back plane output transistor numbering sequence and probably stressed those poor op amps by doing so even on a dbt startup. Stay tuned for the next installment once Mouser gets me the op amps.