Yikes another PL-4000

[Overundr1]

Member Geezer sent his PL-4000 seeing as how I had so much fun with the first one. For those that remember his was the one where he saw about 30vdc on the output jacks. Since the PL-700 still waiting for replacement op amps thought I would dig into it. Nice clean piece albeit no cover or headphone jacks with harness assembly.


This is one of the late models that has the strange molex connector between the tone board and the now eliminated decoder board serial number 11499



Sure enough, pulling 30vdc on the output connectors, not good. Noted several missing back panel rca output jacks so fixed them first before continuing,
Have a solid signal up to the tone board, picks up the 30v afterwards. The PL04 logic board which is fed from the tone board has the infamous rca 4136 quad op amp chip, this one has a small burn mark (hard to see in the picture) and I suspect it is allowing the VCC voltage to leak into the gain stages. Unless there is a better solution have the TI4136N in my Mouser order along with the DIP sockets. All the transistors tested fine and will need to update the caps from 35 to 50v of course.

 

[laatsch55]

Got a handle on that pretty quick..

 

[Overundr1]

Redid the tone and logic board this morning, just waiting on the 4136's. Contemplating ordering the 4739 brown dog replacements as well. Uprated voltages on the caps, mostly Nichicon ES and UKL with WIMA 1uf films. Alll molex removed, cleaned and re-flowed.


Hard to see in the picture but tried to photograph the ring of death around the main boars molex connection for the tone board. Would not even attempt this without a good de-soldering vacuum station


Finished the three control board side connections and sealed with nail polish after verifying traces.
Lots of grumbles about working on these, will say easier by far than most any receiver, especially those with wire wraps or boards that require pinning. Everything right at hand and accessible so to speak.

 

[Overundr1]

Whilst waiting for parts I have been reconditioning the rest of the boards. Noted the rc4739 is no longer made, realize the brown dog adapter with ic is almost $30 each with shipping. Would not the rc4559 work for testing if I fiddle about with the pinouts to match the original 4739 before leaping on the brown dog wagon? I suppose in a real pinch I could pull the RIAA board and transfer the 4739 chip to the tone board if necessary for testing.
In other news all but two boards are finished, the entire control board has been cleaned and re flowed as well. Figures I ordered those op amps for the PL-700 last Tuesday 2 day ups and they now will not arrive till Monday what a racket.
Tempted to KiCad/EasyEDA my own adapters to use the OPA2134 and order 100 of them from JLCPCB just because I can
And while I am at it I have close to 400 blank BA312 adapter boards (meant to order 40 oops) so if any forum members need them they are free other than the minimal postage required. The BA312 was an sip transistor array used in many of the mid 70's japanese stereo gear, especially Sansui and have become a source of static and noise as the packaging deteriorates.

 

[Overundr1]

After full recap and chip removal still had that pesky voltage on the main outputs but only when the logic board was installed. Traced the circuit back to the front panel and phooey ran into this, the right channel bass potentiometer was shattered driving a piece of metal onto surrounding components. Once removed the voltage to the output jacks went to zero vdc. Need to either find another bass potentiometer or figure out what value resistors to install to bring that channel to unity.

 

[Overundr1]

Thanks to help from our fearless leader a bass control pot and headphone harness arrived this morning. Now have two happy PL-4000’s to play with. Full recap done, will install covers after a couple of hours of listening.

 

[Overundr1]

Been running the 4000 for several days now, it currently is hooked up to the main in jacks on my Sansui AU-9900A. Also wired up some bling with my cigar box PL-400 meters. Thanks to Laatch55 for donor parts and others for advice now have two restored units, one really early production and one late production. Both have had every single solder point cleaned and reflowed, all electrolytic caps replaced etc etc and sound amazing. Looking back at the amount of time required to do them right there is no way a commercial shop could break even on labor and even a weekend warrior tech needs a stubborn streak to go through one. Here she is happy as can be.

 

[Overundr1]

One of the 4000’s did not have a case. Along with changing out the bottom plate with the slide in plate from the later versions I also threw together a wood case to protect the innards.