SENCORE PA81 Repair - One Channel Out - IC Replacement

[@lancehughes4335]

Great video sir! As these units get older, we are seeing more problems. My PA81 developed a problem of the monitored audio in each channel would either go low or drop out. I saw the same thing on my monitor scope for each channel. I could delicately push on the board and the issue would go away. Long story short, the relay packs were acting up. I carefully removed the covers and with a small relay file I cleaned each one. All problems resolved. Keep up the great work.

[@Wil_Bloodworth]

Excellent. Thank you for making this video!

[@Tinybench]

I have one of these units so I took notes. I hope mine doesn’t fail, but if it does, I am ready because of you. Thank you very much much! And coincidentally, I am also doing a pair of Marantz model mine and your video was also helpful! What a coincidence! Thank you, thank you thank you so much! I have a few repair videos myself. Nowhere nearly as nice and professional as yours but I love doing them. 🙂🙏

[@user-xc8vw5oq8v]

I have one of these Tnx for sharing

[@maraeht]

Just Picked up A Sencore PA81. It has a right & Left channel VU intermittent pointer jumping thing going on. When you hit the meter select switch it will sometimes stick all the way to the right hand side, As you select from -60- to +40 and then -60 the pointer in the VU will stick to full scale right and sometimes just bob around for a second. You can hear very low static from the right speaker. If you give the side a light tap it will drop down and get quite. Sounds like something with a connectivity problem. If you jiggle the function switch it goes away .I'll have to go thru and check all connections. BTW Fantastic Video. I will be ordering those parts just incase.

[@NovaluxStereophonic]

@@maraeht these have issues with the relays after sitting for a long time. It may just resolve itself during normal use. The most common issues with these are faulty relays and broken switches for the monitor speaker controls.

[@maraeht]

Thanks for the reply just picked it up on my phone I'll get into it probably tomorrow fiddle around in there check the relays get out the old Kremlin D5 and start hunting really enjoyed your video and set up but I'm jealous because you have a full page sheets schematic schematic where can I get one of those I have PDF but they're a pain Patera paint to navigate but they're a pain to navigate love the way you put your video together keep up the great work regards mark

[@maraeht]

@@NovaluxStereophonic Did you take your schematic some where in PDF to get that Full size schematic you were using in your great Video? Several guys on my Sencore forum suggested Staples or a printer to make a one sheet full size for us old guys..L.O.L. Also they said that Adobe now has a service to make poster size anything. But it has to upload in .JPG. and you have to have the full Adobe program $$$$ which would convert PDF to .JPG. I just have the free Viewer. How did you get yours done? Thanks: Regards: Mark

[@Edisson.]

Hi, I saw the PA81 inside for the first time, thanks for the interesting look. The advantage of repairing these devices is the possibility of comparative measurement - that is, assuming that the second channel is functional. Nice day 🙂 Tom

[@johncunningham5435]

I also own the same equipment, and it is indispensable, to bad the PA81 was discontinued, a great piece of test gear.

[@poormanselectronicsbench2021]

Nice repair! Too bad these units are so scarce and pricey when they do come up. I found a video somewhere from Sencore that priced them out about $2K when new, and although they go for less now, its still tough to find a working condition unit.

[@PrimeHiFi]

Great troubleshooting work! IC’s are always tricky. I wonder what originally killed the chip… could have been a high voltage transient, old age, or failing power supply? Either way nice job!

[@NovaluxStereophonic]

I noticed the failure of the left channel shorty after testing a power amp capable of producing about 215W which was going into oscillation. Not sure if it was coincidence or not. The front end protection circuitry seems to be pretty robust so not sure exactly what happened.

[@jim9930]

@@NovaluxStereophonic My guess would be the oscillations killed it. Why? If you look, the design engineer breadboarded the circuitry and then realized all the cascaded stages left droop in the high frequency (20k) - so he added the .01u, 200p, 200p, 270p. Alas, he forgot that .01u around the 10k input resistor allowed spikes to get to the supply rails. The opamps can take the abuse; but only 10 extra volts will pop the cmos - first one died protecting the remainder. Those aren't low noise opamps, a 1k in series would have limited the overvoltage current and he didn't have supply bypass caps! It only took 35 years for power amplifiers to become real beasts - there were very few back in the day. edit: Allow me to clarify a little... He probably originally designed in the .01u for noise performance after initial tests. The 10k diode combination was typical manufacturer recommended on a data sheet. Cmos avalanches and it's written in some documents as max current per pin or chip - I tested a bunch of them just to see the limits of each manufacturer's process before specifying them into a final design years ago. If you add 5.6v zeners on the supply rails in your PA81, add a couple .1u ceramic bypass on the supply rails near the diodes, and series a resistor with .01 at the input (pick value that doesn't degrade the noise performance, probably 3k?), you should never worry again. Did you check the electrolytics while you had it apart?

[@NovaluxStereophonic]

@@jim9930 Thank you for the input. I did not do an extensive capacitor check while I had the board out as I plan to be back in the unit soon to rework the power supply board and come up with an alignment procedure. If I end up pulling the board again I may consider consulting you for advice on how to implement the additional protection circuitry that you recommended.

[@Trucam2020]

Thanks for sharing this repairing video 😊

[@user-yn9cc6bw8b]

Great job! How to find a good quality schematic from this analyser?

[@garygranato9164]

Question: on the schematic why do some of the ground symbols have an 'L' below them ?

[@NovaluxStereophonic]

I am focusing on the L channel here. The right channel has R by the grounds. The L and R channels are completely isolated in this device so that you can test BTL amplifiers and other amps like those form Audio Research that cannot be connected with common ground.

[@garygranato9164]

@@NovaluxStereophonic thank you for the info, my imagination got carried away and i was thinking that is was an exotic virtual ground or something. lol :)

[@jdmccorful]

Great job. Good idea with chip extenders. I always agonize over using them.