A post to an old thread, but one I feel to be relevant. I have 2 CR80 D-SS units here. Both are the deck only, without the STK based amp built in. One is made by AKAI, the other wearing Roberts on its face plate. Both are identical , down to the board level and component selection. They seem to share the common problem, of channel 3 dropping out or loosing gain altogether.
Just as the OP's deck did, as well as some other poster(s), Channel 3 (Left Rear) on both my decks has little or no sound or amplification. Probing transistors TR1 and TR2, the voltages are way off on BCE on channel 3 (Left Rear). The other channels 3 TR 1 and 2 check out matching the SM voltages. The Sanyo LD3141 voltages are also correct on channel 3 as well as the other 3, but they are know to be problematical. I have a dozen NOS here I picked up before prices went crazy. I'll try one of them as well. Transistors 1 and 2 are identical 2sc458's, even though the schematic shows another number. Al 4 channels have them on TR 1 and 2, they are well-known offenders. I keep 200 replacement KSC1845FTA's in stock. They are commonly found, failed with black corroded legs. 1970's SS gear is full of them, Japanese or American, no pattern to the model or brand, every other unit, sometimes every unit I'm inside has them. This is why I keep 200 on hand at all times.
I replaced all the electrolytics everywhere on every card, plus the main filter caps, power supply, and all auxiliary function boards, including and the motor run cap. All caps used were either Nichicon or Panasonic, with Muse or Audio grade 105c caps for all audio and other boards. All caps are low ESR, with high current ripple in the power supply areas. The noise floor and other ancillary noise dropped considerably from the original caps, I'd be willing to bet that the S-N ratio is well below the original numbers, thanks to tighter tolerance better performance components used in already well designed circuits. The 8 Varistors are the only things I am considering replacing going forward. They are carbon and plastic el-cheapo pots that no amount of deoxit will quiet or remove drop outs. 8 multi turn Bourns Cermet replacements in the correct resistance ranges are being considered.The heads and transport have been rebuilt, the AC motor serviced, new lubricants thru out. The heads are mercifully still un-molested. The factory adjustment loctite unbroken. As far as head wear, both heads are like new after a considerable cleaning then polish.No phase or crosstalk issues are present, all specifications checked during calibration after service meet and exceed those specified in the manua. All in all a tidy and quite respectable piece of audio gear once completed.
In case there is any doubt, let me dispel it now. Changing out all the old caps with audio grade 105c low ESR name brands, plus replacing the 4 channel pairs of 2Sc458's, AND the failed Sanyo LD3141(s) gets you THE BEST sounding playing or recording 8 track player ever made, bar none. ThIs model Akai ans Roberts are considered by many others in the industry to be the Nakamichi Dragon of 8-Track players. Research it yourself if you don't believe me, I didn't give them that name. Regarded as the ultimate 8 track deck to use, by those in the business of performing archive, or professional transfer, and duplication services. The better houses, even Library of Congress have and use them, or want them when they do 8 track work.
If you are an 8 track fan, there is no better 2 track (Stereo) or 4 track (Quadraphonic-Discrete) 8-track unit to own, with the added benefit of being able to record, with 4 microphone, single ended, or using 2 DIN plugs, with line level controls, discrete VU meters and more. I came across both by a happy accident, a client had one and found another for parts. I ended up restoring both, and gaining new respect for 8 track, which I considered a marginal format in my classic car at best. With a wide frequency response, a well designed AC powered motor, precision transport and more ,the resulting sound of a decent pre recorded or self recorded tape in the unit or recorded from a good source is jaw droppingly good. Why? Not because it's a record breaker for tape, but because it's 8 track and respectfully close to 20khz on the top end. Quad discrete tapes in good condition 'SOUND" as good as their open pre recorded reel counterpart cousins do. If you find decent Maxell or similar blank tapes, and record on them from a modern CD, high resolution files, even a good LP to tape, the results are worth noting,...hell actually toe tappingly good. Better than many of the cassette decks in that period or any other were. Certainly all the 8 track record decks I owned as a youth in the late 60's and early 70's or had in cars didn't hold a candle to this one..
Before I return these decks to their owner, I'm buying more blank Maxell or decent brand tapes. That way, I can record entire CDs of albums to tape and make mix tapes, plenty of both for taking on trips, to car shows or whenever driving my old car, which still has it's 8-track 4 channel stereo AM-FM 40-Channel CD factory in dash unit. I've also added another deck to my bucket list, something fun, and 8 btracks are dirt cheap hear abouts. After you replace the splice, the foam pads or felt guides, maybe add some food-grade silicone if its tight, your ready to rock. I have Kiss's Double Platinum rocking , something if found cheapo in the shrink, and it sounds, well good, and fun. If you can find one of these for the right price in decent physical condition and like 8 track, get it. Then have it recapped and tweaked. It's, well fun, and sounds better that 8 track has any right to, especially the 8 track sound and ker-chunk of tape eating rejects most of us remember.