Motorola ST-82A stereo AM-FM record player

[William]

Speaking only about the two Motorola's that I have rebuilt, one being a not so portable stereo, and the other a 3-channel tabletop radio, both were designed and built well. The not so portable stereo is the one I have complained about having no bass but will blow you out of the room. The tabletop radio works and sounds good.

Bill

[electra225]

I'm sorry, Bill. It wasn't my intention to cast dispersions on Motorola. I know they built good stuff. They were the "other M". But I also believe many manufacturers built a dud or two. This one may certainly be considered such a dud. This thing is horribly designed. The function switch connections are buried inside a plastic ring around the switch. It is impossible to visually inspect, and nearly impossible to clean. It handles AC power, B+ and the signal. There are two heater strings. When the 12AU6 opened, I noticed that part of the tubes were lit, part were not. I started the changer, and got audio out of one channel. The other channel was dead silent. I got to looking at the schematic and noticed the two heater strings. One output tube and the 12AX7 was on one string, the other output tube was on the other string. The FM tubes were on one string, the AM tube and the audio tubes on the other string. In fairness, I put thousands of trouble-free hours on this stereo when I had my restoration shop. It served as my shop radio. But, boy, when it broke, it has really broke. I'm still not convinced that the function switch is not a part of the problem. I'll take another stab in the morning, and try to determine if it's losing B+ or if it's losing signal. I have a VTVM on the cathode of the rectifier, one on B+ into the function switch, and one on the plate of the mixer tube, the 12BE6. I will also fire up the signal tracer to see if I'm losing signal to the first audio tube, the 12AX7. I also need to run the radio on FM for a time to see if it fades and dies, or if this just happens on AM. I'll get it. At least now I have an avenue of investigation.

[electra225]

I may have found something interesting. Now......how to fix it?

B+ stayed steady on AM and FM. The rectifier and power supply seem to be okay. I even think the function switch may be okay. I decided to try to align the AM band and see if that made a difference. When I adjusted A1, the lower trimmer on the second IF transformer, the radio would go dead. If I withdrew my alignment tool, the radio came back on. I got to wiggling components and found if I move the second IF transformer side to side, I could get the radio to cut in and out. I either have a bad solder joint, a bad IF transformer, or a crack in the board. It will take a magnifying glass to find out which.

[electra225]

This stereo has SMD in the second AM IF transformer. It's not the lightning crashing type. It is the intermittent IF transformer type. I can still wiggle the IF transformer and make the radio quit then start working again. This is a five terminal PCB mount IF transformer. The chances of finding another correct part is not that good. I put everything back into the cabinet for the time being. I found a 17C9 in a parts chassis I had upstairs. It doesn't test that good, but better than the other one. FM works decently. The second IF transformer with SMD is common to FM, but doesn't affect FM like it does AM. If I can't find a correct part, I won't do anything. If I take it apart to fix SMD and break one of the wires, that will kill the tuner completely. AM works sometimes, but not dependably. Never when the chassis is cold. This stereo is not Motorola's best effort. Very disappointing. The record player works well.

[William]

It sounds like you have made progress today even though you still have issues to deal with. This Motorola is giving you fits like my VM with the squeal is giving me fits. With mine, I put it back into storage and will deal with it in the fall.

Bill

[electra225]

I wanted to find what was causing this issue. I don't think I'd ever run across this exact issue before, and I wanted to learn something. Connecting three VTVM's really gave me information I needed. When the AM bands goes bad, B+ goes down about five volts for some reason. Voltages are pretty much spot on when it is working right. I'm going to try and find the right transformer for this thing. There is an outfit the TV guys get flyback transformers and other TV transformers from. I can't think of the name right now, but maybe I can find it on VideoKarma.

[electra225]

Moyer Electronics is the place I was thinking of. They advertise old TV parts, but it's worth a shot. I can get the part number, then also list the crossover numbers listed in Sams. You never know they might have one. IF not, I'll search elsewhere to see if one exists. FM works pretty well, so I don't want to mess that up. I may have no recourse but to remove the old part and see if I can replace the mica cap without totally ruining it. SMD is not something I typically fool with. My eyesight isn't good enough anymore, nor are my fingers nimble enough to work with the fine wires.

[Motorola minion]

I have just one used 6C9, that fixed a Silvertone chassis where FM was as dead. It performs fantastically considering the antenna I have it on, better than the Magnavox tuners.

The oscillator side of the 6C9 tested so weak, it wouldn't work. RF amp would pass some 10.7 MC through when I connected a generator to the FM antenna, confirming it was NOT and IF interstage transformer issue.

Im glad you found one Greg, I have a used 17C9, kept for troubleshooting in case this happens to a portable/table radio. I have two or three other consoles that use the 6C9, four with post-61 RCA's RC1205 tuner and a second Warwick-made Silvertone, also using an RCA tuner.

[electra225]

I am still looking for an IF transformer for this stereo.

[sd7]

Hopefully this schematics I found with a recently bought st82a is helpful

[maxidyne]

Thanks for sharing and welcome to our forum......

[TC Chris]

How about posting a scan of the schematic to the downloads section? I copied it and pasted it into Word so I could enlarge it (fuzzily) but having it in downloads would make it more useful.

Chris Campbell

[sd7]

I will make a higher resolution scan/picture and try to post it there soon. I've got the original sales packet and schematic

[maxidyne]

You are a new member. After you make five posts you could download the schematic to the Downloads extension at the top of the index page....

[William]

Welcome, sd7, and thank you for sharing the schematic. Like both Chris and Greg mentioned, please, after your 5 required posts, add your schematic to the downloads. And if you have not already done it, please take a long look at all that is available here on the forum and please share your projects with us. We all enjoy what is being worked on or even what one has in their collection. Again, WELCOME to the forum.

Bill

[sd7]

I don't have much knowledge or electric engineering skills so 5 posts might take awhile. I'll try to get info about and take pictures of the transformer that's installed in mine soon so it can help the original poster find and install a transformer.

I'm new to classic hifi. A few months ago, I bought a 1959 Motorola Sk33 from a great guy and electric engineer who refurbishes them. Since then I've been keeping an eye out for nice looking and repairable stereos. I saw a wall mounted stereo cabinet for the first time at an estate sale (ST-82A) and bought it despite it only being able to play the radio since the cartridge was missing (Gary helped with that).

Like the SK33 (Drexel built), it's a beautiful piece of MCM cabinetry. I've taken it to an engineer to install a transformer to address the always-hot AA5 issue and BT adapter (I've been advised to upgrade to a Wi-Fi adapter for better sound file quality and will probably try that). I'm hoping to use the rear external speaker connections on the ST82A to connect a wireless subwoofer and give it a fuller range of audio playback. The end use of it will be in a MCM-styled 1964 condo tower to be a pseudo-tv console stand, record player, streamable stereo system, and front speaker system for the tv. I will be wall mounting a tv above the stereo cabinet and maybe place a center channel on top of the stereo so the tv has a 3.1 output.

Long story short, some of these MCM cabinets (Motorola especially) are beautiful and worthy of preservation. I hope to someday develop electrical engineering skills so I can find more and bring them back to life myself but that'll take much longer than this post.

[maxidyne]

Five posts don't take long. You already have three......

[William]

What Greg said, you already have three and if you share a photo or two of your toys you will have 5. I Googled your Motorola SK33, that is a nice-looking unit, something to be proud of and I hope it sounds the way you were expecting it to sound. I have a Motorola SK52W in the restoration process. I am starting with the cabinet, it was totally beat up and full of drywall dust, mouse poop and urine, and just plain dirt. Everything came out so all I had was a bare cabinet, striping was next, followed by lots of sanding, stain, and it is now waiting for a final finish. Once that is done, I will start on the electronic guts and the changer. Please, do not be afraid or discouraged about rolling up your sleeves and tackling something. I am an old, retired guy that always wanted to work on electronics back when I was a kid and never did back then. Now that I am retired, I rolled up my sleeves and went for it. So, if I can do it so can YOU! The only thing I will warn you about, it becomes additive, one leads to two which leads to three, which....We have a saying on the forum, "there is always room for one more and one must learn how to stack".

Thanks for sharing, and please share some photos of what you have.

Bill

[sd7]

SK33 photos attached. Tube sound is nicer than the modern HT amps I use. Nice to have a 15" sub in the cabinet. I'm considering trying to try to frame-in the speakers and sub in the sk33 and add some foam sound mats so they perform more like a sealed speaker. Not sure if that'll help make a tighter sound and/or cause issues with heat buildup.

Speakers don't perform as well as the Martin Logan ESLs I use. Have a small tube amp coming so I can try tabletop vinyl on the ESLs soon.

[sd7]

ST82A photos attached, it'll be interesting to hear how it plays when I get it back. I'll try to copy the specs and paperwork I have for both models and upload it soon.