$5 Riviera Deluxe

[electra225]

For the benefit of those of us who know zipkus about solid state circuits, Chris, would you share your thoughts on the suggestions you just posted? My guess is those caps are used for coupling caps. Am I even close?

[TC Chris]

I know minus zipkus, but I looked at the schematic and saw two caps, one probably paper/wax and the other electrolytic, series with the misbehaving balance control. Caps are a principal mode of failure, and the resistors in the circuit don't look to be heavily loaded, so I'm just guessing.

Chris Campbell

[stbasil]

Yeah, all electolytics replaced, the .1mf in the feedback circuits were not replaced, I didn't have but one handy, but the originals both checked good. While one channel was oscillating, the one with the single larger speaker, I tried substituting my one new .1mf, but no change in the oscillation, so I left the original. The oscillation follows that large speaker.

I've read that sometimes a particular speaker can destabilize a transistor amp due to its internal crossover circuit. If this little amp was designed for a particular set of speakers, I suppose that could be what's happening here.

I'm gonna shelve it for a while, since I just picked up a GE Wellington coffee table (see finds section). I'll start a new thread in the Other Consoles section for the GE.

[stbasil]

After being inspired by a restoration video on ARF, I decided to take another look at the speakers. It turns out that three of the speakers being open is due to corrosion on the voice coil wire as it exits the voice coil and attaches to the feed-through. I took a close-up of one of the tweeters, which happens to still be good, so you can see the green corrosion on the leads. When that stuff is cleaned off, the wire is broken at that point underneath the corrosion. I suppose it's just a matter of time before that one also opens up, too.

I managed to repair the woofer that was not seized by replacing the wires from the voice coil to the feed throughs, and patching up the holes with Post-it paper and Elmer's glue. The other woofer was seized because the pole piece had shifted off center. The only thing holding everything together was glue between the flat magnet, the pole plate and the frame of the speaker. The glue apparently failed, allowing the pole piece to sag out of alignment. Prior to discovering that, in my attempts to free up the voice coil, I damaged it beyond repair.

So, I went on eBay and searched for a 7.5 in woofer, and lo and behold, a guy was selling two of those Magnavox woofers plus four 5-in tweeters. Eventually I ended up paying $17 for the lot plus about $9 for postage. Pretty good deal! Those woofers have the older style magnets and looking at the listing photos it seems that they have no corrosion on the voice coil leads. I'll need to adapt the 4-in mounts to the 5-in tweeters, but that should not be a problem.

Now I have hopes of completely restoring this guy, hopefully can now get the little amp working with the correct speakers, so stay tuned!

[William]

Nice job, and every once in a while one gets lucky on eBay finding what you need. I was lucky a while back when I discovered that one of the 12" woofers, EIA code 619, in a VM was bad and just then one was available on eBay and at a reasonable price. The day was saved.

Bill

[TC Chris]

Make sure to check/replace the crossover caps so only HF material goes to the tweeters. If they're getting all the amp's output, that might lead to burned-out leads. On tweets, the most common problem is a small, over-drawn amp that's clipping badly. (Been there, done that).

You might be able to scrape that glue and solder a mall jumper over the broken lead. I did something like that to revive a KLH Model 17 tweeter.

Chris Campbell

[stbasil]

I'll be sure to replace the crossover caps, but the corrosion issue was present on all 4 speakers. That, plus the glue issue on the woofer, makes me wonder if there was some kind of quality control issue. Or perhaps the moisture exposure that this unit obviously experienced could have contributed. Anyway, after fixing/patching the original speakers I now have 2 functional tweeters and one functional woofer. But I'll probably end up replacing everything with the "new" ones from eBay, assuming they are all OK, they should arrive day after tomorrow..

[stbasil]

Removed the old grill cloth from the speaker baffles and temporarily attached the "new" speakers (which all have the same impedances as the original speakers). Even though the tweeters were 5 in, they fit the two 4-in mounting studs. The woofers are deeper than the originals, that makes the woofer on the left side not quite able to fit in the enclosure. I'll either just use the original working woofer, or cut a hole in the enclosure. Since there is already a hole there, it can just be expanded a little. The hole doesn't show when the turntable changer is folded down.

After hooking up all the speakers to the little amp, everything works great! The feedback oscillation that I was getting on my test speakers does not occur with the factory spec speakers and crossovers. So, the amp must have been sensitive to the crossover components in my test speakers and/or the difference in speaker impedance. The crossover capacitor is one microfarad with a series 4 ohm resistor to feed the tweeter.

Now I'll get the changer hooked up to see how that goes.

[stbasil]

Well, unfortunately, the changer works great as far as cycling and record size detection and tone arm set down, but when playing a 33 record, the tracking is very poor as if the horizontal movement of the tone arm is slightly impeded so that it cannot track correctly, even with the correct stylus pressure. It appears the cartridge is okay at this point, so that means I need to tear into the mechanism and find out where the excess friction is coming from. They're lots of videos regarding working on this particular changer, so I've been watching those and I'm ready to dig in. The service manual mentions a couple of issues that can cause tracking problems, so I'll be looking closely at those areas while cleaning and lubing the mechanism. Another learning curve, so any advice from you collaro experts is appreciated!

[stbasil]

Well after working on cleaning up and lubing, the tracking seems to be much better, but it's hard to tell because the cartridge is shot, sounds terrible. So I've ordered the kit Gary recommended. Wasted money on the new stylus that I bought a while back!

[William]

Been there, done that. I hate when it turns out to be the cartridge.

Bill

[stbasil]

ARG!!!
So I got t the new cart from Gary, started to remove the old cart and discovered that its ground wire had loosened, reattached it, and the old cart sounded OK! But then I noticed some very low frequency rumbling, so I thought I was getting feedback through my plastic workbench top from the downward facing speakers to the changer, since the isolation springs were not in play. So, foam under the changer jig, but oscillation still there! Unfortunately that strange oscillation mentioned earlier has reared it's ugly head again, so the factory spec speakers didn't fix it as I had hoped.

The oscillation frequency varies with different control settings, sometimes high, sometimes low, seems to start up after the amp has been powered for a minute or so. Freeze spray on the left channel first preamp transistor seems to suppress it, maybe. It's there even when volume control is all the way down, which grounds the input.

This will be an interesting sleuthing experience. I hope it's not a defective transistor. ...

[TC Chris]

Are there caps or resistors near that transistor you sprayed, maybe affected by overspray? Seems to me that oscillation or motor-boating is commonly a cap issue or cap & resistor.

My computer-desk radio that I am listening to as I type is a little cheap-o GE receiver that sounds remarkably good with two tiny Polk speakers on stands. No bass because of the tiny enclosures, but remarkably good sound and imaging. Occasionally, when the broadcast program does it, I'll hear something from outside the locations of one speaker or the other. It convinced me of the theory that little speaker can produce very good imaging.

But the little old receiver is running on original caps after about 50 years, and there's a constant low-level put-put-put in the L speaker. I'm pretty good at ignoring it. Why do I use a very modest receiver when I have several less-modest ones to choose from? Because it fits in the space!

Chris Campbell

[electra225]

The schematic doesn't show the speakers that I can see. It shows a terminal strip or plug of some kind that the speakers connect to. My guess is that the impedence of the speakers are a part of the feedback loop. IIRC, there are two caps in series with the feedback loop that may need to be increased/decreased in value to eliminate the squeal.

[stbasil]

I've replaced the feedback caps, but didn't try different values. However, I have since confirmed that cooling one of the second transistors, X2/X6, will stop the oscillation. Cooling the one in the other channel also has an effect, though less so. Leakage is over 2ma when the transistor is tested on my little Chinese component tester, goes down when cooled. So I'm guessing maybe these are the cause of the oscillation, though I don't understand why. One of them is noisy also.

Part number replacement is given as Delco DS-26, or GE-2, or RCA 2N614 (2N2614). Anybody have an idea on a cheap replacement for these? Germanium of course.

[stbasil]

After doing some Googling research, and posting some questions on ARF, and experimenting, I've made a few circuit changes. I added .001uF mylars to the base of each driver which killed the oscillation without affecting the frequency response. Ended up replacing both driver transistors, one was noisy, with 2N3906's which are PNP silicon, worked without any circuit modifications!

The strange balance control, which is somehow tied in to the feedback circuit, performed poorly. On either extreme, that channel became very loud, overall volume was much lower in the center. After some experimentation, I ended up putting a 2.2K resistor on the wiper to ground, vs the original directly grounded wiper. Volume is now consistent across its range. So, here are all the changes I made:

I've cleaned, lubed and adjusted the changer, will install the new cartridge next. Then, another learning curve for refurbing the trashed Tolex-like stuff on the cabinet!

[electra225]

I'm glad to know we have someone on this forum who has worked with transistor equipment. I know zipkus about solid state equipment. Nice job on figuring out what the issue was and for your workaround.

[stbasil]

Well, at this point, I still prefer tubes! I earned my EE degrees at the turn of the 60s to 70s: tubes were passe, semiconductors were the focus, but 1s and 0s and ICs were my specialty. Google has been immensely helpful in reviving my elderly memory when it comes to transistors. When I picked up this Maggie, I didn't realize it would be transistorized, so it was either sink or swim!

[electra225]

My prior post may have sounded like I think everybody on here who hasn't worked with solid state stuff is daft. That was not my intention. It's just that, I think, most of us struggle with that stuff and, when someone figures out a tough one that is solid state, I am inpressed. I must prefer tubes myself, but I have a Pioneer receiver on my bench that only needs parts and my stronger backbone for the project to get started. There are times I think I have a plan, others where I feel totally lost......

[TC Chris]

After doing some Googling research, and posting some questions on ARF, and experimenting, I've made a few circuit changes. I added .001uF mylars to the base of each driver which killed the oscillation without affecting the frequency response. Ended up replacing both driver transistors, one was noisy, with 2N3906's which are PNP silicon, worked without any circuit modifications!

So was the germanium to silicon substitution made just by finding a silicon with similar characteristics to the old one, or just good luck? Maybe stated otherwise, was this a rare instance in which the switch could be made?

And was the oscillation a result of aging components and drifting values, or just bad design?

Chris Campbell