1948 Olympic 8-925 AM-FM phono console

[stbasil]

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I acquired this nice looking Olympic in December 2020 (picture is "after", unfortunately I did not take "before" pictures back then). Originally an estate sale item, nobody wanted it for $300. Gee, I wonder why? Supposedly, according to the sales agent, it "lights up". Yeah, right. The power cord leading to the changer was totally rotten and shredded, exposed copper, completely shorted out. The only thing it would light up is the person who plugged it in! Also, the grille cloth was ripped along with the speaker cone, probably from something stuck through it by Dennis the Menace. It was relegated to a storage locker since it didn't sell, she wouldn't take less than $50, but I thought it would make a fun project for my first vintage hi-fi restoration. The cabinet was in good shape, and had nice brass logos, even one for the cabinet maker!

Phono power cord

First order of business was to replace all the paper, wax and old electrolytic capacitors. I ordered a Sam's off Ebay, which turned out to be for a slightly different model 9schematic is different in several places), but later got the correct schematic from the Nostalgia Air website. One capacitor was obviously shot (the light blue one in lower center, with the white stains on the end) and had leaked a lot of electrolyte onto the wood below the chassis.

I took all the tubes to a neighbor who was a ham operator and he graciously tested them for me. Some were not in good shape, but back then, I thought that might make a difference so I order some replacements for a few of them, but I did not realize at the time that those would be used tubes, too, and may not be much better than the originals! Also, someone had monkeyed with the RF front end, there were some broken wires and misshapen, hand-made inductors, but I eventually got everything working.

The 12" speaker was shot, I did not want to ship it off somewhere for re-coning, so I found a reasonably-priced 4 ohm speaker on Ebay. The original speaker did not have a permanent magnet, it had an electromagnet which was also served as a filter choke in the power supply (a common practice back then). And, the output transformer was mounted on the speaker. So, I just removed the rest of the old cone and mounted the speaker frame, coil and transformer in the cabinet adjacent to the "new" speaker. The grille cloth is some burlap that my wife had in her materials stash. For posterity, I stapled a shred of the old grille cloth inside the cabinet (upper right)

I'll write more of this later, the changer and cartridge swap was a real challenge. And, now, 4 years later, as I'm working on it some more to get it ready to sell, some other issues have cropped up!

To be continued.....

[stbasil]

As I was wrapping up this 4-years-later project, getting it ready to sell, I noticed that it just didn't sound right when feeding it with music from the newly added Bluetooth adapter (much higher-fi than a 78 record!). First, I found that a 100 pf capacitor in the amp section had gotten very leaky, and that was a new one, at least in 2021. After replacing it, I also noticed that the bass got very weak after the unit warmed up for about 5 minutes

Eventually, after a few days of looking for another leaky new cap, drifting resistors, etc, I started swapping tubes in the amp. I had four 6SQ7 preamp tubes but only one tested as decent. (the other 2 are weak, one is strong but noisy). Using the noisy one plus the good one, the warm-up issue went away, So I just ordered 2 more from a guy on eBay, supposedly tested as strong.

I've still got a lot to learn about tube circuits, but this problem took me a little farther along that learning curve! Plus, thanks to a neighbor who heard about my work on tube stuff, I was gifted a vintage Knight tube tester (which was another restoration project)

[William]

I have a couple units that have odd things going on too and I hope one of these days I will.get them sorted. Good luck with your newer tiubes and they solve your problems with the Olympic.

[stbasil]

Well, my replacement 6SQ7s apparently fixed the problem, they both tested as having strong emission, just as the seller claimed! With the original ones, after 5 minutes or so, the plate voltage of one of them would continuously drift down, and the volume of the audio would begin decreasing also. Both of those had tested as pretty weak. BTW, those voltages are wrong on the above schematic, another schematic had them as 95v and 85v, closer to reality.

I decided to just remove the 100pf caps on those plates (one failed anyway) which improved the frequency response above about 6khz and didn't seem to cause any issues (like ringing). Maybe I'll add a cheap little tweeter? Would improve the sound when using the Bluetooth....

Also the 2 meg volume control was very nonlinear and only usable in the upper third of its range (not shown in the above schematic), I added a 1meg resistor from the wiper tap to the input side, which made it much more usable. It's in the lower left of the chassis photo below.

Now that the amp is sorted, I'm working on this flakey FM tuner!

[stbasil]

The tuner knob had always bothered me, because it moved the pointer in the opposite direction expected, and I also realized that the stations were reversed from where they were supposed to be, ie, low frequency stations were on the high frequency end of the dial, and vice versa. I never paid any attention to this before, since I only was interested in playing 78s, and the FM was lousy anyway! But, I'm gonna sell this thing, so I wanted to fix up the FM.

First, I cleaned up the tuning capacitor. I didn't want to attempt to remove it, since the connections were buried underneath the multi gang selector switch, and a mess of other frontend tuner components! I managed to do a tedious but halfway decent job, I hope, using isopropyl alcohol and passing folded paper strips between the plates. I also deoxed all tube pins, and the selector switch (again).

Several components attached to the selector switch were very sensitive to parasitic capacitance when touched, which I suppose is normal, but one mica cap seemed more so. It was about double its speced 47pf, so I replaced it (highlighted below). Maybe it was ok as is, who knows?

Now that I could receive stations, I started looking at the tuning issue.

Turns out that someone had "fixed" the dial cord in the past, it had a knot in it, and they had totally messed up the stringing. So I re-stringed (restrung?) it correctly, using the same dial cord, luckily the knot was located where it didn't cause a problem!

The FM sounds pretty good now, but I also went through all the alignment adjustments, peaking everything by ear, since I don't have the equipment to do a proper alignment. That's probably heresy, but now the FM works great! Time to put everything back in the cabinet....

[TC Chris]

I decided to just remove the 100pf caps on those plates (one failed anyway) which improved the frequency response above about 6khz and didn't seem to cause any issues (like ringing). Maybe I'll add a cheap little tweeter? Would improve the sound when using the Bluetooth....

I've done that a few times on late '40s, early '50s devices. They have been described in parts lists as "quality" caps, which is another way of saying HF-cutting devices. But I've always wondered if maybe they didn't suppress ultrasonic oscillation. I don't have the equipment to observe. But it does improve the HF audio response.

Chris Campbell

[William]

I took a look at the field of capacitor you changed out and from what you have indicated it made a difference in the radio's performance. Good Job.

Bill

[stbasil]

I added a cheap tweeter ($7 from Amazon) with a 2.3 mf crossover cap, so I have a little bit better high frequency response when combined with the 12" speaker. BTW, did you know you can make a non-polarized cap by connecting two electrolytics in series, with opposite polarities connected together? The resulting capacitance is half of the value of the caps, but the combined voltage rating is the same as one of the caps. So, I used two 4.7 mf 50 volt caps for this. I researched this because I thought I remembered reading it long ago, and I have a bunch of low voltage electrolytics (Amazon assortment)

But, I am frustrated that the sound via the Bluetooth adapter has some mild distortion, which I can detect in spite of my crummy hearing! So, I took everything back to the bench to see what gives.

The distortion is the same even if I use the phono input instead of my converted AM input, so there is something else causing it. I could not see any ringing (ripples on a square wave signal indicated that the amp is unstable) on my scope, so that's not the issue. Speaking of square waves, I could tell that the low-frequency response was pretty bad, maybe intentional to cut out turntable rumble? It really rolled off below 150Hz or so. After some experimentation, I changed the coupling cap that runs from the volume control to the grid of the upper 6SQ7 preamp from .005 to .05. Checking it with sine waves and the square wave showed much better low frequency response, now it starts rolling off about 60 Hz. High frequency response really starts rolling off over 9Khz, which I can't hear anyway, but at least there is no ringing on the leading edge of the square wave, which would indicate instability in the amp, so I guess removing those 100pf caps did not hurt anything. The photo is of the input 1Khz square wave on bottom in blue, and upper is output into 4 ohm speaker load. The 100Hz square wave looks pretty good, too, now that I increased that cap. The tone control impacts the flatness of the top of the square wave, I had it turned down a bit when I took the photo of the scope.

That change did not make any difference in the distortion that I could hear, though, so more to come....

[TC Chris]

Have you tried reducing the output of the adapter, in case it's overloading the inputs?

Chris Campbell

[stbasil]

Chris, I wish the Bluetooth output was that high! It's actually pretty low and requires the Olympic's volume control to be almost maxed out. This is apparently common with low-end Bluetooth adapters because of their being powered by 5V USB wall warts. You just can't get much output voltage out of them.

But, I think I have found the solution to the distortion. The B+ on the 6K6's was about 317V, that is well over the voltage listed on my schematic. I always knew the power supply was running high because of our 124V-127V line voltage (spec on one schematic I have says "105-125 V. A.C. Supply"). I looked up specs on the 6K6's and found that the maximum plate voltage, at least design-wise, is shown as 315V. But for the example push-pull amp design parameters in the GE datasheet (https://frank.pocnet.net/sheets/093/6/6K6GT.pdf), it suggests 285V for B+, which is about what my schematic shows. So, I think this amp design is getting out of spec by the time the line voltage gets up to 125, forcing the bias points into a range where higher distortion occurs.

Rather than putting a hefty resistor in the AC line, I decided to put one in the DC power supply at the output of the 5Y3 rectifier. A 150 ohm 5 watt resistor now drops about 20 volts, giving me a B+ of about 290 volts at 124V line voltage. I can't hear the distortion any more, and the tubes should be happier with the lower voltage!

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Higher line voltage causes other problems, though. The 78RPM turntable's speed is more like 81 RPM at 125V line. I put a 100 watt dim-bulb tester on the line just to see what would happen to the turntable speed: it drops to 71 RPM at a line of about 90V. Artie Shaw's "Stardust" actually sounds pretty good at that speed! Maybe I'll add a dropping resistor to the changer motor, too....

Now to put everything back in the cabinet.

[William]

It sounds like you just about have everything the way you want it, give it a good road test before you put it back in the cabinet. If your luck is like mine, once back in the cabinet something will go wrong and back out it comes.

Bill

[TC Chris]

For my GE E86 restoration, I had the same problem with over-voltage. I made a bucking transformer device to drop the line voltage.

Chris Campbell

[electra225]

My stepdad used to routinely replace 6F6 and 6K6 with 6V6's. They are essentially the same tube, but the V runs higher plate voltage. If you have a set of 6V6's you could sub, it might be worth a shot to see what happens.

[stbasil]

Good idea, but I only have a single spare 6v6. Maybe I'll borrow from my moto console at some point, but for now the dropping resistor seems to have cleared up the distortion issue.

The Bluetooth, FM and even AM sound pretty good, except my garage is so "live" that vocals are overpowering, at least I think that's it. 23 ft x 23 ft, 9 ft ceiling, so lots of resonance. I don't have a nice shop, I share with our cars and all the other garage stuff!

The bass response of the cabinet seems to be less than optimal, probably because there is no bottom on the speaker side, it's open to the floor, cheaper to make it that way I suppose. I'm experimenting with that....

[electra225]

I have plenty of 6V6's I won't ever use. I don't have any matched pairs, but the ones I have would be good for single-ended use or for test and subbing. You might want to use caution making mods to the cabinet. The speakers, cabinet and amp are designed to work together. Sometimes you do more harm than good disrupting the trio. I closed up the bottom of the speaker enclosures in my old Imperial stereo. I'm not sure it made any difference one way or the other. My first shop was a bedroom in our house.

[Doug]

Use a 120v to 12 v transformer to increase or decrease
The primary voltage by 12vac.
Use reduce voltage on a variac to make sure you have the secondary 12 winding reducing the voltage.
If u feel uncomfortable try it on another transformer or even use a light bulb to see it reduce ac by 12v.
Doug

[stbasil]

Thanks, Electra! Since it appears that the dropping resistor solved the distortion issue, I'm sticking with that. Switching to 6v6's would probably work and would be a much more elegant solution, but I've already spent too much on this thing, and I still have another issue. And thanks, Doug, for that great idea, didn't occur to me to use an autotransformer approach like that! Unfortunately, I don't have a handy 12v transformer lying around, but I do have a bunch of sand resistors that I could use, so once again, I'll probably go with a dropping resistor under the changer, the motor even has a little fan blade on it to keep things cool. In both of these applications, the power dissipation is less than half the 5 watt rating on the resistor.

But, my other issue is that records don't sound too good, even considering they are 78's. Seems lacking in bass with too much midrange. FM and Bluetooth sound great, so something isn't right with the ceramic cartridge, I suspect. If I run the Bluetooth into the phono input, it sounds the same as the shared AM input that I rigged up. The cartridge signal path into the preamp is identical to the Bluetooth and FM, with the exception of a resistor divider leftover from the original Astatic crystal cartridge, probably was there to more closely match the other input levels. I removed it when I switched to the Pfanstiehl P51-3; but adding it back only affects the amplitude, no effect on frequency response.

I'm going to move the Olympic back into our living room so I can get rid of the terrible acoustics of the garage, and see how it sounds there. BTW, my temporary closing off the bottom of the speaker enclosure seems to improve the bass somewhat (back is still open). Stay tuned....

[TC Chris]

It has always been a mystery to my why so few manufacturers were to cheap to add a tiny bit of plywood to close off the bottoms, the simple expedient to get better bass response from the big speakers.

Chris Campbell

[stbasil]

Well, I tried a dropping resistor on the changer motor, but the speed was still too high. Put the changer on a dim bulb tester with 100 watt bulb, and even at the resulting 107 volts, the speed is still about 80-81 RPM! I had already cleaned the motor spindle and lubed the motor and drive wheel tensioner so maybe that sped it up a little! Apparently, the motor spindle was made just a little large, there does not appear to be any build-up of gunk on it. I think I'll work on it with some emery paper, or maybe something coarser.

Anybody have any other ideas? A dropping resistor isn't going to work because of heat issues given the size/wattage it would have to be to lower the voltage to 95 or so, which is about where it needs to be to get the speed down.

I know that running fast is not a critical issue, given the quality of 78 records, but I would be happier if it ran a little slower instead of faster--I think it would sound better! I worked on this changer (1948 Seeburg Model S) four years ago, but I had forgotten how clean it was! It's the original drive wheel, which works fine, no slippage or serious wow--there are tiny cracks in the rubber, but it is still somewhat pliable. Those motor mounts are hardware-store grommets.

[TC Chris]

Is your measured speed with the tonearm on the record, playing? If not, try that and see if the extra speed when free-wheeling is to compensate for drag when not.

Chris Campbell