VM 102 speaker/amplifier

[William]

Back in January I posted about my new toy, that being a VM 570 console along with its matching VM 102 amplified speaker. The VM 570 started out as a mono unit in 1958 but with the addition of a stereo cartridge and the VM 102 speaker/amp it was transformed into stereo. The VM 102 was not available until 1959.

If you refer back to my original post, I talked about how dirty the electronics were and I showed a few photos. Since then, I have gone through all of my VM amps that have just been sitting there waiting for me to do something with them. When I say gone through, I mean tested for parts that I will need for restoration but not really restored yet until the last few weeks when I picked the VM 102. All parts needed have been here for some time, I have just been lazy and spring/summer stuff has gotten in the way.

As I mentioned, first victim was the amp in the VM 102 speaker/amp. With my previous testing I already knew what had to be replaced so that is what I did. This was actually my first amp that had the "HUM" when I first fired it up so naturally all the electrolytics were replace along with all the paper caps. 6 resistors had drifted so they were replaced, and this amp had 3 wire wound resistors, so they got replaced as well. All controls were cleaned several times and all connections were double checked to make sure everything was tight. I also installed a fuse holder with a 1-amp delayed fuse. Tubes tested strong with no shorts except the 6CA4 rectifier which tests strong but showed 2 shorts. By luck, or not so lucky, I had a NOS Sylvania 6CA4 tube that I have no idea where it came from but there it was in my tube stash. I stuck it in the tube tester and, , it had a short too. WHAT! Since it had only one short, and the short light was weak comparted to the original two short tube with a strong light I stuck it in just to see what I had being very careful with the Variac voltage and a watchful eye for anything that might happen. So far, all is well, and it really sounds great with plenty of bass and sufficient treble. I'm using an RCA 45 changer for testing purposes. I still need to find a "no short" 6CA4 rectifier and with that replaced I need to run voltage checks again just to make sure everything is good. With this unit almost under my belt I can now move onto the main unit, the VM 570. I have all the parts for the amp, and I need to go through the changer as well. I'm not sure when this will happen so stay tuned as I will post when it does.

Bill

[electra225]

I didn't realize that 6CA4's were so short-prone. Nice job on your amplifier. Now good luck on getting your console sorted.

[Hi-Fi-Mogul]

Very nice clean work, Bill.

How about a photo of the satellite speaker ?

[William]

I apologize, my photos never show in the order I want them to. If you start with the last photo and go backwards you will get the correct sequence of events.

Thanks, Mr. Mogul and Greg, and I'm trying to figure out why the photo of the speaker is not showing. I have six attachments showing in the que, but only five showed up in the post. I'm confused. Greg, any ideas? Since the last photo did not show in the original post, here's a photo of the speaker, at least I hope so.

Bill

[electra225]

Bill, when you post pictures, if you will use the "place in-line" selection, the pictures will post in the order you selected them. You can also post content, then pictures, using that feature. Maybe we need a topic on that?

[Firedome]

Nice little amp, should sound good paired up with the main unit - also 6BQ5 (El84)?
What are the speaker units?

[William]

The main amp does use EL84/6BQ5's but has a 5Y3 instead of the 6CA4. VM used a lot of Carbonneau speakers in their products. I'm guessing that is because they could be locally sourced as Carbonneau was a Grand Rapids Michigan company. They also used Jensen speakers in their higher end products.

Bill

[William]

Over the New Years holiday, I decided to work on the VM 102 speaker/amp. The amp and controls have been done for a while now, but there was still a couple of problems. One problem, no matter what was feeding the amp any increase in volume would cause the speakers to motorboat. The other problem, the music coming out of the speakers was clean and crisp.

Problem number one was my first thing to resolve. I checked, rechecked, double checked my work and could find nothing wrong. Shy of replacing all of the capacitor I had replaced one at a time I first tried other things. The interesting thing, when the speakers were motorboating I could touch the amp or the bracket that holds the controls and the motorboating would stop. Take my finger away and it would start back up. Turn the volume control to off and it would stop. I tried running a ground wire from the source to the amp, nothing. I tried running a ground wire from the amp to the controls, nothing. I started inside the amp, using a plastic stir stick, just wiggling wires and components, nothing, until I wiggled the wires going to the power switch. With that, the motorboating stopped. Wiggling again they started back up. There is a protective cover that covers the terminals and incoming wires on the switch so I removed that thinking I was going to find something touching something that it should not be touching. Nope, not the case. But leaving that protective cover off the switch seems to have cured the problem. I have no idea why it stopped, but I'm not complaining and I'm hoping leaving the cover off is a permanent fix.

Does anyone have any ideas why that cover would cause that problem?

Problem number two I'm still working on, so stay tuned. Here's a photo of the controls and the pesky cover.

Bill

PS...for some reason my newest photos of the power switch attached themselves to my other earlier photos so please look at the first two photos at the beginning of this post.

[electra225]

Do I see a disc capacitor on the control that had the cover? Is that cap good and still connected? Motorboating is something backfeeding into a circuit where it doesn't belong, then amplified. Output filter caps used for cathode bypass are known to promote motorboating and squealing. Removing the shield eliminated the coupling, so the motorboating stopped.

[TC Chris]

Amplifying Gregg's comment, so to speak, long ago my Bogen DB-115 amp developed a persistent hum. I had a disc cap from the power line to chassis, and when I lifted that, it disappeared. I was at school and had no replacements so I left it off. One day I'll restore the amp and will add a safety cap there. But you might look to see of that's a line-to-chassis cap and lift one end if it is.

Chris Campbell

[electra225]

To Chris' comment, I have had line isolation caps cause humming, but never have experienced one causing oscillation. It might be a good idea to check or just replace that disc cap on the power switch then note any effect with the shield back in place. It might be a learning experience if nothing else. Be sure to use a safety cap if you do replace the disc cap.

[William]

The little disc cap that is sitting on top of the power switch actually goes to the volume control which is in front of the power switch. The only connection to the power switch is two grey wires.

Bill

[electra225]

Have you checked that cap and know it to be good? Could it have been contacting the cover somehow?

[hermitcrab]

Bill does it motorboat while playing records? ....If so then check your leads to the cartridge , they could be out of phase or reversed... my fisher did that until I fixed the wiring error on the cartridge

[TC Chris]

Motorboating is oscillation. It would be interesting to know where that is happening. Do you have a signal tracer? My first one was a single high-impedance headphone from the 1920s with a .01 mfd cap in series. Now I've got a Heathkit version.

Chris Campbell

[William]

Elton, it motorboats/oscillates not matter what's hooked to the input. I have had my RCA 45 changer, my KLH tuner, and the changer that is in the main console for this speaker/amp. No matter what I use, it does it.

Chris, I do have a signal tracer and I have done some tracing in this amp but never came up with anything that might be the cause. Leaving the cover off the power switch has made it stop and I have not reattached it to find out if it will start back up. I may just leave well enough alone and go onto the other problem.

[William]

So, here's problem number two. Again, no matter what the input is, changer or tuner, the sound is not clean and crisp. It sounds a little fuzzy. If you remember my first thread, which is under VM and it delt with how dirty the main unit and the speaker/amp were (see photo) I am now trying to locate that problem. Because of the filth, especially the controls, I decided to super clean the controls, and unsolder all connections at the controls, clean those connections, replace the right on the boarder resistors and large disk cap and then hook everything back up. All of the solder joints on the controls look bad, green crud and corrosion. I had some of that in the amp as well but took care of it when I was replacing parts in the amp. Today I have some time to reconnect everything and give it a test drive. I'm keeping my fingers crossed, but not holding my breath. Stay tuned!

Bill

[electra225]

Your signal tracer might help you with this one. Is the sound muddy when you start it up, or does it get worse as it gets warmer? If it does it in all modes, then the trouble is likely in the amp, not in a cartridge or a tuner. I know you have tested, retested and swapped tubes, so that is not likely an issue, although grid emissions in an output tube is still a possibility. You might put a volt meter on the grids of the amplifiers to see if you get any positive DC voltage on the grids. That would let you know if you have a leaky coupling cap or a tube with grid emissions. You won't find grid emissions on a tester reading, the only way is in-circuit testing like I mentioned. Leave the meter connected to the grids for awhile, watching as the chassis and tubes get warm. Any DC voltage at all on the grids should be investigated.

[TC Chris]

I'm placing a bet that the putt-putt and the fuzz are related and are the result of an out-of-spec component....

Chris Campbell

[William]

I got back to the shop yesterday and the VM-102 speaker amp.

I decided to try the last simple thing, which was trying a different speaker. The speaker I used, and that would fit, was a 10" modern speaker. My friend Jim gave it to me, along with a few others, before he passed. He thought it might fit my GE trimline with the missing 10" speaker. Nope it would not fit, which is ok, but I was able to try it on this project. Two things happened. One, it played fine, and the fuzzy sound was gone. Two, the bass was gone too. Having said that, and as it was playing the 45 record that I was using for test purposes, I heard a funny noise, like a little "TICK" and the bass popped in and the fussy noise along with it. Before re-installing the original I cleaned it up. It was full of the same dirt that was all over the amp and speakers in both units. After installing the original, I fired the changer and amp back up to see just what I had. It was clear of the fuzzy noise but also no bass. I let it play for a long time, but I never heard the "TICK" noise and the return of bass and fuzzy sound. I flipped the amp over, took a plastic stir stick and moved and tapped everything in the underside. I also tapped the tubes, nothing changed. I looked though my stash of spare tubes, found the appropriate numbers, 6AU6 and 12AU7, and installed them. No change so I put the original ones back. I guess I need to go back through the amp and see if there is a component that is off. Take another voltage reading and see if something has changed. I have no idea or where in the circuit this intermittent "TICK" is and why it kills the bass and fuzzy sound. One more thing, before removing the original speaker and installing the new one, I had bass and the fuzzy sound as soon as the amp came alive. Somewhere from removing the original speaker and installing the new speaker this problem developed.

Bill