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Amp Repair

Amp Tubes





I recently got an AMi C amp in for repair. The usual suspects, bad capacitors and open resistors, the output transformer checked good.  I did the repair and connected it to my test jig. Dead as a rock. The 5U4 tube isn't lighting. A quick check showed that the 5 volts was there for the filament, but it was still dead. It was a nice clean amp, no rust or corrosion. What the heck?


Remembering that AMi seems to have more loose connections and bad solder joints than usual, I resoldered all the wires on the 5U4 tube socket and the amp came to life! Yes, the wires were soldered and it looked like a good job too, but they weren't making contact. I find this more often on printed circuit boards, components that look like they're soldered but aren't. It's not very common to find this on a hard wired amp, but here you have it. So, the moral of this little story is, just because it LOOKS good doesn't mean it IS good.


The same holds true for tubes, transformers, resistors, etc. Some of the worst looking amps on the face of the earth can work out to be great sounding amps, and I've had amps in here that you would swear are nearly brand new that weren't repairable. You never know.


My friend Richard has been spending most of his time on a Seeburg W. This is the one that I bought that was just a bare cabinet, but it was very clean. We've been gathering up parts for it, fortunately most of the early 100 play mechs are the same so we're using repainted C plastics on a G mech. I had to buy mirror frames (they were missing) and one of the grill stars, but I think it's ready to go back together. The W uses a C amp; it was the economy version of the G which was high fidelity. The C and G amps will physically interchange, but the G is a much nicer sounding amp. I'm going to check stock and see if I have a G amp to put in it.


I guess everyone does this at one time or another. I sent several pieces from a Seeburg R off for rechroming. They came back wrapped nicely in paper, so I just stuck them up on a shelf without looking at them. Also, the charges seemed a little high, but it was already a done deal. This was a little over two years ago. Last Saturday I finally needed them and unwrapped them. The first piece, the one that the lid hinge mounts to looked great. Then I unwrapped the "boomerangs" and there was about a two-inch warp in both of them. They looked like a strip of bacon. Also, they had ground the edges down to the point that the metal was way too thin. They also filled all the screw holes with excessive metal. I just stood there for a minute looking at this crappy chrome job and wondered what to do. It was too late to call the plater and complain, it had been 26 months since I sent them in.


Well, let me see if I can make the best of it. As you know, pot metal doesn't bend, it breaks. So I started on one side, matched up the holes with the cabinet as best I could and started installing sheet metal screws in place of machine screws. I carefully worked my way around the edge installing and slightly tightening the screws. Over the space of an hour, I managed to get one of the boomerangs pulled almost all the way down without breaking. I left well enough      alone and started on the other side, same procedure. I then installed the top strip and quit for the day. I let it sit for a few days to give the metal a chance to stretch a little. Today I finished tightening the screws all the way down and it does look OK now. But for a while I thought that I was just out of luck on the side pieces. Patience sometimes helps.


As I sit here writing this, there is a digital control center lurking on the bench waiting to be tested and repaired. It's from an SPS-2, roughly 1974. This is a well engineered piece of equipment, but time is not kind to them. First, some of the components are no longer available, and then they can get intermittent due to age. I wonder what the original in-service time was for this series? Did the factory intend for jukes to be used for 30 odd years? I suspect not, but they sure did a great job on the mechs. I've seen AMI and Seeburg mechs with well over 300,000 plays that still work great. Of course, the problem is worse on some of the early digital jukes that Ami-built, the memory chip isn't available any longer. I think the R- 80 was the first digital AMi, but you could also get a mechanical select R- 80. Perhaps it was a mid year change, or maybe the digital select was optional. It worked reliably at the time and one in good shape will work great today, but when it quits that's probably the end of the line unless somebody works out a way to make new chips.


Speaking of time not being kind to certain jukes, I've mentioned the Seeburg K and L series with the printed circuit boards. Time is not kind to them. The boards start breaking down and can actually blaze up. Keeping this in mind, I remember I had a customer with an L that his father had bought new. He was dead set on repairing this juke and he wanted it 100% reliable. So what to do? At the time I had a Q in the warehouse with a bad cabinet. It was one of the cheaper models, mono amp and no stepper. This was going to be the donor. I pulled the back door off the L, removed the memory unit and proceeded to rewire the mech to run with the 0 components. I had to reuse the L keyboard, so I had to install the large Jones plug on the cable end. Not an easy job, but it had to be done. I had to remove the preamp from the mech and route the signal down the shielded cable as well as rewire the mute circuit, speakers, etc. The whole process took 2 days, but in the end here was an L that would work reliably for many years. I think I did this about 15 years ago and since then he's bought two tubes and a set of needles. It just sits there and works!


The same thing can be done with a K using the TSR-6 receiver and HFMA-2 amp from the series 101 through 201. It needs to have a 200 select receiver and memory unit. I've installed a Jones plug on a K memory unit so that it can be used on a 201 so I know that they are interchangeable. Sure it's a job, and it's not a job to undertake unless you are very familiar with the differences, but this is one way to have a K and KNOW that it is going to work and be safe at the same time.


I've been asked it it's possible to change a K over to stereo using a later model amp. The answer is yes it is possible, but there will be muting problems along with the fact that extra cabling is required for the second channel. I suppose that mute switches could be taken off a 220 or similar juke and installed on the K, but this is not easy and I don't recommend it. Just stick a stereo tone arm on it and tie the channels together to get better sound if that's what you're after.