In Seeburg’s early amps, the MRA and HFMA series, they used several bakelite rotary switches for bass and treble tone controls, wattage, and AVC or Automatic Volume Compensation. On a recent MRA5 amp recap job I could see that the wattage switch had been broken. They all break at the same spot, there’s not much support for the thin somewhat brittle bakelite. These switches don’t grow on trees and it’s either repair them or rob one out of another amp and render the amp useless. I developed a good method of repair for these that works and is fairly easy just time consuming. I thought I would take the time to share it.

The half of the switch with five wires on it is basically floating in space so I just cut the wires going to the it one at a time and tag them as they are removed as to location. I will also make drawings; anything to help get the wires back where they belong. Of course the schematic shows clearly what goes where but with the old cloth covered wires it isn’t very easy determining wire color most of the time. Once that is done I remove the knob, unbolt the switch, and remove it. You will note in the last pictures that the stem got chewed up a bit. The knobs are held in with slotted screws in the early amps and hex head screws in the later ones. This one has a hex head and it was rusted just enough that the hex wrench I used just turned without purchase. I ended up using a Dremel tool with the cut off wheel to get the @3#* knob off. I almost went too far as part of the shaft got chewed up. Note also where the witness mark is for the hex screw. A new screw will still tighten up and securely hold the knob. I got lucky.

I mixed up a small batch of my favorite epoxy, JB Weld, and applied a tiny bit to each piece of the switch. JB Weld has never let me down. I use it for a variety of repairs. This first dab is to glue the two halves of the switch back together. When joining the two switch halves make certain the little selector tab on the round ring, seen at the middle of the five wattage taps at the top in this picture, slides in between the two contact halves. I propped this up so it was perpendicular and let it set overnight.

One more application of JB Weld this time for support. I slather it on top and bottom over the spot where it cracked and let it dry overnight. This switch was clean. Had it been dirty I would have sprayed the bakelite with contact cleaner before using the epoxy to give it the best chance at holding. As it was I used the trusty Dremel with a small wire brush and polished the two brass rings now covered with epoxy for the same reason. One only has to be careful not to get epoxy on any of the moving parts. After this has cured the switch is ready to remount in the amp, carefully desolder what’s left of the wires on the tabs and re-solder the transformer leads back on.

Devil In The Details

Sometimes the devil is in the details. I spent the day taking care of small problems that will keep a juke from working just as well as big problems. Example of a big problem: I broke the idler gear in the HSC1 motor gear box. Irreplaceable unless one is taken from another unit. It’s currently at Sean B’s place hopefully getting 3D’d.

Today I mounted the Gen2 computer in the SMC1. This is the same SMC1 that was at Jeff’s house and now brought home. When I was working on the SMC2 I robbed a lot of parts out of the SMC1 because of all the trouble I was having with the SMC2. So, for the SMC1 I already have a good mechanism, PCC, Gen2 computer and an amp that looks like it was gone through recently enough that the electrolytic capacitors are a modern style. I mounted the Gen2 and kept getting an Error 80. It took awhile but I found the paperwork for this wunderkind computer and the error code translates to Left Reset. What???? Never heard of that switch. I started researching and was actually at the point of emailing Ed the Inventor when it dawned on me that the error code meant the left reset button on the selector buttons was stuck down. There are two resets-left and right. I can remove the computer and the code goes away. I dis-assembled the button faceplate and selector buttons and their springs and tossed the lot into a bucket of soapy water for a soak.

While fiddling around I did get the juke to make a selection and play a record. The bass speakers are out. They’ve been reconed but I’m waiting to put them back in until I’m done crawling around the juke to lessen the chance of putting a knee through one of them again. Yes, I are super tech. Knees and all. The sound I had coming out of the horns was small and tinny. I also had no left channel presence. Out came the amp and onto the bench. No output at all. I broke out the O’scope and set it up. I unpegged the upper driver board and left it hanging to be able to get at the pre-amp board to look for inputs. With a scope one can work forwards or back. I prefer to go from input to output. I fired up the amp and now had sound ! I lifted the driver board and the sound went away. OK, been here, done that. I immediately unplugged the amp knowing something was wrong with the edge connectors. If you have various edge connectors making and breaking contact it can do weird smoky things to the amp. Costly things.

I went inside and washed the soaking buttons and faceplate and put them in the sun to dry and called timeout. Time to relax a bit and catch up on the internet and perhaps get a bit of writing done.

Most evenings I’ll work at the bench about 9pm until 10-10:30. Tonight I started on eight pulse amps I had picked up from the Operator’s Stash. These pulse amps are used from 1959 to 1964 I think and are all the same internally. They mount a 12AX7 tube and have an RCA input from the tormat. These pick up and amplify the pulse from a charged selection and trip the mechanism into Play. I thought to go through them testing resistors and replacing the one .05uf cap in them, get them tested in a juke and offer them up for sale. This is interesting work, something different. Most of them had one to five out of tolerance resistors and these were replaced. I did about five of them and called it a night.