Repair of Adcom GTP 500 II volume pot and motor

[raymondjiii]

I have been working on a repair of this unit. It looks like the unit was dropped on it's face. I was able to repair the on/off button internals, I am going to put some jumpers on the little FM Scan board that cracked in half (only has the two buttons attached to it, plus small ribbon.) The last problem is that the volume knob was pushed in bending the internal metal frame. I straightened out all of the metal and the volume knob is no longer recessed. But there are two board lines that oddly go "around the potentiometer", pus turning the pot is very rough. I could dismantle and put two jumpers on the mini board that goes around the pot but has anyone ever opened a pot to repair it? Finding the full part: pot attached to the motor, does not seem possible. It looks like the motor is still good. I am considering prying open the pot, and see if anything is salvageable. My guess is no. Otherwise it has to live with the volume level set as is. Thanks!

 

[gadget73]

I don't suppose you could sub in some other motorized volume pot? A lot of those things were made by some maker and its possible they still make them.

if the pot is currently smashed and damaged though, you don't have a lot to lose here. I've never been into this specific one, but i have opened pots before. Not all that much in there. I don't imagine a motorized pot is all that complex either, same bits just with an extra mechanical connection to the motor that drives it. Be careful with the metal tabs that hold the sections together, they will only bend open and closed again so many times. There are also usually a couple of loose parts inside that will fall out when you open it up so beware escapees.

 

[raymondjiii]

I would be okay with getting another Alps same model pot without the motor - if I have to. I'm going to de-solder the pot-motor combo tonight. I've opened pots decades ago and a lot of "debris" came out. There is a metal box between the pot and motor, which is not seen on many other pot-motor combos. I am surprised that I have not been able to locate an exact part. Yes, this preamp is from the late 1980's but I would think I could find it somewhere.

The original part was made by Alps (Japanese) and I do see Alps pots out there, so far no 50k X 2 yet. I'm going to keep looking but I'll have a better assessment when I get this thing off the pcb. Thanks for you reply!

 

[raymondjiii]

Just following up on this in case anyone reads this in the future....The volume potentiometer has a small amount of PCB around the outside (which is what broke when they dropped it.) Since this part is near impossible to get, I found another broken GTP 500 II unit with the goal of taking the full volume pot/board out - only to find that this unit is a different revision that what I had. I have all blue PCBs and this one has all green PCBs (clearly different circuits everywhere.) The volume PCBs looked the same so I went about desoldering the ribbon cable from the spare unit and installed it in the my unit only to find out (after it was all resembled) that it was not a perfect fit and it was causing the pot to be tapered...which after all put back together caused the volume knob to bind when the motor was used.........take it all out, find the original circuit board, luckily it's still in decent shape, move everything from the green volume PCB to the blue PCB. A lot of desoldering and resoldering which takes forever as I'm sure you know. It's all back together and now the On/Off button works (it was cracked from the drop), the FM scan up and down works (mini PCB cracked from the drop) and now the volume knob works manually and via the motor with remote. There was good output signal before this, hopefully everything in that area is still the same.