Post by wdln on Jun 23, 2020 19:23:28 GMT
Hello all... I've been working on an MM6. Let me give you some background and then I'll spin a little yarn on the problem I've been having; sorry for the length of this post!
The MM-6 is internally almost identical to the working TI-1 I have (same record mechanism and amp, different layout for the selector buttons but same wiring scheme, same search unit and credit unit setup), so I'll admit right off the bat that I've been referring to the field service book that I have for the TI-1. However, I've found it to be correct, even on wire color codes. On this particular MM6 the credit unit is missing, so a previous owner/mechanic put a jumper between pins 10 and 14 on the credit unit edge connector. According to my TI-1 book, this basically makes it so there is always a premium price credit present.
I started by cleaning all the garbage out of the inside of the 'box and working on the amp. I replaced all the big filter caps, the electrolytic caps, and a few resistors that looked toasty. I removed the selector button assembly, cleaned it out, and replaced the big 5uF electrolytic cap that's on there. While I was waiting for new caps to arrive in the mail, I also repaired a tear in one of the speaker cones, replaced the front grille, and repainted some internal metal parts that were rusty or nicotine-stained.
On first fire-up, the basket rotated once around, but the latch solenoid didn't kick in as it should when there's a credit, so I could not select anything (buttons would not stay down or register). Then I looked down and saw the Magic Smoke coming out from under the plastic cover over the search unit board. The trace that jumps pin 7 (feeds one side of the R1 coil) on the large edge connector over to the connector on the left side had burned up.
Looking at the schematic, I suspected that maybe R1 or R2 in the selector button assembly wasn't firing, or the normally-closed contacts were bad (the plastic cover protecting R1/R2 was missing), or that the latch solenoid wasn't firing. With the assembly disconnected from the unit, I was able to successfully fire the latch solenoid with an external 30VAC supply and R1/R2 with a 20VDC supply (closest I could find to 30 but it was enough to engage them). Checking the relay contacts with a meter had some weird results, so I disassembled both relays, cleaned and adjusted the fins, and got all the contacts to disconnect/connect as designed when the each relay was triggered.
So I put it back in and turned it on. Still no clunk of the button latch solenoid. The trace for pin 7 on the search board was still burned/disconnected, but this time, within seconds the trace on the search board from pin 6 (common) burned up.
When I worked on my TI-1 I replaced all three diodes on the selector button assembly when I was diagnosing a selection/memory issue, and one of them is directly related to R1's operation. So I replaced all three of those on the MM6, and put the button assembly back in again. The latch solenoid still didn't pull in. Nothing else burned, but the traces are still not repaired so there's definitely things that aren't connected anymore.
So now I'm gun-shy about repairing the traces or buying a NOS search unit board, as I'm not sure if I fixed the problem or not. If I fix the traces, it might end up burning something else, or I could burn up a new search board if I replace it. I'm not reading any voltage on either side of the burned traces now, so maybe it's OK. There are also no blown fuses. I'm kinda irritated that a fuse didn't pop instead of burning the search unit board but this must have been the weakest link. Should I repair the traces and see what happens? Is there anything else I can check ahead of time (before applying power) that will prevent potential further damage?
The MM-6 is internally almost identical to the working TI-1 I have (same record mechanism and amp, different layout for the selector buttons but same wiring scheme, same search unit and credit unit setup), so I'll admit right off the bat that I've been referring to the field service book that I have for the TI-1. However, I've found it to be correct, even on wire color codes. On this particular MM6 the credit unit is missing, so a previous owner/mechanic put a jumper between pins 10 and 14 on the credit unit edge connector. According to my TI-1 book, this basically makes it so there is always a premium price credit present.
I started by cleaning all the garbage out of the inside of the 'box and working on the amp. I replaced all the big filter caps, the electrolytic caps, and a few resistors that looked toasty. I removed the selector button assembly, cleaned it out, and replaced the big 5uF electrolytic cap that's on there. While I was waiting for new caps to arrive in the mail, I also repaired a tear in one of the speaker cones, replaced the front grille, and repainted some internal metal parts that were rusty or nicotine-stained.
On first fire-up, the basket rotated once around, but the latch solenoid didn't kick in as it should when there's a credit, so I could not select anything (buttons would not stay down or register). Then I looked down and saw the Magic Smoke coming out from under the plastic cover over the search unit board. The trace that jumps pin 7 (feeds one side of the R1 coil) on the large edge connector over to the connector on the left side had burned up.
Looking at the schematic, I suspected that maybe R1 or R2 in the selector button assembly wasn't firing, or the normally-closed contacts were bad (the plastic cover protecting R1/R2 was missing), or that the latch solenoid wasn't firing. With the assembly disconnected from the unit, I was able to successfully fire the latch solenoid with an external 30VAC supply and R1/R2 with a 20VDC supply (closest I could find to 30 but it was enough to engage them). Checking the relay contacts with a meter had some weird results, so I disassembled both relays, cleaned and adjusted the fins, and got all the contacts to disconnect/connect as designed when the each relay was triggered.
So I put it back in and turned it on. Still no clunk of the button latch solenoid. The trace for pin 7 on the search board was still burned/disconnected, but this time, within seconds the trace on the search board from pin 6 (common) burned up.
When I worked on my TI-1 I replaced all three diodes on the selector button assembly when I was diagnosing a selection/memory issue, and one of them is directly related to R1's operation. So I replaced all three of those on the MM6, and put the button assembly back in again. The latch solenoid still didn't pull in. Nothing else burned, but the traces are still not repaired so there's definitely things that aren't connected anymore.
So now I'm gun-shy about repairing the traces or buying a NOS search unit board, as I'm not sure if I fixed the problem or not. If I fix the traces, it might end up burning something else, or I could burn up a new search board if I replace it. I'm not reading any voltage on either side of the burned traces now, so maybe it's OK. There are also no blown fuses. I'm kinda irritated that a fuse didn't pop instead of burning the search unit board but this must have been the weakest link. Should I repair the traces and see what happens? Is there anything else I can check ahead of time (before applying power) that will prevent potential further damage?
