Technics SL-7 runaway

[Pio1980]

If nothing in the linked thread helps;
After tracing and checking everything from FG coil incl continuity of all connections from coil to main board, check Q202 and anything associated with it incl soldered connections and any small electrolyticaps.

 

[Montycat]

Well, a couple of steps forward, and a step back.

I checked the connections all around Q202 and examined the caps etc visually. C201 looked a bit suspect so I replaced it with no change. Then based on this thread at VE I took out C208 and replaced it. In that thread lensmanMK2 states that the silkscreening on the board is wrong for C208. I wasn't sure I believed that so I matched C208 with the board markings, and I had results!

Yeah, but not a solution.

The motor now would not start at all, but the tonearm operates. Previously the motor would run at Warp 9 but the arm did nothing. Now the arm and cue functions work well but no spinning.

Just for the heck of it I took C208 out and reversed the polarity and everything went back to square one. No arm movement and warp speed motor.

However I think this might be telling me that a more extensive re-cap is in order since one replacement had a significant change. C208 was probably bad and maybe C201 so possibly a bunch are dead.

 

[Sandrews]

Hi, I've had a little experience with Technics turntables and runaway speed. In both instances FG loop continuity was the main culprit. The photo of the SL-7 board looks a lot like the SL-DL1 I worked on. The pins that connected the FG loop to the main PCB had dry solder joints in that case, but were very fine fractures so difficult to see. It sounds like you've reflowed those already. Strange thing, I'd checked continuity on these pins with the board unplugged from the connector, but it seems that pressing on them with the probe had made contact, which wasn't maintained when the board was plugged back in. The other Technics was an SL-1200 with corrosion behind the Delrin FG loop mount, very difficult to see. There was also a blown transistor, Q203. In my experience the motor control ICs seem pretty reliable - I replaced both on the 1200 without solving the runaway speed problem. And recapped both the SL-DL1 and 1200 without any effect. But I understand each case is different! Hope this helps.

 

[Sandrews]

Also, I think the way to test a Hall Effect sensor is to connect a multimeter to its outputs, then spin the motor. You'll get a very small voltage change if it's working - it's something like 5mV from memory.