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Technics SL-7 runaway

Montycat

Montycat

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I picked up this SL-7 today knowing it had the runaway speed problem I've heard about. It has been high on my want list and it's in exceptionally good shape aside from the speed issue. Hopefully I can fix it, and if so I'll pass on the SL-5 I already have. I mean, how many linears does a guy need? ;)

Technics SL-7 3 by Buhduh, on Flickr

Technics SL-7 2 by Buhduh, on Flickr
 
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That's a really nice looking SL-7. I have a couple of them myself. What are the characteristics of the runaway speed problem?
 
When you press start it spins and continues to go faster and faster. The only way to stop is is to pull the plug or press the open button. I don't know if it is actually continuing to speed up after a certain point; it's scary enough to watch that one does not want to wait too long and you cut the power.

I've heard that reflowing solder joints might fix it but I'm open to all suggestions. I paid $50 knowing about this issue, but since I've never seen a functioning one locally for less than $150 I figured the risk was worth it.
 
I think it is worth it as well. Not often I see these in this good of cosmetic condition. I have a service manual. I'll take a browse through it to see if I can see anything that jumps out as a possible cause.
 
I see a few variable resistors that may need some deoxit. One that has my attention is labeled servo gain adjustment. The other part is the speed control IC201 (AN6680) that could be bad. There is a good block diagram in the back of the manual. Do you have access to a manual?

David
 
After looking at this a little more closely, this turntable uses a Hall Effect transducer for feedback to the speed control IC. I highly suspect that the Hall Effect element needs replacing.
 
I have the SM from VE.

Calibrating those pots does not call for an oscilloscope, just a voltmeter, so after cleaning I should be able to dial them in.
 
Pio1980 said:
Check the tach coil trace and related connections.

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Click to expand...

So, what and where is the tach coil trace? Is this on the stator board? And how does one test the Hall Effect sensor or get one if it is bad? The part # seems to be H-300A.

I've opened it up and had a look over the main PCB and stator board. I don't see evidence of bad joints or anything yet.
 
The stator trace should be under the rotor on the board toward the outside if it's like other planar motors.

Sent from my SM-N900V using Tapatalk
 
Yes I have the SM. I have reflowed the ICs that are in the motor control part of the circuit and the Hall sensors, plus a reflowed and cleaned the 2 larger multi pin connectors that go to the motor and stator board. It still goes a million miles an hour, but I think the switches and LEDs in them are responding now where they did not seem to before. Pressing the cue or start buttons will start the motor and power or stop will stop it.

I stuck my ohmmeter on the Hall sensor in circuit but am not sure what I am supposed to find there. What should I see and do I have to remove them to test them?
 
I physically examined the boards really well, reflowed a lot of junctions, tested continuity in a bunch of spots but still no dice. I could be mistaken but it felt like one of the Hall thingies (H1) got hot after only a short time spinning at warp speed. Hard to tell though as I have to remove the platter before I can touch them.

Anyway, I buttoned it back up as I am kind of stumped right now. I could re-cap I suppose but I imagine the ICs are unobtanium as are the Hall devices.
 
I've got one in the same situation.

I was hoping someone had figured out the answer for me . . .
 
Montycat said:
Here's the main board.

Technics SL-7 Main PCB by Buhduh, on Flickr

And the stator board from the top with the Hall devices. I have had the main board off but have not exposed the bottom of this PCB yet.

Technics SL-7 stator board by Buhduh, on Flickr
Click to expand...

The Japanese Hall devices (and control ICs) are normally very reliable, faults with them or associated circuits generally register as failure to start reliably and/or uneven running or starting or running reverse. The spacing of the Hall devices to the rotor can be critical, don't change anything that affects that.
Speed control problems otherwise are usually the fault of the tachometer/ frequency generator and speed control sections, look there for speed/overspeed runaway faults to include cleaning the speed pots and carefully resetting them to the same exact position and checking board grounding screw points as per the linked thread.
 
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Pio1980 said:
The Japanese Hall devices (and control ICs) are normally very reliable, faults with them or associated circuits generally register as failure to start reliably and/or uneven running or starting or running reverse. The spacing of the Hall devices to the rotor can be critical, don't change anything that affects that.
Speed control problems otherwise are usually the fault of the tachometer/ frequency generator and speed control sections, look there for speed/overspeed runaway faults to include cleaning the speed pots and carefully resetting them to the same exact position and checking board grounding screw points as per the linked thread.
Click to expand...

The only pots I see are for adjusting Offset and Servo Gain. Those don't sound like speed pots to me, and should in fact just be for the tonearm positioning if I'm reading this right.
 
Montycat said:
The only pots I see are for adjusting Offset and Servo Gain. Those don't sound like speed pots to me, and should in fact just be for the tonearm positioning if I'm reading this right.
Click to expand...

Right, don't mess with them.

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