Technics SL-1300 MK2 Pitch control broken, possible IC failure?

ztoundas

New Member
Hello everyone,

Found a beautiful Technics SL-1300 MK2 in an abandoned storage unit a while back, finally cleaned it and got it running. Cue arm lift is broken, no biggie, manual operation is fine by me until I get around to wire/epoxy fixing the mechanism. I've been using it and loving it otherwise for about 200 listening hours until the pitch started sounding off, a little warbly sometimes. Then I found the pitch control buttons no longer changed anything (I know they did work as expected for a while). The buttons themselves are fine (I did fully remove them and clean/lube them anyway), and in fact when I press them, the LCD dims a little and the tracking dots clearly waver, this stops once I've release the button. I tested each button with a voltmeter as well, the contacts open and close promptly. Same goes for the start/stop buttons and 33/45 buttons. Anyway, I would simply power cycle it and the problem would go away.

About 40 hours of play time later the pitch has dropped to -10% (the readout still says 0.0% change), I timed the platter rotations to find this out. That, and it obviously just sounds slow.

I've since then gone through the manual, and using my oscilloscope calibrated all the variable resistors to an exact degree. No change. Then I went through the more advanced troubleshooting procedure listed in the service manual, and I think the IC201 (Frequency divider circuit, labeled DN860) might be acting up. One of the first pin the troubleshooting flow chart has you check seems to have a square wave slow by about 10%. I haven't gone past that so far. The chart says if this diagnostic step fails the tests, its either the IC or 6 specific transistors, which I will be checking tonight (there is a handy list of expected voltages at each pin for each transistor). I still need to fully check the actual pitch control IC301, because I feel like the behavior of the buttons and lcd tells me that's whats failed.

Anyone had experience with this problem in this model?
 
I have a parts unit if it comes to that ... With the cueing repaired.

How much? I'll keep it in mind, at least. I'm kind of attached to this model now that I've basically memorized the circuitry and my son has helped me tear-down and rebuild it like 30 times.
 
FO pin on the DN860 is the clock and should be ~262.08kHz, and at 0.0 pitch the FI pin should be the same. If FO is correct and FI isn't the problem is likely the MN6042. If you don't care about variable pitch you can bypass it.
 
FO pin on the DN860 is the clock and should be ~262.08kHz, and at 0.0 pitch the FI pin should be the same. If FO is correct and FI isn't the problem is likely the MN6042. If you don't care about variable pitch you can bypass it.

This is a great lead, thank you!

Where are you getting the names of the pins (FI and FO)? I've searched everywhere and can't seem to find a datasheet anywhere, beyond the simple pin numbers found on the SL-1300 MK2 service manual.

I have zero need for pitch adjustment, provided I can just get it to stay at standard levels. I'll certainly bypass it if the chip itself is dead. I've even toyed with the idea of bypassing half the board with an Arduino, using it to receive the motor's feedback and generate the appropriate wave to send to the motor-driver portion of the board.
 
Table from a service manual, though I can't remember which. 1300/1400/1500/150 MK2, SP-15, SP-10 MK2A, and SP-10MK3 use that chipset, thought the SP-10MK2A doesn't use the MN6042 as it doesn't have variable pitch.
 
Table from a service manual, though I can't remember which. 1300/1400/1500/150 MK2, SP-15, SP-10 MK2A, and SP-10MK3 use that chipset, thought the SP-10MK2A doesn't use the MN6042 as it doesn't have variable pitch.

I'd looked at another service manual for the mk1 of the SL-1300 and it didn't have the same chips at all, so I stopped there. Thanks for the tips, the SP-10MK3 has both the DN860 and the AN660 ICs in great detail so this will help my diagnostics/repair a ton. I'll check the other manuals as well. Plus now I have a wider range of scrap models to keep an eye out for.

Now if only I can find out a better way to prop up the motor/arm assembly so it can turn while still giving me easy access to the board.
 

So the results are in, and I'll be bypassing the pitch control circuit by bridging pin 6 and 7 on IC201 (DN860). Pin 7 is lookin' great, pin 6 is sad an slow. Once I find a scrap board with the pitch selection chip (MN6042), I'll remove the shunt on DN860 and swap the MN6042 with a (hopefully) working one.

I'll post an update once the repair is done.
 
FIXED!! (well... I bypassed the broken part of the board well enough to stabilize play at normal speed)

I first simply bridged the pins with solder (them being adjacent made it easy), then after running the oscilloscope I noticed the wave had a uneven crest:

So I then desoldered half the resistor that led from pin 6 (FO) to the malfunctioning pitch selection chip/circuit, pushing the desoldered side up and straightening it out of the through-hole. This way I can easily bend it back and push it through the hole later if I can find a replacement IC/transistors (I think it actually might be a specific transistor in the feedback loop the pitch selection circuit makes). I tested the circuit, but noticed that while the crest of the wave was now shaped right, it was 0.3v shy. I figured it was partially back-feeding to ground through the jumper leading from the pitch-selection circuit to pin 7 (FI), so I did the same thing to the jumper as I had to the resistor, completely isolating the pitch selection circuit from the rest of the system. Now the voltage hits ~4.1v at peak as expected, bypass complete!

Fun fact, power consumption dropped from 8.7 watts to 7.5 watts post-bypass. After I reattached the motor, it only went back up to 8.3 during use. The malfunctioning portion of the circuit was burning through more power than the motor itself. I was super lucky in that I had plugged the system into a kill-a-watt power meter a month ago out of curiosity.

Now the player is pitch-perfect and steady as can be! My son my insist to play Hey Jude 20 times a day, but now at least it's not at 28 rpm and warbly!

After I detected the problem but before I bypassed it, I tested various part of the malfunctioned circuit, and noticed one of the NPN transistors, TR303, had it's collector pin spiking up to almost 4v vs. the 0.8v It was supposed to be at. It'll be the first thing I replace once I feel like opening it up again (I see you, cue arm lift lever mechanism!).

Thanks a ton to all who helped!
 
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