SL1200Mk5 Speeds Up After Warming Up

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d2b
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SL1200Mk5 Speeds Up After Warming Up

Post by d2b » 20 Oct 2018 22:46

When cold (i.e. been sitting around for a while), my SL1200Mk5 runs fine at correct speed and brakes properly. After being powered up for a while, say 20 minutes perhaps, the platter begins to spin very fast and when powered down, takes a minute or two to slow to a near-stop.

What is the most likely problem—the motor drive IC or something else?

After turning it off for a while, it returns temporarily to apparently normal operation.

Dennis, aka "d2b"

chiz
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Re: SL1200Mk5 Speeds Up After Warming Up

Post by chiz » 21 Oct 2018 15:09

I'm no expert but others have suggested that AN6675 is the most likely culprit.

d2b
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Re: SL1200Mk5 Speeds Up After Warming Up

Post by d2b » 21 Oct 2018 18:41

is the AN6680 a suitable replacement for the AN6675? It looks like the AN6675 may be hard to obtain.

chiz
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Re: SL1200Mk5 Speeds Up After Warming Up

Post by chiz » 21 Oct 2018 21:53

No, these two ICs are not interchangeable.
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Hopefully someone will have some advice for you on how to troubleshoot this.
Like I said, I'm no expert on this issue - just reporting what I have read elsewhere.

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Re: SL1200Mk5 Speeds Up After Warming Up

Post by d2b » 21 Oct 2018 22:13

Thanks. I guess I could have looked on the schematic rather than ask the question. I'm not home at the moment, though. When I started looking online for the AN6675, several vendors listed the other part instead.

If I can find the part, I'll replace it. In the meantime, I'll check all of the capacitors around it to make sure one of them hasn't gone bad on me.

Dennis, aka "d2b"

chiz
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Re: SL1200Mk5 Speeds Up After Warming Up

Post by chiz » 22 Oct 2018 10:59

Good luck.
This might Help?
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Service manuals available here:
https://www.vinylengine.com/library/tec ... 1200.shtml
https://www.vinylengine.com/library/tec ... 1210.shtml

sylvain999
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Re: SL1200Mk5 Speeds Up After Warming Up

Post by sylvain999 » 28 Oct 2018 21:03

I 've seen one or the other fail along with Q1 so it will be hard to tell without a scope and a multimeter. Never had a bad capacitor doing this yet. Check solders and continuity of the FG coil especialy if the turntable had been used for dj 'ing. One bad bump can break the circuit there.

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Re: SL1200Mk5 Speeds Up After Warming Up

Post by d2b » 06 Nov 2018 02:25

Finally think I've narrowed the problem down. Now I need a solution.

1. Turntable will run fine for days before it goes crazy. This makes troubleshooting difficult.
2. Following the suggested troubleshooting chart, checked the power supply voltages. No problem there.
3. Spent a few days pondering ways to make the other suggested measurements. Finally decided to start at the end of the troubleshooting chart and check the voltage and waveform at the collector of Q203. Of course, by this time, I had to wait nearly a week for the table to spin out of control. I got lucky tonight and witnessed the Q203 signal begin to break up, get really noisy and finally disappear. Of course the turntable took off, running at what appears to be about 5x normal speed, perhaps even faster. Here are some questions:

a. When the table was running properly, that is, locked to the control signal(s), the collector voltage of Q203 was the prescribed 2.8+ VDC but with a sinusoidal waveform that was only 40mV peak-to-peak. That is lower than I would have expected, but not having much else to go on, I assumed it was OK. After all, the turntable seemed well under control. The signal also seemed a bit noisy but the strobe indicated the speed was constant. Is that a normal signal level?

b. There's not much in the service manual that describes the FG coil. I assume that FG stands for 'Frequency Generator. Is that correct, and where is it located? Can it be replaced?

c. I have access to a second SL-1200mk2 that is available for parts. Is it possible to use the FG coil from it?

Any Help would be appreciated. Thanks!!

"d2b"

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Re: SL1200Mk5 Speeds Up After Warming Up

Post by w_axlrose1 » 06 Nov 2018 21:01

When the platter starts to spin up fast do both the 33 and 45 LEDs illuminate simultaneously? If so, check the solder joints at Q201.

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Re: SL1200Mk5 Speeds Up After Warming Up

Post by sylvain999 » 06 Nov 2018 21:14

If the problem is heat.related try to heat one component or one zone at the time with heat gun or hair drier to get it runming in trouble. Get yourself a can of freeze mist and cool components one by one beginning with most suspected components to get back to normal operation. Don't use canned air for cooling these could blow some water at components. Freeze mist is made for that. Wait till the temp is back to normal before freezing another component so you dont get fooled.

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Re: SL1200Mk5 Speeds Up After Warming Up

Post by d2b » 06 Nov 2018 22:59

The problem, as I stated recently, is that the FG signal disappears at the collector of Q203. Please comment about that issue.

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Re: SL1200Mk5 Speeds Up After Warming Up

Post by sylvain999 » 06 Nov 2018 23:45

d2b wrote:
06 Nov 2018 02:25
Finally think I've narrowed the problem down. Now I need a solution.

1. Turntable will run fine for days before it goes crazy. This makes troubleshooting difficult.
2. Following the suggested troubleshooting chart, checked the power supply voltages. No problem there.
3. Spent a few days pondering ways to make the other suggested measurements. Finally decided to start at the end of the troubleshooting chart and check the voltage and waveform at the collector of Q203. Of course, by this time, I had to wait nearly a week for the table to spin out of control. I got lucky tonight and witnessed the Q203 signal begin to break up, get really noisy and finally disappear. Of course the turntable took off, running at what appears to be about 5x normal speed, perhaps even faster. Here are some questions:

a. When the table was running properly, that is, locked to the control signal(s), the collector voltage of Q203 was the prescribed 2.8+ VDC but with a sinusoidal waveform that was only 40mV peak-to-peak. That is lower than I would have expected, but not having much else to go on, I assumed it was OK. After all, the turntable seemed well under control. The signal also seemed a bit noisy but the strobe indicated the speed was constant. Is that a normal signal level?

b. There's not much in the service manual that describes the FG coil. I assume that FG stands for 'Frequency Generator. Is that correct, and where is it located? Can it be replaced?

c. I have access to a second SL-1200mk2 that is available for parts. Is it possible to use the FG coil from it?

Any Help would be appreciated. Thanks!!

"d2b"
The FG coil is wrapped around the aluminium dish around the coils. There are two termination where there is a cutout in the dish. If the coil is open , or there is a bad solder, no signal gets to Q203. This transistor amplifies the low level signal picked by the coil and sends it to pin 24 of AN 6680 which is in charge of controling the speed. This signal gets compared to a reference signal generated in the chip and correction of speed is done.

The catch here is you are working on a phase lock loop circuit. Anything wrong in the loop will get the signal sent to Q203 go wrong, so you should start troubleshooting at the beginning of the troubleshooting chart. It could be some component shorting or loading the Dc voltage, or bad signal generated by a chip. You are going to have to check all waveforms AND DC voltages on both AN 6675 and 6680...or do as i wrote above. The problem is very possibly heat related, and you will save a lot of hassle doing what i suggested, but you will comprehend better if you go with full procedure. You could use the 1200-1210 service manual as well, which has a chart of signals and DC voltages found at all pins.

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Re: SL1200Mk5 Speeds Up After Warming Up

Post by d2b » 07 Nov 2018 04:52

I'm sorry, but I'm inclined to disagree with you. I believe that as long as the drive motor is being driven, even though no longer frequency- and/or phase-locked to the reference oscillator, the FG circuit should still generate a sinusoidal signal at a frequency proportional to the rotational speed. The circuitry leading up to the base of Q203 is not particularly frequency-sensitive. Of course, a faulty solder joint or component in the circuitry leading to Q203 could cause the same problem.

I see now that the schematic indicates that the feedback signal should be on the order of 800mV p-p. I'm only seeing 40mv p-p, so maybe something downstream is in fact causing the signal loss.

Note that the DC voltage at the collector of Q203 does not change when the AC signal begins to break up and disappear altogether. Note also that the speed did not increase UNTIL the signal disappeared altogether. That said, it is conceivable that a fault downstream of the collector of Q203 could affect the feedback signal. So, I will check that possibility as you suggested. However, I'm wondering what the best method is to check all of this while the turntable is turning and of course, when the table itself is covering the circuitry.

If there was a consistent way to make the fault occur, troubleshooting would be far easier. Right now, the fault only seems to appear randomly and when it does, it's unpredictable.

Dennis, aka "d2b"

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Re: SL1200Mk5 Speeds Up After Warming Up

Post by Spinner45 » 07 Nov 2018 07:36

C 215, along with C201 and C216 could be the culprits.
C215 in particular, can be leaky, choking off any signal to Q203.
I'd replace all of them with fresh stock.
And add C202 and 203 to the list.

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Re: SL1200Mk5 Speeds Up After Warming Up

Post by sylvain999 » 07 Nov 2018 10:58

Let the top cover loose. Block vent holes with tape to generate some heat. Unscrew the alu plate where the ac line enters to make space for passing clip probes. You could solder a resistor pin to the point you want to test for a good grip.

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