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Old 20th December 2006, 10:01 PM   #11
pjpoes is offline pjpoes  United States
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Thank you. I have the manual actually, and that is what I am using. Its where i got my voltages from.

According to the manual, I need to adjust the pots to get the desired voltage, then adjust a third pot to get the dc offset as low as possible. However the manual is vague, and I have been unable to do what they say.

If you look at the manual, it says to measure the Ac voltage, get it to 8, 5, and 2.5 volts, then measure the dc voltage for each one, and get the DC offset as low as possible. Problem I am having is that, even when I get the correct voltage, the DC offset is too high, and often doesn't stay still at all. I was able to get the 33 rpm speed measuring correctly, low DC offset and correct voltage, however, it is running too slow at the moment, so apparently I need to increase the voltage slightly. That was actually my fault, I forgot to move the thumbwheel adjuster to the middle as the directions instruct, I was hurrying after I started to make some headway. The 16rpm and 45 rpm's are still way off, and I have been unable to get those right.
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Old 20th December 2006, 10:54 PM   #12
AuroraB is offline AuroraB  Norway
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Are the supply voltages across C2 and C4 balanced, stable and at what voltage?? Both 12-15VDC and stable?? Did you change the lamp, and with what characterstics of the new lamp??

The motor volages from each output vs. ground should be balanaced and stable. If one half is stable, the other not, this of course indicates a fault in that half. If both are unstable, a fault in the supply seems more logical. Changing the 709 with a 5534 might be possible, if you compensate for the pin skew, but no guarantees given , at your own risk. I would get a pair of new LM/uA 709s.
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Old 21st December 2006, 06:20 PM   #13
pjpoes is offline pjpoes  United States
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I haven't checked C2 and C4, I will have to do that, thank you. Was that in the manual, and did I just miss that? The bulb is the original, I was incorrect when I said it was blown. When the power supply cap blew it blew paper and oil all over the board, and covered the lamp. As a result I thought the bulb was blown as I couldn't see the filament. However, when I began testing it, the bulb lit up, and once things became more stable, it lit up correctly. However, I did order new bulbs, ten of them, with identical specs, that I could try just in case.

As for the halves of the motor, unless I missed something, the manual doesn't tell you to do that. However, I figured it made sense, but wasn't sure if they should be the same. However, they were the same balanced as you put it, so that seems to be working correctly.

I think this turntable is good enough to warrant putting a little money into. I have contacted a local repair shop that has plenty of testing equipment, and so I will be having them take a look at it if they get back to me. I think I need to see the waveforms on an oscilliscope and get a better perspective of whats going on. I think it will make adjustments easier, and help identify possible parts problems.

At this point, as I said before, the 16rpm and 45 rpm speeds are adjusted correctly for voltage, but DC offset is very high. I also find that if you adjust DC offset until the motor runs smoothly, and smooth is relative here, the DC offset is sometimes in the multiple volt range. I think I got lucky with the 33 rpm, and I'm guessing that the key to getting the right speed and low DC offset has to do with the right ratio of the first two pots, and then using the third to fine tune the DC offset. Somehow I stumbled upon that right ratio, and was able to further fine tune it.

Mind you, DC offset isn't stable. I don't know if it should be, but it goes rom .001mv up to 15mv, but seems to average in the 5mv range. Thats only true of the 33 rpm speed too.

Do you have a better understanding of what each one of the pots does in adjusting things. As I said, there seems to be two pots that adjust the voltage, and onef or DC offset. Is that correct. I couldn't understand fully what the manual meant when it said to adjust them, it doesn't give mention of three, it only talks about two.
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Old 21st December 2006, 11:12 PM   #14
AuroraB is offline AuroraB  Norway
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Have you measured PSU ripple?? Tens of mV is actually quite acceptable for this circuit.
As for changing the film caps, I wouldn't bother. You don't need 1% accurrcy, as the trimpots will fix that for the frequency. You need freq. stability, and that is plenty OK with the original caps. ( I used alot of those back in the 70-80s).
If you get the voltages right, and the strobe looks stable, I think you are OK. I am sorry that I cannot be more helpful at this time, but I have no time to dig into my own till after X-mas..otherwise I could have given you my measurements to compare-
BTW- do you have the original arm..?
Mine came without, but I have an SME tucked a wawy..
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Old 21st December 2006, 11:13 PM   #15
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pjpoes-

Quote:
I couldn't understand fully what the manual meant when it said to adjust them, it doesn't give mention of three, it only talks about two.
My first thought here, is that your manual and contol card are different revisions. You might try downloading the manual from different sources..they might not be all the same. This has worked for me before.

As for your dc offset, It's starting to sound like your oscillator opamp is leaking..before you lay out the lettuce for a service bill (your table is certainly worth paying to get it fixed if you have to), I would throw in a pin-for-pin replacement and see if your problems disappear..I would socket it as well while your at it.

-Casey
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Old 27th December 2006, 06:10 PM   #16
pjpoes is offline pjpoes  United States
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First, thanks all for the help. No I don't have the original arm, I have no idea if my grandfather bought it with anything other than the ADC arm on it. I think he did change it, but I don't recall for sure anymore. Given that the arm came out in the 80's and the table in the 70's, I think he must have.

As for different revisions. I have one source for the service manual, its The Analogue Depot. There are multiple boards, but all of the boards appear to have three pots per speed.

If one of you have adjusted one of these before, please tell me what each pot does. Like I said, mine has three sets of pots, for the three speeds, and each set has three ports. That means I have a total of 9 pots. Is that the same as you? I would imagine as much. Again, as I said before, based on my reading, and trial and error, it appears that the first two change the AC voltage, and the final one changes the DC offset. My assumption is that the first two each change the voltage of each "leg" of the motor, and the final one tries to align the phase of the two voltage swings, so that they are precisely out of phase at the right amount, I assume 90 degrees. However, the instructions aren't clear on how to adjust the first two pots in order to get the correct voltage, while getting a suitable low dc offset. I can adjust it so that the voltage and DC offset are close to correct, however the motor then doesn't run smoothly. If I adjust it so the motor runs smoothly, then the DC offset is way off. I also am yet to get the speed correct for playback yet, even when the voltages are correct.
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