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Disaster! Could use some help.

T

thelanranger

New Member
I got a Technics SL-1950 and cleaned it all up and it was working perfectly. My intention was to take it apart a final time and clean everything one final time and perform the 78 RPM mod per: https://audiokarma.org/forums/index...y-a-technics-turntable-to-play-78-rpm.325483/

I performed this using a 0-100k pot across R7 and fired the table up to test. At first try the switch did nothing at all to the speed (things ran at 45 rpms still). So, I unplugged everything, checked everything again with a meter, and tried again. This time 45 speed did not turn the table on at all!

I removed the wires from R7 to put everything back to normal and I'm in the position where everything seems to work normally except that when you put the switch on 45 speed the table does not turn! What did I mess up here? How can I troubleshoot this? I'm in a little bit deep right now I think.
 
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Super weird!
After several tries it just spontaneously started to work again!

So...is there anyone out there who can give me any ideas as to what might have happened here? Are there some components that I should verify are in good working order? I would like to go forward with this mod but at the same time I don't want to damage the player. If anyone can help me double check what I'm doing and get this sorted I'd be eternally grateful. Thanks!
 
rothwellaudio said:
Have you any experience of electronics? Were you using a good soldering iron? Sounds like a dodgy solder joint.
Click to expand...

^^^

Also, my most recent annoying chase was a bottom mounted film cap where one of the legs snapped at the joint . It would make contact 75% of the time, and was cutting in and out when you touched thy volume knob 5 inches away (which was 100% ok).
 
I’d agree on the cold solder joint. There may be micro cracks within the solder cause intermittent connectivity. Reflow.
 
I'm sorry, I have to disagree. I'd not be surprised if the OP's inexperience had escalated the problem but it's origin seems to me inherently different and it goes back to the 78 mod thread he followed. That mod is based on the circuitry of the SL-D2.
  • The SL-D2 model has R7 (62KΩ) in series with a 20KΩ trimmer and connected to pin #24 (rotary speed adjust. circuit) of the AN630U logic IC.
  • The SL-1950 model has R10 (62KΩ) in series with a 20KΩ trimmer and connected to pin #24 (rotary speed adjust. circuit) of the AN630U logic IC.
The OP had performed the mod on the wrong resistor, which in the SL-1950 goes to pin #10 of the logic IC and is part of the wave form shaping circuit. No wonder it acts up.
 
tnsilver said:
I'm sorry, I have to disagree. I'd not be surprised if the OP's inexperience had escalated the problem but it's origin seems to me inherently different and it goes back to the 78 mod thread he followed. That mod is based on the circuitry of the SL-D2.
  • The SL-D2 model has R7 (62KΩ) in series with a 20KΩ trimmer and connected to pin #24 (rotary speed adjust. circuit) of the AN630U logic IC.
  • The SL-1950 model has R10 (62KΩ) in series with a 20KΩ trimmer and connected to pin #24 (rotary speed adjust. circuit) of the AN630U logic IC.
The OP had performed the mod on the wrong resistor, which in the SL-1950 goes to pin #10 of the logic IC and is part of the wave form shaping circuit. No wonder it acts up.
Click to expand...
That would explain why it didn't work! :thumbsup:
However, it still leaves this a bit of a mystery:
thelanranger said:
I removed the wires from R7 to put everything back to normal and I'm in the position where everything seems to work normally except that when you put the switch on 45 speed the table does not turn!
Click to expand...
thelanranger said:
After several tries it just spontaneously started to work again!
Click to expand...
 
Who knows?! It's a proprietary IC - you screw with it the wrong way and you're lucky if it ever recuperates. The OP added resistance so I assume he didn't fry any internal junctions - but it ain't over yet.
 
tnsilver said:
I'm sorry, I have to disagree. I'd not be surprised if the OP's inexperience had escalated the problem but it's origin seems to me inherently different and it goes back to the 78 mod thread he followed. That mod is based on the circuitry of the SL-D2.
  • The SL-D2 model has R7 (62KΩ) in series with a 20KΩ trimmer and connected to pin #24 (rotary speed adjust. circuit) of the AN630U logic IC.
  • The SL-1950 model has R10 (62KΩ) in series with a 20KΩ trimmer and connected to pin #24 (rotary speed adjust. circuit) of the AN630U logic IC.
The OP had performed the mod on the wrong resistor, which in the SL-1950 goes to pin #10 of the logic IC and is part of the wave form shaping circuit. No wonder it acts up.
Click to expand...

This is clearly the issue. It's not a solder issue. I've been and IT tech for 25 years and I solder things regularly that are FAR more complicated than the couple wires and a switch in this mod (DC jacks in laptops, repairing chip legs, swapping surface mount ICs on hard drives...). Obviously I'm just dumb and connected to the wrong location.

I was worried that I might have blown out a segment of the IC. I started testing components and didn't find anything obvious (no blown caps or missing continuity) but I don't have anything that checks frequency and really reading that schematic to test the motor/frequencies is beyond me. I'm not an electrical engineer, just a repair man. =)

Reading this though it does make a lot more sense that it's not R7. I thought it was weird that it would be changing the R7 value from 33 ohms to 56,000 ohms. But I figured someone did the math and everyone had done it. Shouldn't have had that third beer I guess. =P
 
Something similar unfortunately happened to my SL-1300 when I attempted to mod it for 78 a few weeks ago. I first tried to wire a switch with the resistor in parallel, but I could not get the wires to form a good solder joint. I then soldered a resistor directly to the rear of the board, turned it on, and the speed was a bit fast, but certainly not 78. I thought I likely just had the wrong value resistor (different circuitry than the SL-D2/1950 etc), desoldered it, put the turntable back together, and what do you know - same speed issue. I have a feeling that it’s because of me applying too much heat to the board and damaging a component, damaging a trace, or both. I would like it to work properly again, but it’s not urgent, so I think I’ll just have to wait until I find a replacement board that’s not $65 on eBay. I hope that yours continues to function as it should!
 
More Technics end up with problems due to people doing stuff like this when they dont know what they are doing than anything else.
There was a fashion to upgrade the pop up light on sl1200's so many people posted on VE about frying components after trying to do this mod. These are very reliable turntables, but if you dont know what you are doing leave them alone.
Chris
 
Success! I cleaned everything up and resoldered things to R10 vs R7, verified the ohm ratings and everything is working beautifully. Thanks for all your help!

P.S. This is one of the systems where R10 alters the speed of the 33.3 as well as the 45 when you flip the switch so it's changing my plans for switch location slightly. (Right now I'll just leave it dangling out the back on its little pigtail. =P) Perhaps someone could investigate what the 33/45 switch does exactly on these models? Perhaps we could somehow utilize the 'off' location for 78?
 
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