Hi All,
I'm in the mids of repairing a 238 with the manditory runaway capstan issue. It actually came in not running at all, after someone else tried to recap the capstan board. But didn't get it to work, due to much pcb trace damage, lack of electronical insight or a mix of both and aborted the repair.
After some consideration and finding Denis's (238pro.com) gerber file, I decided to order a blank PCB at JLPCB and migrate all ICs, coils, functioning smd resistors and caps to the replica board. All electrolytics were replaced with smd panasonic caps. Also C2 was replaced by a 4.7nF X7R smd cap, as per 238pro's instructions. Lastly zener diode was replaced.
Migration went without issues; IC's were hot air soldered and other components microsoldered with the use of a microscope. C3 and C4 where preserved, since they measures ok. Few resistors and ceramic caps were also replaced, that didn't met specs.
After installing the new board in the deck, initially all was working fine. Vari speed, pitch control and shuttle seemed to function ok.
But after about 30 min. the runaway came back.
Funny enough runaway started right when I tried to tuck away the FG cable. But since I barely touched it, I'm doubting that could be the source of the issue and this was unrelated to each other.
Off course that is where I started my troubleshoot; resoldered FG cable, measured resistance at FG board and checked for ground disconnections or shorts.
I found some info in @SkywaveTDR replies about measuring TP1 with a scope and adjusting the back screw. My scope shows a sine/triangle wave at tp1 and adjusting back screw did make the wave look better, but didn't resolve the issue.
Scope measured a 1.375Vp-p at 0.33Mhz. I coulnd't find any info if these measurements are within specs. See photo.
Chatgpt says the frequency (0,33hz??) is to low and thus the circuit is compensating this by running at full speed. No idea if the source of the issue is even in the FG signal or the processing of it. Since I have to "argue" or question every analysis Chatgpt gives me without real result, I thought asking someone here with real life experience for some advice would be more productive.
Can anybody confirm that the measurements of the FG signal is correct or not?
All powersupply at IC's VCC and zener diode measures good voltages.
Play, pitch and shuttle don't function any more.
Next steps would be capstan bearing lubrication and checking IC1, which I really could use some advice or pointers on.
So if anyone can help me out, please do!
Much appreciated!
Joel
I'm in the mids of repairing a 238 with the manditory runaway capstan issue. It actually came in not running at all, after someone else tried to recap the capstan board. But didn't get it to work, due to much pcb trace damage, lack of electronical insight or a mix of both and aborted the repair.
After some consideration and finding Denis's (238pro.com) gerber file, I decided to order a blank PCB at JLPCB and migrate all ICs, coils, functioning smd resistors and caps to the replica board. All electrolytics were replaced with smd panasonic caps. Also C2 was replaced by a 4.7nF X7R smd cap, as per 238pro's instructions. Lastly zener diode was replaced.
Migration went without issues; IC's were hot air soldered and other components microsoldered with the use of a microscope. C3 and C4 where preserved, since they measures ok. Few resistors and ceramic caps were also replaced, that didn't met specs.
After installing the new board in the deck, initially all was working fine. Vari speed, pitch control and shuttle seemed to function ok.
But after about 30 min. the runaway came back.
Funny enough runaway started right when I tried to tuck away the FG cable. But since I barely touched it, I'm doubting that could be the source of the issue and this was unrelated to each other.
Off course that is where I started my troubleshoot; resoldered FG cable, measured resistance at FG board and checked for ground disconnections or shorts.
I found some info in @SkywaveTDR replies about measuring TP1 with a scope and adjusting the back screw. My scope shows a sine/triangle wave at tp1 and adjusting back screw did make the wave look better, but didn't resolve the issue.
Scope measured a 1.375Vp-p at 0.33Mhz. I coulnd't find any info if these measurements are within specs. See photo.
Chatgpt says the frequency (0,33hz??) is to low and thus the circuit is compensating this by running at full speed. No idea if the source of the issue is even in the FG signal or the processing of it. Since I have to "argue" or question every analysis Chatgpt gives me without real result, I thought asking someone here with real life experience for some advice would be more productive.
Can anybody confirm that the measurements of the FG signal is correct or not?
All powersupply at IC's VCC and zener diode measures good voltages.
Play, pitch and shuttle don't function any more.
Next steps would be capstan bearing lubrication and checking IC1, which I really could use some advice or pointers on.
So if anyone can help me out, please do!
Much appreciated!
Joel
