ok,
no need for the 4011 then!
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ok,
no need for the 4011 then!
To be exact: the FI pin of the 6682 is connected to the FO pin of the 6680. The FI pin of the 6680 is only connected to an output of the 4011.Quote:
Originally Posted by dimkasta
Both the FO pins of the 6680 and 6682 are connected to the 4011 to be able to select between "direct 6680" or "pitch control 6682".
(FI: Frequency IN, FO: Frequency out)
With the stock deck you can already hear that it sounds more stable with the reset led on (no pitch control).
Yep I am already running mine with the switch shorted at the header, leaving the pitch pcb unconnected.
I was just to lazy to follow the NAND gates. On closer inspection it should be rather obvious even without going through the gate logic, since the FO of 6680 goes both to 6682 and 4011.
Thanks for the clarifications.
looking forward to your impressions of this mod!
For a dedicated Hi Fi TT.
I like the idea of removing the pitch control 6682 and logic switch IC 302- it much simplifies the PSU. sub regs needed.
TT speed would be at the mercy of X201 and its tolerance (Frequency/ Temperature stability.)
High stability clocks or oven controlled crystals are available.
Has anyone researched this.?
I briefly looked some time ago for an industrial grade replacement, but the frequency seems rather uncommon.
Is absolute pitch really an issue For the 1200? Cd players have been running from the start with these low grade x-tals never heard of pitch issues with cd. Even if an issue I'd be much more worried by the effect of the vco on the decks relative pitch stability.
With the same x tal you can build a oscillator with higher q and less Rf disturbance. Given the rather high frequency this is a specialist job to design, any rf engineers here?
OK, I managed to steal some time to actually work on the turntable.
I rebuilt and triple checked all regulators and replaced both 6680 and 6675 (Check here for problem details)
At the same time, I added some female headers so that I am ready to try the mod discussed on the latest posts.
Unfortunately the problem remained the same. So it was either 4011 or 6682. Just to be sure I removed both, and jumpered pins 1 and 4 on the 4011 socket in order to short pins 2 and 3 on 6680.
The turntable now works, but there is another problem.
At 45rpm it seems to work with stable speed (judging from the strobe).
But at 33rpm the speed seems to fluctuate back and forth at a constant interval. Something like 4-5 times a sec. Like something is oscillating.
Could it be the extra unbuffered inductance and the poor connection of the jumper between pins 2 and 3 on 6680 now that 4011 is gone?
I 'm too tired now to check it with my scope. Any input is ofc welcome
Such a low frequency oscillation is probably due an unstable PLL. Prime suspect is the loop filter (R206-210, C210-C212). C212 is a lytic, is still ok? Another possibility is the FG amp input filter (R204, R205, C2020-C206) wich has a lot of electrolytic in it.
Can you see the oscillation with the strobelight dots?