2 Dual C844 decks - 2 questions

At least the question on the 2nd tape deck was already posted in another thread - and there's already an answer...

I already published question 2 but there was no solution.
I already cleaned both capatans, rubber roller, idler. Even I lubricated (silicon lube) back ends of both capstans.
The only thing - I did not clean the surface of capstans which contacts with the belt.
But I suspect it will not help.
There are some suggestions:
1. There is to be some tension of tape between capstans, in other case the tape will not comtact with erase head. So left capstan should move the tape slightly slower that the right one. Is it correct?
2. Difference in speed should not be mich in other case the tape could be damaged.
3. Maybe pinch force of the left pinch roller should be lower than now?

Hi,

the tape counter logic is inside the µ-controller.
Only the driver is external. But your problem does not seem to be related to the driver.

The input to the µ is generated by a photo sensor. There is also quite a bit of signal shaping circuit with OPamps.
Also there the problem could be located!

Do you have the schematic?

ad 1) Correct. If the tape is not tight between the capstans, the contact to all heads is poor, especially for the erase head as that one does not have a felt.

Bernhard

Hi, Bernhard,
I have schematics of C844, as I understood the digital display accepts all signals including degments and decades from something as D-triggers (or D-flipflops). Is it correct? Which OP amps do you mean?

Capstans. This means that both capstans are not identical (though they look alike). As the tape drive mechanism looks tobe opened and disassebbled (at least partially), maybe chanding capstans will sollve the problems?
I mean tho place right instead left and vice verca.
But even in this case the speed gradient seems too high.
The erase head loses contact with tape within several seconds.


Alexander

Hello,

well, this IC407 is only the driver for the LED Display.
Described in the manuals text section.
The counter itself is realized within the µ-controller IC409.
The input to this counter is the signal from Q420, that is part of the "DLL". This DLL is also described in the text of the manual.
The LED 406 is "flickering" with a certain frequency, generated by an oscillator, which is on half of IC404 (I think an Opamp, but may also be a comparator).
The other half of IC404 acts as an pre-amplifier for the received signal from fototransistor Q420.
So I think you should first of all check the signals in this area. Maybe the oszillator frequency of 1050Hz is marginal and shifting a little bit more away with warmup.

If the capstans are not identical, I do not know. I would not recommend to interchange capstans. Anyway this will not be possible I think. I would also not interchange the pinch rollers.
If the flywheel belt is ok (did you check/replace it?) and the pinch rollers are cleaned, it should work.
I never had the problems you are describing.
If the capstan belt is original part from the 80ies you definitely have to change it.

The exact band tension between the capstans depends also from the "grip" of the two capstan, which may vary with lifetime. Anyway the lift drive should supply the tape more slowly than the right one. You cannot adjust this. It is achieved by design. So flywheels belt tracks will have diffent diameter and/or capstan diameter will be different.

Regards,
Bernhard

Hi,
the belts are new. I exchanged all of them.
Here are pictures of capstans.

http://photofile.ru/...03750/#mainImageLink

http://photofile.ru/...03780/#mainImageLink

I had never seen colored pinch rollers before and it is strange that one in "normal" black" and one is pink.
But anything is possible.
The sutuation when left capstand feeds the tape faster than right one can accept could be caused:
1. In case if both capstans (really flywheels)slightly differs in diameter and in case that the mechanism was disassembled - accidently (or not) when assembling the right one was installed instead the left one and vice versa.
2. In case if pinch rollers are not original - the right roller is pressed to the capstan not good. But I was trying to increase this force by hand ((to depress the roller to capstan additionaly). This does not help.
3. Some additional brake should be on supply hub or lubricant penetrated somewhere where librication is not required. When braking manuallty the supply reel in play mode the mechanism operates normally.
4. Dirty surface of right capstan and the belt slightly glides. Visually surface of right capstan does not differ from the left one.
5. Lubrication of the capstans bearings required.

Deck 1.
As I understand the CPU is counting pulses and sends all segment and decade data to digital display via some latches. And these latches are synchonising normally at the beginning and in some time synchronization fails.
There is no diagrams near me now, I will see in the evening.
Maybe exchanging of these latches could help?

Alexander