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Old 11th Nov 2020, 8:54 pm   #1
dj_fivos_sak
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Default AKAI VS-10 (HR-7700/3V23) incorrect speed in X2 mode

Hi there. I have this very old VCR and it worked fine but it recently developed a fault. While the capstan speed is correct during normal playback mode, it's not increasing to double the normal speed when X2 playback is selected. The capstan speed actually increases a bit when the "X2 SPEED" key is pressed but because this model has an audio pitch shifting circuit for X2 playback, I can hear the 440Hz tone tape much lower in frequency, and I can see the tape not moving fast enough. I have measured pin #48 on the SERVO-1 board and it's low (0V) during normal playback and high (12V) when X2 is selected. So I believe the syscon circuit is working as it should. What could be the cause of this issue?

Thanks

Fivos
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Old 12th Nov 2020, 1:35 am   #2
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Default Re: AKAI VS-10 (HR-7700/3V23) incorrect speed in X2 mode

Hi Fivos

It might be worth checking for correct operation of the following.

On the servo-1 board, input X2(12V) is inverted by NAND gate IC11 (4011) with output pin 10 at 0V. Transistor X3 is turned on via R20 (100k). R18 (180k) is bypassed by X3 to reduce IC3 monostable (BA222) time constant to increase capstan speed.

Rich
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Old 12th Nov 2020, 4:09 pm   #3
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Default Re: AKAI VS-10 (HR-7700/3V23) incorrect speed in X2 mode

That's useful info Rich. Thanks a lot! Both R18 and R20 are measuring good as does X3 (2SD636). Pin 10 of IC11 is measuring 12V during normal playback mode and goes down to 0V when X2 playback is selected. So could IC3 (BA222) be bad? Apologies if my question sounds stupid.

Fivos

Last edited by dj_fivos_sak; 12th Nov 2020 at 4:10 pm. Reason: typo
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Old 12th Nov 2020, 6:40 pm   #4
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Default Re: AKAI VS-10 (HR-7700/3V23) incorrect speed in X2 mode

Problems in high speed modes I have sometimes found to be poorly lubricated capstan bearing.
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Old 12th Nov 2020, 6:59 pm   #5
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Default Re: AKAI VS-10 (HR-7700/3V23) incorrect speed in X2 mode

Ben

I measured the voltage going to the capstan motor and it's 5.1V during normal speed playback but it's only 7.6V in 2X speed. Shouldn't it be like 10.2V?
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Old 13th Nov 2020, 5:21 pm   #6
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Default Re: AKAI VS-10 (HR-7700/3V23) incorrect speed in X2 mode

Quote:
Originally Posted by dj_fivos_sak View Post
So could IC3 (BA222) be bad?
Fivos
IC3 may not be bad, What does the waveform and frequency look like at TP-3 in 1x and 2X modes?

Rich
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Old 13th Nov 2020, 11:42 pm   #7
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Default Re: AKAI VS-10 (HR-7700/3V23) incorrect speed in X2 mode

Unfortunately I don't have an oscillosope to measure the frequency but voltage-wise, TP3 measured 2.25V in normal speed and 0.5V in 2X speed. But I don't think voltage measurement tells anything.

Fivos
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Old 15th Nov 2020, 1:07 pm   #8
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Default Re: AKAI VS-10 (HR-7700/3V23) incorrect speed in X2 mode

It does appear that IC3 is working however an oscilloscope is essential for VCR service and repair work.

There are a couple of other areas to check. Going back to pin 10 of IC11, diode D27 is also active in 2x mode pulling pin 2 of IC12 (AN6341) low to select speed. The discriminator output is on pin 7 and would expect to see a deviation from the nominal 5.1V. At the same time compare 1x and 2x speed output voltage on pin 9 of IC11.

The feedback control system, combines speed (pin 9) and phase (pin7) errors together in IC13 (uPC1458). Now is there 20v supply on pin 8?

Hmm...I have spotted horror of horrors, tantalum capacitors back to back in the filter network!! Although reliable at the time, many are now starting to fail in a lot of equipment. I would check C41, C42 (22uF) and C39, C40 (10uF) particularly for leakage. What is the output voltage on IC13 pin 7?

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Old 15th Nov 2020, 3:33 pm   #9
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Default Re: AKAI VS-10 (HR-7700/3V23) incorrect speed in X2 mode

Hello. I tested the voltages and they measured as follows:
IC11 pin 9=0V stop mode, 0V play mode, 11.7V X2 mode
IC13 Pin 7=2V stop mode, 6.9V play mode, 9.5V X2 mode
IC13 Pin 8=25.6V stop mode, 24V play and X2 mode

Is it safe/right to replace tantalum caps with electrolytics on a circuit like this? I also have a replacement AN6341 in case it has failed.

Fivos
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Old 16th Nov 2020, 7:56 pm   #10
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Default Re: AKAI VS-10 (HR-7700/3V23) incorrect speed in X2 mode

Thanks for the voltage info, Fivos.

Voltages look OK, but I suspect IC11 pin 9 voltage is not correct. The schematic I have has labelled speed output from IC11 pin 7 as capstan phase error, which is not the case. Feedback from capstan FG (frequency generator) after amplification is applied to input IC11 pin 16. Under normal circumstances this is used for speed control and will assume convention is followed. IC11 pin 2 selects/deselects an internal divide by 2 to obtain both speeds.

As there are two loops (speed and phase) one will affect the other since both errors are summed by IC13 (uPC1458). It might help to determine where the problem lies by temporarily removing phase error signal. I would suggest disconnecting one end of R76 (270k) and R77 (1M) to isolate the two loops only leaving speed control in place.

Note when playing at 1x the picture display will be free running. On selecting 2x speed if the FG control loop is working correctly at least 10 or 11 volts should be present across the motor terminals.

In this circuit configuration, it is always best to replace capacitors with the same type.

Rich
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