AIWA AD-F770

AIWA AD-F770 Dual Capstan Cassette Deck

[Latest additions/corrections: 24/9/2019]

I've had this deck since about 1999/2000, but seldom used it. About 3 months ago, I decided to play the machine and after a short while realized that the playback speed and stability were not to standard.

Having replaced the belts, the problem of playback 'wow' effect was still problematic - I thought the issue would have been eliminated, but apparently not.

The default 12DC, 2400rpm, CCW (counter clockwise) motor was ageing so I decided to search the web for a replacement. To reduce a long story to a few sentences - I bought several Mabuchi (originally Japanese?) non-brushless motors. At this point it is worth mentioning that the said motors are available on the internet, but could these be Chinese copies, probably?

Rescuing the pulley from the original AIWA motor (spindle ~2mm), I eventually assembled the whole cassette transport together (difficult job) until success was achieved. The transport now runs fine, with the machine running at the correct speed, subject to less than 1% error.

Mabuchi replacement - notibly smaller than the AIWA original.

Both the new Mabuchi (Chinese copy?) and the original motor have voltage regulators inside, and so it would be quite feasible to repair the olden - perhaps another time?

Motor Speed Calibration
So, how did I achieve calibration?  As I don't have a frequency counter I had to resort to other ways. 

My only route for now was to use my ant-audio.co.uk Reference Tape with a recorded 3000Hz test tone. 

I rigged up the AIWA to playback the tape through the left channel of my amp, and through the right channel a digital reference 3000Hz tone. On playback I listened to both. The two sine waves effectively modulate one-another (ie multiply), and the result produces a beat frequency - the effects of which I can hear. In theory two beat frequencies are produced - the sum and the difference in frequency.

Adjusting the motor speed (via a pot at the motor rear) one can hear the low frequency beat effect speeding up or slowing down. Ideally, the beat frequency should be zero, but I managed to get it to around 1-3 Hz. That's an error of about 3/3000 or 0.1%. Not bad! However, as with motors of this kind, adjustment is very sensitive, and likely to oscillate about the set point within tens of seconds, but as long as the temporary drifts aren't excessive, it shouldn't be an issue. In contrast my Sony TC-K61 has superb speed stability combined with ease of precise adjustment.

Playback Level Calibration 
According to the service manual, playback level trimming is adjusted via SFR203 and SFR204 potentiometers.

My ant-audio.co.uk Reference Tape is modulated at 400Hz, Dolby Level, and after cleaning the pots (Servisol) I examined the traces on the oscilloscope. The service manual suggests 580mV from AIWA's calibration tapes - which I assumed were at Dolby Level too? The oscilloscope below now reads ~3.2 divisions x 500mV/per division = 1600mV peak-to-peak. This equates to a rms value of 0.7071x800mV ~ 566mV .... close enough, in fact < 2.5% error. 

The truth is - playback level at Line Out doesn't have to be very precise, ±5% should be sufficient?, as long as both Right and Left channels are identical to within small margins, say ±0.5db?

Left trace -top, Right trace - bottom.

Peak Meter Calibration
Now that the output was calibrated, what about the meters? - they must read DL at this point. Potentiometers SRF701, 702, and 703 were first cleaned and then adjusted from below DL, and then finally 'resting' and stable on DL.

 

February 2022: Note: adjusting the Left and Right channel meters ca be a little tricky!

 

Turning either Left or Right channel potentiometers can result in the meters swinging from high to low, then back up to high again.  

 

My calibration was carried out by finding the minimum point for each meter, then lowering the that single potentiometer (above in the picture) to indicate below dolby level, then turn each potentiomter for Left and Right so that they indicated dolby level on the meters.

 

I hope that isn't confusing!  ;o) 


Record/Replay Head Azimuth
This I already adjusted a while back, but just to check again, I ran the ant-audio.co.uk Ref Tape through the machine and made any (minute) adjustments. Ideally, both left and right channel sine wave traces should line-up (in phase), and display a maximum amplitude or output.

Record Level Calibration
For a deck that supposedly can self calibrate bias, record levels (ie tape sensitivity), and equilization, I found it strange that I could tweak the internal record levels myself, or at least set the machine to reproduce the same output for the same input, ie 0VU in gives 0VU out, or Dolby Level in gives Dolby Level out after invoking the auto-calibration feature. But it seems this was necessary, as this machine was apparently putting too much record level on the left channel by about 2-3dB for Maxell UR tapes, and others.


There are two potentiometers to help the machine calibrate - they are SFR401, and SFR402 located as shown (bottom right half of the overall board) ...

Amendment (May 2021) ....

 

 

I finally got the F770 to record, and playback 400Hz/1000Hz sine waves to the same level, estimated to be within the region of 0.5dB/1dB, according to the meters. Interestingly, not all tapes were so compliant, which I suspect is down to tape flexibility, the tape pad, and a small amount of head wear! Fast dv/dt transients however, can be a law unto themselves; some +2dB above that recorded.


Much of the calibration work I have done on this deck has not been strickly in accordance with the manual - for one reason, I don't have AIWA's original reference tapes.


Power Supply
There's no doubt that this deck is a good machine, but its circuit layout is complex. The power supply was always getting warm, so I decided to clean the underside of the PS board, and replace three of the regulating electrolytic capacitors which take up less space. Hopefully ensuring better air flow than the previous configuration. 

A red marker labels the replaced capacitors.

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16/03/2021: 

 

Hello everyone,

 

This page gets more views than any of the other Blog pages, except for the Revox. 

 

So my question to you all is - why are you especially interested in this page for the AD-F770? What problems have you got?

 

I cannot help you individually, but your feedback would help me to identify common issues with the AD-F770.

 

Thanks.