Fixing an old AKAI GX-F90 Casette/Mic/Pre-amp deck

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Hi All,
So I have an old (1981 I think) AKAI tape deck Model number GX-F90.
Picture: **broken link removed**

Unfortunately, it has recently developed two faults. Firstly, the capstan motor no longer spins, and secondly, it has very recently and suddenly stopped producing any sound.

Issue 1: the DC capstan motor (part number BLM-100 made by AKAI)
See the attached photo. It seems that the PCB 'coil' (on board part number CU-7007) is designed to lift the permanent magnet away from the board for a low friction bearing, but that coil no longer seems to be working properly as the magnet on the rotor simply sticks to the board now (the board is backed in steel).
There is a driver board that is connected to the lift coil (as well as other parts of the motor) and this driver board has the part number CU-5031, which uses a UA7392PC "general purpose motor control IC"
I suppose the 'lift coil' would be AC powered but I can find no AC or DC power a the terminals of the motor control board and my frequency counter reads no freq.

Details about the operation of the capstan motor, as well as board schematics is on pages 28-31, 47 and 48 of the uploaded manual. Does anybody know how to fix this issue with the coil?

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The second issue with the GX-F90 only happened very recently. One day it worked perfectly (except the capstan motor) and then the next day, I turned it on to the sound or buzz (like when you touch the input terminal of an amp). I checked all the connections, but no problem there.
Upon investigation, I found that:
* the bar meter on the front doesnt display the input signal, only a few bars of noise
* connecting headphones to the headphone socket on the front (sound is normally taken from line out on back) yields the same probem
* if i plug a source into the MIC input (either socket) the left channel works faintly, and the right not at all
* when it has been off for a while, and I turn it on again, there is faint music (whatever is plugged into the inputs) but the sound dies away to nothing very quickly (1.5 seconds or so -- capacitor? checked all caps, none *seem* visually bulged)

Can anyone suggest anything to try? This unit is quite complex on the inside and it is a little beyond my normal experiences.

Thank you so much for trying, all input will be appreciated.

 

[tunedwolf]

The "lift coil" you refer to on the Capstan Motor is actually a flywheel/ feedback generator used for speed control. A common problem with some of these early DD motors, particularly the old Sony ones, was for one of the coil ends to fracture and break off the board, so check for continuity of all three coils.
The motor flywheel should not be able to actually get close enough to the drive board to stick to it, so there is a mechanical issue there. Likely the glue has dried and allowed the shaft to slip out of place, or the bushing on the flywheel has lost it's tension. Usually the Capstan Motor shaft has a thrust washer or bearing coupled with a circlip to keep the correct ride height. An alternative method was for the shaft to have a highly polished and domed end that rode lightly on a thrust plate on the motor drive board. Ether way, the flywheel rides a few mm above the drive board, only just clearing the drive coils. If the distance is too great, the flywheel generator coil won't have sufficient voltage induced in it to maintain proper control of speed, nor for that matter will the hall effect sensors be able to properly regulate the rotating motor winding current. It may just be cheaper and easier to replace it if you can obtain one, a new pinch roller probably wouldn't go amiss either.

The Audio Hum that you have could be an earth off somewhere. Also on early tape decks, the wires going to the play/rec head were very delicate and may have fractured and came off the head with getting moved around. Failing that, to troubleshoot, you can start at the preamp and work towards the output, or as I prefer, work from the output back toward the pream stages. At some point you will find the hum either appear or disappear depending on which direction you took. A tone tape or generator and a 'scope would be handy here, but I'll wager you have neither available. A tone tape is easy enough to make, a 'scope is however beyond being reasonable. Maybe try with a sensitive in ear type headphone and a small preamp and "probe" the stages. Just beware of the Mains and power rails

rgds