Sony TC-766-2 Control IC

[BalfRepairCafe]

Hello, I am a volunteer for a repair cafe and someone has brought in a Sony TC-766-2 reel to reel. Many of the bearings had siezed due to lack of use but now all is nice and smooth. Switching on, the take up spool spins and the controls do not have any effect on the indicators or motors. I have checked the inputs and output voltages on the CX738 control IC. The buttons effect the inputs but all the outputs remain at 0V. Would it be safe to assume that the IC is the problem or could it be something else (the vcc is ok on the chip)?
Thanks, Peter.

[greg_simons]

Hi Peter, only talking in general terms but those early CX type chips are rarely the problem, they're ultra reliable, you need the schematic to check for missing voltages and every other possible cause of trouble before blaming the chip.
Greg.

[BalfRepairCafe]

Thanks - I have the service sheets and will look deeper...
P

[DMcMahon]

Hello Peter and a welcome to the Forum.

A lovely looking machine which I have no experience of. Apart from whats in the Service Manual I cannot easily find any technical information on the CX738, it looks to be a Sony proprietary programmed chip.

If not a dead chip, it is difficult to see where the problem may be, the fact that all the outputs remain at 0V and hence no motor/lamp drive etc appears to indicate a common reason.

When you measure the +12V VCC supply at pin 14 of the CX738 make sure you check it wrt the 0V connection at pin 1, i.e. don't just connect the meter negative to metalwork or another 0V/ground connection, just in case there is a break to pin 1 (highly unlikely I know). The schematic also shows the VCC supply going to pin 15 which I assume is the heatsink at one end of the chip.

Are all the 5 tape transport control input pins at logic high with no tape transport buttons selected/pressed ? i.e. if the Stop button was permanently made it may force all the outputs low all the time.

David

[BalfRepairCafe]

Hello David, thanks for the pointers. I am unable to look at it until Thursday but will certainly bear your comments in mind and report back. P

[ben]

Check the switches on the tension arms. Maybe the deck thinks there's no tape. If there's a remote socket (been a few years since I looked at the back of mine!), check there's nothing amiss there - perhaps something is impeding the front controls if it thinks there's a remote plugged in.

[BalfRepairCafe]

Update - Because the takeup spool was running all the time I couldn't put a tape in - the controls do nothing if the tension arms are "relaxed". I tied them together and the controls work (thanks Ben). I just need to locate the motor control problem. I shall go through the circuit next...

[DMcMahon]

That is good, I could not initially see how the tape tension switches had this control but now after more studying can see that the microswitches are S605 & S606 and with slack tape both switches are made, switching +24V through to transistor Q723 which turns Q723 on which then connects through to the IC 701 Stop input enabling Stop.

So now is the motor control problem the Take Up spool spinning issue ?

David

[ben]

You say the TU reel spins. Is it in fast wind mode (i.e.e high torque) or is it just in take up/play mode (low torque)? Identifying which mode will help narrow this down. My guess is a shorted transistor.

You could try powering up with a junk tape and see if it enters play mode.

[BalfRepairCafe]

Getting to know the diagram a bit now! It is in Fast forward mode but the brake is not off (I'v put a rubber band on to keep it off). 0.6v on the base of Q738 which tests ok in cct. 0.42v on R807 and 0.006v on R806 so it is not being turned on via them. I think I will take out Q738 to test out of cct...

[BalfRepairCafe]

Q738 tests o.k. with a meter but I changed it anyway and now the fault has cleared. Now to clean the heads etc. Thanks for all your help. Peter

[BalfRepairCafe]

Cleaned the heads, capstans etc. and tested record. Left ch o.k. but right channel has a serious rolloff starting below 800Hz. I suppose the place to start is head alignment? Any pointers gatefully received.
P

[ben]

Do not touch the heads!! Check caps in the preamp and relay contacts

Edit. Mine has a 2/4 track switch on the headblock, clean that if fitted.

[BalfRepairCafe]

I had a chance to have another look today and now there is something else! Switch on and after a few seconds it goes into play mode. Also, it does not now record. With the record mode swiches on to record their lamps light, but the record lamp does not flash. The control pin to the system board is raised from 0V to 24V by the mode switches. Any ideas? Thanks,
Peter

[ben]

Is the supply voltage to the logic circuit steady? Put a meter on it and see if it drops when functions are operated.

I think these units had a 'timer' switch, the unit will go into play or record by itself after power up if this is set. Check it is clean and not shorting closed.

[BalfRepairCafe]

Sorry for the delay - can only look when I get a moment. The timer was on - The cover plate was off and I thought the switch went the other way (stupid me!). Checking the circuit the voltages showed the record lamp should be on... The lamp has blown! Any ideas where I could get a replacement? Thanks, Peter

[DMcMahon]

If it is Record Mode lamps, PL101 & PL301, the SM lists them as 6V 35mA

David

[BalfRepairCafe]

Final update. The lamp fault was a dry joint on its wire to the pcb. It's all working now but the levels are not good. I haven't the test gear to go any further so it's back to the owner to see what he wants to do. Thanks for your help with this - I may be back if anyone brings in some vintage equipment to be saved from landfill.
Peter

[Analogue man]

Did you clean the heads? Although Ben said don't touch the heads he meant don't play with their adjustments but a good clean may be all it requires. Use IPA or Meths and clean all of the tape path from the tension left arm to the right tension arm.

[ben]

My TC-765A (the younger brother of this unit) had black crud an inch deep on the tape path, someone had obviously played a sticky tape on it and then left it. It took several attempts to shift it all, but in the end I got fabulous sound. So clean - and clean again.