Studer B67 Mk2

[Michael Maurice]

I have been involved with servicing this deck.

The original fault was that when switched on, all the motors spun at full speed.

This was quickly traced to a short circuit capacitor on the -12V rail on the tape transport control board.

I replaced this along with all the 0.47uF Rifa capacitors on the motor boards and also the mains filter capacitor.

I also refilled the dashpots for the sensor arms with Kilopoise.

The problem now is that whichever mode I select, both motors are switched on full power.

I've checked the output from the tension sensors vary but no matter what their position both motors run at full speed/torque.

Can anyone point me in the right direction. I'm fairly certain that the fault lies on the tape transport control board.

[djwells01]

Hi Michael,

Try this thread - https://www.vintage-radio.net/forum/...ad.php?t=65718

post 16, para 4 onwards.

Another hint, after I had re-capped the power supply board on my B67, all reassembled, both reel motors ran on power up. After reading the above thread I removed the main transport control board. Once out, I turned it over and a washer fell out, wondering if this could be the problem, B67 reassembled again
and problem cured!

Hope something here helps,

Duncan

[Michael Maurice]

Duncan, I've been over the transport board with a fine tooth comb, no washers fell out!

When a mode is selected, the outputs to the motor boards go very high (around 6V) Adjusting the pots on the transport control board makes no difference.

Checks I've done is ensure that the three opamps have +/- 12V on them but their outputs are high, should be around 0V

I've also checked that moving the tension arms increases/decreases the output voltage from the modules.

Does anyone have any ideas?

[Vicboduk]

Hi Michael, I’ve never worked on a B67 but I’ve been looking at the circuit.
Have you checked Fuse F1 400mA, the 20v ac line on the PSU? This is rectified by D51-D54 on the control board and is used as a common pulse reference by both drive amplifiers. If this signal is missing you will not get any proportional control.
Scope the junction of R139/R140 you should see rectified 100Hz pulses @ approx. 1Vpk-pk (if I’ve done my maths right!)

Vic

[Michael Maurice]

I've not checked with a scope but there is around 18V DC at that point.

[Vicboduk]

If you're getting 18v dc across R139 it would suggest that it must be open circuit!
I would expect to get >=18v out of the bridge though.

Vic

Edit;
Oops. I may need a re-think. I've just noticed the 20vac line doesn't share a common ground. It is made via the bridge -ve trerminal.

Edit2;
I've looked again and still think my original thought holds true.

[Michael Maurice]

i measured it at the junction of D52 and D54 bridge rectifier

[Vicboduk]

Any progress Michael?
Assuming the drive amps are ok another candidate could be C203 (1uF tant) on the tape tension sub board. It is in the circuit to do with selecting of the tape tension. I think it is there to provide an initial kick to both motors when entering a mode. If it has gone sc/leaky then full power may be provided at all times. I have had quite a few failures of these low value tants in Revox machines.

Vic

[TIMTAPE]

I dont have the circuit so cant help on that front but I agree that suspecting electrolytics whether conventional or tantalum is always a good starting point.

Cheers Tim

[Michael Maurice]

Vic,

I have changed C203 but there's no change.

I've got to persuade the owner that its worth replacing the transport conytrol board

[Vicboduk]

Shame, I had high hopes for C203! But I understand your predicament when it’s a commercial venture, time costs money and will the customer pay?
Obtaining another control board is probably the most economic solution.
However, it would be great to pin this fault down to a component. – One for the log book.

Vic