B67 MKI rewind probleme

[erhard58]

Hallo,

da mein Deutsch ist nicht mehr so Gut ist, so muss ich diese frage in Englisch stellen. Ich hoffe das jemand mir hier helfen kann, Danke!

From what I have been to establish right now, is that there is an issue with the left/supply motor tension circuit.
I can get the left supply motor to actually rewind no probs if I push the left sensor arm down between 5-10mm after pressing the < rewind button.
To start with, I cannot get anywhere close to adjusting the left tape tension sensor as described in step 5.4 in the B67 service manual. It asks for a distance of 48mm, the best I get is about 55mm and then run out of trimmer adjustment.
I can adjust the right tape tension sensor to the 50mm, no problems.
I have version 1.167.759 in my B67, which uses the RC4558 Op-Amp as part of the tape tension circuits.
The next step, 5.5, peak adjustment, I cannot do for either the left or right motor, neither will come to a stop as described in 5.5, and I adjusted each trimmer all the way from one stop to the other, which is even more confusing since I can adjust the right tension as described in 5.4.
I did a good visual check over of the left tension assembly and circuit board, I cannot see anything obvious.
Any input and/or suggestion are REALLY welcome, thank you.
By the way, this deck has been re capped and all electros as well as tanta & RIFA film caps have been replaced.

[benmuetsch]

Hi,

the peak adjustment did not work on my machines as well. Do you have the service manual? Try to measure all the incoming and outgoing signals and compare them with the other working sensor.

Good luck!

Ben

[erhard58]

Hi,

the peak adjustment did not work on my machines as well. Do you have the service manual? Try to measure all the incoming and outgoing signals and compare them with the other working sensor.

Good luck!

Ben

Hi,

thanks for the input. I believe one of the RC4558 OpAmps is faulty. As well, there has been some advise that Q14 & Q15, both BC108B's, tend to fail due to them being somewhat under rated, and should be replaced by a BC141.
I will do some further tests soon and report back.
What was the solution in your case?

[benmuetsch]

Hi, sorry for the late reply,

concerning the peak adjustment, there was no solution as it just did not work, but everything else worked perfectly fine. I called a technician who has way more experience with B67 machines and he told me that he never had one deck where the motors actually stopped spining at some point, like it is described in the service manual. SO I left that as it is. I have two B67 now both in near perfect state, I took everything apart, cleaned, lubed bearings, re capped, aligned and so on. Everything is up to spec, but that peak motor stop does not work on both. It even does not seem to make much of a noticeable difference in tape transport, how the peak potentiometer is set up, so I just set it to middle position.

In my schematics, I don't have OP amps on the sensor pcb, so yes, you maybe should check these components and try to replace.

Good luck,

Ben

[erhard58]

Hi, sorry for the late reply,

concerning the peak adjustment, there was no solution as it just did not work, but everything else worked perfectly fine. I called a technician who has way more experience with B67 machines and he told me that he never had one deck where the motors actually stopped spining at some point, like it is described in the service manual. SO I left that as it is. I have two B67 now both in near perfect state, I took everything apart, cleaned, lubed bearings, re capped, aligned and so on. Everything is up to spec, but that peak motor stop does not work on both. It even does not seem to make much of a noticeable difference in tape transport, how the peak potentiometer is set up, so I just set it to middle position.

In my schematics, I don't have OP amps on the sensor pcb, so yes, you maybe should check these components and try to replace.

Good luck,

Ben

that is interesting, thank you!
The Op-Amps are on the transport control pcb. The early B67's used the TCA561 which are almost impossible to find these days, mine is the later version which uses the RC4558 and the J411 which is a dual J-Fet unit, this J411 is also near impossible to find these days, so I am hoping that those are not faulty. The RC4558 is still readily available these days.
I hope to have it all back together in the next week or so, will keep you posted.

[benmuetsch]

The J411 may be replaced with several alternatives. Have a look at BUX80 / BUX81, they can easily be found and will do the job.

[erhard58]

The J411 may be replaced with several alternatives. Have a look at BUX80 / BUX81, they can easily be found and will do the job.

thanks for that, but the BUX80/BUX81 are NPN power transistors, the J411 as used in the B67 is a dual JFET unit

[benmuetsch]

Oh, just thought you are talking about the MJ411 power transistors.

[erhard58]

Oh, just thought you are talking about the MJ411 power transistors.

I know they are very close, but I am talking about the J411 dual JFET's