Studer B67 MKII speed issue

[Studio Mollan]

Hi,
I have a Studer B67 that is acting up. ITs the high speed version. 7,5/15/30ips.

slow speed always works medium and high are harder. It is weird cut if its been turned of for a day or so it won't play 15/30 its at all. but after fiddling around with it for a while, changing speed a couple of times, scratching ny head and cursing at it, seems to get it to work. Right now it runs perfectly at 30IPS and has done so for a couple of hours. Im positive though that it will be back to not working in the morning.

So, could this be a bad motor run capacitor? I read somewhere that if they are failing the motor will loose torque.
Actually earlier today i turned it around in the trolley and when it bumped into the lower edge the speed instantly lowered? Could also be a bad connection somewhere but i feel i have checked them all now over the last few days.

Ive measured all the voltages and they are fine. Capstan seems to be getting the correct voltages too measured on the capstan controls board. Breaks and tape tension seems to be correct and it spools fast and reliably. 
There is a bit of wobble in the audio, this is probably to do with the weak drive.
Any input is very appreciated!
/
Emil

 

[Bo Deadly]

Has it been recapped?

Personally I favor the "shotgun" approach which is to just replace anything that is known to wear out like all electrolyics and any high power parts like motor caps, snubber caps and big transistors or triacs. Then just see what happens and decide from there.

If it's stuff that deserves to be done anyway, it doesn't hurt. At least if you do it right. If you're not so good with that sort of work then you could just make a bigger mess (when I shotgunned my B77 I somehow torched a transistor on the transport control pcb and it took some extra work to fix).

 

[Studio Mollan]

Hehe, been there.. Thank god there are more qualified people for this.
The thing is that i can't leave it to any of my techs as they are both senior gentlemen. Covid19 etc.

I'm pretty used to fixing stuff around the studio but tape recorders i usually send away. Thought i'd take it as an opportunity to learn something. This particular machine is not the easiest though. Lots of intermittent faults and also some weirdness in the audio circuits. I have recapped all the audio boards, and most of the Stabilizer and oscillator cards.

You are absolutely right about the "shotgun" approach. I'll order what i can find and replace as much as possible and see what that does. 

 

[Bo Deadly]

Audio path stuff is of course not related to your problem. Focus on anything that handles power. Old silicon under high current and stress can get burned up inside and start to become slow to turn off and do weird things. So bridge rectifiers, triacs, power transistors, etc. And obviously anything labelled Frako or Rifa must go immediately. You might leave the tantalums (although I didn't - I replaced them with films). I didn't even turn on my B77 before completely recapping. There's no point in risking stinking up my place with blue smoke.

 

[Conviction]

If the tape path seems fine I would focus on the capstan controller board. Motor caps could of course be the issue, but often - when it comes to B67's - speed issues are related to the controller clock frequency. I recapped the controller board on my B67 MkII when it had trouble engaging high speed. It didn't run too slow, though, just not engaging high speed correctly. Might be a different problem.

Does yours have any problems engaging the different speeds? Or is it engaging high speed but running to slow?

 

[Studio Mollan]

Hi and thanks for your reply! (Eller tack kanske passar bättre!

I will replace what I can, I just need to go through it all and put in an order for the lot. Will take most of what I have to spare of this day I think.
I have recapped the entire capstan controll board but I felt it didn’t do much.
I haven’t measured the exact speed of the deck I watch the light in the speed switches that only goes on when synchronized.
It almost always reaches the speeds in idle but when not having been turned on for a while it will not retain the high speed after I hit play. The oscillator board is giving the correct frequencie output. I have a hard time measuring on the capstan control board when it is in circuit. Really could use a couple of extenders.

It takes a while for it to reach 30ips I would say between 5-10 seconds. 7,5 and 15 are way faster.
[iquote author=Conviction link=topic=75228.msg953870#msg953870 date=1591003375]
If the tape path seems fine I would focus on the capstan controller board. Motor caps could of course be the issue, but often - when it comes to B67's - speed issues are related to the controller clock frequency. I recapped the controller board on my B67 MkII when it had trouble engaging high speed. It didn't run too slow, though, just not engaging high speed correctly. Might be a different problem.

Does yours have any problems engaging the different speeds? Or is it engaging high speed but running to slow?
[/quote]

 

[Studio Mollan]

A weird thing:
I hooked up my scope to the test point on the capstan controls board. I should be reading 800, 1600 and 3200Hz square waves. One for each speed.
I read nothing at all at first and the capstan was idling really slow.
When I turned the deck over so that it was parallell to the floor, as if it was flat on a table top, all frequencies were there. Really strange, something has got to be loose in there. It clearly should be able to work standing up as well as lying down.

I could see from the square waves on the high speed that it sometimes lost synk. The square changes to a pulse with signal as i hit play and sometimes the edges of the PWM met and the unit lost sync. The motors still kept running at close to 30IPS so easy to get fooled there.
I get the feeling its weak in power. Going start replacing stuff as soon as mouser delivers my package.
Thanks for all your input!