Revox B215 stops randomly in FF / REV modes (plays ok)

[david winter]

Hi,

I'm stuck and trying to understand the cause of the following problem: I have a Revox B215 which stops randomy in FF / REV modes.

So far I did the following:
- Recaping the power supply / control board (the one with the three MAB 8440 chips)
- Recaping the daughter board (capstan driver)
- Replacing the four 47K trimmers R15...R18 (before the LM339)
- Cleaning the infrared sensors
- Checking that I get low/high states on the four IR sensors while rotating the reel motors manually
- Adjusting the four 47K trimmers (R15...R18), which adjusted at around 9-10 o'clock instead of the usual near-middle position.

Playback works although there is some wow due to weak pinch rollers. No random stop in this mode.

Any hints welcome, hoping the MAB chip (IC8) isn't faulty.

Thanks !

 

[Tinman]

Rebuild the reel motors. Clean the cassette detect switches.
Or exercise the hell out of the reel motors and hope they loosen up. But I rebuild them with precision ball bearings, which usually cures the problem.
The B215 is a fickle deck to begin with. It also does not like some tapes, period.
I keep a Teac C-3X around for those tapes.

And yes.... pinch rollers are vital for proper playback on ANY closed loop dual capstan deck, so you should do those. Cleaning them never works.

 

[david winter]

Reel motors operate fine as far as I think. The FF / REV stops randomly but sometimes works great for several seconds (even more, like 10-15 seconds). Speed is very fast everytime I engage FF / REV whether it stops quickly or later.
Detection switches seem to work fine although I didn't check their continuity. Playback works, so I don't think there's a problem with them.
Any good supplier for new B215 pinch rollers ?

 

[david winter]

Some progress but still no definite fix. Here's what I did:
- replaced both pinch rollers (shafts cleaned and lubed)
- replaced 8 infrared diodes (they sometimes weaken and I wasn't sure whether it was the case)
- changed the LM339
- changed the four trimmers R15...R18 and the trimmer for end of tape adjustment.
- finished doing the full recap
- adjusted all four signals from IR sensors for symetry (I get 2.8 to 2.95V at the LM339 pins 1 2 13 14)
- checked the two push contacts on cassette transport
- checked voltages at mains transformer, after diode bridges and after all voltage regulators. All ok when I'm on 240V position (mains is 230V there) although I get +21.7V and -22.7V vs +22V/-22V but no big deal.

FF/REV works a lot better, but still stops randomly (FF mostly does it during the first 10% or so of the cassette).
Adjustments were done at 50% of tape spooling. All signals look good.

Sometimes, when I put a fully rewound cassette, play won't engage at first. I have to wait for a little and try again. Maybe something due to the leader ? After that playback is great (through headphones so far).

Any ideas what can still cause that randon stop on FF/REV ? I haven't changed any other IC like the MC14094.
I just hope this is not one of the MAB 8440 microcontrollers having a random problem.

 

[Tinman]

Remember, as you keep throwing parts at it: "Reel motors operate fine as far as I think."

 

[david winter]

I consider them ok as far as:;
- they rotate freely by hand
- playback is fine
- FF/REV is very fast as expected until random stop happens

Question is, how could they still generate the problem ?

 

[perryinva]

As Tinman has explained in other posts, the reel motors are NOT just “motors” as used on other decks, but use current feedback from measurement to the motor, coupled with rotational speed differential between the two, in order to set respective speed for tape tension. There are no slip clutches in the Revox transport, like every single other deck manufacturer used. Revox chose pure analog logic to tension their transports. If any parameter is out of sync, the logic calls for “tape stop”. I’ve rebuilt more than a dozen B215s and 710’s (same transport). When the motors behave this way after a total recap, adjustment, and lube etc, then they require a disassembly, clean and rebuild, and his ball bearing replacements update the motors to what the last gen B215s had factory installed. We’ve seen this over and over.