Revox B795 spinning at 100 RPM

[paki976]

Hi all,

Few days ago I substitued all Frako capacitors on my B795 (found the famous X1 cap exploded ). All went ok until today, when I had to answer a phone call (raised the cartridge with the usual up/down button), and left the turntable spinning for maybe 15/20 minutes.... When i returned, found it spinning like mad (maybe 100 rpm, maybe more).
Now, once turned on the platter starts to spin like crazy totally ignoring the tonearm.
Searching over other forums (english speaking, sorry guys I don't know a word in german!), many suggested to change all the tantalum capacitors with elecrtolytics, so did I, without success.
The tonearm still works ok, and I tested with a DMM the motor coils which are intact.
Any idea? : help:

Greetings from Rome,
Fabrizio

[borchee]

AW: Revox B795 spinning at 100 RPM

Hi Fabrizio,

welcome. I'm not the expert you are hoping for, nevertheless I might give you a jump start.

1) What are you able to do according to your electronics knowledge and which equipment is available?
2) Did you replace all the capacitors (except the ceramic ones) on all boards?
3) What do you mean by "...once turned on..." and "...totally ignoring the tonearm"? Elaborate on the actual behavior a bit preciser. It happens...forgetting to plug the tonearm connector onto the board.
4) Browsing other forums you must have came across the IC socket problems - they corrode so you should check them for contact problems. You might not have sockets as the higher serial numbers (what's yours?) have the ICs soldered to the board.
5) Check the boards for cold solder points and broken traces (there was a speeding report caused by a broken trace on the actual motor board).
6) Check the power supply board.
7) And so on....

Regards
Borut

[paki976]

AW: Revox B795 spinning at 100 RPM

HI Borut,

I will try answer your questions sequentially...

1) I'm not so great in electronics, I'm actually able to build a simple circuit like a chip-amp or a preamplifier. I can use a DMM. I haven't an oscilloscope.

2) when I got the turntable first replaced the Rifa on filer board,all the axial electrolytics on power board, the axial one on motor control board,the axial one on the arm control board. Later I found that the relay on the arm control board wasn't working (always closed), so followed the service manual and replaced also C1 (was a suggested modify). Then the turntable worked flawlessy for about 2 months. I also managed to soften the rubber of the Revox/AKG cartridge!

3) Normally when you turn on the turntable the platter stays stationary until you move the tonearm into play position. In my case,just turning on the power switch, the platter start to spin at maximum speed. While spinning out of control, I can raise/lower/move the cartridge by the front buttons. Also the logic of the speed change appears to work (I can see 33/45 led switching very dimly,as before), and if i slow down the platter with my hand the 33/45 led turns on brightly until it keeps the correct speed.

4) my board is an old one (s/n 4405, board 1.179.252-11), so no sockets at all.

5) I was searching all yesterday for a broken trace, without result,actually. What really puzzles me is that the turntable failed when I left it on for 20 minutes with the cartridge in the up position.

6) I tested the supply board with a DMM for +-6V and +24V.

7) Actually yesterday I bought on the bay a replacement board, but I'd rather prefer to repair this one....

Thank you,Ciao,
Fabrizio

[borchee]

AW: Revox B795 spinning at 100 RPM

Let's hope your detailed post inspires the experts to troubleshoot a bit.

All diodes and trimmpots OK?

Well, with the other board you have at least a comparison...Ebay France, was it.

Ciao,
Borut

[steerpike]

AW: Revox B795 spinning at 100 RPM

Check D2 on the motor control board for short circuit.
Otherwise IC8.

[paki976]

AW: Revox B795 spinning at 100 RPM

Will check as soon as possible.
Today also discovered that, albheit tonearm is working, the stylus illuminator doesn't turn on, but the lamp tests ok and when powering off turntable it blinks for a fraction of second.

Ciao,
Fabrizio

[paki976]

AW: Revox B795 spinning at 100 RPM

Will check as soon as possible.
Today also discovered that, albheit tonearm is working, the stylus illuminator doesn't turn on, but the lamp tests ok and when powering off turntable it blinks for a fraction of second.

Ciao,
Fabrizio

EDIT: OK, tested D2 with with one pin disconnected from board and tests ok.

[steerpike]

AW: Revox B795 spinning at 100 RPM

EDIT: OK, tested D2 with with one pin disconnected from board and tests ok.

Sorry! My eyesigt is giving up! That should be D5 to test, not D2 - I misread it on my diagram.

See how the motor behaves with D5 unsoldered.
If the problem remains, the fault is likely IC8

[paki976]

AW: Revox B795 spinning at 100 RPM

See how the motor behaves with D5 unsoldered.
If the problem remains, the fault is likely IC8

Hi steerpike, tried with D5 unsoldered but the motor kept spinning like crazy. Will get a replacement IC8 later.
Did you notice my post yesterday about the stylus illuminator? Cannot understand the connection between the 2 faults....

Thanks everyone for your help,
Fabrizio

[paki976]

AW: Revox B795 spinning at 100 RPM

Ok, removed IC8 and at least the motor stopped turning .
Stylus illuminator didn't turn on,tough . I'm sure the lamp is good becaude when i turn off the turntable it light for a split second, end tests good with the DMM.
Any idea before I go shopping repacement parts?

Fabrizio

[Zeppelin]

AW: Revox B795 spinning at 100 RPM

I would recommend to follow a logical and systematic approach to fault finding instead of guessing as suggested above. First step, as always, is to check the supply voltages. Do not not only measure the voltages but check for ripple. With the Revox turntables the rectifiers and/or NTCs die quite often. So a 50 Hz ripple is a give-away. Apart from the usual reasons, the supply voltages need to be checked as to ensure that the faults are not connected. Apart form the motor revving a high rate, you are reporting a failure of the light.

The reason for the motor turning at max speed will be a failure in the control loop, i.e. the loop is open. That can be caused by a missing sensor signal or failure in the sensor signal processing, a failure in the reference signal generation, a defective sample and hold circuit (the B795 has two) or a failure of the motor amplifier.

[paki976]

Re: Revox B795 spinning at 100 RPM

Hi Zeppelin,

I was reporting the failure of the target light because it happened simultaneously with the motor failure. Sadly I have no chance to measure much with only a DMM. As told before, voltages are present on the supply board. Anyway, being that the fault now seems to involve 2 boards (motor control and arm control), changing also all diodes/regulators on the supply board seems to be a good idea.

Tonearm working (apart of the light) might mean that voltages are there.

Also, the behavior of the platter (rotate without waiting for the tonearm movement) may help someone expert to guide me troubleshoot the fault.

Ciao,
Fabrizio.

[Zeppelin]

AW: Revox B795 spinning at 100 RPM

If you say you measured the supply voltages ok, why would you change all the parts on the power supply again? Makes no sense.

The tone arm control sends a discrete Start/Stop signal to the motor control, Measure it at anode D5. Start is L, Stop is H. H shuts the motor driver down and the synch detector. If your supply voltages are really ok (incl. -6VDC) and Start/Stop is H, the your problem lies in the area of the motor driver.

[steerpike]

AW: Revox B795 spinning at 100 RPM

The reason for the motor turning at max speed will be a failure in the control loop, i.e. the loop is open. That can be caused by a missing sensor signal or failure in the sensor signal processing, a failure in the reference signal generation, a defective sample and hold circuit (the B795 has two) or a failure of the motor amplifier.

A key point that points AWAY from a servo sensor problem is that the motor runs ALL THE TIME. This means there is a fault further down the line than the servo system - the on/off control signal is being ignored.

The two faults together do suggest a common cause such as the power supply, or something like an open circuit ground somewhere.

[paki976]

Re: AW: Revox B795 spinning at 100 RPM

If you say you measured the supply voltages ok, why would you change all the parts on the power supply again? Makes no sense.

The tone arm control sends a discrete Start/Stop signal to the motor control, Measure it at anode D5. Start is L, Stop is H. H shuts the motor driver down and the synch detector. If your supply voltages are really ok (incl. -6VDC) and Start/Stop is H, the your problem lies in the area of the motor driver.

Anyway I need to buy a replacement IC8 before testing (had to split it in 2 parts to unsolder it neatly). Will report later.

Fabrizio