Revox B795 spinning at 100 RPM

[Zeppelin]

AW: Revox B795 spinning at 100 RPM

The crystal ball says: -6VDC are missing. Without measuring you would not realize any other fault on the tone arm control because you would not like to check the tone arm servos at the present speed of the turntable. However, the light is powered by -6VDC. If the -6VDC is missing on the motor control, the motor driver will not work probably and the motor will spin at high speed regardless the Start/Stop-Signal.

[paki976]

Re: Revox B795 spinning at 100 RPM

Okay, replaced IC8 with JRC4558D (couldn't find the RC4558 here) on motor control board, platter start to spin with less force, still not normal.
Measured anode of D5 with respect to virtual ground on power board (is this correct?) and the reading is 3.13V with tonearm in play position and the same with tonearm on test position.

Ciao and peace for all

Fabrizio

P.S. tested MC7906 for DC, pin 1-2 measures 15V input, pin 1-3 6V output.

[paki976]

Re: Revox B795 spinning at 100 RPM

...meanwhile changed regulators (and insulators) and diodes on power board, then changed all tantalum capacitors on arm logic board. Still no joy.
Also noticed that the platter spins more slowly now. if I change speed from 33 to 45, it spins a bit faster (but too slow to lock to quartz speed).

Fabrizio

[paki976]

Re: Revox B795 spinning at 100 RPM

Ok, new week some new readings.

IC3 +13,6V -6V pin 8/4
IC4 +6V -6V pin 8/4
IC8 +13,6V -6V pin 8/4
IC10 +13,6V -14V pin 8/4
IC11 +13,6V -6V pin 8/4

D5 with tonearm in stop or play position measures always 3,5V on anode. (also the same with tonearm connector disconnected from tonearm control board.)

Ciao
Fabrizio

EDIT: New situation again. Now the platter spins ok until I leave the tonearm control board DISCONNECTED (quartz lock ok on 33 and 45 rpm). If i reconnect the tonearm control board (without tonearm,board only) the platter cannot lock quartz speed and stutters very much.

[paki976]

Re: Revox B795 spinning at 100 RPM

Ok, now investigating on arm control board.

Disconnected grey wire from J3,2 (drive inhibit), the platter spins perfectly at quartz speed, the tonearm works ok (l,r,up,down), still no stylus lamp.

Removed Q1,Q2,Q3,Q4 from the arm contol board after reconnectin grey wire,same situation: motor doesn't spin at quartz speed. Put them back.

Fabrizio

[paki976]

Re: Revox B795 spinning at 100 RPM

Just an update about the Revox:

1) the motor control board works ok (today I got a replacement board, and the turntable behaves the same way with both PCBs). When connected to the power supply, the platter spins at 33rpm, switching to 45 rpm works ok and lock quartz speed perfectly.

2) Tonearm control board is still faulty: No stylus illuminator with or without motor control PCB connected, any other function of the tonearm works ok.

3) when I connect both tonearm and the motor control PCBs to the power supply simultaneously tonearm still works ok, the platter continues to spin slower and cannot reach quartz speed anymore to turn on speed led.
Just disconnecting grey wire (drive inhibit), i can play records correctly (but platter never stops and obviously stylus illuminator lamp doesn't work).

Anyone with an idea?

Ciao,
Fabrizio

[paki976]

Re: Revox B795 spinning at 100 RPM

PROBLEM SOLVED!

The problem was IC3 on tonearm control board. Replacing it solved both stylus illuminator failure and motor spinning out of control.

Thanks for your invaluable help.

Ciao,
Fabrizio