Revox A77 MK2 - capstan speed stuck high, can't find the culprit

[david winter]

Hi,

I'm trying to find why this A77 MK2 has its capstan motor spining at maximum speed.
The main psu has been restored and the +21V is ok.

The capstan board has been checked as follows:
- Electrolytics replaced as usual. No polarity errors, no wrong values.
- RCA 40318 transistor found open. I put a 2N3441 in place.
- Diode bridge replaced (motor almost didn't spin before replacing it).
- Motor capacitor replaced (the original was still ok).
- 0.47 "Rifa" replaced by X2 equivalent.
- BC173/183 transistors with oxidized legs were replaced by BC548 / BC558 (later I put two BC108 for Q206/207 for the sake of testing)
- Diodes D203/204 replaced by 1N4148 just for testing (originals tested ok, I plan to put them back in place).
- 1K trimmer replaced.
- Checked each single resistor with one leg removed (all ok)
- Checked the coil (not open, 60-70 ohms)
- Checked all connections numerous times, did a last check of the 2N3441 (ok).

I get the correct +/-10V "square" signal at pin 1 of the coil (as per service manual) so the phonic sensor circuit works ok.
However, it looks like there is nothign around Q206 which cause its collector voltage to drop down to almost 0V, which causes the following transistors to block, hence the unregulated motor speed.



Any ideas ? Thanks !

 

[david winter]

I finished realizing that I got misinformed by some web site giving the 2N3441 as cross-reference for the RCA 40318 (whih was open).
As the 2N3441 didn't work (it even developed a c/e leak), and as I didn't have the proper replacement transistor, I picked another board in a A77 kept for spares. Luckily, this one was equiped with the same type of board, and with later transistor types (instead of the old BC1xx series).
The only other difference I noticed is the 0.1µF film cap (C201) replaced by a 22nF.
In the table below, Found refers to the original types found in the machine (MK2, 1970). S.M. refers to those in the 1968/69 service manuals, and Later Subs. those later used by Studer.
Hope this helps !

Ref Type Found S.M Later. Subs.
Q201 NPN BC183B BC108 BC318B
Q202 PNP BC213B BC178 BC252B
Q203 NPN BC147B BC108 BC237B
Q204 NPN BC183B BC108 BC318B
Q205 PNP BC213B BC178 BC252B
Q206 NPN BC183B BC108 BC318B
Q207 NPN BC183B BC108 BC318B
Q208 PNP BC213B BC178 BC252B
Q209 NPN 40318 40318 2SC782
C201 Cap 0.1µF 0.1µF 22nF

 

[jamesperrett]

It is always worth investing in a cheap component tester so that you can test the components that you remove and the components that you fit. You would have picked up the faulty 2N3441 that way. I'm also not a fan of changing components without good reason. The only components that need changing routinely on an A77 are all the Rifa 0.47uF capacitors, possibly the motor run capacitors and possibly the preset pots. I'd say that you need to double check your work, make sure that your soldering is good and that you haven't lifted any tracks during all the component replacements. If all those components that you removed were really faulty then I'd say that you have big problems - have you checked the motor?

 

[david winter]

FYI, I've been servicing vintage hifi (including Revox) gear for almost 20 years.
I tested all transistors on that board before considering replacing them, and always test transistors before installing (I have three different testers including a Peak DCA Pro 75).
It's just that the 2N3441 turned bad due to the misinformation I got online, and you are right in the sense that I should have checked the 2N3441 specs before installing.
Although I have seen various problems with BC-xxx types (shorts, open, noise, you name it), changing this particular 40318 transistor is very unusual.

Talking about routine, you omitted several other important steps:
- when present, always replacing the white electrolytics as well as the blue Philips electrolytics.
- always replacing the Frako electrolytics (especially the 1000µF/40V on the psu, they happen to short).
- always testing the relays for bad contacts.
- always checking the red wires (21V etc) near the connectors for potential "green leak". This happens to generate loose contacts. I solder the wire to the connector. Just as an example, on this A77 I checked the red wire going to the 40318 transistor. Still well soldered to the transistor leg. But easily pulled from the pcb although the solder looked perfect. Only what I refer as "leak" eventually damaged the solder from the inside.
- cleaning the linear switch, the rotary switches and pots
- replacing the two fuses (they age and happen to turn open after repeated power-on peaks).
- the three motor caps even if they seem to test good. I have seen some unstable speed issues even with capacitors testing good, and won't take risks.

I also replace the following:
- the tantalum caps (I don't waste time testing each of them, and Revox recommended replacing them by audio grade electrolytics, which I do).
- As per Revox recommendation, the 3.3µF on the 21V regulation circuit by a 4.7µF electrolytic.
- All trimmers (preset pots) with oxidised cursors. Later models with "double contact" cursors may be left in place.

Of course your restoration will depend on how old your A77 is. You don't have the same amount of parts replaced between a ca. 1970 machine and a late manufacture from the early 1980s.

 

[KevM]

I had a similar high speed capstan issue on an early Mk1 A77 (the tacho output was approximately 2800Hz rather than 1600/800) and the resolution was to replace the BY123 bridge rectifier. Incidentally the old bridge rectifier measured fine on an ohmmeter which was strange but I’d removed the board to allow easy diagnosis, powering it externally and feeding it a tacho signal from an audio frequency generator. This allowed easy measurement of the expected test point voltages at under and over frequencies. In the end it just had to be either the 40318 high voltage power transistor or the bridge rectifier and the BY123 was at fault.