Revox A77 MKIV, No play speeds

[budwich]

Re: Revox A77 MKIV, No play speeds

thanks, I will look to see what I can see. However, I can spin the capstan with my fingers so it is not "stuck". Not sure of the linkage between the capstan and reel motors. Revox certainly does put alot into any of their "belt info / setup / replacement". With the limit useage of this machine, all drives / tracks are pretty clean. As mentioned, there is certainly a loss of 21v source so that needs to be addressed and then go from the results there after.

[petehall347]

Re: Revox A77 MKIV, No play speeds

ah so you are missing 21v ? i somehow went off on a tangent looking why those other voltages are wrong .
maybe whatever it goes to is pulling it down .is it possible to pull the red and yellow wires to see if 21v comes back ?
d104 still needs ruling out really .

[budwich]

Re: Revox A77 MKIV, No play speeds

As suggested, I tested the zener. It holds ~12v (using a 15v test setup). That would appear to suggest a short somewhere else, possibly q101.... maybe. IF I tally the BE drop plus the diode (d103), then I get the ~1v that is seen instead of 12v.

[budwich]

Re: Revox A77 MKIV, No play speeds

I tested q101 out of circuit with a transistor checker... it checks ok. Further, I pulled the red and yellow wires (at the spade connector), and then did a resistance check. They gave "charging like" resistances in the 500 plus ohm range. That would seem to indicate no "significant load" on those leads. At this point, I am thinking that c101 (1000 uf) is not good. I have to remove / adjust the board to get access to it out of board which is not easy from what I have figured so far. :-(

[budwich]

Re: Revox A77 MKIV, No play speeds

Got access to c101, pulled it and it measure ~1000uf... so it appears to be OK. I guess that only leaves q102 at this point. Running out of components... :-)

[budwich]

Re: Revox A77 MKIV, No play speeds

I pulled q102 (sdt9201) out and tested it with the transistor checker, it tested OK... :-( :-) which is good, I guess. I guess that now leads to probably the last "component", d101 which is an "integrated bridge package" as a likely candidate. Is that a good guess? :-)

[budwich]

Re: Revox A77 MKIV, No play speeds

It looks like Q102 is not being turned on to lower the voltage to -6v. Check the resistors in that area, like R107 across Q102, R103, & zener ZP12. I would also work the pot P106 back & forth in case it the problem.
The cap C101 & C103 are likely not causing this problem,
Does the service manual I attached seem to match yours? (power supply page 48)

I think we have a winner... :-) I am slow some times and miss some good points. I demounted the bridge rectifier but it checked out fine off board. THEN, I went back and read things again. Even though I had checked p106 for overall resistance, I DID NOT check the "wiper" / "divider".... :-( Not sure exactly why the thing is so bad but basically, the wiper contact is in "free air" as the arm is not strong enough to hold the carbon point in contact with the resistance surface... very flimsy... what were the germans thinking ... :-)

So I probably need a replacement along with figuring out how the adjustment is done... what does it do?

[petehall347]

Re: Revox A77 MKIV, No play speeds

looks damaged from a knock to me . maybe you can carefully bend it back ..not much to lose trying .

[petehall347]

Re: Revox A77 MKIV, No play speeds

p106 sets the 21v for correct playback .

[budwich]

Re: Revox A77 MKIV, No play speeds

looks damaged from a knock to me . maybe you can carefully bend it back ..not much to lose trying .

except the "wiper"... :-( just touching it broke of the wiper.

could be fun finding something that fits readily... might have to do some form of "mod" as most trimpots have a lot different spacing based on a quick look. This one is about 1 cm across the outer legs with the middle offset about .5 cm back or forward depending on how you look at it. It does look like it drives the base of q101.

[petehall347]

Re: Revox A77 MKIV, No play speeds

good description on page 22 of the manual ..
i suppose for now you could select 2 resistors and see how close to 21v you can get it . a small crock clip might hold the wiper down for now

[budwich]

Re: Revox A77 MKIV, No play speeds

The tip of the wiper is what broke off and I haven't located it. :-( I could try a couple of 1k in series to see what happens... hopefully it won't blow up anything.

[petehall347]

Re: Revox A77 MKIV, No play speeds

2x 1.25k should get you midway .

[budwich]

Re: Revox A77 MKIV, No play speeds

I tried 1k/1k and it didn't change anything as the voltage reading was still ~1v instead of 21v. :-( I have not checked c103.

[budwich]

Re: Revox A77 MKIV, No play speeds

ah so you are missing 21v ? i somehow went off on a tangent looking why those other voltages are wrong .
maybe whatever it goes to is pulling it down .is it possible to pull the red and yellow wires to see if 21v comes back ?
d104 still needs ruling out really .

another poor follow thru on my part... :-( I did measure the leads for a resistance check BUT didn't power up with them removed. Once removed, surprise, ~21v shows up. I haven't yet looked at the down stream circuits thereafter. The temporary resistor mod using 2X1k appears to give about 21.3v. Not sure what the acceptable range is at this point.

edit: further test of powering up shows with only red connected, no 21v. With only yellow connected, 21v AND capstan motor starts spinning with power up / speed selected (no reel motors spin as play is not selected at this point... which is good as there is alot of disassembly around one reel motor to get access to the power board).

edit2: following the red path (ad1) to the switch board, it appears that possible candidates of c306 and c307 (250uf) might pose an "short" to ground.

[jons]

Re: Revox A77 MKIV, No play speeds

Those Revoxes are notorious for trimmers losing wiper, also replace all 4 Rifa 0.47uF caps (one in capstan motor regulator, 3 in tape drive board) although I would recap at least whole speed control board while at it, there is Frakos and tantalums all over it. For trimmers Bourns have snap-in replacement.

[budwich]

Re: Revox A77 MKIV, No play speeds

thanks. I am somewhat of a "minimalist", partially because of cost and availability ("back waters" of canada, eh!) but more towards "leaving well enough alone" as I find that sometimes when I disturb things, they can go south fast. At this point, I am hoping to get it functioning again and then depending on where that sits, possibly address any "future service impact" components. I do certainly appreciate the guidance on "weakness areas" especially with the drives as that is my current focus.

Having said this, looking at the boards and component positions, the .47's appear to be readily accessible especially when compared to the two caps on the switch board.

[petehall347]

Re: Revox A77 MKIV, No play speeds

does it have micro switches ? had a reel to reel here i was sent from an expert on them for me to look at .it was tripping mains breakers .turned out a micro switch case had become conductive somehow and shorting to the chassis . .

[budwich]

Re: Revox A77 MKIV, No play speeds

well, its fun getting access to the switch board... I hope I can get this thing back together if and when it gets fixed. The good news is that one of the capacitors (c306/c307) 250uf read .14ohms in the checker (not a capacitor) and the other was good at about 253uf. I am hoping to replace both with 220uf as that is the closest "reasonable" value that I can find and hopefully get in some short timeframe.... probably wishful thinking. Hopefully, that value will work.

[budwich]

Re: Revox A77 MKIV, No play speeds

I got some 330uf caps and replaced both c306 and c307 (250uf/25). I also got a replacement trimpot for the voltage adjustment for the 21v "rail". Baby steps forward... now have voltage on the red output... adjusting I got it to 21.03v. As a result of the work, the unit now has the "end of tape" detector light lit (didn't before). I have to still put things a bit further together to do any further operational checks. One thing that bothers me at this point is the capstan is spinning once the speed is selected but no function has been chosen (ie. no buttons are pressed and reel motors are stopped). I would think that the capstan should stays stop until / record has been selected.... but I don't recall if this happened in the distant past.

edit: the speed of the capstan changes from slow to fast by selecting either the "slow" (3.75) or "fast" (7.5).