Revox A77 Capstan Issue

[cgehansen]

Hi, I have an A77 Mk4 that has an issue with the capstan motor and any advice would be greatly appreciated. I've done some searching around via google and in the service manual and I can't find a fault that matches the one I have. What's happening is, when the machine is turned on, the capstan moves a bit then stops. However given a push it sometimes starts to turn again when 3 3/4 ips is selected (but not in 7 1/2 ips). When the capstan is turning it speeds up way beyond 3 3/4ips. The green and yellow wires to the motor both read 150v ac when not under load (there's no voltage on the grey wire). The speed board is also getting 21v on the red and yellow wires. I've changed the motor cap as the original was out of spec, and also the 4 RIFA supression caps. It's the later speed board with IC201 & IC202.

[DMcMahon]

Have you checked the condition of the speed control pot on the board, they have a habit of falling apart ? although the pot may only be 3 3/4 ips. Also check the tacho connection.

David

[cgehansen]

Yes the pot is in good condition. It’s a replacement from the original. I’ve taken the speed board out and replaced all the electrolytic and tantalum caps. The power transistor reads OK with my DMM as does the bridge rectifier but I’ve read they can fail at high voltage? I’ve also swapped the speed board with a known good one and everything works perfectly so the fault is somewhere on that board.

[Michael Maurice]

Personally, I've only had problems with the pot and the capacitors. It could be that one of the IC's has failed.

Check that you have around 12V on pins 4 & 8 of IC202, If you have a scope, check that you have pulses coming out of pin 3 of IC202 and pin 1 of IC201, the waveforms that you'd expect are in the manual.

[cgehansen]

Thanks for your help. Yes I do get 13V at pins 4 and 8 of IC202. However with my scope I get nothing from pin 3 of IC202. I guess that means IC202 has failed.

[DMcMahon]

Not necessarily, do you get a good input signal at IC 202 pin 2 ? this signal is derived from the first opamp stage of IC 201 pin 13 output which is derived from the tacho signal.

David

[Michael Maurice]

You will only get a signal from the tacho if the motor is rotating, but for it to rotate it has to start from somewhere, you must have an output from IC202.

Look at it another way, if you disconnect the tacho (or short it out) the motor still runs but there won’t be any control over it.

If I were you, I’d replace IC202 and see what happens.

[cgehansen]

I've changed IC202 for a new NE555P timer but unfortunately the motor is still not working correctly. The capstan motor now runs when 3 3/4 speed is selected but it runs fast (at around 7 3/4 ips). When 7 3/4 speed is selected it runs at full speed (as though there is no speed control). I changed IC201 for a new SN76131N and the motor still ran at the slower speed (although too fast) but at 7 3/4 ips it started to run and then stops. At both speeds there is power at pins 4 and 8 of IC202. I get a square wave at pin 3 when at 3 3/4 speed (but it's only 5vpp and T is 500us). There's no signal at 7 3/4 ips which is I guess why the capstan isn't running. Not sure where to go from here so any further advice would be greatly appreciated. I'm thinking it'll be simpler to just buy a new board!

[Michael Maurice]

Check the preset pot to see if it's OK, also check the 3 diodes 203, 204 and 205, If you haven't already done so, replace C212 which decouples the supply to the chips.

[cgehansen]

Yes the preset pot is good - I've removed it and checked it with my DMM. The diodes 203, 204 and 205 are also OK and C212 has been changed. I've checked all the transistors with a DMM.

From stop when 7 1/2ips is selected the capstan turns a bit the stops. At 3 3/4ips it starts up and continues to spin. If I then switch straight over to 7 1/2 ips the capstan starts to spin faster as normal but then slows to a stop.

There's no reading on my scope at pins 2 or 3 of the timer. I've fitted a new NE555P.

I've swapped IC202 for a new SN76131N. There was a TBA931 in there before and with that fitted the capstan just spins to full speed in 7 3/4 ips.

I've swapped the tacho with a spare but that doesn't change anything. With the tacho disconnected it doesn't spin at all.

The 21v supply to the board is fine.

I'm not what else to check. I've looked over the board with a DMM and there aren't any broken traces as far as I can tell.

[Michael Maurice]

I've included a diagram of your board so that everyone can see what is happening.

You wont get anything on pin 2 of the NE55 until the motor is rotating but you should get pulses from pin 3.

Start the machine in 3¾ and then see what happens on test points D, E and F.

That should give you a clue as to where the fault lies.

My money is on the components around the input to IC2. Diodes can go leaky, that BC108 (or whatever they've fitted) could also be playing up.

[Radio Wrangler]

I had a problem with my A77 some years ago. The plastic casing of the tacho pick-up had shrunk and cracked so the pole pieces were too far from the motor rotor. Without god enough pulses, the speed controller slewed to max speed. Araldite to the rescue!

David

[cgehansen]

My readings at those test points don't look right. My scope is set to 5V/div and 1ms/div. I get a square wave at F but T is around half of what it should be. Point D and E look very wrong. I've attached some photos to show what I mean. Does that give any pointers as to what could be wrong?

Changing IC201 & IC202 seem to have improved things slightly as at least the capstan turns (not at the right speed though) and it tries to speed up in 7 1/2 before stopping.

[Michael Maurice]

Originally Posted by cgehansen

I get a square wave at F but T is around half of what it should be.

What is 'T' do you mean G?

I take it the middle trace is point F is the first trace D and the third E?

It might be helpful if you remove C206 which couples the output of the bistable, Q202 and try it. The motor will run on full speed as there will no tacho pulses into IC202 but you should get a good square wave.

It might be an idea to replace the three diodes D203, 4 and 5 as well as C206. They're cheap enough. I reckon the fault lies around that part of the circuit.

[cgehansen]

By 'T' I meant the 1250us shown on the expected traces unless I'm reading my scope wrong. My board is slightly different from the one in the manual. There is no D209 diode and no resistor at the front of IC201.

I removed C206 but this didn't change anything - the capstan behaved in the same way and didn't run on full speed. The scope measurements were unchanged.

Then after resoldering C206 the board started working perfectly. The capstan runs at both speeds with no issues. I didn't notice any broken traces or dry joints around that capacitor so this may just be a coincidence. The board is now hanging loose below the transformer but working. Maybe there's a hidden poor connection somewhere. Not sure I want to touch anything now though!

[Michael Maurice]

I suppose we'll never know exactly what happened but at least you got there in the end.

[cgehansen]

I guess it was probably a combination of things. A new timer did get the capstan partially moving but the last step of re-soldering C206 made the difference between working and not working.

Fingers crossed it still works when I've put it all back together! I've not checked the speed with a tape yet but it looks about right.

Thanks for all your help on this one. I wouldn't have got there without it.

[cgehansen]

So I put it all back together and it worked fine for about 15mins then failed again. I’m going to replace those diodes and C206 and check all the board connections again. If that doesn’t fix it I think it’s time for a new board.

[Boulevardier]

Intermittent fault in C206, temporarily cured by removal/resoldering? Worth trying a new replacement.

Mike

[cgehansen]

Yes I’ll give that a go. It’s a ceramic capacitor and reads OK on my DMM. It may just have been a coincidence. There’s something odd going on because occasionally the capstan doesn’t turn at all. Mostly it’s running at 7 1/2 ips when 3 3/4 ips is selected then stopping when switched to 7 1/2 ips.