Revisiting an old Revox PR99 problem ...

[Journey111]

Hi All,
I just joined here after finding an old thread on a Revox PR99 mkii transport issue. I borrowed one to transfer some old original masters from 10" reel tape to Protools audio (digital). I loaded up a tape and it wouldn't play even if holding down the play button. This problem apparently is quite common. Transport buttons won't latch. Mine was slightly different than the most common symptoms I found in various forums. Mine would FF and REW only IF you held the buttons down, PLAY wouldn't engage at all. So I got all the docs I needed and satred a tear down and checked the most common suspects caps, connections, etc etc. No shorted caps, even removed them and tested with esd and capacitance meters all seemed good. Checked all diodes on Tape drv and Fader brd along with all transistors.
EOT sensor seemed to be working as the voltage on the receiver side would drop when blocking the LED source. Took a good look at the tape drive pcb and finally discovered a blown ( or what I thot was blown) Opto isolater 4n26, replaced with a new one and ran the machine with a new problem ... now the take up reel won't take up ... instead in runs backwards when holding FF down and when tape is limp all shuts down.
Checked all voltages at this point as well ... all looking good unless I missed one.
Next Step
So I replaced all the relevant caps on the tape drive brd and just about to run again to see what happens also put the original opto back in as well. I'm pretty sure the opto was cooked on the transitor side as a diode check test revealed OL on the meter both ways where a good one would have 1.076v or something one way.
So after that long story ... looking like I could use some help ... HELP !!!

 

[Journey111]

Writing an update ... well changed ALL the capacitors save one ( the big 100 uF C2) on the drv brd and that seems to have got the play button working so the pinch roller engages. But at 1st had to hold it down for it to keep running. After a time I let up on play and it kept rolling ... no action on the VU's and no audio though. The FF function still isn't working ... the take up reel runs backwards and looses thetape and all shuts down. Also before I tested the cap replacement I also replaced the 74LS00 IC as well. So no way of knowing if it had a bad gate or not.

 

[Journey111]

Update #2 - Well got everything working again except the FF function ... take up still rolls lazily backwards and tape goes loose and then shut down. Haven't tested the record button yet as I don't have any blank tape and I'm afraid to use one of my old music reels for fear of over writing something.
Of course the transport issue wasn't the only thing I ran into. After I got the play function operational ... loaded up a tape and ... wait fir it ....... NO audio output.
So hunted down 2 Frako's on the output brd and replaced 4 other Frako's on the Repro brd and then we gotz some audio.
So all of this work may have been in vane since I borrowed this unit to transfer some original music I wrote from analog into Protools. And guess what ?? That tape is no where to be found in my box full of tapes. Well I guess I fixed buddies tape machine for free.
So only problem left is FF function ... I've replaced both logic gates on the tape drv brd with no luck and EVERY old electro on the brd with no luck.
Looking at the schematic, I'm having a bit of trouble figuring out what part of it controls the direction and spedd for the take-up motor.
The only other IC left to swap out is the S-R latch which I don't have in my tickle trunk full of old logic chips. Figures ... I suppose I might pop it out and bread brd test it for logic integrity. Thinking maybe one latch is cooked ? but I'm not sure.
Any help would be appreciated on this one as I am stumped for now ...

 

[GPS16]

Hi Journey,

the Reel Motor is an AC motor. The direction is governed by a phase shift in one set of windings in relation to the others. If the rotation is not correct then a wiring error is the first thing to rule out.

The take up motor will have a bleed voltage from the Supply Motor voltage via R1 820 Ohm 9W. This is to keep a bit of back tension during fast winds.

The optocoupler is used as isolation as the gate voltage for the Take Up Motor is elevated to the hot side of the transformer winding which runs the motors. There is a 20 VAC (90 - 110 VAC) which gives a -ve supply across C2. This is used to fire the Triac Q4 via the Opt and Q9.

The opto can only do it's stuff if there is a DC Voltage across C2 (28 V ish).

So measure the voltage across C2. Check the 4 wires for the take up motor are on the correct spades on the PSU PCB.

There may also be problems on the Fader Start PCB where the "Tape Dump" function is configured.

Some pointers to follow anyway.

I am more familiar with a B77 than a PR99 but alot of it is familiar territory.

Cheers,

GPS16

 

[Journey111]

Hi Journey,

the Reel Motor is an AC motor. The direction is governed by a phase shift in one set of windings in relation to the others. If the rotation is not correct then a wiring error is the first thing to rule out.

The take up motor will have a bleed voltage from the Supply Motor voltage via R1 820 Ohm 9W. This is to keep a bit of back tension during fast winds.

The optocoupler is used as isolation as the gate voltage for the Take Up Motor is elevated to the hot side of the transformer winding which runs the motors. There is a 20 VAC (90 - 110 VAC) which gives a -ve supply across C2. This is used to fire the Triac Q4 via the Opt and Q9.

The opto can only do it's stuff if there is a DC Voltage across C2 (28 V ish).

So measure the voltage across C2. Check the 4 wires for the take up motor are on the correct spades on the PSU PCB.

There may also be problems on the Fader Start PCB where the "Tape Dump" function is configured.

Some pointers to follow anyway.

I am more familiar with a B77 than a PR99 but alot of it is familiar territory.

Cheers,

GPS16

Hi GPS16,
Thanks for that info and the pointers.
AS far as the 4 wires for the motor power, that was the first thing I looked at very closely and over and over again. And according to photos and the schematics, I have it wired up right.
Something has changed since I first fired it up and discovered transport issues. WITHOUT a tape mounted the REW/FF functions would spin the reels but of course would not lock which I think is normal because of the IR sensor for the EOT function. Pressing the play button would do nothing ... not even disengage the brake.
One the 1st things I thought I had found wrong was the other opto isolator on the fader brd. So I replaced it and then that's when the take up reel stopped working properly.
After this it would just lazily roll in the wrong direction and the tape would go loose and then shut down. SO I left that and started trying to get the transport working.
I'll look into the voltages again, since I already looked at a few points but ya gotta focus on the area of the Triacs.
I did mention above that I replaced the two logic gate IC's but didn't have a replacement on hand to do the S-R Latch. I was wondering if say the FF latch is failing ?
But looking at how that thing is interlinked ... if one latch fails then the whole thing should go wonky not just one function ?
So the takeup is getting the proper voltage since it plays just fine. Rewind works but noticed that it is pretty weak/slow rolling it back. If I give it a bit of a hand boost it will speed up.
So I'm thinking the take up voltage is maybe putting some drag on the REW reel ? Either that or just the weight of the tape on the source reel is enough to slow things down maybe.
Anyway, gotta work sound at the club over the weekend so I might not get a chance to get back to looking at it til Sunday so I'll get back here sooner or later with another update.

Thank again mon amee

PS>> One thing I have forgotten to mention in all the posts is that for some reason this mkii does NOT have a tape motion sensor ... I looked everywhere it's just not there and it's doesn't show up in the schematics for the mkii either but it is mentioned in the service manual aslo it is depicted in the diagram in the schems that show all the different board locations.

 

[GPS16]

Hi Journey111,

does this PR99 have the Real (reel) Time Counter? If it does then the tape motion is taken from the roller on the right side of the Head Block. There is an interlink from the Counter CPU to the Logic Function PCB.

Between the Function Logic PCB and the takeup motor, the fader start/tape dump PCB has been inserted. The Take up Motor wiring is routed there also to be controlled via K1 on the Fader Start board. Check the motor wiring at that end as well. An AC motor CANNOT fail and go backwards. The supplied voltage is changing direction 50 times a second (60 times maybe in your case). The Capacitor would need to change into an Inductor to reverse the rotation. A wiring error looks to be issue here. If the phase shift capacitor fails then the motor will slow down and just get hot.

The Fader Start PCB could also be the issue with the Play Function as this is also over ridden. Go over the Fader PCB wiring with a fine tooth comb. That is where I would be looking!!

Cheers.

GPS16

 

[Journey111]

Hi Journey111,

does this PR99 have the Real (reel) Time Counter? If it does then the tape motion is taken from the roller on the right side of the Head Block. There is an interlink from the Counter CPU to the Logic Function PCB.

Between the Function Logic PCB and the takeup motor, the fader start/tape dump PCB has been inserted. The Take up Motor wiring is routed there also to be controlled via K1 on the Fader Start board. Check the motor wiring at that end as well. An AC motor CANNOT fail and go backwards. The supplied voltage is changing direction 50 times a second (60 times maybe in your case). The Capacitor would need to change into an Inductor to reverse the rotation. A wiring error looks to be issue here. If the phase shift capacitor fails then the motor will slow down and just get hot.

The Fader Start PCB could also be the issue with the Play Function as this is also over ridden. Go over the Fader PCB wiring with a fine tooth comb. That is where I would be looking!!

Cheers.

GPS16

Hi GPS16,
The fader start pcb is where the initial FF function issue started when I replaced the Opto chip .. it just stopped working the way it did before I fixed the rest of the transport issues. And as far as wiring goes on that brd ... it's correct as far as the schematic goes. IT's possible they made an error in the docs. But I took photos before I disconnected all of that and it's all in order. But ya I think you are correct that the issue is on that pcb. The Triacs all check out as good so I don't think they are the problem.
I'm also still leaning towards that S-R Latch if one of those is faulty ? I'll just have to remove it and set it up for testing on a bread brd to make sure it's good.
It's become REAL personal now ... this often happens when I cannot troubleshoot and issue ... I won't let it go ... it's PERSONAL now ! I will beat it into submission !!!
The thing that pisses me off the most is that I can't find that tape that I wanted to use on this unit to transfer to Protools for remastering. And to boot the original machine that recorded the tape was an
Akai GX-650 or something like that and the heads are all aligned differently on the two machines. So if I want to use the Revox I'll have to do some serious head adjustments and maybe even the tape guide as well. Not looking forward to that. And do not have a test tape to do a realignment after I change it all up. I might just have to dig out that old Akai that's in a million pieces since 1986 and get er working again.
Any ideas where to find parts for it ? That Akai is a sound on sound 4 track where as this Revox is just a two track but they should play a tape pretty much the same as long as the tape is lined up right.
I saw an old pioneer 4 track and another newer Akai unit similar to mine at a vintage audio shop in my city. I practically begged the guy to rent me the pioneer and he just led me on to think he would only to give me grief for bothering him about it. What a dick for leading me on for nearly 4 or 5 months. I would have given him $500 for one day to rent that Pioneer which was more than half the price he wanted for a sale. Should have just bought the damned thing. It almost seems I just am not destined to recover a bunch of original music I wrote and recorded years and years ago.
Maybe someone upstairs wants me to get back to recording and writing some new stuff ? But that old **** was pretty damned good.

Journey
PS>>> Not only that tape went missing but all my cassettes with all my drum machine backups went missing as well. If I could find those I could just transfer the original 4 track beds into protools and redo a lot of it. Someone is screwing with me ! I think it's the CIA but then what do I know ...

 

[GPS16]

Hi Journey111,

the opto on the Fader Start PCB has absolutely nothing to do with the Logic/Motor Drive circuitry. This opto is giving isolation to the PR99 circuitry from the external circuitry where the Fader Start Drive is coming from. There is a lot of Logic Override going on to give priority over internal control but this is all "command" oriented and not close enough to the motor control to cause the fault described.

I say again, check the 3 wires from the PSU (at the PSU). There is one spade on the PSU PCB which will be unused!!! Then check all the individual connectors and the Fader Start PCB for the take up motor. There is a problem here to give the wrong direction of rotation!!!

When this is sorted, the FF/Rew commands should run OK, with the motors at full torque and in the right direction. The play issue can then be dealt with on it's own.

It may be a good idea to check the operation of the Tape Move Logic signal as this will LOCK the logic into Pause Mode until the tape move signal ceases. As a quick check on this one, power off the machine, count to 10, power it up and press play. If this works then the tape move signal is a goodly one to check. The Hold Off Flip Flop will not be set until FF or Rew have been requested (IC3-13,14 and 15).

Some more things to try.

GPS16

 

[Journey111]

Hi GPS16,
Well ... I have successfully bludgeoned the crusty old curmudgeony PR99 into utter submission ! After several days in the ring with only agency (CIA) sanctioned boxing gloves, I pulled out the brass knuckles.
Long story short without the all colorful adjectives and adverbs and ridiculous rhetoric ...I found the problem after I blew one of the gates in the 74LS02 after getting frustrated for the nth time. I replaced the IC and that fixed what seemed to be a couple shorted caps in the voltage divider section just after the button signals come from the fader brd. Then went back to looking at the motor control section around Triac Q4.
It was a broken trace connection on the tape drive control brd. It was only visible under at least 7x magnification ... It turned out that when I had removed a transistor (Q9, the switch that fires Triac Q4 which controls the take up reel) in order to properly test the opto transistor side, I managed to damage the pads for that transistor. I thought I had repaired all the damage. But the trace which runs from the collector on Q9 to R12 which connects to Q4's gate had the smallest break in it. What a moron I am for not inspecting that area much more closely after my repair job. It should have been a no brainer ... damage was done and I assumed it had all been taken care of. But thanks to your suggestions I really spent a lot of time going over that section of the schematic and checking for those voltages you mentioned. It was very weird though ... the 1st time I checked for the -ve (~30V) across C2, it was there so went on to check other points. I removed the brd again to look for more points where I could get at to measure other voltages and when I put it back, the 30v was gone just like that. So took it out one more time and after a couple mores hours of looking, found the broken trace. She's running like a top again !
I've never had any experience dealing with an AC motor control circuit like this before so the schematic was pretty much Chinese to me. But after looking at it for hours on end ... it started to make sense.
Well hey GPS ... thanks for your interest and your input again ... you seemed pretty convinced the motor wiring was amiss but I checked that over and over and over again. And in a way it was the motor wiring since there was a break in a connection that turned the damned thing ON ! Sometimes when I spend too much time on a problem and can't get anywhere with it ... I walk away from it for a spell and come back to later. So I left it alone for a few days ... came down with a pretty nasty cold and just didn't do much for 3 or 4 days while I suffered on the couch.

Now I gotta decide whether to try and adjust the heads so my old tapes play half decently on it. As I mentioned, Chan1 was very quiet and if I grabbed the tape with my pinky finger nail ( It's quite long right now ) and pulled the the tape out just a bit I get Chan1 back. But Chan2 is now not as bright as it was before so maybe my old tapes have dirtied up the heads. I don't have a test tape to realign with if I decide to change them up so it's a tough decision to go ahead with it. I'm wondering if I say took some blank tape and made a test tape the way it is setup now ( heads as is ) then change the heads for my old tapes and use the tape I make to set it all back the way it was. Think that would work ? I'm thinking it would be pretty damned close if I do it right.
Your thoughts ?

Journey

 

[GPS16]

Hi Journey,

glad you have had success at last!. Still not sure about the change in rotation!!! Whatever happens with the Triacs, that should only give you a Torque change.

I would not shift the head settings unless you really feel you have to. The adjustment for the high frequencies is extremely small and CRITICAL!!!! This affects the whole frequency response of the machine. If this mechanical adjustment is not correct then the internal electronic high frequency adjustments don't mean diddly. A bona fide reference tape is the only way to set the mechanical alignment of the heads properly. If you make a recording before any adjustments you can only return to that point. If the machine is not running to spec at this time, then that level of performance is all you will be able to achieve by this method. These machines are quality gear and WILL perform but only if they are treated with the respect they deserve.

I have made this point in quite a few threads now and access to such reference tapes (costs there of) is what makes the services of a "techy" never cheap. The costs of these pricey additions to the "Tool Kit" are passed on. As you have also found out yourself, the time involved can also be lengthy!!! These machines are generally at an age where the faults thrown can be obscure and tricky to get the better of.

That being said, I hope the machine now does as you expect and gives you long service into the future.

I will get off my soap box now before I fall off and get a bruised bum!!!

Cheers.

GPS16