Revox PR99 mechanical and electronic service & potential rebuild - my 1st Revox adventure

[smurfer77]

I did move the rear clip (and hence the bell position with respect to the shaft) that holds the bell on. I moved it so it sat 1 hole clockwise just to see if that had any effect. Just be ear I could tell it was more unbalanced. I did the spin/stop test as in post 58 and now got an even more unbalanced result confirmed what my ears heard:

Top: 30
Left: 14
Bot: 16
Right: 9

I can't feel and 'stick' point, just imbalance.

I took the motor apart once again to triple check. All looked good. There was a tiny bit of gunk of the low-friction washer which I cleaned up. Topped the lube as I hadn't gone too heavy on soaking the sponges last time. Nothing I could see out of the ordinary. okay, at this stage I think I'm going to break it in more earnestly and move on with the head and audio alignment. If the 800Hz doesn't down with more use I will come back to the capstan motor.

 

[GPS16]

Hi Smurfer,

this is reminiscent of the 707 of Soundmig's. Run it in some more!!!

Give it a demag, check the Azimuth and you know what's next!!!

Cheers.

GPS16

 

[smurfer77]

Hi Smurfer,

this is reminiscent of the 707 of Soundmig's. Run it in some more!!!

Give it a demag, check the Azimuth and you know what's next!!!

Cheers.

GPS16

Looks like our posts crossed paths in the internet. Yup, I'm moving on for now.

 

[soundmig]

Looks like our posts crossed paths in the internet. Yup, I'm moving on for now.

Oh boy .... I hope you get this one sorted out smurf. I had to put a new reel motor (LH side) on mine to get it to settle in. Hang in there ... I know how frustrating these "issues" can be.

 

[smurfer77]

Here are my circlip pliers. They are so much better than my old ones which had interchangeable tips but were flimsy.

Before doing the calibration I thought I would check the transport operation a bit more. I had a couple of issues but I will report back on that after discussing with the vendor of the parts a bit. Aside from that I finally listened to a mix tape from a friend and it is my first 2track and first 15IPS experience. Holy crud this thing can have a low noise floor, and bass and treble like I haven't quite heard before from the RT707. The tape handling seems really smooth. And when I FFWD or FRWD the tape on the reel looks (almost) as good as if I had wound it at 3.75IPS playback on the RT707.

So I thought I would do a quick record test since I will be calibrating that in a moment too. One channel wasn't erasing (i was using a 4-track mix tape so it was obvious as I could hear the old music in one channel). Then sometimes one channel would not erase. Then when I record I can hear a crackling noise on the right channel sometimes. At first I was thinking record amp transistors as it was quite consistent, but more flipping the record switch to safe and back eventually did make it better, or worse. The relays on the oscillator board could both cause my noise and cause my fidgety erase. So I pulled them out and I've cleaned them up: here are some pics for reference that could be handy if one takes them apart and can't remember how they go back together.

This pic has the pin layout on the black plastic which could be handy one day.

One pair of contacts cleaned up. Each relay has several pairs of contacts hidden inside to clean up.

And back together.

Just be careful not to break the legs when you put the relays back in their case. Clean the solder off of the legs if there is lumps otherwise you will never get them through the holes.

And.......... no more static on record, and erase now works on both channels. Yet, I just ordered a pair of those replacements I linked in an earlier post, as I know that cleaning up often only lasts so long. I just recorded a few tracks from a digital source to 2track 15IPS.... and boy, when I get that input level right up where it should be, there is no hint of tape hiss at reasonable listening level (and i'm monitoring via headphone!). There were moments when I forgot I was monitoring the recording instead of listening to the input... and this thing has been taken apart and had zero alignment or calibration. okay, a quick break and I will start on calibration shortly.

 

[smurfer77]

Ok, can someone explain how the playback EQ is adjusted on the PR99? The manual says to check playback frequency response but nothing about adjusting it and I see not trimmers for playback EQ. Both channels are down <100Hz; up towards 20kHz levels look great, but just need some L/R balancing.

 

[GPS16]

Hi Smurfer,

you don't!. The manual states to check the Eq against the Ref Tape only. This is for reference only and verifies the operation. If the spec is not met then component failure may be the cause. It should be in spec!!!

This is the same on a B77 and also the A700. Following this, the Record Amps are aligned to it. The difference between the PR99 and the 707 is that the Bias and Eq are separate on this machine. With the 707 you set the Over Bias to get you in the right ball park and then fine tuned to get the HF Response. On this machine the Over Bias is nailed to get to the point of Min Distortion and the the Rec Amps are adjusted to get the HF Response correct.

Make sure that the Osc Freq has been checked and adjusted correctly. If the frequency is too far off then the Bias Traps on the Rec Amp may run out of adjustment.

I like the relay strip down alot!!!! Boy!!! You are a glutton for punishment!!!! Well done that man!!! Again, as mentioned in previous threads. The contacts should be cleaned with IPA soaked PAPER. That is abrasive enough. The contacts will either "be" or "plated with" precious metal. If they are "plated with", then a strong abrasive will strip the plating right off. It will also destroy the polished surfaces and leave scratches which will initiate arcing.

Your doing a grand job.

GPS16

 

[GPS16]

Hi Smurfer,

Additional. There may never be complete erasure of the Right Channel of a 4 track recording by the erase head of a 2 Track recorder. The edge of tracks 2 and 3 (4T Right, Fwd/Rev) encroach onto the guard band, between the Left/Right 2 Tracks, more than would normally be the case for a 2 Track recording. The wider track coverage of the 2 Track erase may not be enough. This is even worse if the Guard band is wider due to time code.

I do not have a bulk eraser so if I am using a tape for a different format I erase the old format first. I put the tape on the outside of the hum guard and this lifts it away from the Play/Rec Heads during the erase to eliminate unnecessary head wear. The 4 Track A700 is handy here as it has 15 IPS.

Cheers.

GPS16

 

[smurfer77]

Good stuff. I might have even heard a bit of the R track coming through, even after the oscillator relay was fixed (but previously it was completely not erased). I will fire up the bulk eraser in the future for such occasions!

I'm following the service manual step by step - oscillator freq is next before record setup.

However, the playback is down 2-3dB at 30Hz (spec is 0,-1dB) - both channels almost identical here. HF might be in spec, but its a bit unbalanced between L/R. And I think we can all guess by now that while some might move on, this will not do in my books. This PR99 is going to be better than spec!

I will check transport/head alignment again when I get another chance. (I followed the manual, starting from input check, not skipping a step, up to playback check (this time engaging the x-y scope feature to finalize the fine azimuth adjustment), but wouldn't hurt to check again). It could happen, but seems unlikely low frequencies would be down due to electronic component failure, to exactly the same level on both channels, right? But hey, I might did into the PCBs a bit deeper. I wonder at what point will I get frustrated with this rebuild? So far, it's only fun. And now I've had a taste of the sound, the desire to do this properly only grows!

 

[jlb2]

Hi!

However, the playback is down 2-3dB at 30Hz (spec is 0,-1dB) - both channels almost identical here. HF might be in spec, but its a bit unbalanced between L/R. And I think we can all guess by now that while some might move on, this will not do in my books. This PR99 is going to be better than spec!

EDIT: where did you read that the PR99 is supposed to be down only 1dB at 30 Hz? I've just re-checked the spec sheet, it says:

• 30Hz - 22kHz +2/-3 dB (50Hz - 18kHz +/- 1.5dB) at 38 cm/s
• 30Hz - 20kHz +2/-3 dB (50Hz - 15kHz +/- 1.5dB) at 19 cm/s

So your deck is perfectly in spec. Don't bother trying to improve it.

As regards the LF loss, this is usually caused by a tape-heads-transport issue that you may be able to alleviate somewhat by mechanical adjustments. It has a lot to do with the design of the transport and the shape of the heads, for reasons that are too complicated to be detailed here. FWIW, my PR99 is slightly off-spec in this department at 19 cm/s (-3.5 dB) but not at 38 (it is ruler flat), and in my experience it is a common issue on the B77/PR99, but it is hardly noticeable at all when you listen to the tapes.

For your previous question, there never was HF PB adjustement on any Revox until PR99 Mk II (the Mk III and the C270 do have one), but I've never missed it. High frequency issues on the playback side are usually caused by one of the following:

• Azimuth error (make sure you have adjusted the heads before you measure the PB response). In fact the whole adjustment of the heads is critical: a laterally or vertically slanted head will cause HF discrepancies as well. I'd recommend a full head adjustment, especially since you don't know the history of this machine - who knows how it was serviced before it arrives in your hands. Always assume that all the alignments are off when you receive a new-to-you machine.
  
• Magnetized head (make sure you properly demagnetize your deck, both heads and tape transport, before you attempt to play a tape, as a magnetized machine can partially erase tapes - guess what, the high freqs are the first to go).
• Wrong tape tension (poor tape-head contact causes a loss of signal, also starting with the highs).
  
• Worn PB head. Yours looks ok but it may very well have been relapped, and a relapped worn out head with a widened gap will still be a worn out head no matter what you do. Unfortunately, relapped worn out heads are common on the auction sites, but fortunately new heads are still available for these machines, at a price.
  
• Bad electrical contact, especially between the heads and the head amplifier.
  
• Failed electronic component, which can be an electrolytic capacitor, a transistor or an op amp - I've seen all 3 cases, unfortunately.

BTW, congrats on your succesfully dismantling those relays. However problems may still exist even after that, so I'd suggest to try bypassing completely the sync amplifier card if you have problems with its relays, by removing it from the machine and plugging the heads connector from the heads directly into the PB amplifier card. Incidentally, I did it permanently in my own PR99, who needs sync recording nowadays anyway?

I will check transport/head alignment again when I get another chance. (I followed the manual, starting from input check, not skipping a step, up to playback check (this time engaging the x-y scope feature to finalize the fine azimuth adjustment), but wouldn't hurt to check again). It could happen, but seems unlikely low frequencies would be down due to electronic component failure, to exactly the same level on both channels, right? But hey, I might did into the PCBs a bit deeper. I wonder at what point will I get frustrated with this rebuild? So far, it's only fun. And now I've had a taste of the sound, the desire to do this properly only grows!

It is good practice - in fact it is usually necessary - to adjust a deck all over again several times, both mechanically and electronically, in order to improve the results in an incremental way. I've never found a way to avoid it, and a deck is almost never very good after the first adjustment. You will find this cumbersome long before the job is finished, but the sound will keep you happy for a long time after that. The next times you adjust your Revox will only be periodic routine checks, and they'll be much easier.

 

[GPS16]

Hi Smurfer,

the low frequency response is more likely to be electronic than mechanical. If the HF side is correct then mechanically the system is fine. As then frequency goes up, the mechanical dimensions fall to microscopic proportions. By comparison, 30Hz is gigantic.

Make sure that you have your test leads as per the spec. I think that is where I have just fallen down on the 4 Track C270 conversion. Where are you measuring the output? At the XLRs? If you are measuring elsewhere then that might be the problem!!!

Just for the record....... A Revox machine will ALWAYS be better than the spec. The spec is a minimum!!! But as said previous, aim for within spec!!! Better than spec may be possible but it will not be long lived!!!

You may need to jump back from the output in steps to try and home in on where the deficiency lies. I would usually be saying electrolytics but you have replaced all them!!!

GPS16

 

[jlb2]

Hi Smurfer,
There may never be complete erasure of the Right Channel of a 4 track recording by the erase head of a 2 Track recorder. The edge of tracks 2 and 3 (4T Right, Fwd/Rev) encroach onto the guard band, between the Left/Right 2 Tracks, more than would normally be the case for a 2 Track recording. The wider track coverage of the 2 Track erase may not be enough. This is even worse if the Guard band is wider due to time code.

Unfortunately you can forget the "may": it is impossible by design. Channels 1 and 4 can be played (if there is nothing on tracks 2 and 3) and erased, but tracks 2 and 3 will be played too low due to be only partly on the head, and cannot be completely erased. Note that the "wide" 2-track machines (2.75 mm-wide tracks) have full-width erase heads that will take care of any situation, and it is the normal layout of the "real" pro radio decks (Studer, Nagra, etc.), but wide-track PR99's are rare as hen's teeth. IASA has made available a very useful list of track layouts there.

OTOH the wide guard band is not related to 4-track, it is located at the centre of the tape while the audio is recorded in 2 tracks with the same layout as the Revox (2-mm tracks), so these tapes are (in theory) 100% compatible. Those machines (usually pilot or TC versions of the Nagras or Studers) have a full-width erase head, so those tapes are not not a problem if erasing on the original recorder. OTOH it is a bad idea to use a Revox to play these tapes, due to the Revox's PB head often being slightly out of alignement and catching some of the centre track. In fact, even with perfect alignment the channel separation is not sufficient to avoid some bleeding of the pilot track into the audio. And then of course, the 2-track erase head of the Revox is unable to erase this track.

 

[GPS16]

Hi jlb2,

thanks for the extra info. I can only go off the experience I have with my machines. I am an Equipment Maint Engineer in the Semiconductor Industry and HiFi is my hobby. Reel to Reels are the bulk of that. A Pioneer RT707 was my first "bought" machine and I wore it out. The Revox machines I have now were procured to enable me to play my "Library" of 707 7" tapes. They were also projects for refurb which I also enjoy. Being a service engineer, the insides of the Revox machines shouted out that they are special pieces of kit.

I do own a Revox C270 and this machine is my HS 2 Track. This one does have the Timecode option fitted and still has the separate erase head.



Not something I am ever going to use but removing it may do more harm than good as I have the CPU on the control PCB on a smaller daughter PCB which piggy backs onto the CPU socket. This gives the added "Parallel Port" for direct control required for the timecode hardware along side the usual RS232 Port.

I am still checking the HF response on a reasonably regular basis, as I am curious as to how far the heads can be pushed. Not a lot of life left in them I feel!!!! At the moment still listening and grinning!!!

Cheers.

GPS16

 

[smurfer77]

What a pleasure to wake up and read several posts on the the thread! I didn't have time to do much today and I will reply more fully to some of the points raised later, but I just wanted to quickly drop one or two comments in.

where did you read that the PR99 is supposed to be down only 1dB at 30 Hz? I've just re-checked the spec sheet, it says:

• 30Hz - 22kHz +2/-3 dB (50Hz - 18kHz +/- 1.5dB) at 38 cm/s
• 30Hz - 20kHz +2/-3 dB (50Hz - 15kHz +/- 1.5dB) at 19 cm/s

I was wrong. The +0,-1dB was me crossing wires from 6.4.7 (Frequency response check without tape). So thanks for reminding me the proper specs. Also, now you mention I do remember reading somewhere else people talking about the B77 7.5IPS vs 15IPS bass response. And, I am using a 7.5IPS MRL tape. So that it all making sense! Thank you. So I will just proceed to realign the heads and sort out the HF properly, when I get a chance. And you are right about it being good to check everything because of the lack of history I have on this machine.... everything is/was out of whack for sure.

I won't have time to press on with the calibration tonight but one quick thing I wanted to ask quickly to the forum readers is about the VU adjustment. The Line Input Circuit was adjusted (by previous owner I assumed) to have about 0.55V at the monitor connect. So I adjusted line input pots to have 0.775 V (and yes, my tape I will use is a 250 nWb/m) at the monitor. However, the VU METER CAL pots now only allow me to get to +2dB minimum. Another common B77/PR99 thing, or just me?

 

[GPS16]

Hi Smurfer,

You are now at the point where my 4 Track C270 is upsetting me. The Line is fine, the Tape Monitor can't be lifted high enough to match it. Go through the manual with a fine tooth comb. The C270 manual states conditions for the test leads with links for unbalanced measuring equipment and termination resistors for 600 Ohm loading. I suspect that this is where I have boobed!!!

Coming along nicely.

GPS16

 

[GPS16]

Hi Smurfer,

just had a peruse of the manual. If you have the 0.775v at the monitor output then that signal goes directly to the Meter Amp. If you have the same problem on Left and Right channels then this would suggest that the 0.775 is incorrect or the Meter amps have been modded. You have not said that this is one channel only.

GPS16

 

[smurfer77]

Hi Smurfer,

just had a peruse of the manual. If you have the 0.775v at the monitor output then that signal goes directly to the Meter Amp. If you have the same problem on Left and Right channels then this would suggest that the 0.775 is incorrect or the Meter amps have been modded. You have not said that this is one channel only.

GPS16

Both channels the same. If the 0.775 V is RMS then i have it right (checking at monitor connector) but of course i could still be doing something silly. Off to bed now... A fresh mind tomorrow may solve it

 

[jlb2]

Hi GPS16,

Wow, you really have a rare machine, I'm impressed! Yes that C270 is an impressive design, I'm jealous with my lowly PR99 (but then my other RTR is a Nagra IV-S).

As regards the lifetime of the Revox heads, for sure they are softer than those of other machines, but in my experience these heads are far from shot (meaning that the gap does not show signs of widening, I check it with a microscope to make sure) when they reach the 4 mm wear limit, and relapping gives them at least a second life. Incidentally it is fairly easy to relap Revox heads, I do it myself using 3M lapping film on a 4 mm thick glass slab, and so far my relapped heads measure like new ones. It says a lot about the serviceability.of those machines.

BTW the experience that led me to Revox is quite different from yours, but interesting as well. I am a research engineer with a background in video and audio, probably one of the very last ones with knowledge and experience in analogue technologies, and I've been involved in archival projects that involved playing old tapes recorded on machines and in formats that are now long extinct, so I am often requested to digitize archives (some of which I recorded myself in my early days), hence my need to gather (who said hoard?) and keep in working conditions RTRs with the different track configurations, cassette decks, DAT decks, early video recorders of a variety of formats, and even 5 1/4" floppy drives. Needless to say, since I have to do it all by myself I prefer things that work well, are not fussy about the condition of the tapes, and are easy to service with a good supply of spare parts - all this on a budget since there is next to no money available to do all this.That's where Revox come into play A Studer would treat the tapes more gently thanks to its far better tape transport, but it wouldn't easily read consumer tapes. Besides, budget and scarse parts supply wouldn't allow it.

 

[jlb2]

Hi smurfer77,

I won't have time to press on with the calibration tonight but one quick thing I wanted to ask quickly to the forum readers is about the VU adjustment. The Line Input Circuit was adjusted (by previous owner I assumed) to have about 0.55V at the monitor connect. So I adjusted line input pots to have 0.775 V (and yes, my tape I will use is a 250 nWb/m) at the monitor. However, the VU METER CAL pots now only allow me to get to +2dB minimum. Another common B77/PR99 thing, or just me?

My gut feeling is that you just need to take the time for a full calibration, particularly since it is your first time - and probably the first time for this deck of yours too. Do the full procedure from the book, not thinking about the particular problems you are facing now since they will probably be solved in the process. Then do it all over again: your deck will be closer to correct adjustment already, and you will have learned how to do it, so you will achieve better results. If you cannot adjust a particular parameter at this point, it will still be time to give it a closer look. No need to hurry: as you said, a fresh mind (and no haste) goes a long way.

 

[GPS16]

Hi Smurfer,

had another though!! Does your meter have an uncal mode? If so has it been disabled. If it has one and it is as per a scope, they usually reduce the gain from the max (cal). This will give you a larger input to the PR99 than you intended, which may be the cause of the +2dB problem.

GPS16