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Relapping Revox heads

SaSi

SaSi

Seriously Illogical
Some time ago I wrote about my Revox PR99 and how it showed very good frequency response but instability in reproduce level.

In other words, while playback of 1kHz tone was constant at 0dB, 1 16kHz tone would fluctuate during playback by at least 1-2dB. At even higher frequencies, there was a drop in response and an increase in fluctuation.

I was told this is a result of worn out heads and a mirror formed.

Last night I took the heads off, took pictures of them and relapped them. They did have a rather large mirror, about 5mm for the playback head and ~4mm for the rec head.

The relapping process isn't yet finished (the final polish hasn't been done yet), but I put the heads back and alligned them to check how they do.

Well, the fluctuation has gone by a large factor. There still is some, but with a quick check using the deck's VU Meters, it is in the range of 0.1 ~ 0.3dB - as far as my eyes can see in the VU meters. I will recheck with the oscilloscope for precice measurements, and continue the polish process, posting pictures of that in the DIY forum.

What I would like to share here now is that the process is far simpler than it sounds and I would never consider sending the heads for pro relapping unless I was actually using the deck for commercial purposes. Having a set of extra heads to try really did help giving me the opportunity to try without destroying.

Also, the metal is pretty soft. I cannot remember sanding anything softer than that permaloy metal other than tin. Actually, permalloy is 20% iron and 80% nickel, so that explains it.
 
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This an interesting subject and I'm glad you raised it. I spent 15 years of my career servicing PR99s. If I never saw another PR99 it really wouldn't bother me! However, I don't need to tell you that they are very good machines which can perform extremely well if looked after. The figures can match those of a Studer A80, and whilst the Studer's mechanics are leagues above the Revox's, the PR99 does not in any way disgrace itself, allowing for a few a few things. A 5mm flat or 'mirror' on the rec and rep heads is equivalent to about 150,000 miles on a car. There's a way to go before you can expect problems. (This is not a Ford, remember :)) On the replay head, you only have to worry when the head gap starts to open up. By that time, the flat will be about 5.5 - 6mm across. You'll have cooked your goose at that point. No amount of lapping can repair a head where the gap is opening. Ironically, the HF phase stability should improve with head wear, since there will be less opportunity for tape weave. (Tape weave can be caused by many other things as well by the way - capstan, pinch roller, guides and azimuth/zenith especially). The bass response should also be at its best as the head approaches the end of its useful life, as the proximity of the tape will be closer to the curved edges of the head - consider the recorded wavelength at a given speed if you're not sure. (This is why using higher tape speeds improves everything except the bass!) Re-lapping a record head that has a 4mm flat may cause you problems. It doesn't need doing yet. The bias circuit relies on the inductance of the record head, which is proportional to the amount of core material in it. If you lap away the pole pieces (core), you will find that [within the limits of the bias trimmers] you can't reduce the bias enough to gauge how much overbias to apply when you align the machine. (Hope that makes sense!) The effect will be that your MOL could suffer at high frequencies (but noise and even-harmonics will be reduced). If you don't have a PR99 service manual already, please do go to the trouble of obtaining one that's right for the vintage of your machine. Mk1: mechanical counter, Mk2: electronic counter, Mk3: as Mk2 but with grey plastc knobs. Good luck, and if you want me to go over any of the above just ask.
 
Thanks a lot for taking the time to provide insight. I do have the service manual for the Revox machines. Mine is a MkII.

I got myself a pair of used PR99 heads as a replacement, and they are much less worn off than the ones on the deck. Before replacing them and discarding the older ones, I decided it was worth it to spend some time relapping them, if only before throwing them away. I did it as part of some work involving replacement of all tension arm rods and head guard assembly. I collected these parts over the past month, and some come out of B77s.

While relapping, I took care to remove as little material as possible, while creating a smooth path for the tape. The result is that the head face is no longer circular arc but more like elliptical. I just removed the edges of the flat part.

I have put the heads back on the deck and tried them out. They are smooth but not polished like they were. I can discern a significant improvement in the sound overall. High frequencies are smoother and better present. I can now measure decent response beyond 22kHz and totally flat and constant up to 20kHz. For whatever reason, low frequency response is now much better. At 15ips I measured -3dB at about 30Hz, now it goes all the way down to 20Hz with -1dB. Drops drastically below that. Practically, frequency response is flat within 1dB between 25Hz and 21kHz at 0dB recording level and 15ips. At 7.5ips it drops down to 16kHz.
 
Quantegy 478, bought new, and Revox 621/631 reels, whenever I find them NOS in the German eBay. Turns out there are a few sellers that have a lot. Whenever I find anyone selling a reel or two, I write to them to check how much they've got. Makes no sense to pay postage for a reel only.
 
Hello
Clever I have a question, a couple months ago I bought a ReVox PR99 NIB.
It had been stored in a heated warehouse since new.
Yes it is as new, I did notice one issue using the deck, although it seemed to be perfect in every way. but I notices a ticking or real faint popping deep in the background of one channel.
I played back a clean tape, no noise, slight tape his, thats it.
So I ran a tape thru it, set to record no signal going in.
I played back this tape and got the noise in the same channel, so I'm thinking there is a weak component in the record board.
This noise is not enough to move the VU's but it can be heard, sounds almost like static/poping
Any ideas or comments ?

bob
 
I have also relapped a few Revox heads with great success. It's easier than the shops make it out, obviously... But requires patience and the right materials. I only have the B77 version, but the results were better than I expected. And I DO have a NOS set of heads in a drawer... just in case.
 
Bumping this up since I'm very interested in the whole relapping procedure.

Sasi, what kind of abrasive film did you use for making the head nice and round again?

Where can I get this material in Europe?

Anyone?

Taneli
 
I used 600 grade sanding paper (wet/dry). This is dark grey and normaly used for finishing lacquer paint. It's easy to find it in any hardware/hobby/diy/woodworking store here.

One trick: I used a large diameter roll of masking paper tape, one with an internal diameter of 8-10 cm. I glued the sanding paper to a segment of the inside of that roll (to achieve a circular and flat surface.

Securing that roll on a flat surface and the head on the same surface, simplifies the "rubbing" of the head on the sanding paper while at the same time ensuring a vertical orientation of the flattening.

(I read what I wrote here and I'm not sure if it makes sense, so bear with me. I may eventually have to add some pictures here. Still have a Revox head that is totaly shot.)
 
perryinva said:
Why so coarse? wet dry to 2000 is easy to get.
Click to expand...

That is what I had available at the time. That paper doesn't give a good glaze - a finish is required.

I am not sure that starting with 2000 would give results quickly. It might and might also negate the need for extra polishing work.

For sanding pursposes, I consider 1000+ papers to be suitable for the final touch, sanding wet with a bit of soap added.
 
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