Head changes on a Revox PR99

[Linnovice]

Hi all, I'm looking for a little advice please. I need to change the heads on my PR99. I have a spare headblock and resurfaced replacement heads. Looking at the SM it does not give any instructions on how to change the heads, only to return the machine to a Revox Service agent.

Looking at the drawings and the headblock (it has worn heads in place) it looks a simple matter of unscrewing the old heads and screens and then just replacing the new heads in reverse order. Obviously they need to be unsoldered/soldered but other than that are there any major pitfalls I should be aware of?

I've done this previously on a Teac 3440 which was pretty straight forward but never on a Revox. Any tips and/or pointers would be very gratefully received.

[Ted Kendall]

Wrap, height and face tilt are the points you need to watch. Replacement original heads would usually be fine without adjustment of height or tilt, because the manufacturing tolerances were held to tight limits. The head is retained by a single screw, so wrap has to be correctly adjusted - the gap should lie in the centre of the wrap.

To check these points, apply some felt tip pen or engineer's blue to the head face and run a (dispensable) tape. You should see a rectangular contact patch with the gap in the middle. If there is, clean up, adjust azimuth, job done. If the patch spreads at top or bottom, the face tilt is out. Be careful with adjustment so that you don't lose track of where you are and can't return to the original state. If the height is out, the patch may miss parts of the head core, but this is unlikely.

[Analogue man]

I assume you intend to replace the whole head block rather than just the heads, if so the job will be a whole lot easier than changing just the heads.

[TIMTAPE]

The adjustment I find the most tricky is head tilt forward and back. The head face needs to be perpendicular or uneven/premature wear will result. I try and visually align the head face to a reference on the machine such as a static tape guide, or the capstan shaft.

In the factory they probably have a special right angled tool which sits on the head block or chassis, and the other surface is made to but against the head face. Any misalignment is seen as a gap at the top or bottom of the head face.

As Ted says if the front and back height screws are left undisturbed, or only adjusted together by the same amount for height, only a minor trim may be needed, if that. I often write reference marks with a small felt tipped pen on the height screws so I know where one is in relation to the other.

For wrap I find it easiest to not tighten the central screw underneath so tightly that it's impossible to rotate the head from the top. I also draw a reference line next to the head to keep track of where I started.

[Linnovice]

Originally Posted by Ted Kendall

Wrap, height and face tilt are the points you need to watch. Replacement original heads would usually be fine without adjustment of height or tilt, because the manufacturing tolerances were held to tight limits. The head is retained by a single screw, so wrap has to be correctly adjusted - the gap should lie in the centre of the wrap.

To check these points, apply some felt tip pen or engineer's blue to the head face and run a (dispensable) tape. You should see a rectangular contact patch with the gap in the middle. If there is, clean up, adjust azimuth, job done. If the patch spreads at top or bottom, the face tilt is out. Be careful with adjustment so that you don't lose track of where you are and can't return to the original state. If the height is out, the patch may miss parts of the head core, but this is unlikely.

Hi Ted. That all sounds fairly straightforward but I think I need to explain myself better. I do have a spare block, it too has worn heads but I do not think it worthwhile putting the replacement heads on this one. I may just as well mount them on the one currently in the machine. The spare block though is good to practice on, which I have been doing this afternoon. I do have a couple of questions.

When I remove the worn heads on this spare block they are enclosed in a two part 'shroud'. Within this 'shroud' there are two lengths of black plastic rod (it is solid, not hollow tube). They dropped out when I separated the two parts of the shroud so I'm not sure how they sit within the it. It looks obvious that they sit behind the head either side of a 'fence' arrangement but I'm not certain and there are no instructions in the SM. I'm attaching a photo which makes it clear (I think). Am I right in thinking this?

Second question. My spare heads have numbers on the top 023 on the Record but I have two kinds for Playback 027 & 029. What would be the difference between the two playback heads? Would you know? I haven't yet taken the machine apart so am not aware of what's already in there but, for warned is for armed.

And finally, as you suggest I've checked the brass guide and it doesn't look as though it has been tampered with. It's mount is held on with two machine screws and the brass nut on the top is locked with red lacquer which looks as if it has never been touched.

Very grateful for any advise as I am uncharted waters here.

[Linnovice]

I’ve now found out that the two plastic rods are fitted either side of the head in it’s shroud. Hold the head central.
Still trying to find out what the 029 head is for . . .

[Linnovice]

Apparently, the 029 is a playback head for a Revox G36.

[Ted Kendall]

Are the two heads half and quarter track, maybe? Revox pattern heads were made in full track, half track, quarter track and occasionally butterfly (narrow guard band half track). Examination of the head face should show you the track layout. The other thing to watch for is that replacement heads were made in this form for the valved 36 series machines, and you may have one of these. Somewhere I have a list...

[Ted Kendall]

Posts crossed. The 029 is of unsuitably high inductance for the transistor preamplifiers, from memory.

[Linnovice]

Would I be correct in assuming a 023 record head is for a G36 also?