Revox A77 doesn't erase

[mole42uk]

I'm struggling to get this A77 completely working. Most things are okay now but it doesn't erase the tape. Voltages on the orange and blue wires are as specified, oscillator frequency is as specified. The machine records a signal but the previous contents are still on the tape. Heads have been cleaned with alcohol.

Has anyone any ideas?

[GeoffB17]

You're not very clear about what you've checked, but sounds like the Record head may be recording, but the Erase head is not.

If you take a tape with something recorded, and 'record' silence (i.e. no new signal being recorded) does this make any difference at all to the original signal? If so, how much?

Another possibility to check, where was the signal originally on the tape recorded? If the original signal is 1/2 track, but you're now using a 1/4 track format, then the original signal may not be being fully erased and may still be heard. You don't say what the relative levels are, merely that the original signal is still heard.

Geoff

[TIMTAPE]

The A77 tape path is difficult to clean, even difficult to see. A small dental type inspection mirror at least allows viewing the heads to check they are actually clean. Also the A77 being older will struggle to erase some very high energy tapes.

[mole42uk]

The record & play heads have been out of this machine and lapped. And tested for continuity and impedance.

Tests today include cleaning the erase head. If I set the machine to record with no input signal the previous sound on the tape is audible but at a much quieter volume than if I play a section that hasn't been through the erase section. I do not know what machine this tape was originally recorded on. I suspect that the erase head is functioning but possibly with less power than needed but the oscillator signal at the erase head is present and at the correct voltages. Tape looks like polyester, in an EMI box and wound on a Grundig 7" reel.

[Analogue man]

Originally Posted by mole42uk

The record & play heads have been out of this machine and lapped. And tested for continuity and impedance.

Tests today include cleaning the erase head. If I set the machine to record with no input signal the previous sound on the tape is audible but at a much quieter volume than if I play a section that hasn't been through the erase section. I do not know what machine this tape was originally recorded on. I suspect that the erase head is functioning but possibly with less power than needed but the oscillator signal at the erase head is present and at the correct voltages. Tape looks like polyester, in an EMI box and wound on a Grundig 7" reel.

Then I suspect that the machine is working correctly, although the erase head could still be dirty, even if it was lapped which I somehow doubt. I would go with TIMETAPE's comment as I have had that problem with my A77.

[GeoffB17]

OK, sounds like the erase head is doing something, but not effective.

Is the tape the correct way round, i.e. the side with the magnetic coating is against the head. Certain tape types are difficult to tell, back coated types are much easier. When you PLAY, does this sound OK/normal? If this seems low level at all, then the tape may be back to front.

Another thing to try - can you erase the tape manually, using a magnet if need be, better a magnet wrapped in paper held against the tape. Even a few minutes. Then PLAY that, and ensure that the old signal is gone (this will remove signal full width). Then try normal RECORD again, and see what difference that makes.

As suggested, even a bit (maybe not visible) dirt on the head may reduce tape/head contact, so clean is important. Also, if other heads have been removed this may have affected the alignment of the erase head so tape is not getting correct tape/head contact?

Geoff

[TIMTAPE]

Are these heads quarter track or half track ? Have you physically aligned the heads since removal or did the head lapping tech do it for you? The erase head can be adjusted for height, and rotation around the vertical axis. Both must be correct. I would temporarily remove the entire control panel fascia plus the swinging aluminium member to allow clear viewing of the tape path. Without removing these panels, physical setup of heads can be a nightmare. This was somewhat improved on the B77 but it wasn't until the PR99 that it was completely sorted.

[mole42uk]

Originally Posted by GeoffB17

Another thing to try - can you erase the tape manually, using a magnet if need be, better a magnet wrapped in paper held against the tape. Even a few minutes. Then PLAY that, and ensure that the old signal is gone (this will remove signal full width). Then try normal RECORD again, and see what difference that makes.

Yes, I’d already done that with a small permanent magnet. The previous recording had gone, and a new recording was perfectly good. Bear in mind that I’m recording test tones, not music or speech and viewing the results on an oscilloscope.

[mole42uk]

Originally Posted by TIMTAPE

Are these heads quarter track or half track ? Have you physically aligned the heads since removal or did the head lapping tech do it for you? The erase head can be adjusted for height, and rotation around the vertical axis. Both must be correct.

They are quarter-track heads. The play head I aligned by matching the phase of a sine wave from a Ferrograph test tape. I haven’t aligned the record head yet. The erase head hasn’t been out of the machine.

[GeoffB17]

OK - various points now seem all OK.

My comment about the alignment of the tape refers to the 'curve' of the path of the tape over all three heads. The tape should cross each head such that the head gap is central to the contact of the tape over the head. If the next head (the RECORD head) is projecting slightly from it's correct position, it may alter this curve so that the tape contact is slightly out, and this may be enough to affect the performance of the erase head.

From what you say, the erase head should be working electronically, but for some reason it's not doing the job magnetically. It is doing something, it seems, but not enough. You say you've cleaned it fully. If the other heads were worn enough to need lapping, another question could be - is there any hint of any damage to the surface of the erase head? Specifically where the head gap is?

Geoff

[TIMTAPE]

Originally Posted by GeoffB17

...From what you say, the erase head should be working electronically, but for some reason it's not doing the job magnetically. It is doing something, it seems, but not enough. You say you've cleaned it fully. If the other heads were worn enough to need lapping, another question could be - is there any hint of any damage to the surface of the erase head? Specifically where the head gap is?

Geoff

My thought also. These erase heads do fail. Often the telltale sign is where once the dark surfaces of the track 1 and 3 pole pieces were highly polished all over, where the tape has scraped against them, the surface is now dulled, and closer inspection reveals a roughness where many small crystals from the Ferrite material have eroded or broken away. I sometimes had to replace such erase heads, usually on machines with high tape mileage. Not saying this is the problem but it could be, or part of it.

[mole42uk]

There's more. The record function does put a signal on the tape, but only if it's heavily over-driven and the output is distinctly iffy. I've abandoned work with the erase head for now, I scrubbed a tape with a permanent magnet so at least there's no previous content, just hash.

I've checked both the input channels with a suitable signal, the VU meters indicate correctly, there's a believable signal at the collector of the output transistor of the record PCB, but the only signal that gets as far as the record heads is the oscillator. I swapped the two record amplifier boards and the fault remains, I checked both channels using the same settings and the results are similar.

There's obviously a common problem somewhere I haven't yet established - the record relay clicks in and out as expected, the contacts look clean. I can't see anywhere on the circuit that might give me an aha! moment.....

[Ted Kendall]

Perhaps the inductors on the record cards are o/c - it has been known...

[DMcMahon]

Originally Posted by mole42uk

I've checked both the input channels with a suitable signal, the VU meters indicate correctly, there's a believable signal at the collector of the output transistor of the record PCB, but the only signal that gets as far as the record heads is the oscillator.

Note - that the record signal that goes to the record head is much smaller voltage amplitude compared to the record bias signal and of course the bias signal is a much higher frequency. Depending upon your scope settings and how strong the record signal is, it can be very difficult to see the record signal which is effectively modulated onto the bias signal.

You generally have to set the volts per division to a much more sensitive setting (compared to if you were monitoring the bias signal) and set the timebase accordingly for the audio record signal.

On some machines the difference can be dramatic, on some machines I have half convinced myself that the record signal is rubbish/nonexistent (because can be so difficult to see) when in fact the machines record very well.

David

[Restoration73]

Ensure you use a x10 probe to look at these signals. The manual quotes RMS volts, and
you need to multiply by 2.8 to derive the peak to peak value (hope this is not too obvious).