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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 10th Jan 2019, 1:14 pm   #21
Mikebee88
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Default Re: Revox B77 not recording

In response to Michael's 6 Jan. post, I would say I'm quite competent to deal with this fault with over 45 years associated in one way or another with electronic equipment. I have a well equipped workshop with lots of test gear and yes, three 'scopes! Returning the B77 'no record' fault, in record mode I can see 150KHz on the erase head and that bias signal also present at the record head. All five audio plug-in PCBs have been proven as working correctly using my other B77. Both B77s are Mk.1 3.75 and 7.5 ips machines. Both are dated at mid 1978. The fault is common to both channels which leads me to conclude I must next suspect the audio interconnection PCB. Continuity from the head block connections to the oscillator board plug has also been checked. I have not yet attempted to monitor the path of the audio input signal via the Aux socket beyond the input amplifier when in record mode.
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Old 10th Jan 2019, 2:22 pm   #22
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Default Re: Revox B77 not recording

Have you checked the voltages coming from the power supply and also the Q8 transistor on the tape control board . This transistor supplys the Y-REC to the oscillator.
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Old 10th Jan 2019, 10:20 pm   #23
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Default Re: Revox B77 not recording

Yes, of course! With the L & R record switches ‘on’, there’s +21.0V on the y-rec line (the orange wire between the tape transport and audio interconnection PCBs). Pressing Play & Record turns on Q10 (it’s the 1.177.315 variant), the y-rec line goes to 0V, the oscillator board relays operate and the oscillator circuit is then active. I thought I’d explained this already!
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Old 10th Jan 2019, 10:41 pm   #24
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Default Re: Revox B77 not recording

Input a signal, set the machine to record and use your 'scope.

Lawrence.
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Old 11th Jan 2019, 10:14 am   #25
Michael Maurice
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Default Re: Revox B77 not recording

Record some tone at say 1Kz at high modulation level and see it you can see it with your scope on the record head. You can see it modulating the bias.

If you can, then the head or associated wiring is the cause, if you can't, again you need to see where its being lost.
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Old 15th Jan 2019, 5:31 pm   #26
Mikebee88
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Default Re: Revox B77 not recording

Many thanks, Michael and others for your assistance to date. I don’t have time at the moment for further investigation but I must be getting very close to finding the fault!
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Old 15th Jan 2019, 11:54 pm   #27
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Default Re: Revox B77 not recording

What is odd is that its affecting both channels.
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Old 16th Jan 2019, 1:39 am   #28
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Default Re: Revox B77 not recording

That's the thing that exercises me, Michael. It's either two coincidental faults, or else the linkage must be a big clue.

Looking forward to see the resolution of this one.

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Old 22nd Jan 2019, 10:58 pm   #29
Mikebee88
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Default Re: Revox B77 not recording

Injecting a 1.0kHz tone from the Aux input, I’ve traced it all the way through with the ‘scope to the K1 and K2 relay contacts on the oscillator board and out again (when in record mode) for both channels. I’ve degaussed the record head and checked the connections from it to the J1 connector plug as well as continuity of the head windings (with a DVM!). Mixing of the record and bias signals is done on the audio interconnection board and I’m next going to remove it completely for a derailed examination and clean all the rocker switch contacts along with the ‘strip switch’contacts - some of which form part of the biasing signal path.
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Old 28th Jan 2019, 2:36 am   #30
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Default Re: Revox B77 not recording

Have you done as Michael suggested and monitored the signal at the record head itself?
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Old 3rd Feb 2019, 1:53 pm   #31
Mikebee88
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Default Re: Revox B77 not recording

My B77 is now recording to tape! The moral of this story is to [I]visually[I] check the condition of the tape heads and not just assume that by giving them a ‘wipe’ (using a genuine Studer Revox tape head cleaning kit, I might add) that any deposit is removed. With the machine on the bench, standing upright, I didn’t actually look to see the condition of the heads; rather embarrassing I know but one never stops learning. But at least I am now very familiar with the record process, signal and logic path! But the whole machine was very dirty and really needed a full strip down and good clean. Re-capping is also complete, new tape guide bearings fitted now, just recalibration required. Many thanks for all your wise words of advice!
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Old 3rd Feb 2019, 2:42 pm   #32
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Default Re: Revox B77 not recording

Never assume that a tape head is clean, even if you can't see any more oxide and dirt coming off! If the fault could be caused by dirt on the heads, then there could be either something still coming off that's just not very visible (especially if you're using "Good Enough for the BBC" methylated spirits), or a big stubborn lump of cp needing some more persuasion to shift it.

With a three-head machine, dirt on the record head will not affect playback and vice-versa; so there is a possible clue to be had there if you have a second, known good recorder to hand.
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