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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 26th Sep 2022, 5:10 pm   #1
ruggb66
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Default Revox A77 record/play question

I have just about fully restored this unit.
New reel brakes
repaired broken PCB socket
Replaced all pots
replaced tape counter belt
greased counter to remove squeal at fast speed
replaced bulb
reconstructed EOT sensor
replaced noisy balance pot
replaced bad transistor
Demagnetized everything

Now -- if I record with no input, I get a ~100kHz signal on the tape. That would be the erase frequency, right?
The problem is, when I play it back, I see the signal level on my VU meters.
Is that proper??
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Old 26th Sep 2022, 8:01 pm   #2
Ted Kendall
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Default Re: Revox A77 record/play question

Something is very wrong somewhere. Only the Dolby versions read playback level on the VU meters, and 100kHz wouldn't come back from the tape at any level.
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Old 27th Sep 2022, 12:05 am   #3
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Default Re: Revox A77 record/play question

I see the signal on my scope at the output TP also.
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Old 27th Sep 2022, 8:46 am   #4
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Default Re: Revox A77 record/play question

Is it possible you have another out-of-band signal being recorded at high level?

I would be looking for both subsonic and slightly ultrasonic (say anywhere from about 15kHz upwards). If you haven't caught it on a scope at 100kHz (which would be amazing, as that is likely to be way up above the replay head turnover frequency), then my guess is that it's probably "clean" mains hum down at 50Hz.

Otherwise as Ted says, something (else) is horribly wrong.

By the way, Revox machines of that era are not the wonderful beasts they're often said to be:

I have a first-generation A700, which has some extremely nasty logic in its record switching. Studer revised the design, so later models don't do this, but the early ones are capable of putting audio without bias on the record head with the track record-enable and input selector switches in the wrong settings.

There's no need to press the actual record button for this to happen, and it both partialy erases the tape and overlays the recording with nastiness if you run the tape past the heads with that combination of switch settings.

Unfortunately I ruined a recording before I realised what was going on.

A77s aren't logic controlled in the same way, but you might just have something odd going on with the mechanical switchery. I would be quite surprised though.
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Old 27th Sep 2022, 9:41 am   #5
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Default Re: Revox A77 record/play question

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Originally Posted by Simondm View Post
I have a first-generation A700, which has some extremely nasty logic in its record switching. Studer revised the design, so later models don't do this, but the early ones are capable of putting audio without bias on the record head with the track record-enable and input selector switches in the wrong settings.

There's no need to press the actual record button for this to happen, and it both partially erases the tape and overlays the recording with nastiness if you run the tape past the heads with that combination of switch settings.

Unfortunately I ruined a recording before I realised what was going on.
This wasn't a logic fault as such - the transistors switching the record audio feed couldn't switch sufficiently "off" in playback, so input peaks could break through to the record head. Studer fitted a plug-in daughter board once the fault became known, which was made available for retro-fit, and the subsequent revision of the oscillator card (267 type no.) eliminated the trouble at source. From memory, the oscillator card was changed around serial number 12000.

There were other niggles on the A700, which have been discussed here at length, but the fact remains that it was head and shoulders above the competition, especially at the price, and remains a useful tool today.

Last edited by Ted Kendall; 27th Sep 2022 at 9:47 am.
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Old 27th Sep 2022, 9:59 am   #6
lesmw0sec
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Default Re: Revox A77 record/play question

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Originally Posted by Ted Kendall View Post
Something is very wrong somewhere. Only the Dolby versions read playback level on the VU meters, and 100kHz wouldn't come back from the tape at any level.
Agree with that. Could the bias osc. be by some means running whilst in play? You can monitor playback on an A77 though, by switching through the 1 > 2 settings.
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Old 27th Sep 2022, 2:25 pm   #7
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Default Re: Revox A77 record/play question

Quote:
Originally Posted by Simondm View Post
Is it possible you have another out-of-band signal being recorded at high level?

I would be looking for both subsonic and slightly ultrasonic (say anywhere from about 15kHz upwards). If you haven't caught it on a scope at 100kHz (which would be amazing, as that is likely to be way up above the replay head turnover frequency), then my guess is that it's probably "clean" mains hum down at 50Hz.

Otherwise as Ted says, something (else) is horribly wrong.

By the way, Revox machines of that era are not the wonderful beasts they're often said to be:
That is it. IT IS A BEAST!!! But not so wonderful.
The signal I see on the scope is definitely ~100kHz, the same as the erase freq. However, I did not think about looking for a lower freq riding on that as I did not hear any audio. I will check that out. I do find it strange that I can see 100kHz coming thru the playback amps.
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Old 27th Sep 2022, 7:22 pm   #8
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Default Re: Revox A77 record/play question

@ruggb66: If you have 100kHz coming through the playback amps it's not likely to be coming from the rep heads - I can't guess more than that, but something seems very peculiar indeed.
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Old 28th Sep 2022, 3:05 pm   #9
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Default Re: Revox A77 record/play question

3 posts moved to new thread: Revox A700

https://www.vintage-radio.net/forum/...d.php?t=194757
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Last edited by Station X; 5th Oct 2022 at 8:30 am. Reason: Link to new thread added,
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Old 4th Oct 2022, 1:09 am   #10
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Default Re: Revox A77 record/play question

I'm back and frustrated
The above problem just went away after moving a couple wires on the backplane and testing components on the board. It was a feedback oscillation about 100Hz.

I now have little to no recording on both channels.

1kHz on Aux input, channels shorted.
VU meter @ -1 both ch
Record level trim pots at max both ch
OUTPUT level set to 200mv with input level controls.
Bias traps adj for min Vpp both ch
set for NAB
OUTPUT ch1 ~ 150mv with lots of 130kHz
OUTPUT ch2 500mv with little 130kHz
Trap TP ~300mv 130kHz and 150mv 1kHz Modulation
Signal at base of Q504 = 600mVpp
Signal at collector = 1Vpp
Signal at base of Q505 = 1Vpp
record head violet wire = 22Vpp 130kHz
record head Green wire = 800mVpp 0f 130kHz and 80mVpp 1kHz modulation.
This troubleshooting info isn't given in the manual so I have no idea what it should be.

21V = 21V
12V = 11.8V

So, why isn't it recording?

Since it is both channels - do I have a bad record head, and if so how can I test it?
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Old 5th Oct 2022, 11:12 am   #11
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Default Re: Revox A77 record/play question

Just wondering if the machine worked (albeit badly) before the restoration? That way we can narrow it down to an introduced fault or an existing one. No offence intended regarding your work, of course, but we've all been there!

I take it the machine is erasing and playing back a good tape normally.

The only real way of checking the record head (other than replacement) is by careful resistance measurement with a digital meter, ideally with a series resistor, followed by demagnetisation. i'd be very surprised if both halves had failed though.

Nothing to do with the fault, but have you replaced the RIFAs? They're always capable of surprising you.
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Old 5th Oct 2022, 5:38 pm   #12
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Default Re: Revox A77 record/play question.

There have been many problems with this unit. I can't say that the record worked b4 I fixed these things;
repaired broken PCB connector
new brake band
new balance pot
new bulb
rebuilt EOT sensor
silenced screaming tape counter
all trim pots replaced
Adjusted most trim pots

It is playing back a prerecorded tape OK.
It is erasing OK
There is 130kHz on the record head
I don't think the audio modulation is adequate on the record head but I have no reference as to what it should be.

Do you have any visibility as to what these numbers should be?
Trap TP ~300mv 130kHz and 150mv 1kHz Modulation
Audio Signal at base of Q504 = 600mVpp
Audio Signal at collector = 1Vpp
Audio Signal at base of Q505 = 1Vpp
record head violet wire = 22Vpp 130kHz
record head Green wire = 800mVpp 0f 130kHz and 80mVpp 1kHz modulation.

They are pretty much the same on both channels.

I would likewise be surprised to have both side fail. Both sides of the record head read 16.5Ω thru the 100Ω to ground. I demagnetized the whole assy with a large tape demagnetizer.

According to the schematics in the SM, which I have found to not be entirely accurate, I should have the same Ω reading with the record buttons in either position (out or in), but I do not. With the buttons in, I read 31.75Ω on the red & violet to grd, and 118.2 with them out. The grn & yel lead read 37.5Ω in, and 101.7 out. That makes no sense.
118.2 - 101.7 = 16.5 head resistance.
How do I get the other numbers
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Old 5th Oct 2022, 11:22 pm   #13
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Default Re: Revox A77 record/play question

Are you trying to use Ampex tape by any chance? Have you thoroughly cleaned the record head? You'd be surprised how many times this fault is down to a dirty record head after someone has tried to use an Ampex tape.

Also, as Glyn says, those Rifa capacitors need to be replaced ASAP.
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Old 6th Oct 2022, 12:34 am   #14
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Default Re: Revox A77 record/play question

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Originally Posted by jamesperrett View Post
Are you trying to use Ampex tape by any chance? Have you thoroughly cleaned the record head? You'd be surprised how many times this fault is down to a dirty record head after someone has tried to use an Ampex tape.

Also, as Glyn says, those Rifa capacitors need to be replaced ASAP.
Not sure what brand tape as it was supplied by client.
Yes, I cleaned the heads a few times.
Which caps are you referring to specifically? None of the electrolytics look like they are going bad.

The resistance numbers are affected by the relay board. It runs the leads thru L501. Now I am trying to figure out if the record relay is working, but I can't figure out where the 24V is coming from to activate it. There isn't any 24V listed on the PS schematic. Any idea?
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Old 6th Oct 2022, 10:41 am   #15
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Default Re: Revox A77 record/play question

Quote:
Originally Posted by ruggb66 View Post
Now I am trying to figure out if the record relay is working, but I can't figure out where the 24V is coming from to activate it. There isn't any 24V listed on the PS schematic. Any idea?
In the schematic I'm looking at the 24 Volt +ve for the record relay comes from the 27 Volt +ve rail (DB1) on the power supply via the record channel switches.

Which means one or the other is marked up wrong in the schematics, the circuit description states 27 Volts.

Lawrence.
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Old 6th Oct 2022, 11:46 am   #16
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Default Re: Revox A77 record/play question

Right - the RIFA capacitors. On the motor control board there are three or four of those. They are easy to identify - they are semi-transparent, yellowish and oblong. Not electrolytics - 0.47 and 0.1 uF. Replace with Class X components. They absorb water and explode giving off a smell that takes a long time to dissipate!

Although many people go on about recapping, the general opinion on this forum is don't - just change either unreliable types (like these RIFAs) or ones obviously in distress.
I would, however, also change the two 220uF capacitors on the control board - they are axial so easy to spot. They tend to go short at the drop of a hat. As they are on the 24v line they are worth a look, though they usually cause no capstan motor rotation.

Unfortunately these probably have no bearing on your fault. I think it's time to get the scope out and see where the record signal stops. DC conditions at this level can lead to confusion.

Bearing in mind how difficult it is to work on the control board, also that you've had a crack needing repair, it might be time to go over your work again. I'm just confused as to how BOTH channels can be affected if the record switching voltage is OK, so maybe that's the best place to start.

Remember the manual was written over 40 years ago so the voltages are a little, er, approximate.
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Old 6th Oct 2022, 12:00 pm   #17
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Default Re: Revox A77 record/play question

Quote:
Originally Posted by ruggb66 View Post
...Yes, I cleaned the heads a few times...
The A77 is a shocker for cleaning the head faces, even seeing what you are cleaning. The later B77 was somewhat improved but still not great. Temporarily removing the front control panel is the only way to gain clear access to the heads for viewing and cleaning.

Otherwise in many ways still a great, useful machine. Replacement heads are a big concern these days. I believe Italian company Photovox is now making new heads for the Revox company but at present only in half track.
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Old 6th Oct 2022, 9:30 pm   #18
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Default Re: Revox A77 record/play question

A little bit more information on the RIFA capacitors.

There are 3 on the Tape Drive Control Board (also referred to as Control Board/Relay Board), namely C113, C114 & C115. If still the original RIFA parts, they should be replaced as they will fail sooner or later.

Replacements should be new 0.47uF /470nF Safety Class X2 ideally rated at 275VAC, do not use second hand or NOS as replacements because they could easily have degraded during storage. Metalized Polypropylene (MKP) is generally recommended but there are other suitable types available.

Each of these RIFA capacitors has a 4.7 Ohm resistor (R121, R122 & R123 respectively) in series with the capacitor, they each form a RC snubber that connects across the relay contacts for suppression. In the event of the RIFA capacitor failing it is possible that the associated 4.7 Ohm resistor may be damaged so check the resistor in the event of RIFA failure.

There is at least 1 other RIFA of the same type/value, it is on the Speed Control/Servo Board connected to the bridge rectifier. It is recommended to also replace this if still the original RIFA part. For the all transistor Speed Control board it is C213, for the Speed Control board that has 2 IC's in addition to transistors it is C220.

David
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Old 6th Oct 2022, 10:09 pm   #19
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Default Re: Revox A77 record/play question

Quote:
Originally Posted by ms660 View Post
Quote:
Originally Posted by ruggb66 View Post
Now I am trying to figure out if the record relay is working, but I can't figure out where the 24V is coming from to activate it. There isn't any 24V listed on the PS schematic. Any idea?
In the schematic I'm looking at the 24 Volt +ve for the record relay comes from the 27 Volt +ve rail (DB1) on the power supply via the record channel switches.

Which means one or the other is marked up wrong in the schematics, the circuit description states 27 Volts.

Lawrence.
not surprised. It is also showing CH I & II swapped on Relay board as compared to Record amp and interconnect. There are other errors also.
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Old 6th Oct 2022, 10:35 pm   #20
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Default Re: Revox A77 record/play question

Remember with an A77 (certainly the Mk4) set to source monitor with the record buttons out channels I & II are reversed only coming the right way round once the record buttons are pushed.
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