A77 Meters

[Linnovice]

Hi all, can anyone advise me how to set the VU Meters on a quarter track A77. For the life of me I can't see it anywhere in the service manual. I tried wetting my finger and holding up but that didn't help 😡

[Restoration73]

Without a calibrated level tape, it will be a bit of a guess. Normally replay is set first.

[KeithsTV]

Section 6.6.1 in the manual covers the meter calibration. Took me ages to find it though!

Keith

[Linnovice]

Thanks Keith, I can well believe that. Not a logical sequence, adjusting after the distortion settings? I've seen elsewhere that the meters are set to a 0.775mV signal which seems a much simpler method. I'm not sure that the result is correct though. Not experienced enough to do the math but it sounds logical. My B77, PR99 and Studer are all set to that.

[Radio Wrangler]

It's the voltage for 1mW = 0dBm in 600 Ohms, and a commonly used reference level in telecomms and audio

David

[mitajohn]

I will give you some brief and fast guidance which works fine for years:

1-A77, in recording position at 15 or 7.5 ips (select your prime recording speed).
2-RECORD LEVEL controls, at max and stereo.
3-VOLUME control, at max.
4-Audio generator at about 30 mV/1kHz in AUX inputs.
The generator must feed both aux inputs in parallel with the appropriate connectors. (Generator’s source impedance: ~600 Ohms or preferably ≤ 50 Ohms).
5-The line out level of A77 must be 2 Vrms per channel. Adjust BALANCE control for equal level/channel to 2 Vrms. To achieve this exactly, make also some micro-adjustment to the generator’s output level.
6-Turning the VOLUME control decrease the line output level to 1.55 Vrms/channel (+6dBu). For equal levels, make some micro-adjustment to the BALANCE control.
Check it by using a DMM and a 2-trace oscilloscope for equal levels and zero phase difference.
DON’T TOUCH, FOR NOW AND THEN, THE VOLUME AND BALANCE CONTROLS.
7-Increase the generator’s output level to 1.55 Vrms and adjust, with the RECORD LEVEL controls, the line output level of A77 to be 1.55 Vrms per channel. (So the recorder is defined as with a gain Vo/Vi = 1). This level of 1.55 Vrms is now the “reference level” 0 dB at 1 kHz.
DON’T TOUCH, FOR NOW AND THEN, THE RECORD LEVEL CONTROLS.
8-With the above conditions calibrate the VU meters by the METER CALIB. TRIMMERS for 0 VU reading. (When recording the real thing, keep in mind that the VU meters are average meters and their indication would practically be 4 – 6 dB less than the actual recording level).

[SteveCG]

I don't have access to my A77 info at the moment but I'm pretty certain that there were two different output line levels over the production run of this machine.

Perhaps Linnovice can help us by narrowing down the possibilities.

There were 4 versions of the A77 I believe and also a Dolby B version. I think the later ones had a lower output level. Others here might confirm this.

[Linnovice]

Hi, thanks for all this info. I'm finding it very useful. The machine is a 4 track MkIII 3.75/7.5ips, I think quite an early one as the S/N is 496??. I've now got it up and running and I'm quite amazed how good it sounds. When I first started working on it I wasn't quite sure. Compared to my other decks it seemed a bit 'basic' but I've now grown to like it a lot! Being very inexperienced in the electronics side of the hobby I've found its simple boards (fairly) quite easy to work on and especially solder. Plus the ease of obtaining parts a big bonus. I've changed all the RIFA caps, the motor run caps plus the three caps on the regulator board. I've also changed all the trimmer pots. I need to service the capstan motor as it has a pronounced rumble (I've ordered a replacement from Germany). I'll gradually work my way through the machine and change all the remaining caps over the next few weeks. I'm finding it a rewarding learning curve.
One thing I have found odd. When setting the bias I've found the oscillator card signal differed from record amp to record amp. It even varies as the hands get close. It's not the record amps, I've tried swapping them and it makes no difference. The amp that is nearest the oscillator card has a bigger distortion than the other. On the B77 there are additional 'shields' that come up from the bottom plate to isolate different groups of cards. I wonder if it would help if I did a similar thing and isolate the oscillator card on my machine.

[SteveCG]

Linnnovice,

I found that you need to have the aluminium shield (the one with the holes for access to the preset-pots) in place to get satisfactory alignment results.

[Diabolical Artificer]

Think your saying your record PCB's don't have metal shields. If one PCB is different, could it be down to the inductor's - L501 being set differently? How about using a plastic screwdriver as used for setting scope test leads?

Andy.

[Linnovice]

Hi Andy, that's right. As you know, the cards are inserted into the switch board and held firm by the bottom plate. On the B77 there are a couple of shields fixed to the bottom plate that go up and isolated the groups of boards. I wonder if that was a fix for this problem with the A77. I don't think there's a problem with the PCB's as the distortion doesn't move with the board if I swap them. It's definitely the influence of the oscillator card that is causing it. I also use a ceramic screwdriver to set the trimmers so it's not that either. Having said all, that it appears to make no difference to the sound on replay. It's only the bias current that is distorted so makes no difference to the record or playback audio signals.
Mike