Studer A807 - head preamp

[jpmorris]

I have recently acquired a Studer A807 and it seems to have a bit of a problem with the repro amp on channel 1. I've been lining the thing up for the 200nWb standard my Revox was using, and while I only need to set the level to about 140 on channel 2, channel 1 needs about 250 to draw level (that's out of 255, since it uses an 8-bit DAC).

I've swapped the audio connections to the channel cards over - something I need to double-check - but the results of that experiment lead me to suspect the repro head preamp might not be doing what it ought to.

The problem is, I haven't been able to work out from the manual where said preamps are actually located. Are they on the 'audio control board', which I believe is the card between CH2 and CH1, or closer to the heads themselves and deeper into the machine's guts?
Either way I may have a bit of a problem, as the middle card seems to be a third-party replacement which I don't have schematics for.

If anyone here has serviced these machines, I'd also be grateful for a little confirmation of how the cards are supposed to be remove. The manual seems to say "Attach a screwdriver to the nuts on the corner of the card and pull", but they aren't coming out easily and I do not want to break this machine. It was not cheap.

[emrr]

I have one, but I haven't been in it enough to be of any help. Have you subscribed to the Studer list? Your questions are right up its alley.

[jpmorris]

I have one, but I haven't been in it enough to be of any help. Have you subscribed to the Studer list? Your questions are right up its alley.

I didn't know there was one, but I'll definitely look for it. Thanks.

[ormej01]

Hi,

How is it going with this problem?

Here are some thoughts:

I would suspect the problem is with the A/D convertors, IC23 (AD7528N) on the Audio Electronis Board, that plugs into the Audio Control board.

Sometimes these go a bit strange, and need to be replaced. Whilst playing some tone from tape, check the level on pin1 of IC25 (RC4559), on each channel. This is the playback signal before it gets to the ADC - if they are about the same level, you know the head and op amps are OK up to that point. The AD7528N is available from RS.

Hope this helps.

[ormej01]

Hi,

How is it going with this problem?

Here are some thoughts:

I would suspect the problem is with the A/D convertors, IC23 (AD7528N) on the Audio Electronis Board, that plugs into the Audio Control board.

Sometimes these go a bit strange, and need to be replaced. Whilst playing some tone from tape, check the level on pin1 of IC25 (RC4559), on each channel. This is the playback signal before it gets to the ADC - if they are about the same level, you know the head and op amps are OK up to that point. The AD7528N is available from RS.

Hope this helps.

[jpmorris]

I would suspect the problem is with the A/D convertors, IC23 (AD7528N) on the Audio Electronis Board, that plugs into the Audio Control board.

Sometimes these go a bit strange, and need to be replaced. Whilst playing some tone from tape, check the level on pin1 of IC25 (RC4559), on each channel. This is the playback signal before it gets to the ADC - if they are about the same level, you know the head and op amps are OK up to that point. The AD7528N is available from RS.

Hope this helps.

Thanks very much. The machine hasn't had an enormous amount of use, so I've been working around the problem for now. However, that's a great lead and I'll definitely investigate it when I can.

Cheers.

[jpmorris]

I wasn't able to easily get to the test points (IC25), though I might be able to arrange some kind of flying lead. I've tried swapping the two DAC chips, which had no effect.

However, while I had the card out I did notice that the electrolytics on channel 1 were substantially down, in some cases 50% of their stated value. This does not appear to be as severe in channel 2, and curiously it doesn't seem to be so bad in the record section of channel 1 - only the playback section. Seems like I might be looking at a recapping job.

[jpmorris]

In case anyone is curious by way of follow-up, I sat down and recapped the playback circuit on the suspect board until I ran out of 47uF caps. Not very scientific, I know - but it seems to have hit the spot as both channels are now more-or-less equivalent when set to 88/255. I'll do a later test on the MRL tape, but it looks like the problem is sorted (though I guess I should replace the rest when I get the parts).