Studer A807 not erasing/recording

C

carlosraj

New member
Hi,

I am new to this forum. My Studer A807 has a problem. On the left channel, when I set to record, it records the new material for approx about 25secs, then I can hear the previous recording coming back. The right channel records for a much longer time before I hear the previous recording coming back. At times, the VU meters peak(red LED) and then the recording returns for a while and then disappears again. I have monitored the erase heads using a scope, At the start of recording, a high frequency and high voltage sine wave is present. It then slowly falls to a low level sine wave and it is at this point that the previous recording is heard. What can cause this problem. Please assist.

Thanks
 
As a starting point, I'll make a generic diagnostic guess: Cracked solder joint that heats up and becomes resistant under load?
 
Might be worth checking for cracks or bad solder joints, but I would have expected that to affect one channel only, if it's a machine with a half-track erase head. (Mine is full-track, i.e. it always erases both channels and is fed off a single channel card).
Assuming that the frequency is remaining the same and it's just the amplitude that's changing, and changing differently on each channel, I would assume that the erase amplifiers are going bad. If it were mine, I'd probably replace the capacitors for that circuit and see what difference that makes. (But I am a hack rather than a professional)

On mine, the capacitors were way out of spec, but bizarrely they were worse on one of the two channels - I assumed that that one gets hotter or something. In my case it was one channel recording at a much lower level than its companion.
 
The measurement that will tell you most is the CURRENT thru' the heads. Insert a resistance in the earthy side and put mV meter/scope in there (or mV across R and scope across bias feed) . Can you scope the main output of the bias osc'? I think that must be holding up otherwise the recording quality would go to ***t.

If you don't have a full service manual you need DESPERATELY to get one! Sweetbeats! Where T.H. are you?

HiFi Engine say they have a manual but you need to register.
Dave.
 
You can get it straight from the source here:
ftp://ftp.studer.ch/Public/Products/Recording_Analog/A807_MkI/Manuals/A807_MkI_Op_Serv_E.pdf

EDIT: Yeah, I would assume that the bias oscillator is working OK for now, since it would affect both channels identically if it went wrong.

Here's a simple thing to test. On each channel card, there's a blue adjustment pot (R139) to set the erase level. Have you tried adjusting it slightly, just in case the pot has gone scratchy or something?
 
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