Hi all,
I have a tape machine that I suspect has a noisy sel-sync preamp. What sounds like white noise is happening when I engage the sync function on the transport.
(Sel-sync allows the record head to act as a play head.)
The machine is an Ampex ATR104. I've eliminated the possibility of the head, firstly because it happens without any transport function (i.e., tape is stopped) and secondly, I swapped out the channel's cards for another and the problem is following the card. Card in question is the Main Audio
How would I go troubleshooting this circuit? I was hoping to signal trace to find the noise. However, the signal is noisy starting right at the input and the way through the output of the preamp.
Any advice/tips?
Some links to give some hints:
Schematic (pdf page: 34, manual page: 6-43/44)
Theory of Operation (pdf page: 52, manual page: 4-48)
Best,
Chris
I have a tape machine that I suspect has a noisy sel-sync preamp. What sounds like white noise is happening when I engage the sync function on the transport.
(Sel-sync allows the record head to act as a play head.)
The machine is an Ampex ATR104. I've eliminated the possibility of the head, firstly because it happens without any transport function (i.e., tape is stopped) and secondly, I swapped out the channel's cards for another and the problem is following the card. Card in question is the Main Audio
How would I go troubleshooting this circuit? I was hoping to signal trace to find the noise. However, the signal is noisy starting right at the input and the way through the output of the preamp.
Any advice/tips?
Some links to give some hints:
Schematic (pdf page: 34, manual page: 6-43/44)
Theory of Operation (pdf page: 52, manual page: 4-48)
Best,
Chris
Hi,
Yes I do. Maybe I am uncertain how to use?
At output, when I try to scope, it adds a huge pop to the signal. My VU meter for that channel then goes crazy. The needle is hitting the peg so hard, it sounds like it's going to break!
Same on input.
Where do I start? It appears the whole preamp is like a sealed "black box" (can't penetrate upstream before the sound, if that makes sense). Any way to isolate parts of the circuit?
Yes I do. Maybe I am uncertain how to use?
At output, when I try to scope, it adds a huge pop to the signal. My VU meter for that channel then goes crazy. The needle is hitting the peg so hard, it sounds like it's going to break!
Same on input.
Where do I start? It appears the whole preamp is like a sealed "black box" (can't penetrate upstream before the sound, if that makes sense). Any way to isolate parts of the circuit?
Well one does not get their first lesson with an oscilloscope from a live circuit. There's just too much to go wrong with that. Moreover, one does not become proficient with one in 10 minutes...
To be honest.... At this point I would suggest soliciting the help of an experienced technician to help you troubleshoot your problem.
These machines are not for the faint hearted.
The section has very high gain and a multimeter/scope probe at the input would get picked up.
Only way to poke around is short the Input Hi at P1 pin 6 to P1 pin 5 (which I think shorts out the head sel-sync winding). That should leave only amplifier noise in the circuit. If you want to formally troubleshoot the section. Or guess at a few parts.
Check TP3 for the DC voltage there or with a scope too. Estimate +7.5VDC
I would first suspect shorted C28 330uF 6V, C31 47uF 6V, they are getting old. C13 is the only other cap that should also be changed for good measure.
I'm assuming A7 is driving mute switch Q6 properly. If somebody used too much B-field to demagnetize the head, Q5 Q6 can get damaged but maybe not if the sel-sync gets switched out when stopped. I'm not sure.
Another way to troubleshoot a (semiconductor) noise problem is using freeze spray on parts. It would not flush out a shorted cap though.
The op-amp might be noisy too.
The section has very high gain and a multimeter/scope probe at the input would get picked up.
Only way to poke around is short the Input Hi at P1 pin 6 to P1 pin 5 (which I think shorts out the head sel-sync winding). That should leave only amplifier noise in the circuit. If you want to formally troubleshoot the section. Or guess at a few parts.
Check TP3 for the DC voltage there or with a scope too. Estimate +7.5VDC
I would first suspect shorted C28 330uF 6V, C31 47uF 6V, they are getting old. C13 is the only other cap that should also be changed for good measure.
I'm assuming A7 is driving mute switch Q6 properly. If somebody used too much B-field to demagnetize the head, Q5 Q6 can get damaged but maybe not if the sel-sync gets switched out when stopped. I'm not sure.
Another way to troubleshoot a (semiconductor) noise problem is using freeze spray on parts. It would not flush out a shorted cap though.
The op-amp might be noisy too.
This is why I recommended that you get an experienced technician to look at it in person... I doubt you're going to fix this online and certainly not by shorting things out.
Wow!! Working on a $5,000 tape recorder with little electronics experience? You must be brave, or .........
These old machines DEFINITELY need a full recap of all electrolytics, as they are ~ 40 years old now. If you still have sel-sync noise problems after recapping, A4 and/or Q5 would be the most-likely suspects.
These old machines DEFINITELY need a full recap of all electrolytics, as they are ~ 40 years old now. If you still have sel-sync noise problems after recapping, A4 and/or Q5 would be the most-likely suspects.
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