Revox A77 calibration. - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

NorfolkDaveUK · 5th Nov 2018, 5:30 pm

Ok so I thought I had it , I got the Bias setting for sure I went down 4db over the hump .

Then we move on to record levels and the difficulties I`m getting is becoming a bit clearer , there seems to be a fault somewhere on either the input or the output of this thing which is screwing with the settings , have a look at the images from the scope .

The first one (normal looking sine if i get them in the wrong order) is from channel one , The other one is from channel 2 and lord knows whats going on there , but something somewhere with this isnt at all right .

The input is 1khz , bridged across the input .

This is the recording output btw

I hope its not the heads !

Problem doesnt change sides by swapping the record cards , so its not them .

barrymagrec · 5th Nov 2018, 5:52 pm

When you say bridged across the input do you mean the two inputs are in parallel which would be normal - bridged infers that your generator is connected across the two inputs so driving each in anti-phase to the other.

Your second picture, assuming it is triggered properly, suggests you have an oscillation in the amplifier.

NorfolkDaveUK · 5th Nov 2018, 6:24 pm

I did it as per the manual (see pic) and can you explain what you mean buy osc in the amp please bud ? and what would cause it and what would fix it.

barrymagrec · 5th Nov 2018, 6:50 pm

Your scope picture suggests that the amplifier is oscillating at a high frequency and the audio signal is carrying this oscillation. - if you reduce the audio level you would still see the oscillation at the same level.

It can be caused by faulty decoupling caps on the power rails, possibly old dried out electrolytics.

barrymagrec · 5th Nov 2018, 6:54 pm

Having re-read your post you say you are looking at the recording output - do you mean the output of the record amplifier or the record head? If that is the output from thamplifier it could be bias feeding back through a faulty bias reject filter.

NorfolkDaveUK · 5th Nov 2018, 6:56 pm

Quote:

Originally Posted by barrymagrec

Your scope picture suggests that the amplifier is oscillating at a high frequency and the audio signal is carrying this oscillation. - if you reduce the audio level you would still see the oscillation at the same level.

It can be caused by faulty decoupling caps on the power rails, possibly old dried out electrolytics.

OK thanks I will recap the power supply and how it looks after that . I have recapped all of the audio boards I will need to get some caps in for the PSU i go through so many I`ve run out .

Would that not be happening both sides if it were down to the power ?

Thanks Barry !

NorfolkDaveUK · 5th Nov 2018, 6:59 pm

Quote:

Originally Posted by barrymagrec

Having re-read your post you say you are looking at the recording output - do you mean the output of the record amplifier or the record head? If that is the output from thamplifier it could be bias feeding back through a faulty bias reject filter.

Ahh ok , that sounds more like it , Yes its the output , with NAB select whilst recording .

I`ll pull the repro cards and have a look .

NorfolkDaveUK · 5th Nov 2018, 7:34 pm

Yeah you seem to be onto something bud . When i hook it up to my usb scope which has better read out but is way less accurate , the output is reading 120khz not the 1khz its supposed to0 read so I`m seeing the oscillator right ?

I guess my next queston is where is the 120khz filtered . Thats going to be the bias trap right ?

ms660 · 5th Nov 2018, 7:46 pm

Can we assume from the photo's that the channel 2 switch was selected for the 2nd photo?

Lawrence.

NorfolkDaveUK · 5th Nov 2018, 8:17 pm

Lawrence I`ve been trying different card and stuff so let me regroup , get it into a position I can describe and then I will leave it along and wait . At the moment I`m in a right old muddle ...thanks mate

NorfolkDaveUK · 5th Nov 2018, 8:26 pm

I know something is amiss thought because I`m recording a 1khz signal , but my scope is reading a 120khz signal , I`ve has this before and when I fixed it , the scope read 1khz again and I think it was something to do with the bias trap coils if my memory serves me last time , but its doing it on both cards this time and it the coils seem fine .

ms660 · 5th Nov 2018, 8:40 pm

I downloaded a copy manual, the reproduction quality's ok but there doesn't seem to be any circuit refs for the set up adjustments and no proper layout to indicate where they are that I can see at the moment....

Lawrence.

NorfolkDaveUK · 5th Nov 2018, 8:53 pm

Can you PM me an email address and I'll send you a copy of my manual it has all of that.

ms660 · 5th Nov 2018, 8:57 pm

I don't do emails, I can manage with the copy I have for following this thread, thanks for the offer anyways.

Lawrence.

NorfolkDaveUK · 5th Nov 2018, 9:13 pm

https://ufile.io/oaqac

This is the HiFi Engine Revox manual if that's not the one you have.

NorfolkDaveUK · 5th Nov 2018, 9:47 pm

OK I will have to do this over a few posts so first here is how I have it set up .....

Pic 1 , input
pic 2 , output read by scope
see next post

NorfolkDaveUK · 5th Nov 2018, 9:55 pm

Next is what I see on the scope. This is the OUTPUT whist recording.

Pic 1, channel 1 trimmer pot turned completely down.
Pic 2, channel 1 trimmer pot turned completely up.
Pic 3, channel 2 trimmer pot turned completely down.
Pic 4, channel 2 trimmer pot turned completely up.

Hope that makes sense.

The oscillation seems to have disappeared, however as you can see the levels are all over the place. One is just about right with the trimmer turned all the way down and one won't go high enough.

NorfolkDaveUK · 5th Nov 2018, 10:17 pm

Oh and this is what I'm reading on the output with input selected as the source, 580mV ish peak to peak (around 200mV RMS), it's ever so slightly high but close enough. It's jumping between 560 and 588. I'm wondering now if the record head is kaput, because I'm getting workable levels on channel one but very low levels on channel two.

NorfolkDaveUK · 5th Nov 2018, 11:33 pm

Measuring the voltages I get the following , ...see the pic its easier than describing what i`m getting where ....

So as you can see in the pic there is a 100mv difference from the top coil to the bottom coil . This has to be the reason for the discrepancy doesn't it ? Tomorrow I will recap the power supply and get back to it . Could be the record relay is borked .

Thanks everyone you know I always appreciate your help .

David

ms660 · 6th Nov 2018, 12:25 am

I would sort out the channel amplifier differences first.

Lawrence.

5th Nov 2018, 5:30 pm	#21
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. Ok so I thought I had it , I got the Bias setting for sure I went down 4db over the hump . Then we move on to record levels and the difficulties I`m getting is becoming a bit clearer , there seems to be a fault somewhere on either the input or the output of this thing which is screwing with the settings , have a look at the images from the scope . The first one (normal looking sine if i get them in the wrong order) is from channel one , The other one is from channel 2 and lord knows whats going on there , but something somewhere with this isnt at all right . The input is 1khz , bridged across the input . This is the recording output btw I hope its not the heads ! Problem doesnt change sides by swapping the record cards , so its not them . Attached Thumbnails Last edited by NorfolkDaveUK; 5th Nov 2018 at 5:47 pm.

5th Nov 2018, 5:52 pm	#22
barrymagrec Octode Join Date: Dec 2017 Location: Morden, Surrey, UK. Posts: 1,557	Re: Revox A77 calibration. When you say bridged across the input do you mean the two inputs are in parallel which would be normal - bridged infers that your generator is connected across the two inputs so driving each in anti-phase to the other. Your second picture, assuming it is triggered properly, suggests you have an oscillation in the amplifier.

5th Nov 2018, 6:24 pm	#23
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. I did it as per the manual (see pic) and can you explain what you mean buy osc in the amp please bud ? and what would cause it and what would fix it. Attached Thumbnails

5th Nov 2018, 6:50 pm	#24
barrymagrec Octode Join Date: Dec 2017 Location: Morden, Surrey, UK. Posts: 1,557	Re: Revox A77 calibration. Your scope picture suggests that the amplifier is oscillating at a high frequency and the audio signal is carrying this oscillation. - if you reduce the audio level you would still see the oscillation at the same level. It can be caused by faulty decoupling caps on the power rails, possibly old dried out electrolytics.

5th Nov 2018, 6:54 pm	#25
barrymagrec Octode Join Date: Dec 2017 Location: Morden, Surrey, UK. Posts: 1,557	Re: Revox A77 calibration. Having re-read your post you say you are looking at the recording output - do you mean the output of the record amplifier or the record head? If that is the output from thamplifier it could be bias feeding back through a faulty bias reject filter.

5th Nov 2018, 7:34 pm	#28
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. Yeah you seem to be onto something bud . When i hook it up to my usb scope which has better read out but is way less accurate , the output is reading 120khz not the 1khz its supposed to0 read so I`m seeing the oscillator right ? I guess my next queston is where is the 120khz filtered . Thats going to be the bias trap right ? Last edited by NorfolkDaveUK; 5th Nov 2018 at 7:42 pm.

5th Nov 2018, 7:46 pm	#29
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Revox A77 calibration. Can we assume from the photo's that the channel 2 switch was selected for the 2nd photo? Lawrence.

5th Nov 2018, 8:17 pm	#30
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. Lawrence I`ve been trying different card and stuff so let me regroup , get it into a position I can describe and then I will leave it along and wait . At the moment I`m in a right old muddle ...thanks mate

5th Nov 2018, 8:26 pm	#31
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. I know something is amiss thought because I`m recording a 1khz signal , but my scope is reading a 120khz signal , I`ve has this before and when I fixed it , the scope read 1khz again and I think it was something to do with the bias trap coils if my memory serves me last time , but its doing it on both cards this time and it the coils seem fine .

5th Nov 2018, 8:40 pm	#32
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Revox A77 calibration. I downloaded a copy manual, the reproduction quality's ok but there doesn't seem to be any circuit refs for the set up adjustments and no proper layout to indicate where they are that I can see at the moment.... Lawrence.

5th Nov 2018, 8:53 pm	#33
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. Can you PM me an email address and I'll send you a copy of my manual it has all of that.

5th Nov 2018, 8:57 pm	#34
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Revox A77 calibration. I don't do emails, I can manage with the copy I have for following this thread, thanks for the offer anyways. Lawrence.

5th Nov 2018, 9:13 pm	#35
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. https://ufile.io/oaqac This is the HiFi Engine Revox manual if that's not the one you have.

5th Nov 2018, 9:47 pm	#36
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. OK I will have to do this over a few posts so first here is how I have it set up ..... Pic 1 , input pic 2 , output read by scope see next post Attached Thumbnails

5th Nov 2018, 9:55 pm	#37
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. Next is what I see on the scope. This is the OUTPUT whist recording. Pic 1, channel 1 trimmer pot turned completely down. Pic 2, channel 1 trimmer pot turned completely up. Pic 3, channel 2 trimmer pot turned completely down. Pic 4, channel 2 trimmer pot turned completely up. Hope that makes sense. The oscillation seems to have disappeared, however as you can see the levels are all over the place. One is just about right with the trimmer turned all the way down and one won't go high enough. Attached Thumbnails

5th Nov 2018, 10:17 pm	#38
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. Oh and this is what I'm reading on the output with input selected as the source, 580mV ish peak to peak (around 200mV RMS), it's ever so slightly high but close enough. It's jumping between 560 and 588. I'm wondering now if the record head is kaput, because I'm getting workable levels on channel one but very low levels on channel two. Attached Thumbnails

5th Nov 2018, 11:33 pm	#39
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. Measuring the voltages I get the following , ...see the pic its easier than describing what i`m getting where .... So as you can see in the pic there is a 100mv difference from the top coil to the bottom coil . This has to be the reason for the discrepancy doesn't it ? Tomorrow I will recap the power supply and get back to it . Could be the record relay is borked . Thanks everyone you know I always appreciate your help . David Attached Thumbnails Last edited by NorfolkDaveUK; 5th Nov 2018 at 11:49 pm. Reason: Add pic

6th Nov 2018, 12:25 am	#40
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Revox A77 calibration. I would sort out the channel amplifier differences first. Lawrence.