Studer A807 - undesirable echo

[anderssvensson]

I have for some time used tape recorders for mixing, I'm recording digitally to hard drive and when I start mixing, I record one (stereo/mono) channel to one of my tape recorders and at the same time I record the tape signal to the hard disk.

The signal goes first to the recording head which record it to tape, and the recorded signal is at the same time played back on the reproducer head but with about a half second delay - this is the signal I'm recording back to the hard drive.

This method has worked great on my two other tape recorders. A while ago I bought a Studer A807 - but it was strange. On channel one everything was like it should be, but on channel two I got an echo - two signals. The tape recorder should only reproduce the signal from the tape, but it reproduces the signal from the tape AND the original input signal - this constitutes in a delay at about half a second because it is the time between the record and the reproducer head. For hours I have tried to find the solution in the A807 manual, but I cannot find what it is.

Does anyone know why it behave in this way?

 

[Tim Gillett]

The fact that the problem is only on channel 2 suggests that there is a fault in channel 2 of your Studer.

Possibly caused by weak capacitors at some point, allowing the record signal to modulate the power supply rails and in turn the play signal chain somewhere.

Is the machine in good aligned condition? Does what goes in come out at the same level through playback? Freedom from noise and distortion etc?

Cheers Tim

 

[jpmorris]

I have the console version so I don't know the control layout for the others, but are you quite sure you haven't got it monitoring sync on that one channel or something?

That aside, yes, recapping the audio cards is a good idea, especially if it's a MK1. Curiously the capacitors in mine were in a worse state on channel 2 than on channel 1.

 

[anderssvensson]

Is the machine in good aligned condition? Does what goes in come out at the same level through playback? Freedom from noise and distortion etc?

Cheers Tim

I believe that the A807 is in good condition, I can record a 1 kHz sine wave to the tape recorder and in a spectrogram ensure that no harmonics are created. Compared to my former Studer A810, I think it has clear sound and a quite low noise floor.

The fact is that there is also an echo on channel 1 - but it's there on the noise floor level only seen on the VU meter in the DAW.

I have the console version so I don't know the control layout for the others, but are you quite sure you haven't got it monitoring sync on that one channel or something?
.

I don't have the sync button on the front of my A807 - I don't have any VU meters or buttons. But maybe there are switches inside? (I think I tried to find this in the manual before..)

That aside, yes, recapping the audio cards is a good idea, especially if it's a MK1. Curiously the capacitors in mine were in a worse state on channel 2 than on channel 1.

Hmm.. you say .. the audio cards. That means I can switch them inside, and see if the echo on the channels switches? If this happens, it means that I have found the problem.

 

[jpmorris]

I don't have the sync button on the front of my A807 - I don't have any VU meters or buttons. But maybe there are switches inside? (I think I tried to find this in the manual before..)

Like this one?

https://i89.photobucket.com/albums/k216/tapewolf/tape/RadioProject2009.jpg

When you power the machine up, what is the OS build it displays in the tape counter?

Hmm.. you say .. the audio cards. That means I can switch them inside, and see if the echo on the channels switches? If this happens, it means that I have found the problem.

Pretty much. It's not trivial, but it can be done. The audio signals are carried by external jumpers in a cable loom rather than the PCB, and there are about 5 such connectors. I believe they are all keyed, but if you get confused about which one belongs to which channel, you're going to have some rather strange effects. When I first did this I got the main output connectors swapped over.

As a rule the connectors have labels on them, however the glue has probably dried out by now, I lost a few and had to stick them back on or write the designation back on with a CD pen.

To get at the cards, use a hex driver to remove the screws from the underside of the machine. You'll see a bronze-coloured chassis with two little tabs - if you pull both tabs it will unlock and the chassis will slide out providing access to the cards.

To remove one, look for the two bolts at each end of the card - you lever these out with a screwdriver, like this:

ftp://ftp.studer.ch/Public/Products.../Technical_Info/A807_Audio_El_PCB_Removal.pdf

However, from what you're describing it's a pretty good chance that the capacitors are on the way out.

 

[anderssvensson]

Like this one?

https://i89.photobucket.com/albums/k216/tapewolf/tape/RadioProject2009.jpg

Yes! But without that homemade VU meter on the top! Some buttons are not the same.

To get at the cards, use a hex driver to remove the screws from the underside of the machine. You'll see a bronze-coloured chassis with two little tabs - if you pull both tabs it will unlock and the chassis will slide out providing access to the cards.

It went great. I have now switched the cards - and the echo switched channels. The echo is now on channel 1 instead of channel 2.

However, from what you're describing it's a pretty good chance that the capacitors are on the way out.

Are you talking about all capacitors or are you thinking of a special capacitor? There are quite many of them!

I looked at Ebay for new cards and they are more expensive than the A807 itself! I guess replacing the capacitors is the way to go..

 

[jpmorris]

Yes! But without that homemade VU meter on the top! Some buttons are not the same.

Yeah, there's a lot of variants. I'd still be interested to know what the OS version is - this is displayed for a second or two on the tape counter when the machine is switched on.

Are you talking about all capacitors or are you thinking of a special capacitor? There are quite many of them!

I know - ideally you should really replace all the electrolytic capacitors on both channels, though I admit I haven't. On mine I replaced most of the 22pF and 47uF capacitors in the playback circuit, which cured the problem I had (very low playback level on one channel). It is a tedious job, though.

There is a way to check - if you have a meter that can read capacitance, check some of the values first. On channel 1 mine were a bit down but roughly acceptable. On channel 2 most of them were around half their printed value!

If you do it yourself, as opposed to finding someone willing to do it for money, do be aware that the electrolytics must go back in the same polarity as the originals. If you get it backwards it could be a problem depending on what the circuit is doing. Worse, they didn't print the polarity on the circuit board. I went through all of it with a CD marker pen first, marking the negative side on all the caps I was planning to replace.

 

[anderssvensson]

Yeah, there's a lot of variants. I'd still be interested to know what the OS version is - this is displayed for a second or two on the tape counter when the machine is switched on.
.

Sorry, missed that; 10.89

I know - ideally you should really replace all the electrolytic capacitors on both channels, though I admit I haven't. On mine I replaced most of the 22pF and 47uF capacitors in the playback circuit, which cured the problem I had (very low playback level on one channel). It is a tedious job, though.

There's a metal cover in the middle of the card, is this the place where the playback circuit is?

At it's side, the metal cover is holding four transistors, bd135, which I think are output transistors; I didn't really understand how to remove the cover without damaging those components.

If you do it yourself, as opposed to finding someone willing to do it for money, do be aware that the electrolytics must go back in the same polarity as the originals. If you get it backwards it could be a problem depending on what the circuit is doing. Worse, they didn't print the polarity on the circuit board. I went through all of it with a CD marker pen first, marking the negative side on all the caps I was planning to replace.

For me it seems like all electrolytic capacitors shows polarity, either at the side or the top.

First I'm going to order a capacitance meter, then I'm going to order components. I will see if there are any component information in the manual, otherwise it seems all information can be found looking at the components.

 

[jpmorris]

Sorry, missed that; 10.89
There's a metal cover in the middle of the card, is this the place where the playback circuit is?

So, it's around 1989-1990 build, just a bit newer than mine.

Holding the card component-side up and with the connector pins at the bottom, the left side should be the the playback circuit, the erase circuit is in the middle and record is on the right. As I recall, anyway.

At it's side, the metal cover is holding four transistors, bd135, which I think are output transistors; I didn't really understand how to remove the cover without damaging those components.

I left that part alone, I wasn't sure how to open it either.

For me it seems like all electrolytic capacitors shows polarity, either at the side or the top.

Yes, but once you've removed it, you need to know which way the new one has to go in. Most PCBs have this silk-screened onto them, this one didn't that I could see. So I had to mark the board.

First I'm going to order a capacitance meter, then I'm going to order components. I will see if there are any component information in the manual, otherwise it seems all information can be found looking at the components.

Good luck, let us know how it goes.