Recapping power supply for Soundcraft 760 series 24 track tape machine

[skaren]

Good afternoon to you all.

My Soundcraft 760 series 24 track tape machine is having an increasing amount of noisy channels. (15 useable channels)
Recapping/swapping around individual channel cards makes no difference and neither does changing the tape itself. The advice I have been given is that the most likely culprit is the power supply which could do with a recap.

Has anyone here, who is in the UK. worked on these before and would be up for a recap job?

Many thanks!

 

[PRR]

I do not know the machine.

Doesn't one power supply feed all channels?

So it sounds like BOTH main supply and per-channel filtering.

Unless it is just dirty connectors....

 

[gar381]

I worked on one of these guys around 1980 or so in my Soundcraft
warranty days.  I thought I still had a full shop manual in the archives
Haven't found it yet but I'll keep looking.  As I remember  this guy was
very modular and easy to work on for being in such a small package.

I did find some poop sheet info however for those not familiar
with the 760 .

GARY

 

[gar381]

Soundcraft 760  2" Page1

 

[gar381]

Soundcraft 760 2" Page2

 

[gar381]

Soundcraft 760 1"

 

[skaren]

Thanks for your replies.

As far as I understand there are capacitors in the power supply that are there for unique channels.

I do not know the machine.

Doesn't one power supply feed all channels?

So it sounds like BOTH main supply and per-channel filtering.

Unless it is just dirty connectors....

 

[skaren]

Thanks for posting those sheets. Interesting reading.

It is very modular indeed. The whole power supply comes out by unplugging a couple of looms and loosening a few screws.
My idea was to ship the whole power supply to whoever we end up getting to do the recap job.

Many thanks

I worked on one of these guys around 1980 or so in my Soundcraft
warranty days.  I thought I still had a full shop manual in the archives
Haven't found it yet but I'll keep looking.  As I remember  this guy was
very modular and easy to work on for being in such a small package.

I did find some poop sheet info however for those not familiar
with the 760 .

GARY

 

[CJ]

a nylon bobbin was substituted for the layered paper approach, so the wire was wound all the way to the ends of the bobbin, the tape keeps the outer turns from digging down past the edge of the insulation,

 

[moamps]

Recapping/swapping around individual channel cards makes no difference and neither does changing the tape itself. The advice I have been given is that the most likely culprit is the power supply which could do with a recap.

If the power supply is faulty, the noise will be present on the all channels, IMO.
It looks to me as a problem with dirty or magnetised heads. Do you do regularly demagnetisaton of the heads and other metal parts?

 

[pvision]

Noisy how? On playback or Sync, or in record? What kind of noise? Do you get noise on the output with the tape stationary?

Is there any link between the bad tracks? Are they adjacent or alternate?

Nick Froome

 

[skaren]

I do use a demagnetiser but not before every session.

I should have mentioned earlier that the noise is only present when recording new tracks i.e. the playback is fine for tapes recorded on other machines.

I will upload a noise example shortly.

Many thanks

 

[skaren]

Noisy how? On playback or Sync, or in record? What kind of noise? Do you get noise on the output with the tape stationary?

Is there any link between the bad tracks? Are they adjacent or alternate?

Nick Froome

Here is a link to an example of a couple of channels with the noise and couple with just lower levels of normal tape noise

https://goo.gl/cGjZnz

This is a snap shot of 4 consecutive channels. For each channel I first sent a tone at about -3db from the desk, then unplugged the input to the tape machine to record the noise only and finally a couple of seconds of a track recorded from pro tools. You can hear the noise on the 2nd and 4th channels.

The noise is there on both the sync and playback heads when recording. There is no noise when the tape is stationary.

No noise when playing back old recordings hence playback seems to work fine.

Changing over channel cards from channels with less noise does not improve the situation
Recapping the noisy channels does not improve the situation

The heads have been demagnetized and cleaned. The patch bay through which the audio is sent to the tape machine and back has been cleaned.

Unfortunately what makes it harder to find the problem is that the noise has been intermittent on certain channels. This seems to rule out damaged record heads. Channel 1 and 24 definitely have worn heads and don't play back anything at all.

When presented with all this evidence the tech who previously helped with this machine suggested that recapping the power supply would be the obvious next step. He claimed that certain capacitors do their thing for individual channels rather then for all of the channels.

What are your thoughts? Any more information that could help?

 

[Rob Flinn]

As previously suggested if it only happens when you record on certain channels but not when you playback on those channels it would indicate that the problem is not the PSU.

 

[radardoug]

See my post on Gearslutz. It definitely sounds like a bias problem to me.
You do realise the Soundcraft is a POS? Thats why there arent many of them round. Even an MCI is a better machine!

 

[moamps]

What are your thoughts? Any more information that could help?

Could you check how the record head wires are connected to the main-board's back side? If there is a connector used (ie. the wires are not soldered directly to the main-board), check it for the dirt and corrosion.  Reconnect it few times.

 

[pvision]

If it only happens in record it narrows the field considerably

What happens when you punch a track into record? The erase head is activated and the sync head becomes a record head on the live track(s). Audio input is switched to the sync output so you hear what you're recording, not what's coming off tape

So there must be one or two relays per track… are they on the channel cards?

I used a few 760 24-tracks when they were new. Very slow transport and sludgy-sounding. An MTR90 would eat it for breakfast

Nick Froome

 

[skaren]

What are your thoughts? Any more information that could help?

Could you check how the record head wires are connected to the main-board's back side? If there is a connector used (ie. the wires are not soldered directly to the main-board), check it for the dirt and corrosion.  Reconnect it few times.

Thanks for your suggestion. I tried to reconnect a few times but it didn't help the noise. There were some dust as expected but they looked in decent condition.

 

[skaren]

If it only happens in record it narrows the field considerably

What happens when you punch a track into record? The erase head is activated and the sync head becomes a record head on the live track(s). Audio input is switched to the sync output so you hear what you're recording, not what's coming off tape

So there must be one or two relays per track… are they on the channel cards?

I used a few 760 24-tracks when they were new. Very slow transport and sludgy-sounding. An MTR90 would eat it for breakfast

Nick Froome

The sound is fine when monitoring what's going in to the tape machine but not going through the tape ("line" on the remote control) if that is what you mean. It's only when the signal is going on tape and then played back that the noise can be heard.

Here are some more recordings of the noise appearing on different channels.
https://goo.gl/UIRA6L
I want highlight the intermittent nature of the noise. For example when recording a bit of it on a channel and then rewinding and recording over the same bit again the noise is somethings gone just to return for the next pass.

I'm not sure what a relay looks like?

Many tape machines can be considered superior to this much maligned machine, for example we have a terrific A80 mark 2 24 track in another studio but the Soundcraft has this mid range studio 80s vibe that is suitable for the music that's being made on it currently. Not the best but 30ips. My bloody Valentine wouldn't be the same band if they used expensive effects units..

Thanks yuo all for your help and suggestions thus far!

 

[moamps]

I tried to reconnect a few times but it didn't help the noise. There were some dust as expected but they looked in decent condition.

Fine, could you now switch the second and third, and forth and fifth  head cables (connectors) and then record something so we can see is it noise for sure to the head channels related?