Nutting associates 1972 desert fox sound.

[Lurch666]

I'm currently repairing a desert fox from 1972 and am having problems with the sound.
It uses something called the simulator system which is a set of PCBs with a transformer and one PCB looks like an amp.
I'm getting no sound at all-not even a speaker buzz.
Just wondering if anyone here has any experience of this sort of old sound system who could give me an idea on where to start.

 

[rod90]

Re-cap would be a good start.

 

[andrewb]

Get an ESR meter and test the caps, before you go shotgunning stuff. It'll be good to know if any of the ones you're replacing are actually bad. (They very well might be, but it's good to know instead of guessing, as you may have other issues as well with the boards.)

 

[ArcadeTechGW]

I see a lot of general corrosion. Start by cleaning the edge connectors. I had a similar box on a Sharpshooter (Williams) - not the same. Cleaning the edge connectors and then wiping the pots got it working.

Find your fiberglass pen. Buff them to a high luster.

Once there, it's the usual drill. Check the transformer - does it have voltage in? Does it have voltage out? Next, the rectifiers - AC in - DC Out or nothing?

This will be fun. For many members, I'd be worried, but not with you.

 

[Lurch666]

I'm guessing some caps would need replacing but I've never bothered with an esr since you have to remove a cap to test and once removed it might as well be replaced anyway.
I think I remember testing the voltages (I've been working on this machine for a while and put the sound issue to one side while I fixed the rest of the machine) but will recheck.
I cleaned the edge connectors on the PCBs for this sound module but haven't cleaned the sockets they plug into since it's hard to get to them. Will check for continuity just to make sure.
Was hoping someone might either have schematics or an idea of which caps would be most likely to be stopping the amp from working.
There's a single chip on the amp board but it's covered with a heat sink so will have to be removed before I have a chance to identify it.
If continuity and voltage is good I'll start replacing the caps on the amp board. No point in moving onto the other sound boards until I at least get speaker hum.

 

[Lurch666]

So this is the amp PCB.
It's single sided so I took the picture in front of a light to show the tracks. I also filled in some gaps where a capacitor and pot have blocked the view.
There's a 1000 mfd capacitor between A and B with + at A but I removed it to show the tracks better.
I've marked where the voltages from the transformer go in (I have verified the voltages are present but have no way of knowing if they are the correct voltages) and by the looks of it those voltages are being fed into a full rectifier but I'm not seeing any voltage at what appears to me to be the output. That pin connects to all the other sound PCBs so I'm assuming it should be powering everything. No voltage output might be the reason this is dead.

I have tried to check the diodes in circuit but am getting nothing.
May look into replacing them all if I can find modern replacements.

 

[Lurch666]

Looked through my spares and found a set of high power rectifier diodes so decided to try them to see if they would work.
De-soldering the old diodes I found they were all just beads held in place by the wires-as soon as the wires were de-soldered the beads just fell off so it looks like that's why I was seeing nothing when I tested them.
Put the new diodes in and now I appear to have full sound FX.
Nearly done with this now apart from replacing all the dead bulbs and putting everything back into the cab.

 

[rod90]

Good job. A good look over before jumping can pay dividends.

 

[Lurch666]

Correct.
Thanks for all the suggestions everyone.