Uniden UST-771 Positioner - Warning

[s-band]

These excellent positioners are only 32 years old and still much better than non-professional modern ones. However, maybe age has caught up. For some time the positioner would go out of synch occasionally. Recently it would not move west and did not count pulse when moving east. It also refused to power up a couple of times.

Checking voltages they did not quite make sense. The design relies on screws connecting the PCB ground areas to each other via the chassis. The screws had become loose. There is no direct ground connection between the PSU board and the main board except via the chassis.The ground area on the PCB to the top right is only connected to the rest of the board via the screw. That's the ground for the pulse input as well. If that screw doesn't make contact, the sensor ground isn't made. That may have been the cause of random mis-counts. The other ground failures must have caused loss of west movement. I'm surprised nothing blew up with the motor voltage possibly ending up where it shouldn't. Tightening the screws and adding the black wires fixed that problem. I checked a couple of spare 771s and both had loose screws.

The failure to switch on was due to the low voltage fuse holder falling apart. This was discovered when probing the voltages.

If you have one of these, I strongly recommend tightening all the ground screws, including those holding earth tags. I think it’s also a good idea to add direct connections between grounds as shown.

 

[Channel Hopper]

There was a modification (beyond the usual cutting of a couple of tracks) offered by Uniden when these positioners were being sold (1991-93), I'll have to open the one(s) here to see if they have had them.

Then there is a lithium battery from memory.

 

[s-band]

Thanks, CH. Here are pics of the other 2. I think I've got them in age order. The latest has a much higher s/n than the one in the previous post, the 'middle' one doesn't have a s/n. I didn't look beyond extra components but the latest one seems to have no mods apart from the ones I did.

My original came from Micro-X. I thought I got it in 1989/90 which sounds too early from your dates. The others were from Ebay.

The Li batteries only lasted about 20 years.

 

[Channel Hopper]

You might be correct. I was involved with Uniden once the 7007 and 771 issues came to light after many were out in the field (optical coupling/2050 wide band modifications/D/D2MAC raw output and subscriber entitlement, and - horror - other decoder loopthrough)

Can you provide serial numbers ?

Ill photograph what I have here over the weekend, mine are from the later batches

 

[s-band]

Original, most mods, 85000xxx
Latest , no mods, 95017xxx

 

[Channel Hopper]

I found two positioners in the lockup, though I know I have an earlier one somewhere.

Pictures of the two mods, both cut track to processor (to reduce Lithium cell current drain), one with diodes, one without.

 

[s-band]

Thanks, that's interesting. It looks like the mods on my early one (85xxc) were tidied up in later ones. My latest one has no battery mod but the one with diodes is similar to one of yours. The one with diodes still has its original battery.

The one I use most is the early one that was the subject of the 1st post. It's now working fine. Looking back, it had been getting worse over the past year or so but I had put it down to the pulse rate from a new jack being too high.

 

[Channel Hopper]

I cannot see the track cut on your diode modded one which is odd.

 

[s-band]

I cannot see the track cut on your diode modded one which is odd.

It's not very clear but it has been cut.