UHF CB Repeater Update

[VK3YNV]

UHF CB Repeater update. The repeater locked up with a transmit timeout yesterday afternoon, cycling the power brought the repeater back on line but continuously un-muted and just kept going into transmit timeout. Kevin (VK3KFM), Josh and myself went up to Mt Wombat today to investigate and also to replace the main backup batteries, ( the failed battery was venting acid fumes, so that's another story)... back to the UHF CB receiver, it kept seeing receive signal even with the Rx antenna unplugged and the mute wound fully up. So the system has been removed from service to investigate further. Lots of marsupial mouse droppings, makes for a fun repair.

More to come later when we figure out what's holding the 828 mute open.

 

[VK3ZYZ]

maybe it was calling for help to clean the mouse house out?

 

[VK3YNV]

Turns out the front panel mute adjustment pot had gone open cct. Tapping it returned the mute to normal operation.
Easy fix, replace the pot.

 

[VK3YNV]

maybe it was calling for help to clean the mouse house out?

The mouse squatters definately made themselves at home.

 

[VK3YNV]

Quick test on the tuning, 0.3-0.4 uV Rx, 38W Tx freq
Tx Frequency 476.47542 MHz 0.48 kHz low adjusted up to 476.47500
Generally everthing looks good, Philips FM828's are very solid reliable radios.

I'll drop the deviation on the ident and tail beep.

 

[BillC]

Might have had a bit of corrosion on the carbon track or commutator wiper of the mute pot - cold moist mountain air possible condensation, I have seen car radio volume control pots sealed at the terminal to body joint, if I remember surgical tape was placed so as to seal the opening in the pot body , could that wok in this case. Just an idea. Cheers.

 

[VK3YNV]

Good idea bill, you are on the right track. The fault was more likely a shorted pot, rather than open, and there was corrosion around the terminals, but once it was disturbed it was impossible to tell if that was the root cause. The replacement pot from jaycar was a different shaft diameter and I cut the old shaft off and filed it to make a tang that fitted into the slot on the new pot. Worked out to be a nice and clean replacement. A bit of superglue and it's nice and solid.

I did search on line for a replacement but the only pot with the correct mechanical dimensions was marked as obsolete. The original pot shaft was a 3/16 diameter, made by soanar according to the marking.

The replacement is a fully sealed type, like this.

 

[BillC]

Hi Ray a fully sealed pot ! something to be said for modern design, it looks really good. Thanks for the discussion Ray, it should be a very successful repair, all the best.

 

[VK3YNV]

I did a quick check up and adjustment of the audio levels, the deviation out is adjusted to be equal to the deviation in, so if your incoming audio is 2 kHz deviation the outgoing audio will be 2 kHz deviation not withstanding the audio level processing that goes on the 828 receive/transmit chain. End to end what goes in is what comes out.

The ident beep and roger beep levels have been decreased to 800 hz deviation, so the beeps will sound softer than previously.

 

[VK3YNV]

Back on line today, no problems. Should be good for the Merrigum car show this Sunday.