Loss of some channels on TX.

[QRZ-934]

As some of you may know I like 934mhz radios, ive got a few of them and have been experimenting with them for a while now.

This issue was noted some time ago but not really gotten into, and it concerns the stabo prs type radios that can do 933 and 934mhz.

Powering them up I figured out how to get the things to connect using the keycodes on the prs mode.
The radios do what theyre supposed to on low band 933 and hi band 934.
For whatever reason the one will not tx on the correct CB channels as follows.

01-Data channel cant be used for voice, is normal.
02 tx and rx ok.
03. tx and rx ok.
o4. tx and rx ok.
05. tx and rx ok.
06. tx and rx ok.
07. tx and rx ok.
08. tx and rx ok.
09. tx sounds off freq, noisy signal to receiving radio.
10. noisy tx no audio readable.
11. Noisy and pulsing audio.
12. tx and rx ok.
13. tx and rx ok.
14-19 no tx to receiving radio, power is being output on the wattmeter however.
20. tx and rx ok.
21-40 no tx to receiving radio.

Note, the radio receives on all the CB channels but as above just doesnt tx on them all.
Obviously its a higher freq than 27mhz but internally it all looks to be quite similar and id expect a similar kind of issue to be the cause if it was on the usual 27mhz bands...as it stands I dont know what it might be.....

Cpu is a upd7508G-784-00.

Something of interest in the spec sheet however thats got me scratching my head.
in the channel assignments section of the manual its specifies that say for channel 1, the tx frequency is 934.0125, the rx frequency is specced as being 992.125mhz....???
At the bottom of the page it states: " RX frequency =TX frequency +58.1125mhz".

I dont understand that at all..... anyone else have a clue?

Any help appreciated.

[Alucard]

I haven't worked on any 934 radios but I would say check the vco is in lock and that the right n-codes are getting to the PLL.
I hope someone with knowledge of these radios can jump in but I'll try to help if no one does.

[ChubbyChops]

A schematic would help if anyone has one?

[Alucard]

Evening Chubbster, we meet again on a possible VCO related incident

Might help to know what the radio model is. I've actually got a 934 somewhere in the loft with no transmit I think it's a Reftec, it has a black front anyway. Dunno.
Edit oh It's a Stabo. Sorry must try to read everything before I post, it's a problem I have
Model number would help me search for a schematic tho.

[ChubbyChops]

Evening fella.
It's good to talk and bounce ideas around.
Even if no direct answer can be given, the chat sometimes help point you in the right direction

[Alucard]

Very true sensei.
On 27Mhz Cb where most channels are working but some in between were not I'd suspect wrong n-codes reaching the PLL either from a dirty contact on the channel selector or dry joints on the PLL..

[ChubbyChops]

Roger that.
Dry joints on channel selector board are another favourite too.

[QRZ-934]

Thanks for the replies guys.

Ok your luck is in, I have the schematic, the service manual and the layouts.
Ive uploaded them here: https://ufile.io/f/luago

Theyll be there for 30 days apparently before theyre deleted so get a copy asap.

Hopefully one of the more technically savvy amongst you might be able to figure this issue somewhat I dont really know where to start with it and my days of "screwdrivering" are long over!!
I dont jump in willy nilly any longer, Ive "matured"...some say Im old tho..

[Alucard]

I downloaded it all, couldn't be bothered to read it all cos headache.
I did note that pin 11 of the upd7508 is the lock detect signal , so the first job is to meter that pin on all channels on receive and transmit to check that the pin remains high and the VCO is locked on all channels.
I'll check back in here later if my headache doesn't get any worse

[ChubbyChops]

The service manual gives a very detailed alignment procedure and you must go through this first to see if it can be aligned before we can move on to any potential fault rectification.
It may not be faulty at all and just in need of a tune up. If the VCO is out, it won't lock on all channels for TX and RX so start there as my learned friend has mentioned above.

[QRZ-934]

Ok guys, thanks for casting an eye over it and offering suggestions.
Havent got much in the way of test gear, decent dvom, wattmeter and a couple of other bits and bobs.
Brothers got a decent scope I can borrow and may have to get a couple of other instruments to follow it as closely as I can.

Will report back when Ive gotten a little further along.
Thanks again.

[ChubbyChops]

DVM is all you need to set VCO.

[Alucard]

my learned friend

Lol are you a barrister Chubbs?

If I may put it to the court, that while servicing the radio is generally a good idea, to do so whilst it is not operating on all the correct frequencies is pure folly. As the honourable fellow suggests get in there with your voltmeter

[ChubbyChops]

Lol are you a barrister Chubbs?

Just a lover of wigs

[Alucard]

And maybe fine lingerie?