SBS FM25

[ehsanspicedigital]

Hi lads, Ive got here a SBS FM25 Transmitter that has been out of service for a number of years.

Ive been mainly using my TX-FM1 as my transmitter for FM, I turned the SBS on after a long time after following a little visual inspection to make sure there wasn't any leaky caps.

I changed the freq. and turned the TX on, RFL power under 200mw on about 29w out, there's a problem though, the transmitter will randomly shut down for a few seconds and then come back on-air. the three indicators on the front, FP, RP and PLL Lock will go from green to red, for about 7 seconds, and then come back on, I thought I'd ask if anyone knows what the issue could be before i go probing around, as someone might have experienced an issue like this before. Its a nice TX, I was planning to use it for the RSL I'm running at the moment, but after the signal start dropping out, I pulled it out of the rack and put the TX-FM1 straight back in.

If you do know what the problem is, or where I should start troubleshooting, then please do let me know.

Thanks Again, Hope everyone is keeping well.

[EFR]

I dont have SBS manual anymore, but you need to tune it after you set frequency, PLL wont stay locked without, and also PA stage needs to be tuned.

[Frequent Lee]

You will need to run it into a dummy load and an accurate RF power meter to properly recalibrate the RF circuit to get the output matched to the new frequency you've selected or the pll won't lock correctly if it senses that there's a mismatch somewhere in the RF path.

What frequency have you changed it from and to?

[ehsanspicedigital]

It was tuned to 102.1, as it was being used for RSL's in Keighley, and I set it to 87.7, Ill have to have a look at the SBS Manual as the last time I tuned it was years and years ago..

[Albert H]

If it's the early FM25, it'll have a BGY33 or BGY133 RF power module, which is broadband and doesn't require any tuning. The module looks like this:

BGY33.jpg

In their day, they were a great solution for 15 to 25 Watts, and didn't need much drive. They were designed for mobile rigs at ~12V, and were slightly biased into conduction (not Class C) and needed a lot of heatsink!