Batboard

I have an M120 (VHF) that has bad RX. I have to turn my generator (IFR1200S) all the way up to get any RX, as soon as I start to back it down, the audio just goes dead on the radio. It doesn't seem to get scratchy at all, just completely dead without full signal. FYI, the radio was the TX unit in a repeater setup, so it probably was not used as a receiver in it's life. I don't have the manual, so I'm not sure what the first transistor in the RX path is, but I've heard that goes bad as a result of TX issues?

Also, has anyone ever tried the "milling machine" trick on the output transistor on one of these mobiles? I put the heatsink on the milling machine, and milled it completely flat. I've heard of teh PC guys doing it for beter heat transfer, so I thought, why not try it? Anyway, wondering if anyone has ever done that, and seen any improvement, although how do you judge that anyway. I'm installing this as a Repeater for our Fire Department, so I thought anything I can do to increase the duty cycle would be great. If it doesn't overheat as fast because of better heat transfer, maybe it's worth it.

I also cranked the TX power down to 20W, any reason lower would be worth a try? I figure I'm already 1/2 rated power, that should greatly increase the duty cycle capabilities.

Any thoughts would be appreciated.

1. This repeater is NOT for Fire Ground, anyone who would think a repeater would be used for fire ground operations is going to get someone killed! In 2004 Phoenix Fire issued a report "Phoenix Fire Department Radio System Saftey Project", in which the summary is basically: Always use simplex frequencies for fire ground communications.

2. Cast materials have voids, which are filled in with heat transfer grease. This grease has a higher thermanl resistance than the metal itself, so the idea is machining a few thousands off the top will make the surface flatter, and minimize the amount of thermal grease between the part and the heatsink. Milling the heatsink removes any voids, which decreases the thermal resistance, thus lowering the die temperature of the device being heatsinked. This is done ALL THE TIME in the PC market, as well as other markets where thermal transfer is critical.

3. The City has NO budget, and NO radio system for our own internal communications. We have to use our Ground channel for day to day operations, as well as during time that dispatch is overloaded. Last year we had a major storm, and had to talk using NEXTEL phones, as dispatch asked all departments to self dispatch. The department can't afford a repeater, so I am donating one to them. I have installed MANY of these style repeaters (15 years ago when I was a two way tech), and they are reliable if properly maintained. If it goes down in a storm situation, we can always revert back to cell phones. It's better than what we have now. So I think ISO, and NFPA would prefer a low duty cycle repeater to NOTHING.

The plan is to get SOMETHING installed, and once we see how valuable it is, we can work on the budget issue of upgrading it. Also, since our Central Dispatch has no idea what they are doing when Narrow Band hits, why invest in an expensive repeater if we might switch bands in a few years?

On the rx problem - the first mix usually results in 1uV sensitivity, not near deafness. I suspect an IF stage or xtal filter is the problem in your case. OTOH, as you said, it is the xmit portion of a rptr, and may not be super important.

On the milling - the rfpa casting is already milled. You can see the machining where they cleared casting flash and flattened the important areas (at least in the rfpa's I've worked on). I don't think you need to worry about this.

On the overall project - I totally understand your no budget problem, and applaud you for trying. Give it a whirl. If you can get the licensed station high enough with reasonable antenna gain and good HAAT, 20w will be a good starting point. With 2013 closing in, I would hope someone in the state/county/city/township is working on a grant to fulfill your future needs. It sucks, but that is the reality of many small agencies.

Thanks for the replies.

It's been about 15 years since I've done much component level troubleshooting on two-Ways, so I'm a little rusty. I seem to remember injecting the IF, and the 455 with my IFR to narrow down where the issue was. I recall a 10.7MHZ too, is that the 45.1MHZ in these new radios? I don't recall where in the circuit you inject it. I put a scope on the 455 filters, and I don't see much activity on either, wether or not the IFR is generating.

Fixed it! It was the 44.645 crystal...

Jon