Quote:
Originally Posted by SiriusHardware
It is possible that with new output devices fitted there was a need to readjust the bias, but as we do not have the service procedure or manual, knowing what to set it to would be a bit of a guess. I prefer to leave things exactly as factory set / undisturbed until there is absolutely no alternative.
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Here is a bit more background to the fault.
I suspect that this issue with SSB running away when keyed even before any audio was present in the mic has been happening before, but cant be certain.
The first time the finals blew I noticed by the fact that the OP power had dropped to just 3 or 4 watts on FM and AM. I only replaced the one mosfet that time, I think it was one of the OP pair.
The second time I had to replace both the OP mosfets, again the symptoms were loss of power.
The last time I changed them was as a result of what appeared to be a short in the radio. The current was so high and L27 (ferrite link) got so hot it melted its solder connections and dropped out.
Following replacing the link and fitting the new Vishay IRF520 devices the radio seemed to be working very well. (in fact slightly better than before on FM, power wise)
It wasn't till I tried some SSB contacts over a week later that I noticed the PSU display was reading in excess of 11amps. This was also causing the radio to alarm DCLO indicating the voltage was lower than the set alarm value. During the alarm conditions the OP is normally cut (the same happens if the radio detects a high SWR) so I dont know if there would have been any RF on the OP or not.
But in this instance the supply current was still showing as 11 amps.
The low voltage was purely internal in the radio as the PSU was still reading 12.29v, which was normal for it as it normally gives 12.3v. It could have been volt drop over the radios supply cable and internal losses in the radio.
This is only guess work, but the original IRF520 devices that were fitted were unbranded and unmarked and may have been less able to survive the heavy current, the same may possibly be said for the replacements which I bought off ebay.
I finally decided to go for some branded types that were sold as Vishay and obtained from
Farnell.
Mike