But Why? TS-940 driver

I found yet another TS-940SAT at a flea market. Receives fine, won't transmit. A five minute inspection showed that the MRF485 driver transistors are burned and fractured. I've seen in a few times before. I have the devices to replace the drivers (12V), so I don't think it'll be a problem restoring it to service. But I have to ask, what is the weakness with this design that causes these driver transistors to blow, or is it possible operator error of some kind?

On a side note, this particular example has so much thermal compound layered under all the devices, 1), I know it didn't come from the factory that way, and 2), I suspect the drivers were previously replaced and popped again. Maybe replaced with some of those rebranded who knows what that come from China. Anyway, I've replaced the drivers on other 940s that I have and don't believe I'll have a problem getting this one back to operational using the advice and direction of members of this forum. Clearly the drivers are a weak point on this rig, and I'd like to better understand the root cause.

arnie - W1GCI

 

I don't think that is completely true.
I use a (30 + year old?) 940 and have no issues other than a loose connector at times, but, I don't drive the s$%^ out of it either.

The output on the 940 can be set to well above 100 watts.

Good Luck

 

Thanks for the comment. I too have a 940 that I've had for years and it's near perfect in all respects. Also, all the 940s I've acquired since then have been broken -only two out of those five had a final assembly that was good. I'm suspicious that the previous owners used them on AM and had the key down for extended periods of time at well in excess of the recommended 40 watts? is that right? Anyway - just curious about the failure analysis.

 

It's total junk! It's so bad, you should send it to me just like it is.....

Actually, it's a great radio. I think the fault lies in the SWR protection, and not the finals. For some reason, some of the 940's (but apparently not all of them) will let you transmit into a high SWR antenna.... which then kills the finals. IF you always make sure your antenna tuning is good, and you use it responsibly as noted above, then they tend to last forever.

On the same note as above, I've heard from a few folks that they tend to get hot on digital modes like BPSK-31 if you run more than ~25-30W continuously.... so that may be another cause of the final failures.

 

Curious about the failed rigs: Did they all have their original power supplies, or were some of those supplies reworked/replaced?

Failure of the 28V regulator can put enough collector voltage on the drivers to kill them, and this is a known Achilles Heel in the '940.

 

I've got a early TS930..... PA and driver transistors are the original ones.

As N8YX mentioned these transistors are only rated at 28V should the regulator go s/c anything can happen... especially when transmitting. Looking at the PA module in situ there is a 30V zener in the top right hand corner of the compartment, supposedly to protect the PA

There was a guy who replaced the PA with FET's , the page has been moved to DK mods. Here's a clip -:
One day (a sleepless night and one bottle of 1998 French wine) later…however, broken rig number two ( J …) was modified using the even cheaper but well available P4NA80 by ST. The lower gate capacity of this 0.25 $ device gave 100 watts on all bands except 80 watts on 10 meter YES !!! (oops, bottle again empty….)

I used these very FETs in TV power supplies, and I crossed referenced them to a BUZ80 which has lower internal C.

Under no circumstance buy from Ebay, there are a lot of fakes knocking about...! Always buy from a reputable source.

Dave

 

Since I have 12V transistors to use as replacements, I'll add the 12V regulator that G4COE described a while back. I expect that it would also protect the drivers from a 28V loss of regulation. The currently acquired 940 has good 28V. I can't say if it's always been that way or if parts were replaced to get it that way. But good point, I know that the AVR board has been a sore spot for this radio and very well may have been the cause of some driver failures.

 

It wasn't me, pretty sure it was Roger G3YRO that suggested it.... he had PA trouble with his TS930, which is the same PA.


Is this the thread?

Dave

 

Perhaps just a bad batch of transistors?

In my experence, transistors that are connected to the outside world (finals, audio outputs, keying) are subject to "mistakes"

Semiconductors that are not connected directly, (drivers,if amplifiers, display processors,etc) seem to fail in batches, and stop failing all at once.

Either the parts were "good" and the engineer goofed, or something went wrong when the part was made.

Rege

 

Are the connections to the Transistor soldered? Or are they slip on (ie: spade)? Slip on connectors can loosen up and cause heating issues.

Dan WQ4G