IC-28A transmit section carnage

The damage wasn't hard to spot. Q17 has overheated so badly the case is charred and cracked. C54 is burned beyond recognition, with a matching smoke spot on the inside of the top cover.

_________________

Service manual is here if you need it:-

Colin

"David Brodbeck"

** The service manual is here:

Q17 switches the output level from the RF power amplifier module SC1019 ( a custom Icom part ) from 5 watts to 25 watts. Almost any low voltage, general purpose PNP transistor in the same pack & capable of a few amps current will do to replace Q17 - eg BD436 / 438.

If the failed electro really was the cause of Q17 failing, then the unit should work after replacing it and Q17.

However, if Q17 still overheats then suspect the SC1019 module is blown.

... Phil

Thanks, Colin. I did find that, and it's been very helpful. I also realized the block diagram had transistor numbers in it, which tells me that Q17 is part of the automatic power control circuit, which would explain the lack of power output.

I may just go ahead and change C54 and Q17 and see what happens.

Q17 is the APC control tethered directly to the PA IC-6 /SC-1019/ (SC-1022 in 45 watt models). Q-17 is controlled by the APC amp (IC-7 and Q-18). APC amp circuit gets its drive from the phase detector a pair of

1SS97 diodes (D12,D13) which one I don't know. The other 1SS97 drives the RF-Indicator amp.

The pinout of the SC-1019 PA amp if you look at it from the front with the 4 pins down is, input, V1, V2, output. The heatsink tabs are ground.

Your reverse polarity idea is plausible. I've worked on a lot of amateur and profession mobile equipment and have seen them reversed.

The 28a was one of the first rigs I talked on many years ago. I've had many different since then now settled on an Icom 706 MkIIG

Thanks, Phil. That's also the conclusion I've come to, so it's nice to get a bit of a sanity check that I'm not overlooking something obvious. :)

Right now I'm working under the theory that the SC1019 module is OK, because I otherwise can't explain why C54 would be burned to a crisp. A bad PA module could cause excessive current through Q17 but it shouldn't have affected that capacitor.

I'll let you know what happens when I put it back together.

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Thanks for the circuit rundown. I followed along on the block diagram and it's a lot clearer to me now.

I don't actually think it was reverse polarity, because there's a large polarity-protection diode that should have caused the input fuse to blow before any damage could be done. (Hmm, maybe I should check that diode and make sure it's not open, though...)

My current working theory is that C54 developed an internal short (maybe due to age) and cause excessive current in Q17. Seems like it's pretty much got to be that or a bad PA module, and a bad PA wouldn't explain why C54 burned. I've never seen a capacitor so thoroughly immolated. It left a big smoke spot on the inside of the case, and it just fell apart when I poked at it with a screwdriver.

"David Brodbeck"

My current working theory is that C54 developed an internal short (maybe due to age) and cause excessive current in Q17. Seems like it's pretty much got to be that or a bad PA module, and a bad PA wouldn't explain why C54 burned. I've never seen a capacitor so thoroughly immolated. It left a big smoke spot on the inside of the case, and it just fell apart when I poked at it with a screwdriver.

** Is C54 actually a * tantalum* bead cap ????

If so, no special explanation is needed for its fiery demise.

... Phil

Yeah C54 destroying like that is a bit of an enigma. Check the crowbar diode. Some people like to do stupid things when they blow a fuse like wrap it in foil or replace it with a 20 amp. The radio could have been powered up without a proper fuse and unattended when the failure occurred. That would have caused a cascading catastrophic failure. I've seen this happen so many times I've lost faith in my fellow hams. :(

If you have some way to limit current, use it. I have a supply that I can dial in from 0 to 35 amps and it comes in real handy

Could be. It's 10 uF, polarized, which could be either. It was too badly charred to tell what it used to be, but I didn't find the remains of an electrolytic can rattling around in the case, so a tantalum does seem likely.

Will do. I don't have a variable-limit supply, but I do have one with a fixed 1.5A limit that I plan to use for initial low power transmit testing.

"David Brodbeck"

"Phil Allison"

Could be. It's 10 uF, polarized, which could be either. It was too badly charred to tell what it used to be, but I didn't find the remains of an electrolytic can rattling around in the case, so a tantalum does seem likely.

** FYI - tantalum bead caps look like this:

They are known to catch fire and emit smoke and bits of the tantalum oxide "slug" for no good reason.

If you found a "can" then the cap is an ordinary electro that overheated, the electrolyte boiled and burst the seal.

... Phil

That will help you not fry some of the bigger stuff but there is nothing like a 0 to 15 volt, 0 to 35 amp supply with volt/ammeter.

Even better if it has OCP. If the current drain reaches a user programmed setting (IE those BJTs are about to blow!), the power supply will shutdown its output completely, instead of just dumping continuous current to whatever the current is set to. I have an Agilent 6643A which does this. While they cost $3K new you can pick 'em up for a 10% of that on Ebay. I paid just a bit over $300 for mine.

That's great if you know within a decent tolerance how much current what you are working on draws at inrush or at an idle.

True. Start low and work your way up. :)

And put your nose close to it and inhale. I do a lot of troubleshoot with my senses. After 30 some years I can distinguish just what's hot or burning, smell ozone from HV, hear things snap crackle pop etc...

Also a good tip, but when doing so I wear safety glasses. I've had a few things go critical mass just as I lowered my nose for a sniff! Gotta love those - practically fell out of my seat.

How true :) I've had my fair share of electrolytic out-gassing up the nose. Never had anything with the potential to injure blow up in my face.