Is there a way to test these transistors still in the amp with a multi-meter to see if there good or bad without taking them out ? thanks
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1446 X 2 TRANSISTORS ???
- Thread starter Switch Kit
- Start date
Not usually.
In the base circuit, the low resistances of other parts in parallel with the transistors' base terminals makes it impossible to 'see' whether or not the transistors' base-to-emitter junctions are okay. Even if you take one leg of each 10-ohm resistor loose and the input secondary's center-tap ground loose, both base leads are still effectively wired in parallel. As far as your test current is concerened, anyway. If BOTH are blown, you could now see that, but if either one is good, they are still wired in parallel. Keeps you from 'seeing' if only one is blown.
Simply lifting the base lead of each transistor will permit you to observe a blown base terminal, once the base leg of each is unhooked from those parallel resistances in the amplifier's input circuit.
Most transistor failures show up in the base circuit, but not all. A damaged junction on the collector lead will only show up with the collector lead unhooked as well.
73
In the base circuit, the low resistances of other parts in parallel with the transistors' base terminals makes it impossible to 'see' whether or not the transistors' base-to-emitter junctions are okay. Even if you take one leg of each 10-ohm resistor loose and the input secondary's center-tap ground loose, both base leads are still effectively wired in parallel. As far as your test current is concerened, anyway. If BOTH are blown, you could now see that, but if either one is good, they are still wired in parallel. Keeps you from 'seeing' if only one is blown.
Simply lifting the base lead of each transistor will permit you to observe a blown base terminal, once the base leg of each is unhooked from those parallel resistances in the amplifier's input circuit.
Most transistor failures show up in the base circuit, but not all. A damaged junction on the collector lead will only show up with the collector lead unhooked as well.
73
Thank you Nomad. A friend hooked up one backwards to a lot of juice apparently and there was no fuse inline ? (or so he says) well , my knowledge of amps is far and in between , This was just a run of the mill Chinese made Blackcat 300HD , I opened it up and the main diode and a 220uf cap were exploded !! everything else looked fairly clean (I know , looked doesn't always cut it) I changed the cap and the monster diode , The amp fired up , I put 2 watts in to it and nothing , no power out ,not even the 2 watts I was putting through it , The receive amp did work ,but IM guessing that has nothing to do with the transistors ? and the little relays were clicking off and on when keyed.When i shut the amp off ,I got my 2 watts back from the radio. This is really poking and hoping for me here Nomad ,I have changed transistors before in amps with success ,but IM just not sure enough with this one ? Getting the 2 new transistors is not a problem , it's just the fear of wasting my time and money here,it looks (IM not real sure) like the board is double sided, and it looks like the only way to get the board out of the heat sink is to remove the transistors in the first place )-: if the cap and diode were distoryed , IM now thinking if the board is double sided there could be some damaged traces on the bottom ? Looks like there's more exploratory that needs to be done , or just can the thing. Thanks again Nomad.
I repaired one not to long ago that sounds similar to what you have. After un-bolting the pills, I found a trace burned apart, after fixing the trace everything was working. It may not be the problem with yours, but worth a look, as those traces are very thin and it don't take much to cook em'. Hope this helps. 73s, Pluto
Thanks Pluto , I did unbolt the trans and the board slid right out the heat sink ,looked pretty good. Next would be to find out if the trans or good or bad. thanks
Reversed power-supply polarity nearly always zaps them. Fast. Faster than you can cut the (reversed) power. Faster than even the correct-size fuse can blow.
Tends not to leave smoked resistors. No need to test the 1446s, unless you're just interested in doing it for its own sake.
Other, more popular methods of popping them, like excessive drive or SWR or power-supply voltage DO tend to smoke the base-circuit resistors.
You might consider how large a reverse-protection diode will fit in that box. 6-Amp rated rectifiers will usually protect two transistors. By the time a 20-Amp (or so) fuse has blown, a 6-Amp rectifier will overheat inside and short. But it won't go "snap!" and explode the way small 1-Amp rectifiers will do. If there's room for a 25-Amp bridge rectifier, you can probably blow that 20-Amp fuse without damaging anything BUT the fuse.
Sounds to me as if the protection diode exploded and took itself out of the circuit, leaving the RF power transistors unprotected.
Dern...
73
Tends not to leave smoked resistors. No need to test the 1446s, unless you're just interested in doing it for its own sake.
Other, more popular methods of popping them, like excessive drive or SWR or power-supply voltage DO tend to smoke the base-circuit resistors.
You might consider how large a reverse-protection diode will fit in that box. 6-Amp rated rectifiers will usually protect two transistors. By the time a 20-Amp (or so) fuse has blown, a 6-Amp rectifier will overheat inside and short. But it won't go "snap!" and explode the way small 1-Amp rectifiers will do. If there's room for a 25-Amp bridge rectifier, you can probably blow that 20-Amp fuse without damaging anything BUT the fuse.
Sounds to me as if the protection diode exploded and took itself out of the circuit, leaving the RF power transistors unprotected.
Dern...
73
Dern is right !! )-: Thanks again Nomad , I can get a couple of 1446s for about 20 bucks if he wants to cough up the cash ,
Good info, Nomad. Also SK, if you haven't already done so, check close where the pills are bolted in place....feel for high spots where the bolts pulls up the soft aluminium (I believe this was shown in a total rebuild in the forum) I did a "Palomar" 250 about a year ago that was getting hot quick. Everything checked good, so after un-bolting the pills....the first thing I see after sliding out the board, was heatsink grease only where the flanges were! The rest was on the bottom of the pill and after cleaning, sure enough.....four good sized high spots. I told the guy how lucky he was 
I did change them ,after cleaning up the old mess and re-greasing the new ones slightly , I did a small prayer before I hooked it up . I turned it on and did one last prayer to the powers that be ,I then closed my eye's and keyed the radio , I heard the amps relay kick in and opened my eye's and looked at the meter NO SMOKE !!! .....Hell-lay-lu-ya !!! it worked !!! Was doing about 40 to 160 with a 1.5 DK ......honestly knowing only a little Pluto , it was surely a gamble for me (a 20 dollar gamble) I feel I got somewhat lucky this time only knowing very little to done when it comes to amps , ,most amps surely have the look of not being that hard to understand if they were explained and why. Thanks again for the input as well Nomad , I was told a long time ago that the better tech's will know amp's just as well as they do radios.
I'm glad you got it going! Remember....everyone starts from the begining. I started in radio when I was 15, started tinkering when I was 17 (now 47) I learned alot from some good Ham operators and continue to learn to this day. There is a wealth of knowlege here and when I run into a dead end on a repair or a project, I hit the search feature because there are people on here that know thier stuff.
I use a simple home made curve tracer with my scope to make fast checks on in circuit/out of circuit transistors,diodes,resistors,capacitors,grounds,connections..sometimes it is best to just lift one end of a diode to check it or lift the transistors leads for a better check..... a good curve tracer on the bench is "worth it's weight in gold" to the electronics technician....
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