Testing resistors should be done by removing one leg and using the ohms scale.
you cant test them by measuring voltage drop without knowing what the current through the resistor is as well, and then crunching the simple
R=E/I formula.
So your testing the 2N3904's?
Testing PNP transistors is very simple. You should ideal use a meter with a diode forward voltage test.
It will basically be like testing two diodes with their cathodes attached together.
By placing the negative lead on the base, and the positive lead on the collector or emitter, you should get
the forward voltage reading of the PN junction, which for silicon should be right around 0.7v give or take the
tolerance of your meter. Reversing these leads should give you a overload or OL reading.
emitter to collector in any direction should give an OL reading unless voltage is applied to the base of the
transistor. If you get a 0v reading or, in most cases the meters will beep at you. It means you have a shorted
junction in the transistor. If you get an OL reading when you should be expecting to see the forward voltage reading, you have an open junction. In these two cases the transistor is bad.
This test must be done with the transistor removed from the circuit as any biasing resistors between base to collector or base to emitter can cause false readings. Capacitors will also cause false readings as your
meter will charge them up, giving you a brief reading until the voltage stabilized to the voltage applied by
the meter.
On the note of the relays. There are two, one is for the preamp, and one is for the RF. The preamp relay
will click when the preamp switch is flicked. This controls the RF pathway to either pass through the
preamp or be routed around it. The transistors in the preamp always have supply voltage present, they just dont output a signal if no
signal is applied to their base. Its just a simple re-route with a relay that is used.
When the mic keys up, the amp senses the carrier of the radio and activates the RF relay, this changes the path of the RF from the preamp (which flows in one direction, from the antenna to the radio) and routes
it to the amplifiers finals. The finals in the radio essentially become the driver transistors for the finals in
the amp, and they pump up the power.
Keep in mind, if your amp was not designed with an SSB delay in it, and you plan to operate SSB on it,
due to sidebands absence or suppression of the AM carrier, the relay will chatter as you speak into the
mic as the circuit in the amp is expecting to see a constant carrier all the time. If this is the case, adding
a 1000uF capacitor across the RF relays terminals will add a delay to the amp. You may want to have a
switch to enable/disable it, as it will add a delay of around 1 - 2 sec. This added 1.5 sec delay on my
amp, and works very well for SSB, the amp does not cut out or have the relay chatter from the lack of
carrier. Down side is, you loose instant receive upon releasing the key, there is a 1-2 sec delay before the
amp switches back over to receive.
EDIT:
I also must have missed the reverse polarity deal. That diode is a reverse polarity protection diode. Replace it as it is sure to have failed and should not be trusted as a future reverse polarity protection device. Now that said. Resistors and capacitors are probably fine, as the diode SHOULD have reduced the reverse polarity to 0.7v (the forward voltage drop of the diode) resistors wont care about polarity, and I'm sure the
electrolytic capacitors barely blinked at seeing reverse polarity of 0.7v for the short period they did. (you would know otherwise as they would
have went pop). Though, depending on how long you left the power applied, as the junction in the diode breaks down, the forward voltage will
increase above the 0.7v until finally going open and the full 12v will then be present reverse polarity to the system.
Check your small signal and switching transistors first, as these are off the shelf cheap parts to replace.
Based on the schematic provided (although not guaranteed 100% accurate to your model) they all appear to be PNP types. See my above
described testing method. If any have failed, replace, but do not yet power the amp. Next test the finals. Again, same method. But because the SD1446 is an NPN transistor, the readings will be opposite to what I described for the PNP transistors.
It doesn't take much reverse voltage to kill a transistor, and i'm betting unfortunately, that that is what happened when you hooked it up incorrectly initially is that its the finals that have opened up. Had the protection diode had not done its job, you would have got a nice fireworks
show. The amp was "probably" in working order when you got it.
If the finals are the case, now is your deliberation, to repair or replace. The finals as mention may run you $80 for a matched pair. (yes matched is critical), it means they are in balance so they share the load equally, so one doesn't over heat, cause non-linearity in the amp, cause
distortion to the signal or worst case, break into oscillation and go out in a fireworks show. So even if only one is bad, expect to replace both for
optimum performance. I picked up my older Palomar 200W amp for $75 used and working from a reputable local seller. so replacing with a known
working unit may be the best route in this case.
Luckily, these things are pretty simple, not a lot of possible parts to fail. Best of luck on the repair.
Cheers and 73's
Let the testing begin.
