Since one transistor position has the same incorrect voltage on all
leads, I would look for a short. It might be solder splash on the under
device side of the board. It might be a faulty coupling capacitor to a
collector on a previous stage. A leaky or shorted cap could be
delivering a higher voltage than normal to the base of the stage you
have replaced.
Incidentally, seldom do variable caps go bad sitting in a circuit, if
this one is a calibration rotor and plates, set and forget type. There
are some faults that can develop in a trimmer type cap, but it was not
explained what class of variable this is. Stator and rotor multiple
plate capacitors may rarely get shorted by a stray wire fragment from
other repair / construction, landing between plates. Mechanical faults
in larger variables include loss of ball bearings; frame loosening, to
the extent rotor and stator start to rub and short out; and wear
problems where spring contacts lose connectivity to the rotor plate
assembly. But all pretty rare.
Another thought came to mind. You may have a compound stage with the
two transistors, and one is PNP and the other NPN. Are you sure the
transistor has been returned to the proper socket in each case, and you
have the leads alignment correct re Collector and base and emitter, for
each of the two transistors? Many DVMs include a Beta checker position
on the scales of the meter, and out of circuit, you could verify the
transistors from these two positions exhibit useful gain.
Other rare problems are if a transistor lead breaks internally to the
plastic case, or header of the transistor.
Another idea is to look on the schematic for other stages arranged with
two transistors similar to the one you are testing. Use the as found
voltages on other working stages to judge what differential you should
find on the EBC of the pair of transistors. Most base terminals are
going to normally be 0.25- 0.5 to 0.6 Volt higher than the emitter when
amplifying normal signal levels.
I think early on, someone mentioned checking for correct DC bus rail
voltages feeding the stages.
On radios this old, if there are carbon composition resistors, larger
values may drift much higher over time.
I have seen a 1 meg resistors got to 10 megohms, which really caused
a weak signal in a TV gain chain!
If you suspect resistor troubles, fire up the circuit for a period of
time, then measure the heating, (or non heating) of resistors around the
malfunctioning circuit. Right after powering down but before everything
cools, measure the resistance of individual resistors.
You may see the value changing back to normal on one, before your eyes.
Since this gain problem seems close to the front end of the radio,
consider if the radio was left connected to an antenna before the fault
appeared. You may have suffered static discharge damage, which can take
out semicondutors, but also front end antenna and band coils. I infer
that the gain problem is on all bands. I think someone mentioned to
check any antenna switching if included on the back panel of the radio.
But the biggest clue that stands out to me are the similar voltages on
all pins of that one transistor.
Good Luck,
Stuart Rohre
K5KVH
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