Why am I getting a drop in volume, and what is causing the problem????
I will start off by stating the problem with my Philco 48 Radio. After the
sets has been on for a short time. I
hear a crackle, and static sound come from the speaker, then the volume
drops significantly. The volume
returns sometime later. Just before the volume returns, I again hear the
crackle and static then the volume returns and the radio sounds great (good
volume). This process repeats it's self often (no time pattern).
I checked for leakage on the all capacitors. First, I removed all tubes
except for the Rectifier then turned the set on and measured all caps with
my VOM. All seem to have the proper voltage. On the coupling capacitors, I
got the proper B+ voltage on one end of the cap(s) and zero on the
I then put all tubes back into the set and powered it up again.. I then
measured (and recorded all cap voltages). Again, each capacitor appeared
fine (I am getting the proper voltages). The radio volume during these
tests was fine.
I took my insulated tool and moved the insulated wire(s) around to see if
any of the wiring could be cracked or broken. Nothing.... The set
continued to play well throughout this process.
Next step was to try some freeze spray on the caps (after the drop in
volume occurred. This has no results. So I eliminated heat sensitive
components as my problem.
I then tried the "tap test" on the tubes to see if there was any
intermittent tubes. Nothing......
I then disconnected the leads to the field coil and checked the continuity.
My schematic showed that the field coil should be at 1700 ohms, it actually
reads 2700 ohms, is that too high? I didn't isolate the field coil from
the from the audio transformer, so I don't know if that makes a difference
and is the cause of the slightly higher ohms reading? I reconnected the
field coil and fired up the set again. Now remember the drop in volume
comes and goes.
So I determined that I needed to measure the socket voltages (and cap
voltages) after the decrease in volume since I now had the voltage
readings when the set was working fine. So the next time the volume
dropped I was ready to probe (check) each capacitor and B+ voltage at the
various points in the set. Here's where it gets a little interesting. I
placed one probe (-) on the chassis and the other (+) on the B+ voltage
connection coming off of the filter ***(the big cap). The moment my
probe touched the cap, volume returned! When the volume drop again later, I
repeated the same step, and again the volume returned.
Each time the volume dropped I placed the probe (+) on a different cap and
no matter which capacitor or B+ connection I touched, the volume returned
One other interesting item that I noticed during the my probing. One of
the coupling cap's between tube sockets 6Q7 and 6K6 (both ends of that cap
are no voltage which is proper) When the set was working correctly and I
touch the probe (+) on one end of the cap, I got a slight "increase" in
volume. When the sets volume drops due to my intermittent problem. I again
touched that same cap and the volume significantly decreased to a point
where the volume could hardly be heard ( ?!?!?).
So, am I missing the obvious, did it head me in the face and I still don't
Any ideas, suggestions, next steps that would help me identify what is
causing this problem would be very much appreciated...
P.S. I tried replacing the tubes with other known good ones to see if a
intermittent tube could be the cause. Obviously with no results. The volume
continues to drop and return....