[R-390] R-390A trouble
Roger L Ruszkowski
[email protected]
Thu, 28 Mar 2002 10:09:43 -0800
Hi Roger and list,
OK, I want to fix this receiver. Here goes. Jim 73, KF4ICZ
Jim, your on it now. Roger.
{The unit, at first alignment, aligned well}
{I am positive the RF deck timing is correct. I have triple checked the
timing and am positive it is correct. The deck did work fine for a couple
of hours.}
[yesterday this receiver worked]
{At some point, within a couple of hours of operation, the receiver
developed a screeching/motorboat putt putt sound}
[now it is broke]
{Above band 03, the motorboat sound will vary in amplitude and putt, putt
frequency with adjustment of the antenna trim control, and becoming more
intense as band increases}{ All I can surmise is that one or more of the
ceramic caps are bad. I can
shotgun the caps, but it would be helpful if I started with the most
logical
stage. I would pick the V201/6DC6 and go from there. What do you think?
[any thing that lays out on the RF deck band switches and get changed with
the band switch (.5-1, 2-3, 4-7, 8-15, 16-30) is not the problem. Forget
all those cans on the tops side of the RF deck.] [ any thing that change
over under 8 with the band switch is not the problem. Forget those 1st If
mixer and 17 mHz osc and.] [all those caps are out of bounds]
{{Yeah, but ... the screeching/motorboating kicks in above the 3 MC band
only. Doesn't that suggest the RF deck?}}
[[Naw, you just got to get up that high so the stray capitance or large
inductance of the low bands do not swamp the putt, putt, On those bands the
time constant may be slow and the AGC action covers it very well.]]
{I checked the power supply voltages, and I get 260V at F102 and 225V at
F103. The 150V regulated is 150.4V.} [good enough]
{The carrier level meter indication increases as the amplitude increases,
the strongest coinciding with the antenna trim peak prior to the problem
developing.} {The sound goes away when V202 1st mixer, V203 2nd mixer, and
V204 3rd mixer are individually removed. I would expect this. Concerning
V202 1st mixer, the sound returns at 8mhz and above, again as expected.
Also, when V201 RF Amp is removed, the sound goes away.} {I tapped the
tubes when the putt,putt sound is occuring, and when the antenna trim is
detuned, when there is no putt, putt and I hear no harmonics or any
change.}
[That circuit is in the IF deck, so between the antenna jack and the end
of the IF chain (where the meter circuit is) some thing makes noise. If the
antenna trim is peaked to let more noise in to the 6DC6 it will mix with
the internal noise and manifest on the meter. (all these actions are the
same, dare I day it? Dare! Dare! There is more than one way to skin a
cat.)]
{The AGC control will change the frequency of the putt,putt, depending on
the position of the control. Fast=fast. Slow=slow, or right on the edge
of putt,putting. Medium=medium frequency.}
Does this putt, putt occur when in the MGC function?
If no, then its most likely an IF deck AGC voltage generation problem and
not the RF deck at all. If yes, then it is not and AGC problem at all and
we are looking for a bad CAP.
Bill Smith adds, Sounds like one of the stages controlled by the AGC
circuitry is oscillating for some reason. High probability of a leaky cap,
or off-value resistor. AGC drops, stage oscillates, AGC is boosted, stage
stops oscillating then AGC drops.. in a cycle. You might try manually
controlling the AGC voltage and look for the (unstable) stage that is
charging the AGC circuit.
Bill
Dave wise adds, I suggested the IF deck only because it's so easy. It's
still possible that IF (fundamental or harmonic) is getting out of the deck
and back around to the RF, with enough sneaking through to regenerate. Does
the motorboat change with IF bandwidth? If not, you've eliminated that
possibility, one down, ??? to go. This one's going to be a booger. You'll
feel *so* good when you nail it. 73, Dave Wise
Has any of the other feed back here hit this problem square for you?
I have Thursday AM here at the end of the day am out of here until Monday
AM for Easter. I hope we can get you going for the week end.
The question please.
Does this putt, putt occur when in the MGC function?
If no, then its most likely an IF deck AGC voltage generation problem and
not the RF deck at all. If yes, then it is not and AGC problem at all and
we are looking for a bad CAP.
Roger.