[R-390] AGC Problems
Flowertime01 at wmconnect.com
Flowertime01 at wmconnect.com
Thu Jul 14 00:52:53 EDT 2005
Dennis,
If switching over to AGC is not just plain killing the signals or you cannot
tell the difference between AGC and MGC modes, you are likely OK.
You only have one receiver (yours) to listen to. If you were an O5H, 33B,
31E, or any other MOS that used the receivers and had a chance to listen to
several hundred receivers, you would know when you were listening to a bad one.
So your AGC questions are a wise pondering.
A full review of the subject starting from some recent post follows.
Remember first that in the MGC position, the AGC line is simply shorted to
ground in the function switch. The line at ground in MGC pulls the grids of the
AGC controlled tubes toward zero volts. Cathode resistors in the tube stages
provide some positive cathode voltage relative to the grid voltages. AGC
voltage is negative. When the AGC voltage is applied to the grids of the tubes, it
drives the tube stages toward cutoff. A couple volts on the grid of a tube that
is working on micro volt signals go a long way in signal reduction.
If you have no signals in either the AGC or MGC mode, you expect you have a
tube stage problem and have no reason to expect it to be related to the AGC
line. You trouble shoot to find the stage that is not making the grade. The found
problem may be in the AGC line, but it will not be the symptom that leads to
a logical troubleshooting progression.
If you have an AGC voltage in the MGC mode, you should check the wire harness
and function switch. In MGC mode the AGC line is not being held to ground by
the function switch.
If you have strong signals in the MGC mode you may have just one more bad
item pulling the AGC line to ground. No Problem.
When you switch on the AGC you expect the negative AGC voltage to reduce the
receiver gain a little bit. If you get no AGC gain reduction then you have to
wonder if the AGC line is shorted somewhere and the receiver acts as if it is
still in the MGC mode.
The carrier level adjust has been a sorry circuit since the day it was
implemented. Unless you have changed some of the parts, to improve the performance,
what ever your carrier meter shows you when switching the AGC is not a real
inspirational troubleshooting step.
You likely expect you have an AGC problem when you switch to AGC and you
loose all your signals. Or a lot of the signals. How does opening a line from
ground, and placing almost no negative voltage on it cause a loss of so much
receiver gain?
You likely expect you have an AGC problem when the receiver starts acting
intermittent with gain coming and going. The receiver appears OK in the MGC mode
but looses signals in the AGC mode.
Now you are not looking for a shorted item. A shorted item in the AGC line
looks like MGC.
You are not looking for an open resistor. The resistors work fine in the MGC
mode.
_______________________
Barry - N4BUQ wrote, "When listening to a moderately strong station in the
15mc band, if I switch to AGC, the signal drops significantly from the MGC
position, regardless of the AGC speed."
This may not be bad. If the signal in the MGC position is strong, the
receiver is being over driven and when the AGC is switched on, the strong signal
produces an AGC voltage and a resulting signal level that is smaller than the MGC
level but not over driven or distorted.
However, if the receiver seems to loose lots of signals in the AGC mode,
there is a problem.
If the IF gain is not set to high (-7 volts) and the receiver looses it in
AGC expect a problem.
-------------------------
A most common AGC problem is with the mechanical filters. The AGC line
crosses R507 (22K) and is filtered by C512 (5,000pf). From there the AGC voltage is
applied through all four filters, all the time. If one filter is pulling the
AGC voltage to ground this is a problem.
The filters may appear to have good band pass and performance so a simple
check of bandwidth and sensitive with a signal generator and meter on the diode
load may not find the filter problem. However this is a good first test if you
have a signal generator. 200- 250 UV at 455Khz into the IF deck and rock the
generator both sides of center to find the bandwidth of each filter. Note the
diode load voltage for each filter and see if one of them has more loss than
the others or has a very wide response.
The next step is too place a tube extender under V502. You want to look at
the control grid voltage. The AGC is applied to the control grid through the
selected filter. As the AGC is applied to all filters, and if a filter is sucking
AGC, the "bad" filter will pull the AGC all the time.
If you have a bad filter, you would expect zero volts on the grid, as opposed
to several volts negative. The TM shows the grid to only be .4 volts
negative. So this test is not a real sure bet either.
Ohmmeter checks may not get you anywhere either. On one side all four filters
are tied to the AGC line. If the short were near that end of a filter coil
winding in one filter, all four filters would read the same meter value. Thus
you have no clue which filter is bad.
Good trouble shooting practice never supported unsoldering wires just to see
what will happen. Heating mechanical filter terminals is not a high item on
things I would do this week.
But, if we open R507 and remove all AGC from the stage. Pull the AGC wire
from the resistor and let that wire hang open. Short the end of R507 to ground,
so the stage thinks it has zero volts AGC. Now review your AGC and MGC
operation and retest the filter band pass with the signal generator and see how the
receiver behaves. Missing AGC on one stage should give about normal performance.
If all of this work has not isolated one of the filters as a candidate for
concern, I would give them a passing grade and go on to the next likely items.
__________________________________________________________
Second most likely AGC problem after the filters is the bypass caps on the
AGC line. Locate the green screw extractor, Bristol wrench, schematic and meter.
Go down the AGC line and just meter every cap and resistor.
----------------------------------------------------------------------------
Barry N4BUQ wrote, "With a signal generator as input and the RF gain all the
way CW, I can adjust the signal generator to get -10V on the Diode Load in
MGC. Switching to AGC, the Diode Load drops to about 3.4V to 4V (depending on the
AGC speed position)."
This is not a problem, -10 volts on the diode load is over driven by 3 volts.
This should produce lots of AGC. The balance point is where going from AGC to
MGC to AGC produces no change on the diode load. This may not be -7 volts.
This may also not be the optimum operating point for the IF gain adjustment
setting. So we never consider what this balance voltage point may be.
-------------------------------------------------------------------------
Roy Morgan offered the following to help deal with leaking caps on the AGC
line
Put an ohmmeter capable of reading high resistance on the AGC line to ground
(notice if it reads the same with the negative polarity on the line as with
the positive polarity on the line). Then unplug each module (RF, and IF) in turn
to see where you might be getting leakage. Knowing how the AGC wires run from
the source in the IF module to the rear panel terminal and the AGC switch and
to the RF module will help figure out where any excess leakage is.
Pull the AGC jumper off the back panel and hang an amp meter across the
terminals. Observe the correct negative voltage polarity. You'll be able to detect
micro amperes of leakage and normal AGC line currents. Most DMM and analog
meters have low amperage ranges that will handle the AGC current range.
We do not know the current of a good AGC line so until someone makes some
test and offers some values the number your meter produces is not going to help.
Will someone please make a test of AGC current and offer up the values they
get?
Inquiring minds want to know.
------------------------------------
Roy Morgan offered
(One way to measure low levels of leakage is to hook up a 9 volt battery to
simulate AGC voltage with a DMM on low voltage (or current) range in series
plus side to ground, negative to the AGC line. You'll be able to detect
microamperes of leakage and normal AGC line currents.
Start with a cold receiver.
If the tubes are all cold, you may not detect a leaky tube, but you will find
any leaky caps or cable leakage. Cable wires can be leaky, too, so don't
discount that possibility. Teflon wire in the AGC line would be a good idea. Roy -
Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
Then turn the receiver on. A warmed up tube may leak more than if it's
filament is cold.)
If the problem is in the RF deck and you use a battery, unplugging a RF deck
that is good should produce little change in current.
The AGC stages are all in the IF deck. Unplugging that deck opens so many AGC
lines coming and going you have no idea which end of the circuit is being
offensive. This is the real problem of trouble shooting the AGC line in the
receiver. Remove each of the tubes involved to see if you have a leaky tube. If you
find one, you are lucky. If you don't, remove the module and find the
(likely) leaky cap in there.
--------------------------------------------------
Dennis wrote, "Good afternoon, got some time to look into the AGC trouble in
my Motorola R-390A. First thing I wanted to do is satisfy myself that there
was indeed a problem. I looked at the AGC line with a strong signal and the
Calibrate sig. The most I could get was about -4.9 volts at the AGC jumper. Under
no signal conditions the AGC line is about +0.15 "
OK, these are not bad numbers. Once the -4.9 volts get back to several tube
grids it is on the order of less than -1 volt. At no signal we expect no AGC
and a value of .15 plus is typical.
----------------------------------------------------------------------------
Dennis,
I took out the IF deck and measured resistance from pin 6 to ground, and I
see about 7 meg with no charging behavior. It says I should see infinite
resistance.
_____
If you were doing this in 1970 with a TS 505 or a TS 352 on the K ohm range,
7 meg would be infinite resistance. You are likely OK on this test.
TM 11-5820-358-35 8 Dec 1961 Para 72 b. Alignment of Z503 Page 113.
says AGC should be in the range of -1 to -2 volts
Thanks for the reply Roger. Good point. 7 meg may not indeed be a problem.
But why don't I see the 350K I saw from the AGC jumper? So let me review why
think there is a problem: 1. With all modules connected, from the AGC jumper to
ground is about 350K. When the RF deck is disconnected, it remains the same.
When the IF module is disconnected, it goes to about 1.8M to ground.
Does this isolate it to the IF deck?
No not really. When you unplug the IF deck you open so many wire in the AGC
line coming and going from the rear panel, front panel function switch, from
the AGC diode, to AGC controlled stage, you just never know what you are
checking. If you do not have a good schematic in front of you and consider what you
have disconnected at any give instance of measurement, you can run your self
ragged.
---------
I don't see more than about -4.9 vdc at the AGC jumper on very strong, or the
cal, signals.
OK.
No signal is +0.15
OK
There is distortion on strong signals.
OK. R390/A are known for this aspect.
Two basic questions:
Do I have an AGC problem given the measurements above?
And, is it isolated to the IF deck? Dennis
----------------------------------
So we come full circle to your original question.
The part not really dealt with here is, If the AGC is killing lots of signals
and is related to one of the mechanical filters, how do we determine good and
bad filters.
First I will go read the archives and see what I find. I'll kick out a new
post when I get there.
Give me a few days to read up.
Dennis, I would say you are likely OK with your receiver. Go on with your
other troubleshooting and cleaning. If you do have an AGC problem, operating the
receiver will not cause more damage. You can always operate in MGC if you
think you may have troubles with the AGC. You can always come back to the problem
if it is still nagging at you. Having one receiver in isolation to care for by
your self is a daunting task. The military trained us guys in groups to take
care of bunches of receivers together with support, and this scared the hell
out of most of us. Most of the tecks never got conformable about being
repairmen. They did one four year enlistment and got out of the trade altogether. It
was just not the military they did not like. They gave up electronics in
general. Being a fellow willing to work on a glowing receiver puts you in a special
class. Just being willing to give it a go earns you respect. Do not worry that
you are working from a disadvantage. You have to start some where and you are
down the road of glowing tubes. Just because some of us ran ahead and come
back with tales does not make us any better. Do not let our tales worry you.
Roy Morgan added his best thoughts. Barry put in with his findings. No one
has taken us to task for being way off base on this, so we are likely in the
park.
Roger L. Ruszkowski KC6TRU
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