[R-390] AGC Problems

[JMILLER1706 at cfl.rr.com]

There is supposed to be a jumper on the rear panel termial strip for a 
remote AGC line?  What would happen if it was missing?

----- Original Message -----
From: Flowertime01 at wmconnect.com
Date: Thursday, July 14, 2005 0:52 am
Subject: Re: [R-390] AGC Problems

> 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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