Author Topic: RCA 9-X-572 Weak Reception  (Read 243 times)

electra225

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RCA 9-X-572 Weak Reception
« on: September 03, 2019, 09:29:17 AM »
This thread is a continuation of the AVC thread I started on "Chat".  I figured in the interest of forum integrity, we might oughta move our discussion to this board.

The victim here, a circa 1952 RCA 9-X-572, chassis RC 1079F, a five-tube "All American Five" (AA5) broadcast band radio with phono input.  The schematic is in Riders 20, pages 17 and 18.  I don't see any reference to suffix "F", however.  The schematic is also in Sams 107-7 and it makes reference to the "F" suffix.  I have the Riders, I don't have the Sams.  It's main claim to fame is that it has an 8" speaker.  It has three controls, a combination function/tone control switch for radio and phono, on-off volume, and tuning.  It has a 12SA7 converter, 12SK7 IF amp, 12SQ7 detector, audio amp and AVC amp, 50L6GT audio output, and 35Z5GT rectifier with the heaters in a series string.  This particular set was in my parts department, a victim of its ugly cabinet.  It is one of those with a tan painted bakelite cabinet with those big horns on the front.  The faux wood finish on the front of the cabinet is worn, I knew it would never be allowed into the house, so onto the parts pile it went.  My practice is to establish a baseline on parts sets so I know what parts are good.  This set had SMD, silver mica disease, silver mica migration in the second IF transformer.  I replaced it with one from a parts set.  This set has been recapped with the exception of the power supply with uses 120UF caps that I did not have on hand.  I performed an alignment for diagnostic purposes and found its performance to be weak.  I want to establish another baseline and make notes so they can be referred to without delay as we go along.  I'll do this and try to answer a couple questions already asked, then we can start from there.  If we can get this thing where its performance is acceptable, maybe I can get Ron the Refinisher to squirt something on the cabinet that will allow its entry into polite company.
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Motorola Minion

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Re: RCA 9-X-572 Weak Reception
« Reply #1 on: September 03, 2019, 08:18:02 PM »
I will save the AVC discussion for later, as I just solved the AVC issue in a 1938 Crosley Dynatrol 11.

After alignment, my AA6 Zenith from 1949 sounded like new in the detached garage but is pretty unusable ( for the station I want at least) in my kitchen, which has LED under cabinet lighting. I am looking to put my '37 Zenith tombstone there after relocating at work, to a spot where I cannot hide an antenna. Back to FM I guess..

How does this radio work if removed from the shop, outdoors, etc. The loop antennas are a limiting factor if you use the radio anywhere indoors.

At work the '37 Zenith (the old AA5 tubes) needs a wire antenna, just 10 feet of it is under my carpet ;). That is the only radio that picks up two good but low-power AM stations 25 miles away, in a cubicle. ANYTHING else just picks up the closest EMI source instead>:(
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Dave

electra225

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Re: RCA 9-X-572 Weak Reception
« Reply #2 on: September 03, 2019, 08:54:34 PM »
I may have made some progress with this radio today.  While establishing the baseline, I got to tinkering with RF alignment.  I could not get L3, the oscillator adjustment to peak.  In fact, I could not get a signal from the generator, wide open, thru L3 at all.  I turned about three full turns before I got any response from the generator.  I finally got it to peak.  This radio has a rather handy feature when it comes to alignment.  There are notches in the panel behind the dial.  (The dial comes off with the cabinet).  Left to right the notches are for maximum capacity, 600KC, 1400KC and minimum capacity.  Makes it easy to find the tuner setting for alignment procedures.  I got the RF aligned pretty well.  I did it three times.  At night, after dark, this radio may still be a little weak on output, but the volume was really quite adequate.  Tuning seems sharp, with no birdies between or on stations.  I get all the stations I should.  WSM was really weak tonight, but I could not get it at all on my Halli SX-110 bench radio.  I experimented with AVC a little as well.  If I ground the AVC connection on the loop antenna to chassis, the sound distorts, just like Ed was talking last evening.  If I put my hand behind the antenna or add an external antenna to the tuning cap, it kills the radio and increases noise.  I believe the volume could be stronger, but it is certainly better.  I'm going to redo the IF alignment next and check for improvement.  I'm not there yet, but I'm on the right track.  I'll see what it does tomorrow in the daytime.

Dave, I believe I'll try the take it outdoors trick you mentioned.  My garage has a metal roof.  AA5's work exceptionally well at night here, even with the metal roof.  I'd be interested to hear your AVC story.  I'm not sure I have an AVC problem, but I'm also not sure I don't.   :-[ :-\ :) :)
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electra225

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Re: RCA 9-X-572 Weak Reception
« Reply #3 on: September 04, 2019, 03:51:21 PM »
Last evening this radio played well.  Maybe not 100%, but acceptable.  Today, it is back to "normal".  I need to put my ear right up to the speaker cone to be able to hear the strongest stations.  This is exactly the issue I'm trying to figure out the cause of.  Why will this radio play fairly well at night, yet barely play at all during the daytime?  It makes no sense.  I have another RCA much like it, a "bullhorn" radio, but without the phono input sitting beside it on the bench and it plays like it should.  It's not the environment, it's the radio.

I have made resistance and voltage checks and have made notes.  I still don't see anything out of the ordinary.
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ed from Baltimore

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Re: RCA 9-X-572 Weak Reception
« Reply #4 on: September 04, 2019, 05:18:12 PM »
             I think that what might be happening here is something I have seen happen with other systems-electronic, mechanical,-anything that uses feedback to self adjust and compensate. It could be anything from a car's electronic fuel injection, to a central air conditioner or anything that is constantly monitoring some end result--room temperature, air-fuel ratio, AM radio detector output, and adjusting to keep that result constant throughout all operating conditions.
              As long as the adjustment range is within the system's capability to adjust, everything seems fine and the user-radio listener, car driver, whatever-thinks the system is working perfectly. Meanwhile some fault is causing the system to "bend over backwards" in attempting to keep everything normal.  Say an air conditioner with a clogged filter, a auto engine with a partly clogged filter, or--a radio with some strange problem. Everything seems normal until the point where the system can no longer compensate for the faulty condition, and then suddenly everything falls apart and regulation ceases.
             A car with a clogged fuel filter will seem totally fine until you try to climb a steep hill. A central air conditioner seems fine until that hotter than normal day, or--the radio plays fine until one extra demand is made on it--and then it can't keep up any more.
            Anyway, I wonder if this is what's happening here. The radio plays "OK" most times because the AVC can adjust for the lower gain in the tubes (or tuned circuit or whatever) until the stations are weaker during the day.
           Just putting that out there for discussion.
                 Just curious, how close are the "local" stations transmitting antennas, Greg and are they all medium power, say  10 kW ? 

TC Chris

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Re: RCA 9-X-572 Weak Reception
« Reply #5 on: September 04, 2019, 08:05:33 PM »
Following up on Ed's inquiry, one way to check the day/night difference would be to use the old trick of listening to another radio's local oscillator on the device under test.  That ought to be the same signal strength, day or night, and so it ought to sound the same, day or night.

Chris Campbell

electra225

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Re: RCA 9-X-572 Weak Reception
« Reply #6 on: September 04, 2019, 08:21:36 PM »
The two closest stations are 40 miles away.  I don't know what their wattage is. 

I've been digging around with my signal tracer.  This radio does not play as strong as it did last night, but the other radio I checked is not as strong tonight as last night.  And I may have tried an hour earlier tonight than last night.  The one thing I found that I consider to be odd is that I have audio on common negative!  I found this, quite by accident, at the "ground" terminal on the phono input jack.  I also have audio across the AVC resistor, which, by the way, is a 3.3 megohm.  The strength of the audio on these points is approximately the same as that found on the plate of the 12SK7 IF amp tube.  There is also a component that looks like a dogbone resistor that goes to common negative in the first audio stage.  I have not traced it out yet, and don't see anything made to look like a dogbone on the schematic anywhere.  I'd think this radio was too new to have a dogbone resistor in it from the factory.  I'm not sure what I should make of all this.  Should there be audio on common negative?  I don't reckon I ever checked for that before, never thought about it, really.  Is it possible that something is bleeding RF and audio off in the front end somewhere, and sending it to common negative? 

I have aligned this radio six times.  The last couple times, there was no change, so I'm reasonably sure I'm not aligned to a harmonic of some kind.  Selectivity still seems good, with no birdies or oscillation anywhere.  Just not enough volume and the thing BARELY plays at all during daylight hours.  This is an AA5 for heaven's sake.  These are supposed to be simple............

Voltages are what they are supposed to be.  I don't have any positive grids.  I have a couple of drifted resistors, but I don't believe they are the cause.   

Sorry, Chris, I reckon we posted at the same time.  I have checked the oscillation squeal and it seems the same day or night.  The only thing I'm missing is volume, particularly during daylight.  I know, it makes no sense at all.   ::) :-[

Another tidbit of information.  The trimmers in the IF cans and the other places all seem to peak fairly sharply, not overly broad.  They were considerably out of adjustment, but once I got them close, they peaked nicely, for what that is worth.

I just found something else.  If I stand the chassis up on end, the volume hops up.  If I lay it back down on its bottom, the volume goes down.  Why do I get all the good ones?  First the Zenith with green crud, now this.  We are fixing to learn something on this one....... ::) ;)

One more thing, then I promise I'll go away.  The strength of audio is about a fourth as strong at the speaker as it is at the plate of the output tube.  I mentioned this before, but now it may be more significant.  All the output transformer does is to couple the plate of the output tube to the voice coil of the speaker.  It should not cut down the volume I wouldn't think.  I'll have to check this on a radio in known good operating condition.  The OP transformer is the typical center tapped primary job that RCA liked so well.  The center tap goes to the screen of the output tube and to the plates of the rest of the tubes.  That part seems fine.  Could there be a shorted turn in the OP trans that would not only cut down the audio, but send some of it to common negative?  It checks okay, both by resistance and by voltage. 
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Motorola Minion

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Re: RCA 9-X-572 Weak Reception
« Reply #7 on: September 05, 2019, 02:05:22 PM »
Ed makes a great point about feedback control systems. AVC is a pretty "tight" loop and a few drifted high-value resistors will limit gain of an IF amp noticeably. I suspect all the AM band is stronger at night, regardless of most stations having to drop output at sundown.   

What I found on a 1938 Crosley is that just one resistor in the AVC voltage path (like R4-5-6-7 in Greg's RCA) was off by 20%, compounded by other resistors drifted, resulted in a constant -4 volts, regardless of what it was tuned to. Never mind the circuits in this Crosley are nothing like a 1938 RCA I am also tinkering with. For instants, Crosley has the low end of the volume connected to ground while the RCA has the low side connected to the AVC bus. RCA did defend its patents rigorously. ;)

Just one very strong station helped to confirm AVC was not varying as it needs to. Grounding the AVC "bus" made the moderate-strength stations louder and of course, the strong station was clipping the audio amp or output.

After changing the culprit resistor and some others, the voltage varied between -2 to -4, tuning from strong to moderate to no station. Now, grounding the AVC bus to a voltage even less negative had no effect on volume of the moderate-power stations. Success  ;D

The bonus with the Crosley is that the tuning "eye tube" actually closes on that one strong station, before it did not do much - just wide open. The 3x high-drifted 1 meg resistor in the eye tube socket should fix that, but it made no difference until the AVC issues were resolved to see it actually work.
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Dave

electra225

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Re: RCA 9-X-572 Weak Reception
« Reply #8 on: September 05, 2019, 02:42:27 PM »
R4 is rated at 56K ohms, reads 70K ohms.
R5 is rated at 3.3 megohms, measures right at 3 megohms.
R6 is rated at 220K ohms, measures 240K ohms.
R7 is rated at 220K ohms, measures right at 220K ohms. 
R12 is rated at 220K, measures 272K ohms.
R13 is rated at 470K ohms, measures 753K ohms.

Element voltages are close, and I'm running the chassis on the Variac and isolation transformer at 115 VAC.

R4 is well above 20%.  R13 is a grid resistor for the output tube.  I need to find some new resistors for a start.

Can I use a "regular" VTVM to measure AVC voltage or do I need something different?  We are dealing with a rookie when it comes to AVC circuits.   ;) :)

I see the schematic calls for 10% resistors in this chassis.  I have more drifted resistors than I realized.  I see on my notes that R14, the cathode bias resistor has drifted up 50 ohms, so it's no good.  R17 is rated at 82 ohms, measures 90 ohms, so it's high.  And R5 is probably off more than 10% as well.  Am I getting warm or chasing my tail?

Anybody care to comment on why I have audio on common negative? 
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electra225

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Re: RCA 9-X-572 Weak Reception
« Reply #9 on: September 05, 2019, 05:26:09 PM »
I found some wiring I don't like.  Too much red wire.  I'm going to pull the chassis out of the 9-X-561 I have and do some comparing.  The 561 has the same basic chassis an RC1079B.  It does not have the function switch like the "F" version has.  I need to verify the wiring scheme in this chassis.
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electra225

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Re: RCA 9-X-572 Weak Reception
« Reply #10 on: September 05, 2019, 07:36:08 PM »
The AVC on this radio is stuck on -.3 volts (three-tenths of a volt negative).  It does not vary.  This is during daylight hours when even the strongest station can barely be heard with my ear up against the speaker cone.  I took this reading with my VTVM, which is all I have.  IF I try to ground the AVC bus, it kills the radio reception completely.  I'll see what happens when it runs at night time.
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electra225

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Re: RCA 9-X-572 Weak Reception
« Reply #11 on: September 05, 2019, 09:01:09 PM »
The AVC varies between a little over 1 to a little over 2 volts negative when this radio runs at night.  I have the 561 with its chassis out and on the bench for comparison.  It is all original except the filter caps.  I know they have been replaced since RCA did not use electrical tape on the connections at the factory.  The thing plays well, good volume.  I'm going to measure some of its components and compare those to the 572.  I'll get some resistors to replace those that are drifted, particularly in the AVC circuit.  The components that look like dogbone resistors must be mica caps.  Someone has kluged another filter cap can in the chassis and wired the high voltage section with all red wires.  Not very neatly.  It appears to be wired correctly.  If I can get the thing to work right, I'll dress that stuff so it looks better.
I don't need Google.  My wife says she knows everything.