crosley: Ch= 1217; Prestotune 12: Volume jumps

Thanks to everyone for their help with this radio.  I ended up replacing all of the electrolytic and paper capacitors.  That fixed the excessive current draw but there was still no output. I then replaced the audio amplifier tube which restored normal operation.  The radio now works and sounds great.  Except for one problem: occasionally, at some random times, the volume will suddenly drop almost in half.  Then it will play that way for a while and then it will pop back up to the original level.  That cycle continues to go on as long as it's on.  What on earth could be causing this?  It's hard to trace, because most of the time it is working OK.  I have checked all of the tube connections and switch contacts.  They seem to be OK.  I'm not sure where to begin and would appreciate any advice.  Thanks und danke schoen.

          - Michele

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Dear Michele,

in "Ghirardi, A.A.: Radio Troubleshooter's Handbook, Murray Hill Books inc., 1943" there is in paragraph "Case Histories of Receiver Troubles and their Remedies" only a remark to "motor not stopping" for model 1217, however, for model 1117 which is hopefully somewhat similar, a remark to "Fuzzy" tone is posted:

"Install special shielded lead (No. G122-34403) between center arm of volume control and the coupling condenser."

It seems that fuzzy tone is a known problem since 1943.

I hope this information will be helpfull.

Best regards,

Dietmar

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Thanks, Dietmar, for the reply.  However, the problem is not a fuzzy tone.  The problem is that the volume simply jumps randomly between loud and soft.  I'm thinking it might be a cold solder joint somewhere, or perhaps a tube problem.  Could a defective tube cause sudden changes in volume?

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Hi Michele,

This kind of problem is often not so easy to track down.  It sounds like you already looked into some of the most likely causes, which would be:

-  poor contact of tube pins in a tube socket (test by wiggling tubes while radio is operating)

-  dirty switch contacts (test by rocking the switch during operation, or testing resistance while rocking)

Other candidates might be:

-  unreliable chassis ground connection (try tightening/cleaning any connections with screws, or hard wiring connections between things relying on rivets)

-  dirty potentiometer contacts (wiggle the control while operating)

-  dirty or rubbing tuning capacitor plates (test by inspecting very carefully by eye as plates are meshed/unmeshed, and bend to adjust anything that might rub; clean off any physical contaminants)

-  poor contact of tuning capacitor rotor ground contact (spray contact point with contact cleaner and/or physically polish or scrape)

Your hunch about a bad tube is another possible cause.  Once in a while you can find tubes that have intermittent internal shorts, so substituting tubes is worth trying.  Note, however, that almost any tube in the radio has some potential to cause the problem, not just the audio stage tubes.

In many cases, you simply have to take a very methodical and patient approach, trying to isolate stage by stage what the problem is.  One strategy for this would be to work your way backwards from the speaker, using an external signal source.  At each step, you wait long enough so that the problem should have occurred, and if there is no problem, move back a stage and repeat the test.  The sequence can be something like this:

1.  Test the speaker with audio from another source (pretty unlikely to be the problem, but worth confirming)

2.  Test the output transformer and audio output stage by feeding in an audio signal from a signal generator to the grid of the output tube.  If no volume variations, assume good.

3.  Test the first audio stage by feeding in an audio signal to the grid of the first audio stage.

4.  Test the IF stage, second IF transformer, and detector by feeding IF signal (audio modulated 455 kHz) into grid of IF tube.

Continue working your way back toward the antenna in this manner.

If the intermittent problem occurs only rarely, this approach can take almost forever.  A strategy for speeding up is to spray things with "Freeze-It" spray, although you don't want to hit a tube with that spray.  It is only marginally successful for inducing intermittent problems in general, so don't count on it really helping much.

Others may have some good ideas on how to attack this kind of problem.

Best regards,

Tom

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Thanks Thomas, for the very clear reply.  I powered up the radio, and put meters on the 6Q7 audio amp. grid, the 6K6 audio output cathode, and the 6K6 audio output plate.  Then I waited.  And waited.  And waited.  After over an hour, much longer than the problem usually showed up, I got a brief dip in volume and then it popped back up.  I saw no change in any of my metering points.  For the record, I was seeing -1.1 v. on the 6Q7 grid (which seems about right), 22 v. on the 6K6 cathode (which is exactly what the specs call for) and 264 v. on the 6K6 plates.  The 264 sometimes varied from about 259 to 267.  After another hour, it wouldn't repeat the problem so I called it a night.

Today I tested all four 6K6's and found two indicated weak emission on my Eico 625 tube tester.  I replaced them and tried again.  Now the output plates were reading 276 v.   The radio sounds OK but the specs call for 245 v. here +/- 10%.  276 is over 10% - should I be concerned?  What could be causing excessive plate voltage?

Another possibility is that the fault might be in the auto-quieting circuit that mutes the audio when the "prestotune" buttons are active.  I may take a look at R42B, R40, and R41, though I'm doubtful.  Can old resistors have some failure mode that results in intermittent operation?  If so, they may be sometimes allowing the 6Q7 audio amp. to be partially cut off.  Just a thought.

As for testing the speaker alone with another source, I don't really understand the operation of these field coil speakers and I'm not sure which two of the 5 pins I should hook up.  So the whole thing is still a mystery.  If anyone has additional thoughts, I'd like to hear them.  Thanks.

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Hi Michele,

Sorry to see that you're dealing with long waiting times to get the problem to appear.  That gives you very few chances to make measurements and parts substitutions.  A real test of patience!

Resistors can be intermittent, as can capacitors.  However, it is more common for these parts to show up as "noisy" rather than intermittent.  If you are successful at some point in isolating which stage is responsible for the problem, I would consider every component in that stage as suspect, and worth investigating.

Honestly I doubt that it pays to check your speaker, but if you want to, you would break the connection between the output transformer and the speaker voice coil, and connect the voice coil to another source.  You have to leave this radio turned on, however, so that the field coil remains energized.

Testing by the voltmeter approach could actually take longer than testing by the stage-by-stage approach, since there are quite a few different points to measure in each stage (cathode, grid, screen, plate for each stage would be the most complete).  After you identify which stage is at fault, then probing several points can help further narrow down the problem.  Anyway, since you've already starting probing voltages, I can add some comments on that approach.

The particular points you chose to test were OK for trying to find the problem, but these points are not sufficient to guarantee that the problem is not in the audio stages.  For example, the plate voltage on the 1st audio stage was not checked, and a change in its reading would not affect any of the other points you measured, but could still affect the audio output. 

The phase inverting transformer 61 is another thing to suspect in the audio stage.  Check the grid #1 voltage on both pairs of 6K6s; they should generally be about the same (both should be zero volts DC).  If the DC voltage on either grid moves away from zero when an intermittent event is heard, the transformer is highly suspect (could be open intermittently in part of the winding).  There are various failures (e.g. shorts) that could occur in that transformer, however, and you won't necessarily see a DC voltage change on the grids.  If stage-by-stage testing points to this transformer, we can come up with some further ways to check it. 

Regarding the high plate voltage on the output tubes:  Not necessarily a problem, but one possibility is insufficient plate current in one or more of the output tubes.  If you confirm that cathode, grid #1, and grid #2 voltages are correct on all four output tubes, then I wouldn't immediately worry too much about the somewhat high plate voltage.

As I mentioned above, don't rule out the IF and RF sections as the source of your problem.  A bad tube or component in those areas is just as likely to be your problem as a problem in the audio stage.

One DC measurement that I might suggest to help "divide and conquer" would be to measure the AVC voltage.   Here's what it can tell you:

-  If the AVC voltage (ungrounded side of capacitor 19B) does not change when the volume changes, the problem is not in the RF, IF, or detector stages, but should be in the audio stages.

-  If the AVC voltage does change, the problem is in the RF, IF, or detector stages, and is not in the audio stages.

Note that AVC voltage should be several volts negative on a strong station.  The more negative the AVC voltage, the stronger the signal in the RF and IF stages. 

B+ should be monitored at all times, since a change there can affect all stages.

Best regards,

Tom

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Thomas, you are a *GENIUS*.  I set up the following probes and got these voltages:

1: B+ = 281 v. (note that my line AC is at 117 V. this evening)

2. T61 = 0 v. (both sides)

3. 6Q7 AF amp plate = 180 v.

4. hot side of C19B (AVC): on a very strong station = -0.2 v., does not change with volume

I was watching these numbers for about 20 minutes when suddenly the speaker volume jumped *up*.  As it happened, I was looking right at meter no. 4 at the time and saw the needle jump up to 1.6 v. just as the volume changed.  It hung up there for just a few seconds and then went right back to -0.2 v. as the volume went back down.  None of the other voltages changed.

So that certainly seems to suggest that the problem is in the AVC circuit itself, would you agree?

The question now is - what next?  C19B is connected to one of the diode plates of the 6Q7 AVC tube via R35 and R43.  It also connects to the grid of this tube via R35, R42A, and R42C.  The AVC tube, perhaps?  I look forward to any further advice you might be able to offer.  Thanks again.

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Hi Michele,

You've probably significantly overrated my capabilities; hopefully we'll get this one figured out!

First, I just want to confirm that when the volume went up, the AVC voltage went to +1.6 V (as opposed to -1.6 V).  Is that correct?

If so, then these AVC voltages are generally not correct.  We would normally expect on a strong station to see several volts negative, and a smaller negative or near zero voltage on a weak station or no station.

In interpreting the AVC voltage, keep in mind that there can be a malfunction in the AVC circuit itself, or it may be an indication of a malfunction in the circuits which drive the AVC.  The AVC voltage is generated by rectifying the RF (actually 455 kHz IF) signal on the plate of the 6U7 IF amplifier (this is not the usual configuration for an AVC circuit, but should work fine).

If the AVC circuit itself were OK, then we would expect to see the voltage go more negative on a stronger signal (i.e., when the volume jumped up, the AVC should have jumped more negative).  If that happened, we would assume that something intermittent in the RF or IF stages caused the signal to increase.

Since the AVC voltage is not in the right range, and it seems to have jumped more positive when the volume went up (if that's what you confirm is correct), then it is quite possible that a problem in the AVC circuit itself is actually the root cause of the volume jump.

First, the AVC voltage should never be positive, so if we analyze the circuit, we might suspect perhaps intermittent leakage in capacitor 18A.  This is a 250 pF mica capacitor; usually they're pretty reliable, but sometimes not.  Leakage in this capacitor can pull the AVC voltage towards positive, since the other side of the capacitor has a high positive voltage on it (plate voltage of 6U7 IF amplifier).  You might try swapping this component with a new one.

Resistor 35 is also a good suspect, since it is a high value (2 megohm) resistor, and these are often unreliable in old radios.  If it is intermittently open or jumping up in value, the AVC could float to some unexpected voltage.  Try replacing it with a new one.  Resistors 42A (500 K ohm) and 43 (300 K ohm) could also be suspect.

An intermittent short in the diode of the 6A7 AVC/squelch tube (55A) might be a possibility, although it should not cause the AVC voltage to actually go positive.

An intermittent short from screen grid to control grid in either the first 6U7 (52A) or the 6A8 (53) tubes could also cause the AVC to go positive, so you might try substituting either of those with a different tube.  You could also have grid emission, an unusual type of tube defect, in either of these tubes which can cause the control grid to go positive.

I'd suspect capacitor 18A or resistor 35 first.

If none of this helps, then try shorting the AVC to ground (short across capacitor 19B).  Wait and see if you still get a jump in volume.  If you don't, it further confirms that the problem could well be in the AVC circuit itself.  If you still get a volume jump, then the real problem is in the RF or IF stages, and what we are seeing in the AVC voltage is merely a symptom.

Good luck with your detective work,

Tom

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Hello Michele,

If the intermittent "volume fluctuation" problem has not yet been resolved, I would offer a similar suggestion as posted by Herr Dietmar Rudolph:
Throughout my textual descriptions, I use the term elko, in place of the lengthy "electrolytic condenser" - my apology.
Please note that I have reviewed all 4 schematics pertaining to the chassis 1117 (early and late, and for the Super 11, in addition to the combination 1217, 1227 models chassis).

In the Sylvania News Technical Section, Volumes I and II there were 2 instances wherein service technicians reported similar anomalies as in your Crosley Prestotune, chassis 1217.

In the first report (Mar-Apr 1940 Vol 8 No 8) for  chassis 1117 the problem was described as low persistent, steady or intermittent hum which would sporadically increase or decrease in volume. After extensive fault isolation procedures, the technician determined the culprit to be related to the 40Mfd elko in the power supply - a poor ground connection. He installed a lockwasher on the top of the chassis under the elko can, and to ensure absolute best ground, soldered a short piece of heavy copper braid from the securing nut to the chassis. Problem disappeared. Curtis W. Winegar, Salem, Oregon

In the second instance  (May 1945 Vol 11, No 12), the fault was related to the Crosley Super 11, model 1117. There was considerable distortion in the audio. After trying "Everything else" in the words of the service technician, he replaced the 1 Meg ohm volume control and the set played as good as new. Green Radio Service, Muncie, Indiana.

I realize that it is quite possible, or probable, after more than 70 years, that the original elkos may have been completely bypassed with full under the chassis replacement. If the elko was "restuffed" and replaced, the possibility still exists that the poor ground (or oxidizing contact) will have NOT been resolved.

If the problems you are experiencing is not related to the elko, then the fault may very well be in the volume control itself.

I consider somewhat important your own observations that the B+ is showing fluctuations which may be attributed to a mediocre ground connection.   

One other lesser possibility may relate to a failing C13 elko across the screen and suppressor grids of the RF amp tube 6U7G.

Respectfully,
Robert
 

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Well, the problem is fixed.  I tried posting this yesterday, but I couldn't send any posts to the Forum.  The Forum Editor window appeared, but no text box to enter any text.  It must be my computer because I'm typing this from a different machine and it seems to work here.

Anyway, to end the suspense, the problem was simply a defective 6A8 modulator tube.  After reading your last post, I decided to take your advice about not ruling out the IF or RF stages.  The easiest thing to try was a little tube tapping so that's what I did.  I wiggled the grid cap on the 6A8 (one of the tubes you mentioned) and instantly got the problem to show up.  The volume jumped up and down predictably when I moved the cap.

I pulled the tube and carefully cleaned both the cap and the connector and then tried it again.  To my surprise, that didn't help.  I pulled the tube again and tested it.  It showed up at the top of the "Replace" arc on my tester and also lit up the "tube short" lamp when I moved the no. 2 lever.  I replaced the tube and the problem was gone.

And to answer your question, in my last post I did indeed forget the minus sign when I gave the AVC voltage.  I was getting -0.2 v. most of the time.  When the problem appeared, the needle jumped up, but that means the voltage jumped lower, down to -1.6 v.  Anyway, with the new 6A8, I am now getting -1.3 v. on a very strong station.  Would you call that acceptable?

I'm still not sure what is wrong with the old 6A8, but it clearly was not simplly dirty contacts.  It took me a while to figure out because in general I am loath to condemn a tube before anything else.  However, since I had already found one bad and two weak tubes in this radio, I should have been more willing to suspect there might be more.

So thank you again Thomas, for all of your help in fixing this problem.  I do really appreciate all the time you obviously took to read my posts, look at the schematic, and write illuminating explanations.  Your suggestions were right on the money and led me directly to the trouble without a lot of "wild goose chasing".  The radio is now playing like new.  I have it hooked up to my little Vectronics AM transmitter and I play the 1920's Radio Network feed on it from my laptop PC.  It sounds great!

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Hi Robert!  As you can see in my delayed post above, the problem is now solved.  As it happens, I had already replaced all of the electrolytic and paper capacitors in this radio.  The electrolytics had already been replaced once by some amateur (the 30 uF C13 had been replaced with three 10 uF's in parallel).  The paper capacitors were all original and all quite dead.  Not one of them showed any capacitance action at all.  I was really hoping I hadn't gotten a defective new capacitor in there somehow.

Anyway, it's great that you provided those tech notes because they are sure to help someone else down the line.  I will also print that out and keep it with my folder on this radio  Thanks very much for your help and for taking the time to add this information.  The Prestotune 12 is a great old console and it's nice to have it running like a champ again.

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Hi Michele,

Great to hear that you got it fixed.  Your 6A8 may have an internal short, and these can indeed be intermittent and influenced by external tapping, etc.

The AVC voltage on a strong station should probably be a bit more negative.  If the radio performs well enough, you don't need to do anything about it.  However, the following are the most likely cause:

-  antenna not long enough

-  radio needs alignment (if you are interested, there are some simple things that can be done without a signal generator without trying to do a full alignment)

Cheers,

Tom

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Hi Thomas.  You are probably right about the new AVC voltage.  My "very strong station" is my little AM transmitter kit with its antenna wire placed right next to the radio's antenna terminal.  At this point, I think I'd like to let it run for a week just to convince myself that it's really fixed and that nothing else goes wrong.

One strange thing I noticed was that one capacitor you mentioned, C18A (called out as a 250 pF mica) is actually a 100 pF unit in this radio.  In fact you can see a photo I took of that very part in my previous thread on this radio about mica capacitor color codes.  I'm not sure if this was a mid-production run mod, if the assembler just picked the wrong part, or if it had already been incorrectly replaced at some distant point in the past.  Would that affect the AFC voltage?

Anyway, I'd love to hear your no-signal generator alignment tips.  I've never done an alignment before and I'm afraid of giving my radio diddle stick-itis :-)

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Hi Michele,

I noticed that the value on one or more of your mica caps didn't match the schematic back when you first showed the photos.  This may have been a production change, so I would not advise changing the value to match the schematic until some attempt has been made to improve the alignment.

Normally, I would advise touching up the alignment on the IF transformers using a weak over-the-air signal, but because this radio has a triple-tuned 1st IF transformer, as well as some feedback for neutralization, there is too much potential to create problems.  So I would advise against touching the alignment of the IF transformers on this one without a signal generator and following the manufacturer's alignment instructions.

You can, however, touch up the alignment on the antenna and RF stage trimmers, and this often leads to substantial improvement in sensitivity.  If and when you're interested in giving it a try, see below.

Before adjusting any trimmers, please check the following:

-  Make sure the dial pointer is in the correct mechanical position.   When you tune to the two ends of the dial, does the pointer properly travel to both ends of the scale?

-  Is the dial calibration on the broadcast band pretty close?  Tune in a station on the broadcast band near the bottom of the dial, another station in the middle of the dial, and one near the top of the dial.  Does the frequency read properly in all three parts of the dial?  "Close enough" would be within about 30 kHz, although closer would be better.

If you notice problems with either of these, I would stop and report back here.  I'll provide instructions on what to adjust to fix the dial calibration.

If the calibration is pretty good, then take a look at the chassis diagram shown in Fig. 2 of the "chassis" diagram available here in RMorg for this model.  Find the two trimmers "Antenna - B.C." and "Radio Frequency - B.C."  (Note there are three "Antenna" trimmers and three "Radio Frequency" trimmers -- one for each band.)

Before adjusting, mark their original positions, so you can put them back in case things don't work out properly.

With the local/distant switch set to "distant" and a typical wire antenna connected to the antenna terminal (about 10 feet connected to "A1" is fine; you should also have a jumper wire between "G" and "A2"), tune in a weak station on the broadcast band at around 1000 - 1300 kHz on the dial.  Adjust both of the trimmers mentioned above for strongest signal.  After adjusting, retune the station on the dial and adjust them one more time (sometimes there is a little interplay between the dial tuning and the adjustment of these trimmers).

You can adjust for strongest signal either by listening with your ear (which works quite well for most people) or by monitoring the AVC voltage while adjusting.  Remember that a more negative voltage on the AVC indicates a stronger signal, so you will be adjusting for maximum negative AVC voltage.

Did your weak station get stronger after adjustment?  Most likely it will.  Did you get a clear "peak" when adjusting both trimmers?  If you don't have a clear peak within the adjustment range of the trimmer, be sure to report back here, since that can indicate a problem.

If you were successful in adjusting both trimmers, now check to see if sensitivity is reasonably good for stations at both the top and bottom of the dial.  If OK, you're done.  If not, report back here with what you find (or simply put the adjustments back to their original positions and do nothing further).

Do you get at least a few volts negative on the AVC now for a strong station?  If not, the IF may need alignment, but I would recommend getting a signal generator to align the IF on this radio.  For most AM radios, you can touch up the IF alignment without a signal generator, but this one's a bit more complex.

I'm also not suggesting making alignments of the bands other than the BC band for now.  Best not to try too many things on your first attempt.

Naturally, the best way to align a radio is with a signal generator and following the manufacturer's instructions.  The simple procedure mentioned above doesn't replace a full alignment by any means, but it can be helpful in cases where a more elaborate alignment is not possible.

Best regards,

Tom

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