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 Post subject: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Wed 19, 2019 1:25 am 
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Joined: Mar Thu 29, 2012 5:22 am
Posts: 177
Location: Lincoln City, Oregon
I am tinkering with an H500 T/O that I previously restored a few years back, and there’s one issue that’s driving me nuts.
As a preface, all paper/lytic caps were changed, as well as a resistor check/replacement process. Also the mica caps inside the IF cans were replaced with dipped mica caps under the chassis and aligned again afterwards. Also tubes in the effected stages have been swapped with no change.

The symptom- distorted audio on weaker stations.

I assume that this issue is within the last two stages, the 1U5 det/AVC/1st audio being most likely, since the distortion isn’t present on all stations which I think would indicate that the final audio 3V4 stage is up to snuff.

The only issue I can find through voltage checks is that pin 4 of the 1U5 is reading positive .05V, when the schematic states that it should be negative 0.7V. I’m not sure where to look for this one, and it’s really throwing me through a loop. No change in measurement with signal strength variances.

Would anyone be able to point out a component or two that could cause this pin to go positive?

Here’s the schematic I’m going off of, which is proper for the chassis variant I’m working on:
http://bama.edebris.com/download/zenith/h500/H500.pdf

Thanks!

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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Wed 19, 2019 2:02 am 
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Joined: Nov Tue 14, 2017 5:09 am
Posts: 1711
Location: Austin, Texas
Have you verified that the feedback winding on the output transformer is OK?
The volume control resistance is also important. If it has been replaced, the resistance may be too large.
There are some resistors such as R27 and R28 that are trying to pull the AVC bus positive. You need to be sure that all the resistances to ground are in place. Measurements in the AVC circuit are difficult because the resistances are so high that the meter will have a significant effect on the voltages.

Jay


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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Wed 19, 2019 2:44 am 
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Joined: Mar Thu 29, 2012 5:22 am
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Location: Lincoln City, Oregon
JnTX wrote:
Have you verified that the feedback winding on the output transformer is OK?
The volume control resistance is also important. If it has been replaced, the resistance may be too large.
There are some resistors such as R27 and R28 that are trying to pull the AVC bus positive. You need to be sure that all the resistances to ground are in place. Measurements in the AVC circuit are difficult because the resistances are so high that the meter will have a significant effect on the voltages.

Jay


Thanks for the input Jay. The feedback winding measures right on 13 ohms per schematic, and the volume control is original and measures correct as well. As to R27 and R28, R27 was replaced with a new film 10M, and R28 tested within spec. I have been looking over chassis ground leads on resistors and checking their continuity to make sure they’re all kosher. No issues found there as of yet.
I figured that a digital meter would introduce the least load to the circuit, and I verified that pin 4’s measurement was indeed positive by swapping the leads.

Sean

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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Wed 19, 2019 4:10 am 
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Joined: Nov Tue 14, 2017 5:09 am
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Location: Austin, Texas
OK.
If you have a small signal diode like a 1N914 or 1N4148, you could try disconnecting pin 4 of the 1U5 and substituting the SS diode. I have had some radios that would not work correctly with any of the tubes I had on hand including the one that came in the radio. I think the problem is that the diode current modulates the filament voltage and that appears as signal input to the triode section. Putting a diode and triode together works OK when the tube has a real cathode but not as well when the two sections are using the same piece of resistance wire as the cathode.

Jay


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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Wed 19, 2019 4:22 am 
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Joined: Mar Thu 29, 2012 5:22 am
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Location: Lincoln City, Oregon
One more update, I was messing around with it again and decided to swap the 1L6 for a 1R5 for the troubleshooting duration to avoid stressing the unobtanium with all the on/off cycles. After the swap I found that sweeping across the band I AM getting varying voltage on pin 4 of 1U5, no signal sits around +.02V, and a strong signal knocks it down to -0.4V or so. A weak station will make the voltage sit at about +0.01V, which would explain the distortion on weaker stations. Whatever this problem is seems to be slight, now I’m wondering if it could be that one of the megohm-plus resistors is a bit higher than the 20% allowance and could be throwing the AVC above zero volts?
This is a problem I’ve never encountered, so AVC troubleshooting obviously isn’t my forte. I’m starting to wonder just how many possible trouble spots there are, considering this could be happening somewhere on the AVC line farther upstream than where I was looking.

Sean

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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Wed 19, 2019 4:49 am 
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Joined: Nov Tue 14, 2017 5:09 am
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Location: Austin, Texas
Battery sets are always tricky since each filament/cathode is at a different voltage as you go up the string. The AVC needs to get more positive toward the RF stage and there appear to be a lot of resistors to create voltage offsets. You may have to Increase the values of one or more of the resistors that are pulling the AVC positive or reduce some of the resistances going to ground. I would try to get the voltage to zero at the detector with no signal present.

Jay


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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Wed 19, 2019 2:01 pm 
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Location: Long Island
If you look at the VI curves of detector diodes—tubes or solid—they are only relatively linear for a small distance in the middle. At low signal levels and high ones they’re very nonlinear and a lot of distortion will result at either end. So radio designers counted on good AVC action to keep the signal levels on the linear portion of the detector. This was one of the reasons diode detectors did not enjoy much popularity outside of crystal radios until AVC was developed in the early 1930s.

So not only could out of spec resistors, bad tubes, or bad capacitors in the AVC circuit be the cause of the problem, but the RF and IF stages of the radio have to be doing their jobs right too. Otherwise there won’t be enough AVC voltage for proper action. So now those tubes and parts, as well as the state of alignment, have to be checked over as well.

One hint on radios like the TO series. The AVC circuits contain a number of multi-megohm carbon resistors. Aside from considerable drift, there is always a possibility with resistances that high of leakage paths through the film of dust, humidity, and fried electrons that accumulates in old electronic devices. Old or new solder flux can have a significant impact as well. Making sure all components and terminals associated with high value resistors are scrupulously clean is good practice. I use denatured alcohol or acetone on cotton swabs for this.

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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Wed 19, 2019 6:17 pm 
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Joined: Mar Thu 29, 2012 5:22 am
Posts: 177
Location: Lincoln City, Oregon
Thanks all for the input!
I started back into it again by trying tube substitution one at a time while monitoring the detector voltage, and no significant change was noted. In swapping the RF amp 1U4 for another I noticed that the voltage with no signal went from +0.05 down to +0.03. Not a huge difference, but it could show that the one in place was very slightly leaky. Sadly not enough to fix the issue.
I now plan on going back through the meg-plus resistors and changing the originals one at a time that tested good (again, these were tested about 7 years ago and the set has seen a lot of use since then). I would ohm them out but I don’t have a meter on hand at the moment that can handle anything over 2M.
It’s a shot in the dark, but hopefully will yield some results. Plus, it’s probably in the neighborhood of preventative maintenance since any high value carbon resistor is bound to just keep on drifting as the years go by.
While I’m in there I will scrub all the phenolic terminal strips that are associated, as well as the tube sockets. Another possible spot for issues I can think of would be on the phenolic bases for the IF cans, where a little tiny bit of B+ leakage could ever so slightly bring the AVC up.

I’ll keep you posted with my progress on it.
Sean

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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Wed 19, 2019 8:41 pm 
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Joined: Dec Wed 25, 2013 7:57 am
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Location: USA
I never had a meter that can handle more than about 2 meg ohm; if in doubt, it's just a few minutes of labor to swap out those resistors.
I would also suggest checking everything surrounding that IU5 tube, not just resistors, including voltage on all of the pins.

Not knowing exactly how you "tinkered" with the set, is there a chance that you inserted a resistor along with a new rectifier diode? If so, double check to make sure there's sufficient filament and plate voltages...it just maybe a few millivolts low enough to cause an older 1U5 or other tube(s) not to provide full performance. If it still has the original selenium rectifier...same thing.

Just a hunch; also check your AVC circuit to make sure it's working correctly.


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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Wed 19, 2019 9:00 pm 
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Joined: Mar Thu 29, 2012 5:22 am
Posts: 177
Location: Lincoln City, Oregon
AJJ wrote:
I never had a meter that can handle more than about 2 meg ohm; if in doubt, it's just a few minutes of labor to swap out those resistors.
I would suggest checking everything surrounding that IU5 tube, not just resistors.

Not knowing exactly how you "tinkered" with the set, is there a chance that when you restored the set, you inserted a resistor along with a new rectifier diode? If so, double check to make sure there's sufficient filament voltage...it just maybe a few millivolts low enough to cause an older tube not to provide full performance.

Just a hunch; also check your AVC circuit; perhaps AVC is somehow pushing down a the signal level below the threshold needed for good audio detection on weaker signals.



Tinkering is what I’m doing now, the restoration part is pretty much done lol.
Yes, the selenium stack was replaced with an added 50 ohm wirewound to get the proper voltages off the voltage divider. But, that said, I mainly use this radio with battery power. Naturally B+ is a little higher when I run it on AC, but the issue persists no matter if it’s on AC or battery.
I am going to swap the higher value resistors as soon as my new pack of resistors gets here on Monday. Odd how all the meg resistors seem to fly out of the parts stash quicker than they can be refilled...

I have been putting the circuits around the 1U5 under scrutiny, and one of the major suspects that turned out to be innocent was one of those famous Zenith leak-o-matic wax-dipped ceramic disc caps. In fact there were two in the radio in the 1U5 vicinity, but after swapping with new film caps there is still no difference. Again, preventative maintenance justifies chucking them I suppose..

Sean

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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Wed 19, 2019 9:10 pm 
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Yes, check -everything-; make no assumptions of anything being good or bad. In my short experience, whenever a the cause of a problem doesn't reveal itself immediately, a full 100% inspection/test always turned up a mistake/defect. Sometimes it literally required a 10X loupe to inspect every single connection to find the fault.

Could the problem be the speaker?


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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Thu 20, 2019 4:06 am 
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I have been dissecting the AVC line, referencing the voltage on pin 4 of the 1U5 as I go along to look for changes. Disconnecting the lead coming from the bottom of the 2nd IF sec. from the volume control makes the voltage go negative! From there we go downstream.
I ruled out anything in the RF 1U4 and 1L6 conv. stages through disconnecting their AVC supplies, and found that two other connections made a difference. A small change originating in the secondary area of the 1st IF, and the biggest change came from disconnecting R27. R27, as Jay suggested, is trying to pull the AVC positive, so if it is under value it may be causing this. I will replace that one once my new parts come in to rule it out, seems unlikely to me considering it is a new carbon film resistor. If no change, I’ll look around the 1st IF secondary to see if anything is amiss. Up there is another resistor Jay mentioned, R28 (4.7M). This one is original, but got the sharpie mark of approval when I tested them 6-7 years ago. I’ll go ahead and swap this one just for good measure.

I think we’re getting close!
Sean

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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Thu 20, 2019 4:10 am 
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Joined: Mar Thu 29, 2012 5:22 am
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Location: Lincoln City, Oregon
AJJ wrote:
Yes, check -everything-; make no assumptions of anything being good or bad. In my short experience, whenever a the cause of a problem doesn't reveal itself immediately, a full 100% inspection/test always turned up a mistake/defect. Sometimes it literally required a 10X loupe to inspect every single connection to find the fault.

Could the problem be the speaker?


I doubt the speaker could be causing this distortion, since strong stations sound fine. I did the push test on the voice coil and could feel/hear no rubbing. I will give it a shot with headphones to rule it out entirely, but I’m 99% sure this is an AVC issue considering the positive voltage on the det. plate lead in low signal situations.

Sean

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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Sun 23, 2019 12:25 am 
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Joined: Mar Thu 29, 2012 5:22 am
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Location: Lincoln City, Oregon
Alright, it’s back in service! A combination of new resistors and a fresh set of batteries greatly improved the sound quality. I began to notice that the sound quality got worse the more I worked on it, then I checked the A battery voltage. Floating at 8 1/2V! D’oh.
Gave it a good burn-in today on AC, and another odd problem popped up. After a few hours the shortwave reception faded out slowly and quit. When this happened I decided to check the broadcast band, and it worked fine, but only in the upper half! At first I thought the original sand resistor or candohm finally gave up the ghost, but seeing that it still partially worked made me think it must’ve been a tuned circuit problem. Swapped a couple tubes, and with another 1L6 it’s working well again. Odd failure, maybe it just slowly became more leaky until it finally dropped out. Also didn’t seem to happen on battery. Possible that as the power supply warms up the associated power resistors climb in value a bit and reach the point where the mixer voltage drops enough to kill the LO.
But, if it’s the tube, this wouldn’t be the first finicky 1L6 I’ve dealt with. I had another that would just randomly start feeding back and causing heterodynes all over a little while into operation.

Anyways, thanks all for the contribution and help! Glad to have the T/O firing on all five again.
Happy Summer, Sean

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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Sun 23, 2019 3:46 am 
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Have a look into this odd 3 terminal capacitive device.
Attachment:
Zenith H500   IU5 PIN 4   PROBLEM   X MARKS THE SPOT.JPG
Zenith H500 IU5 PIN 4 PROBLEM X MARKS THE SPOT.JPG [ 17.67 KiB | Viewed 285 times ]

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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Sun 23, 2019 4:05 am 
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Joined: Mar Thu 29, 2012 5:22 am
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Location: Lincoln City, Oregon
radiotechnician wrote:
Have a look into this odd 3 terminal capacitive device.
Attachment:
Zenith H500 IU5 PIN 4 PROBLEM X MARKS THE SPOT.JPG


That was the first thing I suspected, so I went ahead and tore the cans open and replaced the mica wafer caps with new dipped mica caps under the chassis. Interesting idea they had in doing that, but unfortunately failure-prone.

Sean

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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Sun 23, 2019 5:28 am 
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Location: Powell River BC Canada
How did you know what capacity to make them ?

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 Post subject: Re: Zenith H500 Trans-Oceanic odd distortion
PostPosted: Jun Sun 23, 2019 6:32 am 
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Joined: Mar Thu 29, 2012 5:22 am
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Location: Lincoln City, Oregon
radiotechnician wrote:
How did you know what capacity to make them ?


I had asked on another forum if anyone had an approximation for these, and 100pf was the ballpark value.
The second IF peaked well with 100pf across the windings, and the capacitor on the very bottom is the IF bypass which, through trial and error, worked best with .001mfd.
The first IF was better with lower values across the windings, and I was able to get it to peak at 82pf. If I go back inside at any point I’ll probably lower the value a little bit since the slugs ended up pretty far out before they peaked.

Sean

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