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Old 14th June 2007, 01:50 PM   #1
AC439 is offline AC439  United States
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Default Zenith K731 AM/FM radio audio random cracking sound in FM mode

Hi...I have been battling with this problems trying to fix my old radio for a while and searched over the internet to no avail.

The radio has developed into a random cracking / popping sound only in FM mode. The radio usually starts up fine and after 30 seconds or so, the randon cracking sound starts. Usually when this happens, the sound level decreases and the tuning dial would be off a little bit and needed to be slightly adjusted. Then sometimes, it pops again and have to put the tuning dial to the previous position. When it first turns on, the station seems to be positioned correctly on the dial and it's loud and clear. After it starts making cracking sound, the station is off and is usually peaks at two adjacent locations on the dial. The position between the two peaks will gives a low and distorted sound.


At first, I thought it was a cold solder joint or something and I have done lots of cleaning (tubes + sockets) and tapping lightly around to try to find the weak joint. But all efforts failed. The cracking sound is not from loose components. I found one info from internet about taking apart the I.F. transformer and clean the mica capacitor within. I took apart one IF trans (which took me 3 hours) but the design of the trans is not the same as the author described so I don't think that is the source of the problem.

My impression is the first RF stage in FM seems to be causing the problem and since this tube is not shared with the AM RF stage, maybe it is going bad? I can't find the *exact* schematic of this radio. I found one previous posting in this forum referring to a schematic but it is a different chassis and I already found some differences with my radio. I don't have an OSC scope so I'm limited to basic home tools for troubleshooting.

Sorry for the long posting. Any ideas for troubleshooting will be highly appreciated.

Thanks. - Alex
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Old 14th June 2007, 03:23 PM   #2
kevinkr is offline kevinkr  United States
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Without basic tools it is going to be hard for you to diagnose the problem, but abrupt changes in tuning frequency usually relate to the oscillator circuit not the rf amplifier. I would replace this tube first. Check for bad paper or ceramic power supply bypass caps in the oscillator stage. (Do not replace anything else as quite often other caps will be NPO types or even have a specific temperature coefficient to compensate for drift in other components.) Beyond that I would find someone who has the requisite tools and knowledge to look into this deeper.
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Old 14th June 2007, 03:35 PM   #3
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An intermittent connection in the IF transformer would also shift the tuning off center... silver migration (common is Zeniths) would probably affect AM and FM, as it usually results in intermittent shorts to ground. Maybe a bad cap or connection to it. Unfortunately, they are all inside the IF cans...

My Sams index says it's in 653-12 - see if you can get a copy (maybe your library?).
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Old 14th June 2007, 03:35 PM   #4
anatech is offline anatech  Canada
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Hi Alex,
I agree with Kevin on this. These faults can drive you nuts. I'm thinking of an intermittent capacitor since the tuning is affected.

You could hang a 'scope and / or freq. counter on the LO and just watch it. If the frequency changes, your fault is in the LO, if not your fault could be RF or converter. Possibly in the IF as well if it's both 450 KHz and 10.7 MHz. I've seen this done before. Same tubes for AM and FM, but different IFT's.

-Chris
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Old 14th June 2007, 03:36 PM   #5
w5jag is offline w5jag  United States
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Based on your description, I would probably look at the FM detector circuit before I looked at the front end. You probably have a coupling capacitor breaking down or maybe a bad tube. After checking the detector for proper operation, I would look at the AFC (automatic frequency control) loop.

You need a schematic to be able to do much of anythng other than basic component checks. Don't waste your time taking IF transformers apart - it will be something simple; likely a bad cap.


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Old 14th June 2007, 03:38 PM   #6
anatech is offline anatech  Canada
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Quote:
Don't waste your time taking IF transformers apart - it will be something simple; likely a bad cap.
Yup.
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Old 14th June 2007, 03:41 PM   #7
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" ... When it first turns on, the station seems to be positioned correctly on the dial and it's loud and clear. After it starts making cracking sound, the station is off ... My impression is the first RF stage in FM seems to be causing the problem and since this tube is not shared with the AM RF stage ..."

Assuming you have tested all of the tubes first especially in the power supply and the rest of the FM section, I would guess this is thermal related ("after warmup" related) like maybe a resistor or capacitor in the FM tuning section is gradually getting hot and undergoing some thermal related changes, effecting the tuning frequency time constants.

It would be unusual that a "cold joint" would show up after years of operation, but hitting those joints in the FM section with a "fresh" iron and a little bit of new solder might not hurt = a very little amount of solder of course, as a big goobber would probably knock the whole FM tuning circuit way out of alignment ... ... beyond that, then maybe a little "out of circuit" measurement of those resistors and capacitors in the FM tuning section would reveal the culpret. (A good voltmeter / Ohm meter (VOM) should do it.)

kevinkr: " ... Check for bad paper or ceramic power supply bypass caps in the oscillator stage. (Do not replace anything else as quite often other caps will be NPO types or even have a specific temperature coefficient to compensate for drift in other components.) ..." " ... it will be something simple; likely a bad cap." ... "Yup. ..." " ... I agree with Kevin on this. ... I'm thinking of an intermittent capacitor since the tuning is affected. ..."

Kevin, anatech, et al, seem to be on the right track here ... bad cap somewhere, probably power supply or around that first stage tube ....

I would also say that Zenith / HeathKit didn't change their schematic much from year to year, so a schematic that falls into the right part of the decade of manufacture would probably not have changed much from your design.

(I rummage through the flea markets here and in upper Michigan, seeking out very old tube radios for repair and upgrade .... kind of a hobby ... If you can't get it fixed, I want it ... I did find this: http://www.diyaudio.com/forums/attac...amp=1060383242 ... but can not verify the validity relating to you set.)
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Old 15th June 2007, 12:10 PM   #8
AC439 is offline AC439  United States
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Thanks for all the ideas. Forgot to mention ---- When I tune off a station to an empty spot on the dial, it doesn't seem to give the cracking sound and the level of white noise remains the same. So, I'm still leaning to problems at the front end.

As for other things I have done on it, I have replaced the selenium rectifier with an 1N4007 with added resistor to keep the B+ the same and the voltage stays solid at the same level over time now. The old selenium rectifier's output decreases as it warmed up. However, this doesn't fix the problem. I have looked and looked but not really finding other paper caps. I think ceremic caps should last forever?

I do have a freq counter so I can monitor the local OSC for changes. I'm going to check the bypass caps. I do think this is somewhat temp related but I think if components drift after warming up, it should drift off one way instead of making cracking sound and bouncing back and forth. I'm also thinking about a possibly leaking tube (12DT8), especially after it has warmed up that the dimensions and distances of elements inside have changed.

FastEddy: I have that schematic and is slightly different than my radio (different chassis). I don't think local library has the Sams index (we have few resources in library locally). I have recently started to get into old tube radios as a hobby too after years of dealing semiconductors.

Thanks again. Will post result/progress here. Any additional ideas are highly appreciated.
-Alex
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Old 15th June 2007, 01:35 PM   #9
anatech is offline anatech  Canada
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Hi AC439,
Have you checked http://www.nostalgiaair.org/ for your schematic? I'm sure they should have it, or something very close.

Just replace all those paper caps. It's not worth troubleshooting them one at a time. Simply replace them all in one go. I have had the odd mica and ceramic failure. Lead bond failure, probably due to them being twisted around by an earlier service guy.
Quote:
As for other things I have done on it, I have replaced the selenium rectifier with an 1N4007 with added resistor to keep the B+ the same and the voltage stays solid at the same level over time now.
Perfect! Most people forget to use a resistor to drop the B+.
Quote:
I'm also thinking about a possibly leaking tube (12DT8)
Possible, but not the most likely. All radios drift, especially the small chassis types. That would be last on my list to check and the check would be via substitution.

-Chris
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Old 11th April 2008, 06:51 PM   #10
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I know this is an ooolllld thread, but just in case anyone else is faced with these exact same symptoms, I have just been through the same thing on the same chassis and when I heard it play I suspected the same thing that Tom Bavis suggested. I did a bit of troubleshooting and found silver migration in the first FM IF transformer. I had it apart, measured the inductance at about 5.5uh and then calc'd the caps at about 40pf for 10.7mhz. There was no room inside the can for them, so I put them on the outside right at the wiring lugs. The set plays like a dream. The day after I did this I found this thread, complete with schematic and if I read it correctly, I got the secondary cap correct, but missed on the primary cap - doesn't matter, it plays very well, and I'm sure not taking apart again - the primary cap is under the soldered-in tuner cover and buried under other components.

So, for what it's worth - check the IF transformers when you have the symptoms described in the OP.

cheers eh?
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