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Heathkit FM-4 channel drift issue.

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tcmtech

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Since I haven't posted anything here for a while I figured I would toss this one out for some of you old school vacuum tube radio guys to ponder on.

Earlier this year I picked up a heathkit FM -4 tuner from a scrap pile and got it working. Mostly working anyway.
The only issue I have now is the AFC doesn't seen to either work or if it does it does not have the range of self adjustment needed to keep it on channel as the old radio warms up.

What it does is when cold the channel has to be tuned in on the high side then as it warms up (20 - 30 minutes) it has to be tuned lower and lower on the FM band to stay on the channel. That implies the issue is with the RF amp/Osc mixer tube and it related circuit having the issue but so far I have not been able to determine what exactly would be the heat sensitive part of it that would cause so much drift.

Best guess is the capacitors are old enough to be that thermally sensitive now and if so which ones are most likely the problem point or is it something else entirely and just normal for a tube based FM radio? It's the first complete and working tube radio I have never had so it's new territory to me.

Here's a link to the radios specs and schematics.
http://www.w6ze.org/Heathkit/Heathkit_045_FM4.pdf
 

tcmtech

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Thanks. That makes sense. I didn't consider those old 1950's resistors to have that much of a temperature drift issue but it does seem like a plausible explanation.

BTW, got any tips on how to bring the frequency response of a 1950's tube amp down lower? :p

I set up the old Heathkit tuner with a equally old Harmon Kardon DA-35 tube amp (dumpster dive rescue :cool:) for my workshed sound system and the low end frequency response is pretty weak below 40 - 50 Hz.

I'm going for sort of a retro steampunk man cave theme for the workshed (~70 - 80 year old repurposed grainery building) by using as much antique gear as I can where I can but still with modern capabilities like the 42" LCD flatscreen and computer and whatnot.
 

crutschow

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The low end of a tube amp is often limited by the output transformer.
They needed to be large and expensive to avoid core saturation and get a good low frequency response.
So you may have to add a powered subwoofer to get down deep. ;)
 

tcmtech

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I rather figured that much but thought I would ask. I haven't played with tube amps for near 20 years so I need a few refresher course on their operational limits.

I put an spare 13 band equalizer in ahead of the old tube and and put the 32 and 56 Hz bands to +12 dB so it's doing okay now.
At normal volumes it's fine but I just cant crank the AC/DC up without getting low frequency induced blurring in the high end with I suspect is due to the core saturation issue.

Still for ~60 year old tech it does surprisingly well! :cool:
 

JimB

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So you wanted a valve amp for it's low quality, and now you're complaining about the same low quality?
It is just like when you enjoy driving an old car, but you do sometimes miss the aircon, ABS and built in SATNAV.

JimB
 

schmitt trigger

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JimB; well said.
Part of the allure of using older technology (and can be applied to anything; radios, computers, cars, etc) is its rather simple, crude and unsophisticated way of achieving a particular task.

This goes so far as an acquaintance of mine took an old rotary phone, and designed an interface circuit to actually convert the pulses that to DTMF!! He actually uses it at home.
 

schmitt trigger

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Tmtech
you may want to measure the AFC voltage and its drift as the unit warms up.

Since this is a high impedance node, even a 10 Meg DMM will load it some. But what you want to see is how does the AFC voltage varies from cold to warm. That could give you a clue.
 

tcmtech

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So you wanted a valve amp for it's low quality, and now you're complaining about the same low quality? :D
No, but I just cant leave 'good enough' alone when it comes to old stuff. ;)
 

tcmtech

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It is just like when you enjoy driving an old car, but you do sometimes miss the aircon, ABS and built in SATNAV.

JimB
Pretty much. I have collection of half a dozen old tube amps so I am just playing around with them to see what they are capable of and so far other than the low end frequency response limit the one I have hooked up now is surprisingly good. ;)

But yea, to be fair pretty much all my life when it came out audio gear if an amplifier couldn't go down to <10 HZ and bottom out a speaker, no matter how big it was, it wasn't 'good enough'. :woot:
 

dr pepper

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I havent messed with any fm sets for a while, from memory if its a ratio detector type receiver then there is a balance cap on the end of the diodes (eabc80 or whatever), if this goes leaky then you'l get distorted non linear audio and weird agc goings on, have you monitored the afc bus to see if its low or up & down?
 

tcmtech

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Unfortunately no. Busy week since I got invited over to the Red RIver Valley to drive semi for beet harvest again this year and now have to leave on Saturday.

Too many priorities to take care of before I can play with the old radio again. :(

Odd thing is today I left it on for a few hours while I was in and out of the shed and never once had to adjust the channel. :confused:
 

dr pepper

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Is that Beer harvest or Beet harvest, I can see one being more popular than the other, I gave up Beer years back.

If there are any wax paper caps in the set get them outa there and put some plastic film ones in, if a cathode bypass or a couplong cap shorts you might loose a tube or a transformer.
 

tcmtech

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Sugar beet harvest. Auto correct seems to like to toss the odd replacement word in now and then even when their not needled so if you saw a beer where there should have been a beet that's why.
 
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