EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: intabits on June 27, 2018, 02:41:44 am
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As a kid back in the '60s, I pulled apart a lot of valve radios, but never really worked on them, or tried to repair them, until now.
I'm trying to restore an Astor Mickey (the HNQ "breadloaf" model with permeability tuning).
(https://www.eevblog.com/forum/index.php?action=dlattach;topic=119519.0;attach=463883;image)
Model pics and overview: https://www.radiomuseum.org/r/astor_mickey_hnq.html (https://www.radiomuseum.org/r/astor_mickey_hnq.html)
A very nice tech bulletin with schematic and parts list: https://www.kevinchant.com/uploads/7/1/0/8/7108231/hnq.pdf (https://www.kevinchant.com/uploads/7/1/0/8/7108231/hnq.pdf)
(https://www.eevblog.com/forum/index.php?action=dlattach;topic=119519.0;attach=463907;image)
But I'm unsure about the status of the two front end valves.
I've re-capped the radio (2 electros & 10 paper), and replaced about 5 resistors that had drifted high.
(https://www.eevblog.com/forum/index.php?action=dlattach;topic=119519.0;attach=463913;image)
I get high tension voltages, but they are low at 163v/135v, vs 195/175v on the schematic.
The two voltages are across R37, a 450R (460R measured) resistor. The low values and higher current (61mA vs 44mA) indicate a possible undue load on the supply. But the voltages are enough to get some scratches, clicks, hum and tones out of the speaker.
(more static right at one end of the tuning, but no radio sounds)
Clearly, the 6X4 rectifier and 6AQ5 audio amp are working, but I'm sus of the 6BE6 front end and 6AD8 IF amp.
The 6X4 and 6AQ5 are too hot to touch after a few minutes operation, but the 6BE6 is only slightly warm, and the 6AD8 not at all.
The filaments appear to be glowing, but not brightly. I bought a "new" 6AD8, which makes no difference (haven't found a source for a new 6BE6 yet)
Filament voltage is good at the sockets (a dial illumination bulb connected to the socket pins lights up brightly)
As best as I can tell, the filaments of these two tubes measure about 3 to 3.5 ohms, and the two good ones 2 to 2.7 ohms (at cold).
The unit draws about 38W from the mains, which agrees with the spec of 40W.
My questions are:-
Shouldn't the 6BE6 and 6AD8 be too hot to touch after a few minutes?
(that's how I remember it with valve gear)
Do valve filaments die by just going colder as they wear out?
Are the higher cold filament resistances of the the two cool tubes significant?
Could my "new" 6AD8 be a dud?
Can anyone in Melbourne assist me with tubes to try/swap/sell, or with another HNQ for comparison?
Any help would be most appreciated.
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Perfectly normal for the 6AQ5 and the 6X4 to get hotter than the other two. They are the audio power output and rectifier. The 6AQ5 would be dissipating something in the order of 10-12 watts on the anode and IIRC about 3 watts from the filament.
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Wow, that was quick!
Yes, I appreciate that those two are at the power end of things, and must be expected to run hotter.
But almost imperceptible heating on the other two just seems wrong...
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Do you have anode voltage on the first two valves? Just in case the primary of either IF transformer has gone open cct.
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Yes, 6BE6: 138v and 6DA8: 98v, (and 113v on the 6AQ5 audio output anode)
Schematic says 175v, 42v and 165v.
The 6BE6 and 6AQ5 look right, given that the 175v rail is low at 139v. (still no idea why that is)
But the 6DA8 anode is more than double what it should be. Hmm...
6AD8 anode is supplied via IF47 and R28 from 175v (139v actual ) rail.
R8 was one I replaced, and now measures 48.5K, in circuit
The IF coil measures around 16 Ohms, no idea what it should be, but the other 3 IF coils are the same.
So all seems nominal there.
Voltage at junction of R28 and IF coil is 97V, about the same as the anode on the other side of the IF coil.
To me, it seems 6AD8 anode is high because valve is not conducting to pull the anode down to more like 42V..?
(voltages are similar with "new" 6AD8, except 97v points drop to 89v)
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6AQ5s are well known for getting bloody hot!
Considering they are pretty much a 6V6 shoehorned into a very much smaller envelope, it pretty much stands to reason.
Same goes for a 6X4---pretty much the same thing done with a 6X5GT.
The 6AD8 & 6BE6 are not "power" valves, so won't get very hot.
There are several things you can check:-
(1)Monitor the HT while removing valves one at a time.
If the voltage increases, that tube may be drawing excess current.
(2)Inject some audio at the volume control & see if you hear anything out of the speaker.
(3)If you have an Oscilloscope, have a look at pin 1 of the 6BE6 to see if your local oscillator is operating, or alternately, place another MW broadcast radio alongside the one you are fixing.
Tune the DUT to the bottom of the MW band, & the other one to the top.
Tune it down & hopefully, you will hear a "rushing" sound.
This is the Astor "Mickey's" local oscillator.
(4) If that works, try the other way around--- you should be able to hear the other radio's local oscillator.
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"6AQ5s are well known for getting bloody hot!" - now also known by me.
Thanks for your interest.
(1) The "175v" (138v) HT increases only 2-3v when either the 6BE6 or the 6AD8 is removed.
But jumps up to 238v when the 6AQ5 is removed!
(2) Injecting an audio signal (via a cap at non-grid end of R26), produces tones in the speaker.
As expected, since the speaker was always producing cracks and pops.
(3) No signal at pin 1 of 6BE6, tuning dial has no effect.
Dead local oscillator..?
On the large voltage jump on removal of output amp. I wouldn't have expected that much of a jump. So, thinking the could 6AQ5 be biased on too hard, due to bad -8v rail at non-grid end of R24.
But that is -12.5v, a fair bit out of spec also (pulled lower by same mechanism as the HT?).
R38, the 200R resistor that sets up the -8v rail is 227R in circuit.
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Do the tuning coils have continuity? Also check for shorts to ground in case that trimmer cap failed.
What's the voltage on pin 6 (6BE6)? (This is the oscillator power)
You need to get that oscillator going before you worry about the IF.
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Good & Bad news
I think you've found the problem (or at least one of them).
The smaller coil (upper in the schematic) reads about 14 Ohm, but the larger one, for the local oscillator is open...
The coil wires solder to lugs at the left in the images below, the lugs are held in place by rings and the other side of the lugs have wires going to the rest of the tuner assembly.
The fine coil wires look intact from the lugs on to entering the wax covering of the coil. I tried re-soldering these connections, but to no avail.
I think my only hope is that the excrescence on the underside of the coil (3rd photo) might be a rupture where the coil burnt out, and that it might be repairable.
BTW: 6BE6 pin 6 is 31v (started at 43 but dropped over about a minute),
and no shorts to ground on any of the tuning coils
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Do you have a heat gun or something to melt the wax? (Maybe boil it) Scraping it might break more turns.
If you can't find the break on the outer turns, you might need to rewind it. If there's just one break, you might be able to use the original wire, if it's really bad, you may need new wire. Make sure you count the turns either way in case it snaps or falls apart.
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Thanks for the encouragement to do this - I would have written it off as unrepairable, but I can now see that it is very doable.
Using the hot air tool of my soldering station, I've melted off most of the wax. I can't see the fault just yet, but I still need to get it under good light and higher magnification.
(https://www.eevblog.com/forum/repair/unsure-about-two-valves-in-a-1950s-astor-mickey-radio/?action=dlattach;attach=465833;image)
From the photo, a few (mostly) good points:-
1/ It appears to be only a single layer winding - it just uses a thicker former than the other coil.
2/ The photo is just about good enough to count the turns (I must get a macro lens...)
3/ Even just replacing the wire and rewinding should be quite possible
4/ The winding pitch varies over the length. (probably not significant as the 38mm ferrite slug will overlap with a lot of coil - I hope)
5/ I even might be able to locate the break by connecting an RF-ish signal and following it along with an oscilloscope probe?
Will get onto that tomorrow...
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Thanks for the encouragement to do this - I would have written it off as unrepairable, but I can now see that it is very doable.
Using the hot air tool of my soldering station, I've melted off most of the wax. I can't see the fault just yet, but I still need to get it under good light and higher magnification.
(https://www.eevblog.com/forum/repair/unsure-about-two-valves-in-a-1950s-astor-mickey-radio/?action=dlattach;attach=465833;image)
From the photo, a few (mostly) good points:-
1/ It appears to be only a single layer winding - it just uses a thicker former than the other coil.
2/ The photo is just about good enough to count the turns (I must get a macro lens...)
3/ Even just replacing the wire and rewinding should be quite possible
4/ The winding pitch varies over the length. (probably not significant as the 38mm ferrite slug will overlap with a lot of coil - I hope)
5/ I even might be able to locate the break by connecting an RF-ish signal and following it along with an oscilloscope probe?
Will get onto that tomorrow...
Re (4):- The winding pitch variation is to make the frequency coverage spread out across the range, instead of being squashed up at one end, so it would be better if you can find the fault & join it up.
A similar thing is done with variable capacitors.
Old books used to talk about " straight line capacitance", straight line wavelength, & "straight line frequency" curves.
In this case, it would be "straight line inductance", etc.
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If you have to rewind it, measure the length of the wound part of the tube and make sure the new winding has the same number of turns and covers the same length. You'll have to recalibrate the radio if you do though.
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Re (4):- The winding pitch variation is to make the frequency coverage spread out across the range, instead of being squashed up at one end, so it would be better if you can find the fault & join it up.
Thanks, I was afraid that the variance was intentional.
...You'll have to recalibrate the radio if you do though.
Yes, the two slugs are factory adjusted and locked (to the coils and each other presumably)
But anyways, it was a lovely winter's day here. The sun was low in the sky, but bright, and there was no wind.
So sitting outside with coffee and a loupe, and after cleaning the glue from the ends of the winding with Xylene and cotton buds:
(https://www.eevblog.com/forum/repair/unsure-about-two-valves-in-a-1950s-astor-mickey-radio/?action=dlattach;attach=466371;image)
The open winding (third from the end) was revealed. The two sides of the break are splayed outwards here.
Using a piece of wire-wrap wire to join the break together:
(https://www.eevblog.com/forum/repair/unsure-about-two-valves-in-a-1950s-astor-mickey-radio/?action=dlattach;attach=466377;image)
Then held in place and protected with hot melt glue:
(https://www.eevblog.com/forum/repair/unsure-about-two-valves-in-a-1950s-astor-mickey-radio/?action=dlattach;attach=466383;image)
Then re-waxed:
(https://www.eevblog.com/forum/repair/unsure-about-two-valves-in-a-1950s-astor-mickey-radio/?action=dlattach;attach=466389;image)
And reinstalled to the radio:
(https://www.eevblog.com/forum/repair/unsure-about-two-valves-in-a-1950s-astor-mickey-radio/?action=dlattach;attach=466395;image)
Then the moment of truth! Turned on the power and waited...
turned the tuning shaft, and ....MUSIC! She works! Astor Mickey rides again!
Thanks so much to you guys for guiding me to this happy resolution, I'm very grateful.
I might have got here eventually myself, but not before messing around with new valves, and only after no joy with that would I have started testing these coils. There's still some more thorough testing and checkout to be done, but this is a great result.
Video of the experience coming in the near future...
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My video of the restoration and repair process.
It's rather long, and mostly concerned with applying power in stages to avoid damage, but other issues that arose along the way are covered.
https://www.youtube.com/watch?v=Xau0YIIZepY (https://www.youtube.com/watch?v=Xau0YIIZepY)