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  #61  
Old 06-19-2010, 05:41 PM
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electroking electroking is offline
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Many ceramic cartridges produce an audible acoustic signal if you bring your
ear close enough, no need to worry. Good to hear of you again!
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  #62  
Old 06-20-2010, 10:57 AM
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Well, in that case, the turntable now works!

Back to the chassis... The oscillator got lunched all right. It's well beyond repair.... Off to source a replacement
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  #63  
Old 06-20-2010, 02:17 PM
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Would it be an idea to re-build the coil? I've calculated the length of damaged wire to be 5m, and I have the same fine stuff on hand.

Could it work, or would the new specs be different enough to require adjusting the set anyway? (what I'm hoping to avoid)
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  #64  
Old 06-20-2010, 07:46 PM
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Quote:
Originally Posted by VintagePC View Post
Would it be an idea to re-build the coil? I've calculated the length of damaged wire to be 5m, and I have the same fine stuff on hand.

Could it work, or would the new specs be different enough to require adjusting the set anyway? (what I'm hoping to avoid)
It will require alignment of the trimmer on the oscillator section of
the variable capacitor, but that's no big deal. IF
alignment (the slugs in the IF transformers) should not be affected.
Rebuilding the coil on its original cardboard core will probably
yield better results than trying to fit a 'universal' replacement coil.
By the way, the exact number of turns matters more than the length
of wire, so try to stay as close to original specs as you can. Also the
wire must be enameled (thin coat of varnish rather than bare wire
or wire with plastic insulation). This is often sold as 'magnet wire'.
Good luck!
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  #65  
Old 06-21-2010, 11:12 AM
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Unfortunately the coil was so chewed up I couldn't reliably count the number of turns.

However, I did piece all the lengths together and I would assume that with the same total length of wire, wound in the same manner (or as close as possible) would result in the same number of turns.

I have numerous low voltage transformers that I'm never going to use for anything else- I'd thought to salvage the wire from one of those, provided it's not potted. I also have a huge copper coil (magnet wire) from a TV yoke, but I think that's too thick to use. Fortunately, the secondary coil was untouched, and so I have it for both a reference in wire size and winding pattern.
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  #66  
Old 06-21-2010, 01:33 PM
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Looks like you did your best to gather all the available info. It's time to start
experimenting! As long as you get the correct phasing (relative polarity of
the two coils), it should oscillate. Personal message to follow within one day,
bye for now.
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  #67  
Old 06-21-2010, 01:47 PM
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Yep... We'll see how it goes.

I'm waiting for a nice sunny (and insect-free) day so I can finish derusting the chassis and power transformer casing, and I hope to get in an order for some replacement capacitors once I've tested for out-of-spec resistors.

Thanks for all your help!
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  #68  
Old 06-23-2010, 05:48 AM
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Just finished re-winding the coil (pic below) and I'm waiting for the wax to harden. The CRT yoke had perfectly salvageable wire (and more than enough on just one winding- I nearly filled an 800 yard thread spool!
Attached Images
File Type: jpg IMG_1181.JPG (61.2 KB, 9 views)
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  #69  
Old 06-23-2010, 10:58 AM
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I reinstalled the coil and looked at the circuit diagram- it seems that there are some differences- (or someone has already "serviced" the radio.

Particularly, C25 and C26 are 0.01 µF (vs .03) and one of the filter caps in the can is 100 µF as opposed to 40 on the schematic.

R5 and R31 are completely absent.

Regarding replacements, I take it aluminum caps are fine for the electrolytics, and ceramics to replace the plastic and paper ones?

Also, can someone confirm whether C12,22,16, and 17 are electrolytic? They have the same schematic symbol as the filter caps, but like kinda like the plastic jobbies. (or just leave them alone?)
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  #70  
Old 06-23-2010, 07:36 PM
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>Particularly, C25 and C26 are 0.01 µF (vs .03) and one of the filter caps in
>the can is 100 µF as opposed to 40 on the schematic.

Those 2 capacitors bypass the power line to the chassis to filter out some
interference. They are sometimes called 'death caps', because while
they are useful for their stated purpose, they also increase the leakage
between the power line and chassis, and bring up some shock hazard.
The lower value is possibly a production change, you need not worry,
but make sure good capacitors are in those spots. The 100 µF can
is probably fine for C23, but at C24 it may overstress the rectifier tube.
Better limit this one to 40 or 50 µF.

>R5 and R31 are completely absent.

Maybe they were removed by someone who wanted to disable the
radio section; these resistors provide the proper voltage and current
to tubes 6BE6 and 6DC8. The node between R3 and C5 should
definitely not be connected directly to the function switch, or the
tubes may be overloaded. You could replace R5 and R31 by a single
10 kohm, 3 to 5 W resistor. (note added afterwards: maybe the
resistors were moved to the part of the supply line on the other
side of the switch, somewhere to the right of C11)

>Regarding replacements, I take it aluminum caps are fine for the
>electrolytics, and ceramics to replace the plastic and paper ones?

Well electrolytics and aluminum electrolytics are essentially the same
(although there are also tantalum electrolytics, not very often used
in tube gear). Ceramic capacitors don't usually come in large values
(0.1 microF) and reasonably high voltage ratings. But in this radio,
probably anything with correct voltage and capacitance rating will do.

>Also, can someone confirm whether C12,22,16, and 17 are electrolytic?
>They have the same schematic symbol as the filter caps, but like kinda like
>the plastic jobbies. (or just leave them alone?)

Those cathode bypass capacitors are definitely electrolytic, but they
are low voltage units, that is why they are smaller. 6- to 15- volt
units would be fine for C12/C22, and 25-V units for C16/C17. You
can read the cathode voltages on the schematic, respectively 1.1 and
12 volts. These should be replaced, as any leakage will screw up
the bias on the tubes. P.S.: modern plastic units for C12/C22 would
be fine if not too physically big; some audiophiles will even tell you they
will improve the sound!

Good night.

Last edited by electroking; 06-23-2010 at 07:46 PM. Reason: improved layout; added info
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  #71  
Old 06-25-2010, 04:57 PM
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The 100 µF can is indeed C23, so I'll see what I can get and may change it for a 40 if circumstances require.

I'll also look in to good replacements for the death caps; chassis ground is also turntable ground, so it is a big issue.

It turns out that R31 and R5 are present after all; they are before the function switch and tucked away, so I did not see them until I searched thoroughly.

I'm placing an order with Digi-key for other electronics supplies, so I'll get as many of the caps as I can there... if all goes well, I may be able to do a preliminary test next week! *crosses fingers*

Not much other radio progress, but after an afternoon with some steel wool and rust remover, I think I've eliminated all the rust. The line cord has also been replaced (the prongs on the old one were bent up and corroded.)
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  #72  
Old 06-25-2010, 07:20 PM
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Death caps, leakage to chassis, etc.

Hello again,

Regarding the death caps and shock hazards, it might be a good idea to
install a polarized plug (maybe cannibalized from a dead TV) with the
neutral (wide blade) connected to the lower side of the power
transformer and the live (narrow blade) connected to the upper
side (the one with the power switch). This would ensure that the
resistive link to the chassis (R55) would remain on the neutral side
of the line. R55 is part of the filter along with the death caps.
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  #73  
Old 06-26-2010, 02:38 PM
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I found the polarized line cord I had; and installed it.

I'm currently re-painting the transformer case and have removed the filter cap can (and gutted it) for re-stuffing.

Capacitors have been ordered and are on the way!
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  #74  
Old 06-29-2010, 12:46 PM
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VintagePC VintagePC is offline
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There's a bit of a lull in activity now, since I'm waiting for my caps to arrive. More photos have been posted and updates made to the restore log on my site.

http://vintagepc.co.nr/site/index.php/stuff/radio/

Hope it tides everyone over!
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  #75  
Old 07-02-2010, 04:58 PM
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*Twiddle Twiddle*

Still twiddling my thumbs. I'd hoped the caps would arrive today, but probably will on Monday (because of the holiday Thursday).

Can't do much else until I have those, but I did rig up an external RCA input using a dual-pole dual-throw switch and some RCA jacks, plugs, and shielded audio cable... so when the set is done, I'll be able to switch between the internal turntable and an external source at the push of a button!

The only issue I could see is with the set's floating ground- Would it cause an issue if I plugged in an audio source with a hard-wired ground?

Anyway, once I've recapped, what would be the best way to power up the set? I don't have a variac (alas) but I'd thought to pull all the tubes, check the transformer output voltages, and then put the tubes back in one by one, testing voltages (indicated on the schematic) as I go.
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