[EdHolland]

Here is the Candohm substitute, seen on the back wall of the chassis in the same location as the original. The screened lead to first IF can also be seen - it is the copper braid near the centre of the picture.

   

[Eliot Ness]

Looks good ED

[EdHolland]

Almost there with this one. At the weekend, I attended to a last few cabinet issues. There is a thin ridge on the front vertical edges to give more shape and form to the corner moldings. This vulnerable detail was made from a soft ply and had suffered several dings over the years. I filled and stained the repairs, and the cabinet looks better still without the tatty edges. The finish on the sides was also quite battered and very dirty, so these were stripped with acetone, revealing wood of beautiful colouring that will look very good on refinishing. I also finished toning the base molding in the darker walnut hue that seems a good match to the original.

On the bench, I played around with alignment a little more, although there is not a lot to it. Band C has a single adjustment to set the 15 megahertz dial point and peak the antenna circuit - the latter adjustment is very vague. Then the usual two point adjustment for broadcast band at 1400 kHz and 600 kHz, rinse, repeat. This leaves band B to wherever the coils and fixed capacitors in the band-switching happen to land, and the dial is off a little. Our Town's emergency radio station on 1680 kHz shows up with the pointer at just over 1700.

Another fun aspect of this set is the 252.5 kHz IF, and the joy of short wave images adding confusion

[EdHolland]

Coming up to having the cabinet colouring looking respectable and will probably start oiling the wood this weekend. Then the new speaker cloth can be added. Does anyone have tips for applying this to the curved grill?


I've solved one mystery on the voltage measurement discrepancy - something to which I should have paid attention earlier. My Fluke is considerably higher impedance than the 1000 Ohms/volt called for use in the service sheet. When I do the calculation for the first audio 6K7, and include 250,000 for meter resistance, this would drop the measured anode voltage right down to the 60V called in the data table, and not the 140 V indicated with a modern DMM.

Something is still behaving oddly with either the mixer or first IF. On the broadcast band, if I switch the two 6K7s between IF and audio duties, with one combo, the set whistles as stations are tuned in - something is oscillating. With the swap reversed, no whistle, but it sometimes it seems on the verge of it, as if there were some regeneration happening, and the tone of the audio changes strongly with tuning, as if bandwidth is limited. The IF is peaked correctly.

Any thoughts? All valves were bought used, so they could be of questionable condition.

Ed

[MrFixr55]

Hi Ed,

Couple of things re the 6K7s.  The original tubes on these are the "G" types, and require shielding.  In addition, the 6K7 1st Audio tube probably gives a lot of gain being a pentode, so lead dress is important.  If metal 6K7 tubes were substituted for the glass ones, it is important to ensure that pin 1 of the sockets is grounded.  Some Zenith tubes were advertised as being "metal glass" - having an internal shield (aquadag?) The schematic seems to show a grounded shield pin for the 6H6 detector, 6K7 1st IF and 6F6 Output tube but not for the RF and IF tubes.  If Pin 1 of the socket is grounded, the metal 6K6 tubes should be OK.  if not, use {G" or bakelite GT Tubes and ensure that the shield is in place.Try shielding the grid lead (and cap) of the 6F6 1st audio tube.  In addition, do the Phono mod; it is documented as a shop mod so is legit.  If the oscillation occurs using the chassis as audio only, then the issue is AF.  If clean audio when used as an amp but issues as a radio, that is the telling.

BTW, a really pretty job on the underside of the Chassis!!

Best Regards,

MrFixr55

[EdHolland]

Thanks MrFix,

Sorry I didn't see this earlier, sound advice. The issue is fixed, but first let me respond to your excellent notes. The chassis is wired to ensure connection of metal tubes. Being that the shields were missing from the chassis, I used metal tubes throughout while bringing it back to life. The oscillation certainly seemed to be in the IF, rather than in the audio stages. I chased down exactly the same things you had suggested.

With this in mind, I went back through the schematic, concentrating on the 6A8 and 6K7 first IF sections, tracing each component. Something looked odd, so I counted all the fixed capacitors (excluding trimmers and large cans) - 16 on the diagram, 15 on the chassis. Sure enough, the decoupling cap on the supply line to the mixer and first IF pentode was missing. Since I'd restuffed each cap one-by-one it can't have been on the chassis when it came to me. I added the indicated 0.1 uF at a convenient point close to the IF transformer and retested the set. Sure enough, all odd behaviour was gone, and the radio works even better on AM than it did before. In fact it is a real performer, excellent reception, sound quality and volume, a superb tuning dial, and handsome appearence.

After the electrical work, I pursued some more gentle rubbing of the cabinet with tung oil - the finish really is looking nice. The last steps will be adding grillcloth and replacing the dial escutcheon and glass. This one is practically finished. pictures tomorrow.

Ed

[EdHolland]

Progress:

   

[Eliot Ness]

From trash to treasure

It wears its dings and dents very well for an 85 year old and has a great story to tell.

[EdHolland]

Thanks John.

While the major, ugly damage was repaired, such as the broken grille fretwork and major chunks out of the cabinet, I elected to leave some character as a deliberate nod towards 85 years of age, which is part of it's originality. I hope I've maintained the set's essence, and ensured that it continues to be a representative example of it's type.

[klondike98]

Looks great.
Leaving a few dings and flaws is something I think is perfectly fine for pieces that have experienced as much as these sets have. I always enjoy taking the really beat up set and bringing it back to workable condition with a reasonable finish.

[mikethedruid]

I agree completely, Ed. The cabinet looks beautiful ! Congratulations.

[EdHolland]

Grillcloth added. A very fiddly task, but it turned out to my satisfaction:

   

   

[morzh]

Gorgeous radio.

[MrFixr55]

Hi Ed,

Yeah, missing that decoupling cap would do it!!

Words and emos cannot do justice in praising the job you did on that radio; it is absolutely gorgeous and museum quality!!!!

Funny how this is only a 5 tube radio (well 6 tubes, but when you get down to it is there a big difference between a 6Q7 / 6SQ7 and a separate detector and 1st AF? Maybe there is.), but the performance on these things is outstanding!!

Again, fantastic job!! Now you SHOULD hold with furniture that talks!!

[EdHolland]

Thanks for your kind words. I really enjoyed working on this one.