[EdHolland]

I still have a couple of things to chase down, before it is really satisfactory.

After tackling the bad section of the Candohm with a temporary patch, I retested with full connection to AC. The set would work well for a while, but eventually go quiet, still receiving, but very low volume and sensitivity. Now the HT was high (335V rather than 305 V in the service notes), thanks to modern line voltage . The poor 6F6 was also getting far too hot under these conditions. If the set is turned off for a minute, it recovers and operates for a while before going quiet again. Returning to the dim-bulb tester, the radio is stable, but with only 180V HT it is weak as would be expected.

Next step will be to check resistors and voltages more thoroughly, and see if something funny is happening with bias and AGC lines. I may also have a transformer that can be used to "buck" the AC down towards the 115 V the radio expects. Our line was around 118 - 119 yesterday, according to my DMM, but I have seen it range higher.

[Arran]

It could be that another section of the candohm is failing, it could also be a problem with the cathode bias resistor on the 6F6 tube, unless this set uses fixed bias as many from then do.
Regards
Arran

[EdHolland]

Arran,

Thanks - yes certainly the Candohm is still on the watch list... Some time watching the voltages it provides, and also monitoring what is happening to the AGC should tell all. At the moment volume reduced, it sounded as if AGC had suddenly increased its action, but it could also be a bias problem. Careful measurement will provide answers. The 6F6 runs at a fixed bias from the Candohm.

Actually I like this stuff - one really learns how the circuits work in practice, and see the things that can go wrong, and the symptoms they present.

Cheers,

Ed

[Eliot Ness]

You need to be careful with those candohm units if you leave them physically in place and tack on replacements.  Sometimes the open section can temporarily come to life, or worse yet, one section decides to short to ground 

There are different schools of thoughts on how best to replace them.  Some use terminal strips, but these turret strips I found on eBay work well for me:

https://www.ebay.com/itm/123172532819

You can remove unneeded turrets and then cut to length:

   

[EdHolland]

John - that's a tidy solution. What care, if any, is required when mounting to the chassis to prevent accidental shorting? Do you use standoffs?

5W resistors have been ordered to replicate the Candohm. I have read about these in other restoration accounts but this is the first time one has popped up in a project here. I was wary from the outset.

[Eliot Ness]

You could use some standoffs, but I just glue some thick fish paper on the bottom of the strip.

[EdHolland]

OK, thanks

I am looking into various options, but those particular tag strips are very nice quality.

Yesterday I had another chance to poke about on the radio. The 11K section of the Candohm had mysteriously repaired itself (for how long?) so I disconnected the temporary resistor, and started measuring voltages. It was at this point I realised that the service sheet info I have is just plain wrong in this department. Looking at the 6F6, it called for -2.5 V on the control grid. The circuit was producing ~-22V, far more in agreement with the datasheet value given for a 300V HT operating point. This time, the radio stayed on, no intermittent problems. The sheet also calls 5V ac for all the filaments, which is just plain nonsense.
Other values around the radio seemed to make sense, and nothing was a cause for alarm. More testing this weekend while I wait for resistors and tag strip. I need to clean up the chassis and replace the dial before proceeding with alignment.

[EdHolland]

After checking the general health issues and a bit more switch cleaning, the radio seems to work nicely on full mains without "dropping out" like before (while the Candohm allows..). A connection patched a connection through to the outdoor aerial "long wire" allowed for a bit of play-time. With this setup, it really came to life on medium and short wave, and sounded rather good, even with the speaker just lying on the bench.

[EdHolland]

The turret strip arrived over the weekend while we were away - still waiting on the resistor kit. The strip is excellent quality, and will make a nice replacement for the Candohm.

Out of curiosity this morning while sizing up the turret strip against the available mounting holes, I measured the Candohm resistances once more. It appears to have failed again - The 4920 Ohm section was reading ~7K. and the 11k section was open. At least it worked sufficiently to let me test the radio, but will now be retired from service.

[EdHolland]

Today I built a new Candohm using the neat turret strips recommended by John. This went well, and the radio came up with voltages where they should be. Attention then turned to resistors around the 6A8 mixer, in the hope that some of the oscillator quirks on SW could be straightened out. the 11 kOhm had drifted to 22 k, so this was replaced. As I turned on the set to test, there was a frazzling sound, and smoke from the shielded wire leading to the 6A8 plate from the IF transformer. The shielding had been badly soldered, and this had probably contributed to the breakdown of the cloth covered wire running within. I have made up a replacement, using some braid recovered from a piece of coax, but am just shy of making the final connection as it is dinner time. The IF windings survived (thank goodness) so I think it should all come back to life.

More tomorrow, hopefully with some pics of the work.

Ed

[EdHolland]

After dinner, I completed the IF wiring replacement and put the radio on the long-wire. Very good reception was the result on all bands. The odd behaviour on SW was restored and medium wave (AM) very lively indeed. Of particular note, was very good reception of BBC World Service on 12015 Khz, coming in like a local. Also WWV stations on 5, 10 and 15 MHz were all present and well received.

[mikethedruid]

Congratulations !     Always feels great when hard work pays off !

[RodB]

Ed, we all feel your joy, congratulations!

[Arran]

I didn't know that the BBC World Service was still on the air? Many countries dropped their national short wave services over the past 20 years, except for the C.C.P propaganda stations (which are everywhere) , and I think Japan. One common one I used to get was Radio Ecuador, haven't heard them in a long time, now it's WRMI, and WTWW.
Regards
Arran

[EdHolland]

New Zealand still have a service, as well as the BBC. Both still offer some decent programming.

Today, I replaced the dial assemembly, including a new belt. This works well, and I like the two pointer system. Alignment was straightforward, and the dial reads with reasonable precision.

I'm very impressed with this radio. For a 6 tube design, there is considerable economy in design and component count, and yet it has great sound quality and good reception.