Ultra U405 restoration

[John_Dw]

Split from this thread https://www.vintage-radio.net/forum/...d.php?t=173384

Very nice 'resurrection' job . I'm currently restoring this Ultra U405.

Fortunately, the seller pre-empted any possible miss handling damage, by packing it first wrapped in bubble wrap in a box, then in a second box, twice the size, with polystyrene chips all around the first box...
So far removed all of this lot and replaced with new, and now just waiting for one last part before introducing volts to it.
As I haven't got access to a Variac I put this together yesterday so that I will have a 100W light bulb in series with the mains for initial power up...
The case has cleaned up 'quite nice'

[Beardyman]

That's cleaned up well, I do like the burl walnut effect. Hope all goes well with the initial power up.

[Nickthedentist]

That's what I was about to say! It would have been a slightly dull set had it just been plain brown or black Bakelite. Quite nice is an understatement.

To be honest, for this kind of thing, a lamp limiter like you've made is as good as a Variac.

The only other thing I'd do if possible is to re-form the electrolytic capacitors with a current-limited supply like this, before applying power to a set that's been out of use for a long time: http://www.vintage-radio.com/project...-reformer.html

[mark pirate]

For a radio like this, I would use a 60W bulb for the first power up.


Mark

[John_Dw]

Originally Posted by Beardyman

That's cleaned up well, I do like the burl walnut effect. Hope all goes well with the initial power up.

Thanks, looking promising so far. Someone had been in it before (a long time ago) and changed a couple of caps, and initially I thought that when moving things around to make room for a smoothing cap, had mis-wired the panel light, and it's corresponding shunt resistor. The set has a fairly late serial number and I now suspect that there was a factory circuit change that removed the panel light and shunt from the heater chain, but wired the mains input from the on/off switch, through the 22ohm panel lamp shunt to the chassis. When I got the set the lamp was blown and the somewhat cooked resistor only read 2 ohms resistance. I'm just waiting for a replacement 5W ceramic one, as the modern 2W one I initially got, does not look man enough for the job as now all of the set's current will be going through it, not just the heaters...
Still looking for a set of original cream knobs for it, will have to make do with some period black ones that I have 'in stock', for the time being.
I did manage to get these eight period cream knobs, but they are a bit big (35mm dia) for the U405...

[SteveCG]

Just a quick thought:

In your photo of the bits you have replaced, there is one item ( black body and silver end contacts) that looks like it is a thermistor and not a standard resistor. Is that the case?

[John_Dw]

Originally Posted by mark pirate

For a radio like this, I would use a 60W bulb for the first power up.


Mark

Cheers Mark, will do

[John_Dw]

Originally Posted by SteveCG

Just a quick thought:

In your photo of the bits you have replaced, there is one item ( black body and silver end contacts) that looks like it is a thermistor and not a standard resistor. Is that the case?

Well the circuit diagram calls for a 100ohm resistor (R15) and as it currently reads 53ohms I was replacing it with a 2W 100ohm one...any suggestions gratefully received.

[ms660]

100 Ohm rated at 2 Watts should be ok, power dissipated approx. 1 Watt.

Lawrence.

[orbanp1]

Hi John,

Really nice work!
I did find some documentation on the elektrotanya web-site on the radio, it had a note on the second page for V5 (rectifying tube) saying:
"Cathode to chassis, 212v DC".
I suspect they meant the anode to chassis voltage, and that is the B+ voltage.
I would set the value of R5 to get that B+ voltage, but the exact voltage is probably not that important.

Regards, Peter

[Herald1360]

No, they do mean rectifier cathode-chassis for B+. Anode-chassis is AC in to rectifier.

[John_Dw]

Originally Posted by John_Dw

Well the circuit diagram calls for a 100ohm resistor (R15) and as it currently reads 53ohms I was replacing it with a 2W 100ohm one...any suggestions gratefully received.

Typo, for 'R15' read 'R16'

[Gabe001]

Originally Posted by mark pirate

For a radio like this, I would use a 60W bulb for the first power up.


Mark

Mark, how did you work out what bulb wattage you'd need for the limiter for this set?

Gabriel

[Richardgr]

Originally Posted by Gabe001

Mark, how did you work out what bulb wattage you'd need for the limiter for this set?

The bulb wattage needs to be equivalent to the device under test.

Too great, and the device under test is an easier path fpr the current, so the bulb stays dim and the circuit sees the brunt of the supply.

Too few watts and the bulb always lights, with a small supply to the circuit, masking a possible short in the device under test.

I have a pair of 40watt bulbs in my one, one partially unscrewed, so I can switch between 40 or 80 watts.

[Gabe001]

Thanks Richard

[orbanp1]

Originally Posted by Herald1360

No, they do mean rectifier cathode-chassis for B+. Anode-chassis is AC in to rectifier.

Correct!
Thanks for pointing it out!

Regards, Peter

[John_Dw]

As this is my first foray back into valve radios for about 60 years, (as a kid I used to buy old radios at Church jumble sales for pennies), the one thing that I was not really ready for was the extent that the rubber insulated wires in the chassis had deteriorated! .Virtually every wire that I moved to get access to a component, to remove it for replacement, started shedding its 'insulation' (I use the term loosely), Most it was just a matter of tracing through and replacing the wire with new modern wire, but the one that gave me most grief was the speaker wire that ran near the mains dropper and disappeared into the speaker coil. With this one I disconnected it at the output transformer end and carefully fed some heat resistant sleeving over it, right down to the speaker coil then sealed it there with Araldite, to immobilise it and insulate it.
I do wish I still had some of the sets that I had as a child, especially the round Bakelite Philco and even more the so Ediswan crystal set, identical to this one. (which was surprisingly easy to get a signal on...)

[John_Dw]

Bit of an update.....
On initially powering the set on (after doing a test power on with the lamp load in circuit) there was a lot of background noise with a couple of medium wave stations heard, distorted, in the background. Further investigation showed that the HT voltages were very low (about half of what they should be). Turns out that the HT reservoir cap (C27), which was the one half of the original double smoothing cap that was still in circuit, was open circuit. Replacing this, initially with an axial electrolytic, restored the HT volts to 'normal' as stations could now be heard above the noise. At this point I decided that the couple of high value electrolytics that appeared to have been replaced in the past needed to go as well so I sourced new 35mm dual 16uF/16uf 500Volt cap to mount back in the chassis where the original cardboard cased one had been. Now the original spec had been 24uf/16uF, which appears to now be made from 'unobtainium'. Could the lower value of the HT smoothing cap that I have fitted be the cause of some of the background hum I'm still experiencing with the set?
I had some 'odd' voltage readings around V3 (DH76) and I managed to pick up some spare valves and swapping the DH76 really bought stations up out of the noise and the voltages looked correct again.
Other than the previously mentioned background interference the set seems to work well, picking up strongly Five Live and Talk Radio on MW, on the 3 feet of aerial wire that remains from the original 20ft the radio should have had. (I'm only about 12miles as the crow flies from Brookmans Park), and a couple of stations on Long Wave.
Other than a set of the correct knobs and an IF alignment (which I haven't got the necessary equipment to do) we are pretty much finished, other than getting a suitable 8uF cap to parallel across the current 16uF smoothing cap to bring it up to the original value.
The replaced parts collection now looks like this:-