EKCO U122 - Restore or spares?

[DonaldStott]

Originally Posted by ms660

I think some folks on the forum have shielded an ordinary replacement by various means, copper tube etc.

Thanks Lawrence - does it need some form of shielding then?

How about wrapping a modern 0.01uF 630V polyester capacitor with copper foil (sticky backed) and then connecting that 'shield' to the chassis with a drain wire?

Or I could re-work and re-use the existing metal band?

[ms660]

Originally Posted by DonaldStott

Thanks Lawrence - does it need some form of shielding then - how about wrapping a modern 0.01uF 630V polyester capacitor with copper foil (sticky backed) and then connecting that 'shield' to the chassis with a drain wire?

I think someone on the forum might have done that, I assume with the joint soldered, have a search on the forum, I'm sure someone made a shield for a typical modern replacement and posted a photo of it.

Lawrence.

[DonaldStott]

Originally Posted by ms660

I think someone on the forum might have done that, I assume with the joint soldered, have a search on the forum, I'm sure someone made a shield for a typical modern replacement and posted a photo of it.

I've found a few Threads that discuss shielded capacitors, mostly in German sets, but nothing to indicate that any screening is essential. General consensus appears to be that it makes no noticeable difference with or without any screening. As usual it's a case of try it and see (or listen) ... so I'll try a replacement without screen first, and if not happy, add a screen later.

I've watched this very enjoyable and informative 'Mr Carlson's Lab' YouTube video which includes discussion of a shielded capacitor. Of course after watching this video it would be difficult not to get obsessive about identifying the outer foil end in modern capacitors!

https://www.youtube.com/watch?v=Nzutvfbucr0

[ms660]

Originally Posted by DonaldStott

I've found a few Threads that discuss shielded capacitors, mostly in German sets, but nothing to indicate that any screening is essential.

It might not be essential.

The manufactures would have fitted them for a reason.

Lawrence.

[Radio tirana]

Hi Donald
I have on occasion done as you suggest and wrapped a modern cap in copper tape in order to screen it, probably not strictly necessary in most cases. This photo is of a Philips set.
Kind regards RT

[DonaldStott]

Originally Posted by Radio tirana

I have on occasion done as you suggest and wrapped a modern cap in copper tape in order to screen it, probably not strictly necessary in most cases. This photo is of a Philips set.

Thanks RT - as I said I'll try a replacement without screen first.

[DonaldStott]

Apologies for a bit more 'navel gazing' in relation to that shielded capacitor but I'm just curious!

If I connect up the high value resistor R12 as per the circuit diagram it will link to the red end (nearest to V3) of C20 - accordingly R12 will be directly in-circuit.



If, however, I connect R12 onto an appropriate shield around C20 it would seem to me that it's not directly in-circuit but is the outer shield of this cap connected to the case as well as the black end? Is R12 just acting as a drain for any potential RF hum between V3 Control Grid and the Volume Control?

[ms660]

R12 should be connected between the grid and cathode, the reason for that is that the triode has no cathode bias and no bias from any other source other than itself (see Post#66) Also see "Grid Current Bias" towards the end of this article:

http://www.r-type.org/articles/art-129c.htm

A more detailed explanation can be found here:

http://www.r-type.org/articles/art-171.htm

Remember that all circuits are potential dividers.

Lawrence.

[DonaldStott]

Lawrence - thanks for the explanation and links, all part of my ongoing education!

I just got side-tracked/confused by the capacitor shielding issue.

[DonaldStott]

I replaced about 50% of the wiring and 80% of the caps and resistors so today was the big test - first Live turn on with the Lamp Limiter of course.

Nothing more than a loud, continuous hum - independent of volume and tuning, nothing else!

Lamp Limiter cycled correctly and hum started once the valves heated up.

Where to begin ... ??

[DonaldStott]

Started off with some basic valve pin readings on V5 (UY41):-

Pin 2 Anode 226V AC (Spec: 215V AC)

Pin 7 Cathode 240V (Spec: 218V)

Something far wrong there so swapped in a known working UY41 but same readings!

Stopped measuring and switched off set ...!

[Gabe001]

All part of the fun.

If you're confident your smoothing caps are fine, try swapping out the UL41 for a known good one from the DAC90a you're currently bluetooth-ing.

Either you've got an AC ripple problem or a heater-cathode/grid short in the valve, with the most likely culprit being the UL41

[DonaldStott]

Originally Posted by Gabe001

If you're confident your smoothing caps are fine

Thanks Gabe, see Post #24 above - the HT smoothing can with C23 and C28 contains two (newish) 50μF caps. Measured values shown and Trader spec values in brackets: -

C23: 41.2μF (20μF)

C28: 42.4μF (50μF)

I'll try swapping out the UL41 for a known good one.

I haven't replaced C29 (Mains R.F. Filter) but don't think that it's absence could be causing the problem(s)? If required it will be replaced with a suitable X2 type capacitor and relocated across the on/off switch tags - on the correct side of the switch of course.

[DonaldStott]

Yeah - it was a duff UL41!

Replaced with a NOS valve and the loud hum disappeared - indeed with those two 'newish' electrolytics there is barely any hum at all. Good reception on both LW and MW with a little background hash but a good performer so far!

And here comes the BUT - the audio output from the speaker is warbling and sounds like it could be underwater! My first instincts are that this is a physical problem with the speaker rather than something in the electronics/circuitry? I have experience of repairing a detached suspension so I'll have a look at that later today. I will first connect up another speaker as a test to narrow down the problem area.

As Gabe001 said "All part of the fun."

[Gabe001]

Glad I could help. I have a UL41 that also hums slightly. Keep us updated.

[DonaldStott]

Thanks Gabe for your support and continued interest.

Guess what - I connected up another known 'good' speaker and that warbles as well! So it's something in the electronics/circuitry causing this problem?

A new one on me so any help would be appreciated.

[ms660]

Make sure the mixer and IF and audio valve control grids aren't floating.

Lawrence.

[Gabe001]

Time to use that Velleman tracer Donald

[paulsherwin]

This sounds like instability, which is normally down to capacitor problems. There will be 2 or 3 caps decoupling the HT line in various places - have you changed them yet? There will also be a cap across the AGC line.

(I don't have the circuit to hand.)

[DonaldStott]

Originally Posted by ms660

Make sure the mixer and IF and audio valve control grids aren't floating.

Thanks Lawrence - for the valves involved would that entail measuring between the control grid and the chassis or the control grid and the cathode?

Originally Posted by Gabe001

Time to use that Velleman tracer Donald

Agreed Gabe - just need to remind myself which valve pins to connect to, especially with V1 and V2?

Originally Posted by paulsherwin

This sounds like instability, which is normally down to capacitor problems. There will be 2 or 3 caps decoupling the HT line in various places - have you changed them yet? There will also be a cap across the AGC line.

Thanks Paul - the waxy paper caps decoupling the HT line have been replaced along with the the AGC decoupling cap. I've also replaced the electrolytic HT decoupling cap with a modern radial type?