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Old 25th Mar 2021, 1:13 pm   #81
paulsherwin
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Default Re: EKCO U122 - Restore or spares?

OK, you have changed the relevant caps.
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Old 25th Mar 2021, 1:17 pm   #82
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Default Re: EKCO U122 - Restore or spares?

Quote:
Originally Posted by DonaldStott View Post
Thanks Lawrence - for the valves involved would that entail measuring between the control grid and the chassis or the control grid and the cathode?
Control grid to cathode, V1 mixer approx. 2.5meg, V1 osc. approx. 47k, V2 approx. 2meg, V3 approx. 10meg, V4 approx. 1meg, it should only take a minute or two and it should say yes or no as to whether the grid return circuits are in order or not.

Lawrence.

Last edited by ms660; 25th Mar 2021 at 1:36 pm. Reason: missing info
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Old 25th Mar 2021, 1:54 pm   #83
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Default Re: EKCO U122 - Restore or spares?

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Control grid to cathode, V1 mixer approx. 2.5meg, V1 osc. approx. 47k, V2 approx. 2meg, V3 approx. 10meg, V4 approx. 1meg
V1 Mixer 2.79M
V1 Osc 46.6k

V2 2.32M

V3 1.24M

V4 886k
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Old 25th Mar 2021, 2:37 pm   #84
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Default Re: EKCO U122 - Restore or spares?

Whilst waiting for Laurence to reply (I'm also curious as to how to interpret the figures) apply your injector and tracer at volume control and report back your findings
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Old 25th Mar 2021, 2:40 pm   #85
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Default Re: EKCO U122 - Restore or spares?

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Whilst waiting for Laurence to reply (I'm also curious as to how to interpret the figures) apply your injector and tracer at volume control and report back your findings
No problem, I wasn't going to reply, no need to if those measurements are correct.

Lawrence.
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Old 25th Mar 2021, 2:49 pm   #86
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Default Re: EKCO U122 - Restore or spares?

Of course I forgot that the Control Grid on the UBC41 (V3) is Pin 3:-

So V3 Pin 3 (Control Grid) to Pin 7 (Cathode) is 9.45M

Apologies to all!
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Old 25th Mar 2021, 3:05 pm   #87
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Default Re: EKCO U122 - Restore or spares?

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Of course I forgot that the Control Grid on the UBC41 (V3) is Pin 3:-

So V3 Pin 3 (Control Grid) to Pin 7 (Cathode) is 9.45M

Apologies to all!
I'd be lying if I said I wasn't expecting that.

Lawrence.
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Old 25th Mar 2021, 4:08 pm   #88
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Default Re: EKCO U122 - Restore or spares?

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i'd be lying if i said i wasn't expecting that.


Lawrence - very early on in this Thread you advised me in relation to the Output Transformer connections and that the Primary hadn't been connected for optimum anode load impedance.....9k as opposed to 3k. As a consequence the existing connections were moved to Tags 2 and 3.

Any chance that this could be the source of the distortion/instability?
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Old 25th Mar 2021, 4:38 pm   #89
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Default Re: EKCO U122 - Restore or spares?

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Quote:
Originally Posted by ms660 View Post
i'd be lying if i said i wasn't expecting that.


Lawrence - very early on in this Thread you advised me in relation to the Output Transformer connections and that the Primary hadn't been connected for optimum anode load impedance.....9k as opposed to 3k. As a consequence the existing connections were moved to Tags 2 and 3.

Any chance that this could be the source of the distortion/instability?
The optimum in the valve data is for when Vg2 = Va, as explained, Vg2 and Va won't be the same in your receiver so select by experimentation as suggested earlier, that would have been the first thing I would have done... Also be aware that any residual hum level from the output stage might be higher than it would have been with the original type of output transformer fitted, unless some extra HT filtering has been incorporated.

Lawrence.
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Old 25th Mar 2021, 8:53 pm   #90
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Default Re: EKCO U122 - Restore or spares?

Not sure if I'm looking in the right place by experimenting with the connections to the Output Transformer Primary but this is what I'm measuring for V4 (UL41) when connected to Tags 2 and 3:-

Pin 2 Anode 210V (Spec 213V)
Pin 5 Screen 56V (Spec 76V)
Pin 6 Control Grid 0.14V

Moving the connections to Tags 1 and 3:-

Pin 2 Anode 181V (Spec 213V)
Pin 5 Screen 63V (Spec 76V)
Pin 6 Control Grid 0.26V

Two issues appear to stand out, firstly there is a low voltage reading for the Screen Grid and secondly there is a small, positive voltage on the Control Grid. For the record, moving the connections didn't appear to have any impact on the audio warbling!

Simple questions I hope - what is the technical name for the warbling I am hearing and where is the probable source of this distortion? And finally, is this the same as instability??
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Old 25th Mar 2021, 8:57 pm   #91
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Default Re: EKCO U122 - Restore or spares?

Can you post a sound/video clip of the symptoms?

Lawrence.
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Old 25th Mar 2021, 9:18 pm   #92
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Default Re: EKCO U122 - Restore or spares?

Donald, you need to try to narrow down the fault imo. As I mentioned earlier, place your velleman tracer probes on the volume control and check whether the output from the velleman speaker is distorted or not. If it is NOT distorted, this narrows the fault to the AF stage. Confirm this by switching to the injector.

If it is distorted when you place the tracer probes on the volume control, the problem is with the RF/IF stage. Once you know where the fault is, you can focus on investigating the appropriate stage

If you think the fault is with the output transformer at least use to AF tracer probe to confirm the sound is clear up to that point.

Gabriel
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Old 25th Mar 2021, 9:40 pm   #93
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Default Re: EKCO U122 - Restore or spares?

Quote:
Originally Posted by DonaldStott View Post
Not sure if I'm looking in the right place by experimenting with the connections to the Output Transformer Primary but this is what I'm measuring for V4 (UL41) when connected to Tags 2 and 3:-

Pin 2 Anode 210V (Spec 213V)
Pin 5 Screen 56V (Spec 76V)
Pin 6 Control Grid 0.14V

Moving the connections to Tags 1 and 3:-

Pin 2 Anode 181V (Spec 213V)
Pin 5 Screen 63V (Spec 76V)
Pin 6 Control Grid 0.26V
According to those measurements there's no grid bias, so did you measure the g1 voltage correctly or has the grid bias disappeared?

No grid bias would cause the sound to be distorted.

Lawrence.
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Old 25th Mar 2021, 9:56 pm   #94
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Default Re: EKCO U122 - Restore or spares?

Donald, re the small positive voltage on control grid see here.
https://www.vintage-radio.net/forum/...ad.php?t=86050

Also check r20 and r18 they're probably both a bit high.

Gabriel
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Old 26th Mar 2021, 10:29 am   #95
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Default Re: EKCO U122 - Restore or spares?

I double-checked the voltage reading for the Control Grid (Pin 6) of V4 (UL41) and it remains at +0.26V - so clearly problems here with the bias as Lawrence states.

Checking the Control Grid on the other valves I am reading:-

V3 (UBC41) -0.30V
V2 (UF41) -0.01V
V1 (UCH42) -0.20V

Looking closely at the circuit and the set, the likely culprit could be C24 the A.F. Coupling capacitor (!) although this is the first cap that I changed?

I'll also check R18 and R20 as Gabe suggests and will dig out my Velleman injector and tracer.
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Old 26th Mar 2021, 11:27 am   #96
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Default Re: EKCO U122 - Restore or spares?

A new plastic film capacitor is very unlikely to be bad.

Do you have another UL41 to try?
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Old 26th Mar 2021, 11:57 am   #97
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Default Re: EKCO U122 - Restore or spares?

Just for completeness, in particular as you set has been got at to some extent, can you remeasure the grid bias with reference to cathode rather than to chassis (unless you've done this already). This may give you exactly the same reading but worth a quick check IMO.

The UL41 positive voltage is likely due to internal leakage (deposits), as Paul said your brand new capacitor is unlikely to be leaky.
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Old 26th Mar 2021, 1:35 pm   #98
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Default Re: EKCO U122 - Restore or spares?

As expected, the new plastic film capacitor isn't the problem (thanks Paul) and is measuring correctly. R18 is also new and not a problem although R20 is another 'bodge' comprising two large 1.5k ohm resistors in series - R20 should be 3.3k ohm. One of these resistors is measuring 1.53k while the other is measuring 2.56k - total resistance across these two (in series) is measuring 4.09k ohms. These resistors wired in series across the Output Transformer are shown on the third picture in Post #1 above. Probably should be replaced?? Voltage across R20 is 61.2V so using our old friend Ohms Law I calculated 0.91W, so using 2W resistors should be ok or would 5W be more suitable at this location?

Found my third and last UL41 valve and fitted that - the warbling has gone and the Control Grid voltage now reads -0.5V - much happiness!

Still got a bit of residual background hum but remembered that back in Post #89 Lawrence said "Also be aware that any residual hum level from the output stage might be higher than it would have been with the original type of output transformer fitted, unless some extra HT filtering has been incorporated."

The speaker now has a bit of a flabby bass but I'll try and address that by an application of 50/50 Water/PVA to tighten up the speaker cone.

My stock of UL41s has now gone so what's the current thinking on alternatives, ideally without changing the valve base?

Thanks to everyone for their help and assistance in getting through this but I didn't expect to have two duff UL41 valves!
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Old 26th Mar 2021, 1:59 pm   #99
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Default Re: EKCO U122 - Restore or spares?

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Found my third and last UL41 valve and fitted that - the warbling has gone and the Control Grid voltage now reads -0.5V - much happiness!
The actual voltage on g1 WRT the chassis should be approx. 4.6 volts -ve (Ohms Law) so either your meter had a low input impedance when the measurement was taken, or you measured the bias voltage incorrectly or you still have a fault...

To measure g1 voltages use a DMM, preferably one with a 10meg input impedance.

Lawrence.

Last edited by ms660; 26th Mar 2021 at 2:05 pm.
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Old 26th Mar 2021, 2:44 pm   #100
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Default Re: EKCO U122 - Restore or spares?

See this writeup for info on replacing the UL41 with a UL84: https://www.vintage-radio.com/recent...bush-ac11.html

That was the 'official' way to replace a UL41 in the 60s as warehouse stocks started to dry up. You can also fit a UCL82 or UCL83 if you have a good one to hand.

A Mazda 10P13 can be used as a plug-in UL41 replacement, but they are no easier to find. A UL46 can be used with a base rewire, but again they're pretty thin on the ground.
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