EKCO U122 - Restore or spares?

[Gabe001]

That makes sense Lawrence.

Donald, I'd try the same bush pin 4 mod then and see. Maybe it works, maybe it doesn't, but it's a quick and easy test

[Sideband]

Originally Posted by DonaldStott

The cathode of V4 (UL41) in the EKCO U122 is connected back to the chassis, no cathode bias resistor. If we also consider what Lawrence highlighted again in Post #109 "Bias voltage for the Mixer, IF and output valves is developed across two resistors in series that are connected between the chassis (HT-ve) and the switched Neutral" then any shorts/leaks in the UL41 will be very apparent?

The cathode of V4 (UL41) in the Bush DAC10 (and the DAC90A) has an 150Ω cathode bias resistor which may help to ameliorate this problem but I'm just guessing? It wouldn't have cost EKCO that much to bung in a few cathode bias resistors, a few shillings at most, but no doubt that adds up over the course of a particular models production run?

Would a simple fix be to bodge-in a V4 150Ω cathode bias resistor in the U122 or is that too simplistic?

The difference between the Ekco U122 and the bush DAC 10 etc is that the Ekco uses grid bias and the Bush sets use cathode bias. That is the reason why in the ekco set, the cathode is returned direct to chassis and a negative bias is applied to the grid of the UL41 from the aforementioned resistors, whereas in the bush sets, the cathode is returned to chassis via a low value resistor and a grid leak resistor is connected to the grid of the UL41 and chassis. The grid then becomes negative with respect to the cathode (since the cathode becomes positive by a few volts above chassis).

So no....you can't add a cathode resistor to the UL41 in the Ekco since the bias arrangements are completely different.

[DonaldStott]

I'll Post an update later on the electronics issues discussed above but meantime I've moved on to address some of the cosmetic challenges with this set.

The case is slightly warped at one of the top back corners (heat damage) while the case grilles are all very much the worse for wear!

.

The dial glass printing is very basic but in good condition and I am intrigued by the strip of brown paper crudely attached by some black tape.



The same brown paper had been used to wrap the dual electrolytic can? Does this brown paper have any particular properties or characteristics, is it needed and if so what would be a suitable replacement?

I still have the problems of what to do about the control knobs:-

https://www.vintage-radio.net/forum/...15&postcount=1

[DonaldStott]

I've been a bit preoccupied with Bluetooth challenges (!) over the past few weeks but managed to find some time to do some work on this EKCO U122. Some Forum members may remember the problems I was having with the bias on the Control Grid of V4 (UL41) which seemed to creep to just above -ve 3.0V to -ve 4.7V but the readings weren't stable and dropped dramatically when the set clicked or crackled?

As I now know, thanks to Lawrence and others, this EKCO U122 uses grid bias and although the resistors responsible for developing the bias voltage for the Mixer, IF and output valves had been replaced there was no improvement?

With the U122 I couldn't work out why the cathode bias on V4 suddenly appeared and then disappeared - this seemed to coincide with some clicking/sparking from the set and I was led to believe (?) that this came from internal shorting within the UL41, a common problem. Further detailed investigations with much wailing and gnashing of teeth showed that the bias issue often coincided with me taking measurements at the neutral tag on the electrolytic dual can! Initially I thought I had a dry solder joint but I reflowed that and still no joy. So disconnected the electrolytic dual can and temporarily bodged-in two 47uF caps and everything worked fine!

So what I've done is to re-stuff the dual electrolytic can with 47uF and 32uF Rubycon Minature electrolytic capacitors and the Control Grid bias on V4 is now steady as a rock at -ve 4.38V (MW off-station).

How does it sound - well reception on both MW and LW is very good, what little there is up here in Glasgow, the audio quality is more than acceptable when on-station. When off-station, however, there is a lot of background hash along with that almost ubiquitous hum, no doubt caused by the UY41 Halfwave Rectifier?

Before I sign off on the electronics side of things I'd to revisit the issue first raised by Lawrence way back in Post #15 - "I might be wrong but looking at the transformer and the data sheet photo's it looks like the primary hasn't been connected for optimum anode load impedance.....9k as opposed to 3k." The connections I currently have to the Output Transformer Primary are to tags 1 and 3. The valve data for this set lists Va = 213V and Vg2 = 76V and I am measuring Va = 218V and Vg2 = 77V - near enough or do I need to try the connections to tags 2 and 3??

I'd still like some advice and guidance in relation to the cosmetic case and knob issues raised in Post #143 above - thanks.

[ms660]

Originally Posted by DonaldStott

When off-station, however, there is a lot of background hash along with that almost ubiquitous hum, no doubt caused by the UY41 Halfwave Rectifier?

Before I sign off on the electronics side of things I'd to revisit the issue first raised by Lawrence way back in Post #15 - "I might be wrong but looking at the transformer and the data sheet photo's it looks like the primary hasn't been connected for optimum anode load impedance.....9k as opposed to 3k." The connections I currently have to the Output Transformer Primary are to tags 1 and 3. The valve data for this set lists Va = 213V and Vg2 = 76V and I am measuring Va = 218V and Vg2 = 77V - near enough or do I need to try the connections to tags 2 and 3??

Don't forget that the original transformer was a hum reduction type and that the one that's fitted isn't.

As I said earlier, the optimum anode load impedance given in the valve data won't be the same in your receiver because in the valve data Vg2 = Va and this is not the case in the receiver so as also said earlier use the taps that give the best result, the various taps should have little effect on the valves DC conditions.

Lawrence.

[paulsherwin]

You may well be picking up electronic hash when the radio is detuned. Almost anything with a switch mode power supply or a processor can generate this.

I don't know of any radios of this type which are completely hum free.

[DonaldStott]

Originally Posted by ms660

Don't forget that the original transformer was a hum reduction type and that the one that's fitted isn't.

As I said earlier, the optimum anode load impedance given in the valve data won't be the same in your receiver because in the valve data Vg2 = Va and this is not the case in the receiver so as also said earlier use the taps that give the best result, the various taps should have little effect on the valves DC conditions.

Thanks Lawrence

Just for completeness I decided to connect up tags 2 and 3 on the replacement RS Standard Output Transformer - this resulted in a slight reduction in hum and the relevant V4 (UL41) valve pin voltages were measured as: -

Pin 2 Va = 230V

Pin 5 Vg2 = 88V

So higher than the measured voltages when connected to tags 1 and 3 (Va = 218V and Vg2 = 77V) but slightly less hum.

Is it possible to source and fit an Output transformer of the hum reduction type?

Originally Posted by paulsherwin

You may well be picking up electronic hash when the radio is detuned. Almost anything with a switch mode power supply or a processor can generate this.

I don't know of any radios of this type which are completely hum free.

Thanks Paul - after several years I have become accustomed to the expected level of background noise in my 'work environment' so my ears are now tuned to the electronic hash.

I appreciate that a certain level of hum and background noise is to be expected but it just seems a bit worse than usual with this set? Not going to lose any sleep over it so I'll probably just move on to tackling the cosmetic case and knobs challenges.

[Gabe001]

What is an output transformer of the "hum reduction type"?? I thought this was the role of the HT choke or smoothing capacitor. Are we talking about AC ripple hum here or something else?

[ms660]

Originally Posted by Gabe001

What is an output transformer of the "hum reduction type"?? I thought this was the role of the HT choke or smoothing capacitor. Are we talking about AC ripple hum here or something else?

AKA a hum cancelling transformer, it's a type of output transformer where the effect of the ripple current flowing through the primary of the transformer is cancelled out so it doesn't appear in the transformers secondary, if the HT filter capacitor's in good shape then for the system to work properly the ratio of the valves anode resistance to the added external resistance that feeds the HT to the rest of the circuit must be the same as the turns ratio of the two primary sections of the transformer, this method was used by many manufactures in various models.

Lawrence.

[Gabe001]

Thx Laurence

[DonaldStott]

To hopefully wrap up the issues associated with the Output Transformer tags and the optimum anode load impedance we have ended up with the following:-

1. Service Sheet : Va = 213V and Vg2 = 76V

2. Tags 1 and 3 connected : Va = 218V and Vg2 = 77V

3. Tags 2 and 3 connected : Va = 230V and Vg2 = 88V

Although 3. gives us less hum, the valve pin voltages for V4 (UL41) are higher. So to what extend would that be over-driving the output valve, knowing how problematic the UL41 can be?

[ms660]

Originally Posted by DonaldStott

To hopefully wrap up the issues associated with the Output Transformer tags and the optimum anode load impedance we have ended up with the following:-

1. Service Sheet : Va = 213V and Vg2 = 76V

2. Tags 1 and 3 connected : Va = 218V and Vg2 = 77V

3. Tags 2 and 3 connected : Va = 230V and Vg2 = 88V

Although 3. gives us less hum, the valve pin voltages for V4 (UL41) are higher. So to what extend would that be over-driving the output valve, knowing how problematic the UL41 can be?

It could be giving less hum because the output power is less, you should be able to tell if the valve's being over run by measuring the anode current and multiplying that by the anode voltage, you could set it back to the higher impedance tap and try some RC filtering between the reservoir capacitor and the load.

Lawrence.

[DonaldStott]

Originally Posted by ms660

It could be giving less hum because the output power is less, you should be able to tell if the valve's being over run by measuring the anode current and multiplying that by the anode voltage, you could set it back to the higher impedance tap and try some RC filtering between the reservoir capacitor and the load.

Connecting the V4 Anode to Tap 1 of the Output Transformer I am measuring:-

218V x 26.4mA = 5.75W

Connecting the V4 Anode to Tap 2 of the Output Transformer I am measuring:-

230V x 26.6mA = 6.11W

So it seems that for the higher impedance tap the output power is less which I assume is better for the UL41? But somewhat counter-intuitively this configuration is giving us more hum?

Does it matter which way round the connections are at the Primary of the Output Transformer? I have the V4 anode connected to Taps 1 or 2 while the +ve from the smoothing capacitor is connected to Tap 3?

@Lawrence - what would you recommend in relation to ' ...some RC filtering between the reservoir capacitor and the load' ?

[ms660]

Originally Posted by DonaldStott

@Lawrence - what would you recommend in relation to ' ...some RC filtering between the reservoir capacitor and the load' ?

I would try a 220 Ohm and a 33uF and see how much that reduces the ripple voltage, you might or might not notice the effects of the DC voltage drop.

The anode dissipation isn't the same as output power.

Lawrence.

[DonaldStott]

I've not yet fitted a replacement Mains RF filter so it will be interesting to hear what difference that makes, if any, to the hum?

X2 type capacitor of course!

[DonaldStott]

Originally Posted by DonaldStott

I've not yet fitted a replacement Mains RF filter so it will be interesting to hear what difference that makes, if any, to the hum?

X2 type capacitor of course!

Fitted an X2 type capacitor as a replacement Mains RF filter and guess what - it made no difference to the level of hum?

[Gabe001]

Originally Posted by DonaldStott

Fitted an X2 type capacitor as a replacement Mains RF filter and guess what - it made no difference to the level of hum?

In my limited experience, it only made a slight difference in one set, the TRF Defiant, where there was a loud sharp interference spike from the mains whenevet I switched on the lights, or an appliance. It didn't remove it, but attenuated it to a degree that doesn't cause me to jump up in fright. Otherwise it has never made any difference at all, to an extent that I mostly don't bother with it.

[DonaldStott]

Originally Posted by Gabe001

.... it has never made any difference at all, to an extent that I mostly don't bother with it.

Thanks Gabe and I totally agree - I've replaced/ignored this component on numerous sets and have never found it to make any difference.

Solving the problem of hum is a bit like chasing unicorns!

[ms660]

Originally Posted by DonaldStott

Thanks Gabe and I totally agree - I've replaced/ignored this component on numerous sets and have never found it to make any difference.

Solving the problem of hum is a bit like chasing unicorns!

The mains bypass capacitor has been known to kill mains derived modulation hum, I know that for sure.

How did you get on with the RC ripple filter?

Lawrence.

[DonaldStott]

Originally Posted by ms660

The mains bypass capacitor has been known to kill mains derived modulation hum, I know that for sure.

To what extent is that related to the 'quality' of the local mains supply and house wiring?

Originally Posted by ms660

How did you get on with the RC ripple filter?

Next on my list - just looking at the circuit diagram to decide where to fit the 220 Ohm and a 33uF? I'm assuming the 33uF is an electrolytic?