EKCO U122 - Restore or spares?

[DonaldStott]

Originally Posted by Simon Gittins

Those circular resistors in picture 3 of post 16 were intended as a cheap solution to one section of a dropper resistor having failed ...

Thanks Simon - Trader Service Sheet 995 lists that the dropper (R17) is tapped at 670Ω + 90Ω + 128Ω from R13 (Brimistor, type CZ2) which seems very close to what I have measured - see my diagram in Post #16 above?

Total resistance across the dropper is listed as 888Ω in the Trader and I am measuring 845Ω - that will do for me?

[Simon Gittins]

Yes, that looks fairly close.
The circular resistors in this instance look to be a substitute for the UY41 heater; they have been used together with the flying 150 ohm resistor and silicon rectifier as a replacement for the UY41. This might have been done around 1980 when a UY41 wasn't readily available but the resistors and rectifier were.

[ms660]

Originally Posted by DonaldStott

Understood, thanks. Tags 2 and 3 it is then.

You might have to experiment with taps, in the valve data the optimum load impedance is given for when the anode and screen grid voltages are equal, looking at the voltage table in the manual for your receiver the screen grid voltage would normally be lower than the anode voltage which might affect things.

Lawrence.

[DonaldStott]

Thanks Simon and Lawrence

I've moved on to look at the large HT smoothing can with C23 and C28.

Measured values shown and Trader spec values in brackets: -

C23: 41.2μF (20μF)

C28: 42.4μF (50μF)

High measured value for C23 can be explained as this replacement dual can actually contains 2 x 50μF caps - is this high value a problem?

[ms660]

Originally Posted by DonaldStott

High measured value for C23 can be explained as this replacement dual can actually contains 2 x 50μF caps - is this high value a problem?

It shouldn't be.

Lawrence.

[DonaldStott]

On the Trader sheet, R8 is listed as the Scale Lamp Shunt at 150Ω but a Note points out that 'Two 330Ω resistors are used in parallel'.

What's the purpose of this arrangement and why wouldn't a single 150Ω resistor suffice?

[ms660]

Originally Posted by DonaldStott

On the Trader sheet, R8 is listed as the Scale Lamp Shunt at 150Ω but a Note points out that 'Two 330Ω resistors are used in parallel'.

What's the purpose of this arrangement and why wouldn't a single 150Ω resistor suffice?

It gives the scale lamps an easier life, also if one of the scale lamps goes O/C there would be no heater current so no workie workie for the receiver without a shunt resistor, the two in parallel could be because of availability and the power dissipation required of the shunt resistance, a single 150 Ohm resistor would do if it's rated to dissipate the power.

Note that on this receiver the chassis is connected to the switched Neutral via two resistors from which the grid bias voltages are derived, hence the -ve's of the HT reservoir and filter capacitors being returned to the switched Neutral and not the chassis.

Lawrence.

[DonaldStott]

Thanks Lawrence for the explanations.

I had already noticed something 'different' about the switched Neutral and chassis connections but I am now a bit clearer.

The picture below shows the location of resistors associated with the electrolytics dual can (temporarily removed) which has prompted more queries:-



1. The pair of resistors on the left are the two 330Ω resistors used in parallel as the Scale Lamp Shunt (R8). Should measure 150Ω but are actually reading 214Ω and showing signs of previous distress! Question is should they be replaced like for like or with a resistor of the appropriate power rating?

2. The pair of dog bone resistors on the right measure 61Ω but should be 68Ω (R7 Common G.B.). How reliable are dog bones and should these be replaced by a single resistor of the appropriate value and power rating?

3. The resistor at the bottom is R6 (Common G.B.) and should measure 39Ω but is actually reading 9.5Ω - needs to be replaced but given its location what's the advice on type and power rating?

I've removed C29 (Mains R.F. Filter) as it was just 'floating' under the chassis with some dubious connections! This 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.

[ms660]

Originally Posted by DonaldStott

Attachment 228630

1. The pair of resistors on the left are the two 330Ω resistors used in parallel as the Scale Lamp Shunt (R8). Should measure 150Ω but are actually reading 214Ω and showing signs of previous distress! Question is should they be replaced like for like or with a resistor of the appropriate power rating?

2. The pair of dog bone resistors on the right measure 61Ω but should be 68Ω (R7 Common G.B.). How reliable are dog bones and should these be replaced by a single resistor of the appropriate value and power rating?

3. The resistor at the bottom is R6 (Common G.B.) and should measure 39Ω but is actually reading 9.5Ω - needs to be replaced but given its location what's the advice on type and power rating?

1) Two 330 Ohm in parallel = 165 Ohms not 150 Ohms, that said if they are out of tolerance I would replace them.

2)Two 120 Ohm in parallel = 60 Ohms not 68 Ohms, that said if they are the original resistors I would leave them.

3) Any modern type.

Power dissipation.....I²R applies.

Lawrence.

[DonaldStott]

Made some good progress today in relation to rewiring and replacing components associated with the Output Transformer, the Scale lamps (both dud) and the electrolytic dual can.

I may have to replace the valve base for V4 as a couple of pins are very loose?

Looking again at the Dropper, I mentioned in Post #13 above that it " ... is cracked in a couple of locations and some of the insulation has dropped off - is this a problem?'

See here:-



The resistance readings for the various taps seem ok but I'd appreciate advice on whether or not the Dropper can be repaired and/or stabilised?

[ms660]

I posted the power dissipation formula earlier, didn't have time to do any calcs. because I had to get ready to go out at short notice, anyways.....

First I found this under chassis view of the same model of receiver:

http://www.richardsradios.co.uk/ekcou122.html

It shows the two 330 Ohm resistors (R8) in parallel and also shows a 68 Ohm resistor (R7) in the bias chain along with the 39 Ohm resistor (R6) so the two 120 Ohm resistors in parallel in your receiver might have been replacements, if so then replace with a 68 Ohm.

Power dissipation:

In the 330 Ohm resistors...If one of the scale lamps goes O/C then the 330 Ohm resistors in parallel will have approx. 0.1 Amps flowing through the pair, that's approx. 0.05 Amps flowing through each of them so using the 1²R formula we get approx. 0.825 Watts dissipation in each resistor so I would recommend a 2 Watt power rating for each 330 Ohm.

For the bias resistors we need to know the current flowing through them, that's relatively straight forward, it's just a matter of adding up all the valve electrode currents that are given in the valve table and adding a few extra milliamps for the current flowing through the bottom half of the potential divider (R19) that's connected across the HT, that gives a current of approx. 0.045 Amps, bunging that into the 1²R formula gives a power dissipation for the 68 Ohm resistor as approx. 0.14 Watts and for the 39 Ohm resistor 0.08 Watts so resistors rated at 1/2 Watt will do fine.

Lawrence.

[DonaldStott]

Hi Lawrence and thanks for the link to the Richards Radios site which is another online source that I'm not familiar with.

Thanks also for taking the time to walk me through the power dissipation calculations - very helpful.

I do smile , however, when you type phrases such as " ... that's relatively straight forward,..." - it may be for you but usually not for me!

[ms660]

Originally Posted by DonaldStott

I do smile , however, when you type phrases such as " ... that's relatively straight forward,..." - it may be for you but usually not for me!

Don't tell yourself that....always remember Ohms Law and that all circuits are potential dividers, basic theory is the name of the game and it's not that hard to grasp, without it you'll struggle.

Lawrence.

[DonaldStott]

Progress so far today includes removal of the two RS 'polo-mints' and associated wiring to Pins 1 and 8 of the V5 valve holder.

As mentioned by Simon previously, these were used as a substitute for the UY41 heater - values are:-

250R J 0.3AMP CC

47R J O.3AMP BA

Although the heater connections to Pins 1 and 8 on the V5 valve holder are still in-situ there are no connections to Pin 2 (Anode) or Pin 7 (Cathode) as expected due to the 'polo-mint' changes. Pin 4, however, has been used as a bridging link to connect to the Neutral tag of the On/Off switch, set side. Pin 4 has also been used to connect R22 1MΩ (V4 C.G.) to Pin 6 (G1 Control Grid) of V4!



I'm no expert but that arrangement can't be right - there doesn't appear to be any rectifier or maybe I'm missing something?

If I'm going to reinstate the UY41 then where should the connection to Pin 7 of V5 be made - this is a missing link?

[ms660]

Originally Posted by DonaldStott

Progress so far today includes removal of the two RS 'polo-mints' and associated wiring to Pins 1 and 8 of the V5 valve holder.

As mentioned by Simon previously, these were used as a substitute for the UY41 heater - values are:-

250R J 0.3AMP CC

47R J O.3AMP BA

Although the heater connections to Pins 1 and 8 on the V5 valve holder are still in-situ there are no connections to Pin 2 (Anode) or Pin 7 (Cathode) as expected due to the 'polo-mint' changes. Pin 4, however, has been used as a bridging link to connect to the Neutral tag of the On/Off switch, set side. Pin 4 has also been used to connect R22 1MΩ (V4 C.G.) to Pin 6 (G1 Control Grid) of V4!

Attachment 228745

I'm no expert but that arrangement can't be right - there doesn't appear to be any rectifier or maybe I'm missing something?

If I'm going to reinstate the UY41 then where should the connection to Pin 7 of V5 be made - this is a missing link?

R22 connection looks ok to me, pin 4 is used as an anchor tag:

https://www.vintage-radio.net/forum/...6&d=1243009202

Where does pin 7 of the rectifier connect to.....pin 7 is the rectifiers cathode so look at the schematic.

Lawrence.

[DonaldStott]

Currently there are no connections to either Pin 2 or Pin 7 of V5?

The picture in your link shows a red wire running from Pin 7 (Cathode) but not connected anywhere - looking at the schematic I think it should go C28 and T1?

Similarly the picture shows a red wire running from Pin 2 (Anode) towards V4 but not sure where it ends up - looking at the schematic I think it should go the top of the Dropper?

Still pondering the connections that use Pin 4 of V5 as an anchor tag?

[ms660]

1) Correct.

2) Correct.

3) Correct.

4) It's your call as to whether you want to remove it or not, removing it is easy, just unsolder and remake the connection from the resistor to the wire and insulate the joint with some sleeving.

Lawrence.

[DonaldStott]

Originally Posted by ms660

1) 4) It's your call as to whether you want to remove it or not, removing it is easy, just unsolder and remake the connection from the resistor to the wire and insulate the joint with some sleeving.

For me the issue is not whether or not the V 5 Pin 4 anchor tag should be retained, it's more to with understanding that the Neutral tag of the AC mains input is connected to V4 Pin 6 (Control Grid) albeit via R22 in series?

On a similar issue I also note that V4 Pin 4 is used as an anchor tag linking C22 and R18. We've all done the DAC90A UL41 Pin 4 Mods so I'm assuming the same would apply here?

[Radio Tech]

Hi Donald

If the set was mine I would go ahead and restore it and bring it back to life again, like others, I have seen worse and just as challenging. It will not matter how long it takes you to restore it as time scales do not matter and think of the pleasure it will give you knowing that you restored it.

Good luck with the restoration

Best wishes

Ken

[ms660]

Originally Posted by DonaldStott

For me the issue is not whether or not the V 5 Pin 4 anchor tag should be retained, it's more to with understanding that the Neutral tag of the AC mains input is connected to V4 Pin 6 (Control Grid) albeit via R22 in series?

No problem.....Remember I said that the bias voltages are developed across R6 and R7, remember that almost all the valves electrode currents flow through those two resistors, that's why the bias for g1 of V4 is taken from the switched Neutral side of the on/off switch and it's junction with R7 etc via the 1meg resistor R22 because at that point the voltage is -ve WRT the chassis, and therefore the cathode, which is what's needed for grid bias, grid -ve WRT the cathode. If g1 of V4 was connected directly to the bias resistor R7 then the signal at g1 would in effect be shorted out by R6 and R7 which only have a total resistance of 107 Ohms.

Lawrence.