Newbie questions Ekco U245

[AndyP-UK]

I grew up in the transistor generation so having a valve set in front of me is a novelty. Apologies for the basic questions but I would rather ask than break something through lack of asking.

1) When valves are in holders that have a wire spring clip at the base, should the clip be removed or pressure relieved before removing/installing the valve or are the valves designed to push/pull past the clip?

2) I have read on here about 'that capacitor' and believe on this set it is C27. My picture below shows this as an aluminum can type 0.002uF. The other two caps of this value on this set are Hunts types with tubular paper covers seen below the can. Does this indicate that C27 has been replaced in the past or would it have been a can from new? if so then why use a different type? I was planning to replace all 3 with polypropylene types. The traders data gives capacitance values but tolerances are rarely mentioned it seems. There is no sign of previous work on this set apart from my own.

3) I read the 'capacitor page' and ordered up an X2 and Y2 capacitor last week. Now I have read that the type I ordered 'Rifa' have a habit of exploding! Is it just 1980's vintage Rifas or are the new ones sold nowadays just as likely to go bang? Do they carry date codes for manufacture?

Thanks for any advice.

[Nuvistor]

1.If these are the 40 series valves then remove the spring clip, there is a glass pip on many of these valves and a stuck spring will break the pip and ruin the valve.

2. Those metal clad capacitors were commonly used in the audio circuits of Ekco radios. Why is another matter, perhaps they had better specifications and deemed more reliable. It may well be good unlike those Hunts, still personally I would replace it, not worth damaging the output transformer or valve.

3. If the RIFA caps are new they will probably last the rest of the sets life, especially if the radio gets little use.

Edit. Some of those metal coupling capacitors had the metal shield soldered to the chassis, that one does not look like that though. This would act as a screen to reduce hum.

[paulsherwin]

Remove the spring clips, as otherwise the glass pip may break off the valves. You don't really need the clip to hold the valves in place.

The cap you mention may be original. Hunts Mouldseals had a reputation for becoming leaky even in the 50s, so manufacturers sometimes fitted a better quality cap in this critical position. If you change it and the Mouldseals (and I would), do them one at a time testing to make sure you haven't made a wiring error. This is particularly important for a beginner.

Cap values aren't critical so you don't need to worry about tolerances. Any modern plastic film type can be used if the voltage rating is adequate. Restorers normally select them on either cost or appearance.

I wouldn't worry too much about Rifa caps, especially if you've already ordered them. You don't really need the X2 cap across the mains anyway.

[ms660]

Some Ekco sets used those TCC types that had the metal case connected to chassis as a screen.

Lawrence.

[AndyP-UK]

Ah splendid, thanks very much.

Yes caps are being done one at a time, old ones measured as they come out and noted down for future reference. Those paper/wax sticky toffee ones are just awful things. I mentioned tolerances as I noticed that a couple of the wax dipped tuning caps has 1% on it ! must be quite tricky to get a replacement for that if needed along with the odd pF values.

I just noticed that my speaker cone has a tear in it.. Doh!

[dazzlevision]

Metal cased paper dielectric capacitors (whether branded TCC, Hunts or Dubilier - the three most commonly used types in UK made sets) are almost certainly faulty (leaky) at this stage in their lives and should be replaced.

[Nanozeugma]

Originally Posted by AndyP-UK

Ah splendid, thanks very much.

Yes caps are being done one at a time, old ones measured as they come out and noted down for future reference. Those paper/wax sticky toffee ones are just awful things. I mentioned tolerances as I noticed that a couple of the wax dipped tuning caps has 1% on it ! must be quite tricky to get a replacement for that if needed along with the odd pF values.

I just noticed that my speaker cone has a tear in it.. Doh!

Don't worry too much about a tear in the cone, nothing a bit of carefully applied PVA or Copydex and some tissue paper can't repair.
The low value (pF) caps are normally fine and shouldn't need replacing. If you do, you will disturb the alignment.

[paulsherwin]

Yes, the low value ones will be mica or ceramic and are unlikely to be bad. Change the ones between 0.1uF and 0.001uF.

[AndyP-UK]

Indeed Paul, once I found they were tuning caps I knew I wasn't going to change them as I don't have the gear to realign it.

Valve clips are now removed.

I've just seen some printing on the main HT smoothing can and another electrolytic beneath that says 'Mar 57'.. that'll be alright then probably dates the set.

[raditechman]

The audio output valve UL41 is prone to internal leakage, you may have +ve on G1 even with a good C27 coupling capacitor, hope yours is OK.
Be careful working with an AC/DC set, make sure chassis is not live.

John

[AndyP-UK]

Thanks John, G1 will certainly be monitored on initial power up along with the HT voltage, I shall be keeping one hand in my pocket and certainly away from the chassis

C27 has now been changed and can be identified as a TCC TCB M/L Type CP30S "Metalmite" labelled as 0.002uF 500V but measures 0.1uF on my Fluke with a DC resistance of 14M Ohms across it.

[Nuvistor]

Originally Posted by AndyP-UK

C27 0.0002uF 500V

Hopefully replaced with a 0.002uf 500v

[AndyP-UK]

Well spotted Frank, too many zeroes there

Replaced with a 2n2 630V Poly

[AndyP-UK]

An update.

I applied power this morning to this U245 for the first time since I have had it after recapping all waxies, TCC tin cans and the main smoothing caps. I constructed a series lamp unit as per this site and was perturbed when the lamp didn't light at all and I measured no HT DC on my set. I confirmed that mains was getting to the on/off switch and rectifier valve OK so felt I had to 'go for it' and switch the lamp out of the supply circuit. After allowing several seconds for the valves to 'warm up' ah yes forgot about that, the rectifier kicked in and it was alive again with all valves glowing.

I have measured all the anode voltages which are within +/- 3v of the 1309 service sheet, it even started receiving a 'Smooth Radio' signal on MW as I was doing the rounds. I have been monitoring G1 of the output valve V4 like a hawk on a digital fluke throughout all this and noticed a gradually rising/lowering negative voltage which I allowed to reached -4v before I switched off to move probes. Later after a short time operating, G1 seemed to settle at -3.6v. So now I am wondering if this is something that I should investigate further with a view to getting it nearer 0v? When switched off, this G1 voltage seems to drop off slowly to 0v.

To say I am delighted with the result so far would be an understatement, all thanks to the good tips and advice on here. I'm so glad it didn't blow up in my face

[ms660]

g1 should be -ve with respect to chassis, it's the grid bias voltage.
0 volts on g1 would be bad news.

Lawrence.

[AndyP-UK]

Thanks Lawrence, oh good that seems alright then, I must have misread that about 0v somewhere then or I just got paranoid about it.

One thing that surprised me is the initial over voltage swing until it all settles down to its operating voltages. Is this usual on valve equipment? for example one anode where I was expecting 157v DC swung up to about 220/230 on start up before dropping back to 160. I know my new components can handle this, I just hope the older ones can.

[paulsherwin]

The increased HT voltage is there because the rectifier is supplying power but the other valves haven't warmed up so aren't drawing any current. Ideally the rectifier and other valves should all warm up at the same time, but UL41s are notoriously slow, particularly the later ones where the heater/cathode insulation was beefed up. It's nothing to worry about in itself.

[Station X]

Originally Posted by AndyP-UK

Thanks Lawrence, oh good that seems alright then, I must have misread that about 0v somewhere then or I just got paranoid about it.

0V is correct where auto bias is used. ie where the bias is derived from a cathode resistor. However the U245 does not use auto bias. The cathode of the output valve is connected to chassis.

The important thing is that the bias must never be POSITIVE with respect to the chassis.

[ms660]

Originally Posted by AndyP-UK

Thanks Lawrence, oh good that seems alright then, I must have misread that about 0v somewhere then or I just got paranoid about it.

One thing that surprised me is the initial over voltage swing until it all settles down to its operating voltages. Is this usual on valve equipment? for example one anode where I was expecting 157v DC swung up to about 220/230 on start up before dropping back to 160. I know my new components can handle this, I just hope the older ones can.

No problem, the golden rule is that the grid bias voltage is the voltage between grid and cathode, the grid should be -ve with respect to (WRT) the cathode. With your receiver It's just a different biasing set up than you would usually see, the most common being cathode bias....if there was a resistor between cathode and chassis and a resistor between the grid and chassis we would expect to measure 0 volts on the grid when measured WRT chassis but the grid is still biased because the bias voltage is developed across the cathode resistor, it makes the voltage on the cathode more +ve than the voltage on the grid which is the same as saying the voltage on the grid is more -ve than the voltage on the cathode, ie: we have -ve bias.

In your receiver there's no resistor connected between the cathode and chassis for any bias voltage to be developed across, instead, the bias voltage is developed across a resistance of 86 ohms (R13 and R14) that's connected between chassis and one side of the mains (preferably the Neutral side for safety reasons) that resistance passes all the current that flows through the valves and the potential divider for the valve screen grids and the voltage developed across that resistance will be -ve WRT chassis and thus the cathode.

Anode/screen voltages falling to a settled value as the valves start to conduct is normal, cathode emission is not instant and warm up times can vary between valves.

EDIT: Post crossed.

Lawrence.

[AndyP-UK]

Thank you all for the information on biasing, I shall have to swot up on that when I find a suitable book.

I have had it on for a bit longer now and am pleased with it picking up 2 LW and 7 MW stations with the internal aerial. The strongest signal is from my local Hospital Radio! Only one undesirable is a low frequency hum which is present all the time and does not increase with the volume control so I am guessing it is mains. A strong station will drown it out quite easily but..

Two of the TCC non electrolytic tin cans that I replaced had grounded cans to chassis, C23 & C26. The poly replacements clearly don't have cans so might this induce hum? I was wondering if I should wrap them in foil/aluminium or something and ground them like the tin cans, providing this doesn't cause other issues.

"the bias voltage is developed across a resistance of 86 ohms (R13 and R14) that's connected between chassis and one side of the mains"
Yes I saw that voltage check in the service sheet, mine measures within 0.3v of the book figures, 1.9 (R13) and 4.1 (R13+14). I had already measured 94 ohms for R13 & 14 during cold checks, so a small difference from the 86 ohms.

Couple of before and after pictures.