Murphy A188C repair.

[Leon Crampin]

Heaters not on 2 & 7 on Mazda octal valves.

Leon.

[Boater Sam]

Thanks Leon, silly me, should be pins 1 & 8 for the UU6 which has an M-O base.
But for the 6P25 it is pins 2 & 7 because it has an Octal base.
Can't tell my octal from my elbow some days.
Glad someone is awake!

[sobell1980]

Didn't get around to it today. Hopefully tomorrow will involve some radio time. The 6P25 turned up today . I've fitted it and it glows nicely for the few seconds I switched on. Ran out of time today so hopefully tomorrow will be more productive on the wireless front. It needs to be if this is going to be ready for the big day.

[sobell1980]

Another question. I will be replacing the mains cable for some braided, twisted 3 core. I have found some rated at 3 amps. For previous sets this is normally more than adequate and I fit a 1 amp fuse in case of even a sniff of trouble. I noticed reading the valve data that the heaters can draw over an amp themselves. This set is fitted with plenty of valves, two of which are 6p25's. There could be over 2 amps on just two valves? Or is this the maximum rated current draw the valve can cope with before being damaged and in reality will not draw anything near this?
Many thanks
Dave.

[Nickthedentist]

Hi Dave,

The power consumption of even the hugest valve radios will never be more than a couple of hundred watts, absolute maximum, so the current consumption from our 230V mains will be under an amp. So 3A flex will be more than adequate.

The usual advice is that the plug fuse is to protect the flex, so a 3A fuse would be fine. But like you, I feel that a 1A one provides slightly better safety for negligble cost.

Good luck with it,

Nick.

[ms660]

A valves heater current data and voltage data are as stated, the 6P25 will draw 1.1 amps with a voltage of 6.3 volts that's the current the valves heater will draw during normal operation, but don't confuse that with the current drawn by the receiver as a whole from the mains.

Lawrence.

[Station X]

The valves will be supplied with 6.3VAC. If they draw 2 amps this will be 12.6 Watts.

With a 230VAC supply feeding the set, a current of only 55mA will give 12.6 Watts.

The flex and fuse will be fine.

[Nickthedentist]

P.S. The manufacturer's data for the A188C specifies a power consumption of 67W from the mains, so crudely speaking, the current drawn will be 67/230 or about 0.3A.

As Graham and Lawrence have shown you, the current delivered by the low-voltage secondary windings of the transformer can be in the order of several amps, but that's neither here nor there when selecting a flex to supply the high-voltage primary winding.

[Boater Sam]

I'm waiting with bated breath to see if the 6P25 and the UU6 respond to pin resoldering.
I bet they will, the 1.1 amp heater current is a lot for a small blob of solder on the end of a pin, for nearly 70 years.

[sobell1980]

Apologies. This is basic ohms law. I should have, like yourselves looked at the sets wattage and I could have done the simple calculation. I think it just rang alarm bells reading the valve data .

[Nickthedentist]

No need to apologies, we all need things clarified now and again - that's what the forum's for!

You could also put your meter, set to its 10A~ (AC) range, in series with the mains input (either L or N wire) and see what it reads - usual mains precautions, of course.

[Boater Sam]

If you consider what happens to the bulb in a lamp limiter when in series with a radio, you realise that they use surprisingly little current.
And all those battery sets of yore, they ran for quite a long time on an accumulator and a dry HT battery.

[sobell1980]

Just thought I'd drop a few lines before the working week starts again. As Sam recommended I soldered the heater connections/pins on the UU6 valve. I can see what you mean now about how the wires lead out the glass envelope and terminate inside the pins. I soldered all the pins, not just pins 1 and 8 for the heater. I now have continuity across the heater element, about 0.8 ohms. However I still have no operation from the valve when it's plugged in. It doesn't glow or get warm like the others do. I have no HT when checked either. So I think sadly the valve is dead. With the uu6 removed I have 4 volts ac to the heater testing across pins 1 and 8. I have 240 volts a.c. to both anodes. That checking each anode pin to ground as the transformer is centre tapped. Everything is there to make the uu6 work.
I've yet to try and solder the pins on the duff 6P25 but as I have a working replacement I want to continue work on the set and get it working, but I will get around to this.
Dave.

[ms660]

0.8 ohms doesn't sound that unreasonable for the cold resistance measurement of a UU6, some of that resistance will probably be made up by the resistance of the meter leads, I would check the valve holder sockets for tightness to the valve pins and the valve pins and holder sockets for muck (oxidization)

Lawrence.

[Nickthedentist]

Lawrence is right, but if all else fails, don't forget:

Originally Posted by Boater Sam

You could always hide 2 silicon diodes and a 120 ohm resistor under the rectifier base if the UU6 is definitely duff.

[Techman]

If you've got 0.8 ohms across the heater pins then that valve obviously has a good heater and should 'glow' when powered from the heater winding of the mains transformer. If it doesn't glow then there can only be two possible causes - 1) There's a bad connection at the base on the chassis, or 2) The glass of the valve is cracked somewhere and the vacuum has been lost, but if this were the case you'd be able to see whiteness inside the glass, so long as it's not of a type that's metalised on the exterior, also it would still start to feel warm even with no vacuum.

This problem with the solder connections within the pins of these type of valve with heaters in excess of an amp is VERY common - I've had to repair loads!

[sobell1980]

No whiteness indicating loss of vacuum. I've cleaned the valve pins with servisol super 10 and wire wool. The valve pins are a tight push fit into the socket. I guess I could extend the wires from the transformer straight to the valve heater pins on the valve and see what happens?

[Techman]

Exactly - that's what I should have added in the previous post. Connect it to an external power supply with crock clips to the heater pins, or as you say, do the same from the heater winding of the transformer.

Just to add for clarity - No HT, just lay the valve on the bench and crock clip to the heater pins from a suitable power source and look for the heater glowing.

It's all about logical thinking. If something isn't working as expected, don't just jump in and blame the most obvious part. Take a step back and try to focus your mind and start to logically think of all the other possible causes of the fault. It may seem absolutely ridiculous that there could be no connection from the valve base sockets on the chassis to the valve base pins on the valve, especially as you've cleaned them thoroughly, but these type of faults are more common than you think - always expect the unexpected when faultfinding an old radio.

[sobell1980]

I've had plenty of the unexpected and some steep learning curves thinking back through my various projects.
What I would like to add is this. Continuity checks are all very well but only show the circuit is continuous or not. It does not tell you the condition of the wiring or how it performs with current passing through it. Let me give you an example I've experienced. Let's say you have a piece of wire that has 6 strands. 5 of those strands are broken and you are left with 1 strand completing the circuit. You get your meter and test continuity through it. You will get the same reading as with 6 strands or 1 because your meter is passing no current. It will read as a good wire and continuous. Try and pass some current down it and it will simply break down and unable to pass it or give the situation of a slow running motor or dim bulb but your meter says that piece if wire is good! I'm wondering if that's what scenario I'm having. Poor connection on the valve heater performing badly under load but testing good on a continuity test.

[Station X]

I've been caught out by something similar. When testing some valves on an AVO CT160 the heaters showed as having CONTINUITY. However subsequent tests showed they had no emission. Problem was dirty valve pins. Subsequent study of the CT160's circuit show that even a very high resistance would show as continuity.

I think you've already cleaned the valve's pins, but ""nip up" the sockets on the valve holder too so that they grip the valve pins firmly.