Murphy A188C repair.

[electrogram]

had a similar problem on a Fidelity uy85/ul84 record player, looked as if someone had had a look but given up. both heaters ok for continuity. valve holders looked ok, put meter between top side and under side of heater pin sockets, measured ok - but of course doing this the meter prods pushed the two halves of broken heater pin socket together to give a good reading... a gentle pull with a pair of pliers and one heater wire with half the socket attached came away, the pair of socket splines still in the valve holder

[Boater Sam]

The pins have to pass 1.1A, the fact that after resoldering you have continuity means the valve heater is there. I suspect that due to the poor contact with the socket pins, the socket connectors in the holder have got hot in use and are corroded or weak in the spring department. Pick up the 4 volts off the underside of the socket or transformer on 2 crocclip leads direct onto pins 1 and 8 of the valve. It should light, now you can test the voltage on the actual pins if it doesn't.
Its too valuable a valve to give up on too easily, same with the 6P25.

[sobell1980]

I have news! We are receiving stations loud and clear although not out of the woods yet.
Using the croc clips I bench tested the heater on the uu6 and it glows beautifully. This was a direct supply straight from the transformer 4 volt winding. Do the same test from the underside of the valve socket and nothing. I've soldered all the joints again on the uu6 valve socket and where it joins L18 . Still nothing. And this is no supply from the socket tags and not the pins. I fitted the valve again back into it's socket and gave a supply to the heater socket tags pins 1 and 8 direct from the transformer. Loads of HT and after fixing a broken speaker wire the set burst into life. So between the mains transformer 4 volt winding and the socket connections I've a broken or bad connection. Possibly something to do with L18 but I ran out of time tonight.

[Boater Sam]

L18 has nothing to do with the 4v heater supply to the UU6, it is the smoothing choke on the HT.

I am confused as to why you have no heater voltage on the underside of the valve socket at pins 1 and 8 yet the transformer has the correct voltage.
Either you selected the wrong pins or the wire/s between the transformer and the socket are open circuit.
I would suggest that it is likely that it is the valve socket failing to make contact with the heater pins. Inspect the actual metal sockets in the holder, they may be broken or have no spring left. Try gently closing them up. If as a last resort you have to change the socket, it is a Mazda-Octal socket not an Octal, a bit harder to find. Depending on the type of socket fitted you should be able to pinch one or more of the 4 or 5 unused contacts from the socket and refit them in place of the duff ones. Straighten the solder tag underneath and they should push up and out.
Great work saving that valve, how is the 6P25 original?
Its really good to hear that the set is working.

[sobell1980]

I don't think it can be the sockets. If I leave the valve plugged into it's socket and supply the socket solder tags underneath with a supply straight from the transformer winding it works immediately. If I measure the voltage at the solder tags on the base with these supply wires in place i get 4 volts. Take the supply wires away and measure again i get zero. Remove the valve from socket and measure again, I get approx 4 volts. Install the valve without my own wires it disappears again.

[TonyDuell]

Sounds like a high resistance connection somewhere between the 4V winding on the transformer and the valve socket. With the valve removed, your meter draws a very small current so the high resistance connection doesn't drop much voltage, you measure the 4V. With the valve in place, the current drawn by the filament causes a voltage drop across te bad connection.

With the UU6 removed and the radio unplugged from the mains what resistance do you measure between pins 1 and 8 of the UU6's socket?

[Boater Sam]

There has to be a wire not connected. The 4v comes off the transformer to the valve filament, as it is a directly heated valve the cathode is the filament to all intents.one of the 4v wires also goes to L18 as this is the cathode connection of the valve to the HT line.
You have the Murphy official sheet?

The tiny numbers on the physical chassis layout Fig 2 are connection numbers, I'll put them in brackets.

So, you have 4 v AC on the transformer, (106 & 107) follow those 2 wires. They go to the 2 valve socket filament pins 1 & 8 (106 & 107)and ONE of them (106) also connects to L18 (106) and the same to the capacitor reservoir C65 (106)

OR are there are 2 wires on the same terminal of L18, 1 to the valve, 1 to the transformer? Or even 3, 1 to C65? Or 3 on the same tag of C65?

Either way the result is the same, the valve gets 4v AC to heat it (106 & 107) and the DC HT from the cathode (106) goes to C65 (106) and L18 (106) and on to the rest of the set from the other side of L18 (44)

So, unless the wiring has been changed or a wire broken off or a really bad soldered joint, it should work! You have all the bits that matter working, the valve, socket & the transformer.
I can't look at which way mine is wired, its 8000 miles away at the moment!

Follow the circuit diagram, the wires may take different routes but the result is the same connections, make the set agree and you are done, well done.

[ms660]

The UU6 is indirectly heated, the cathode is not the filament, the cathode just so happens to be connected to it.

Lawrence.

[Boater Sam]

I'll get my coat and leave it all to you.....................

[Boater Sam]

Out of interest the UU6 cathode is pin 1. The metallised coating on earlier production is on pin 6 and presumably should be grounded.
Now one of the anodes is right next door on pin 5, I am going to be more careful in future near these valves on a powered up set.
If the set is a later model with no ground on pin 6 because the later valve was originally fitted without metallisation, and then a metallised valve is fitted, there could be a considerable risk that there is 350v AC on the loose to catch the unwary.
Is there any good reason why this valve was originally metallised? Are there any other rectifiers with a coating?
UU6 is not known for having a long life. Neither is the octal version UU4. The UU5 is a better valve, on a B4 base, and UU7 is a higher current version of UU6 again on a M-O base.
UU5 and UU7 take a whopping 2.3 amps heater current, no wonder the solder connections on the heater pins fail.
Mazda did have problems making rectifier valves for many years.

[ms660]

The reason for the metallised coating is given in the valve data:

http://www.r-type.org/pdfs/uu6.pdf

Another example with a metallised coating is the U403 which is a half wave rectifier:

http://www.r-type.org/exhib/aag0145.htm

Lawrence.

[kalee20]

Originally Posted by ms660

The UU6 is indirectly heated, the cathode is not the filament, the cathode just so happens to be connected to it.

That's my recollection too, of the UU6. I had one with a loose base.

It is more than academic - the heater has two wires (of course) and the cathode has its own leadout. The cathode leadout is shoved down inside one of the pins, together with the heater leadout, so this pin will actually have TWO wires soldered to its end.

If you clean it and just see one wire, you might be tempted to solder it up and think there's a job well done, not realising that the other wire should be there (maybe cut a bit short in the factory).

If the wire not connected is the heater, it's not going to work and you might mistakenly bin it then. If the wire not connected is the cathode, it might possibly work if the heater/cathode insulation then fails, establishing a connection to the cathode tubes. But it's hardly going to be reliable.

[ms660]

Not sure about the UU6 but others in that series such as the UU4 and UU7 with a cathode to heater connection have the cathode spot welded to one side of the heater lead out above the pinch as do other rectifiers such as the R2, 5Z4G, GZ32, 5V4G, MU14 etc so far as I can make out.

Lawrence.

[kalee20]

Interesting - mine definitely didn't, I can remember the two wires!

Maybe manufacturer dependent?

[ms660]

In that case I wonder if Mazda used the same UU6 assembly for a rectifier with separate cathode pin out, or whether it's just a peculiarity with the UU6.

Lawrence.

[sobell1980]

Nailed it! Disconnected both heater wired each end (from the mains transformer and the socket). Traced the fault back to where there is a common joint for three connections, L18 HT, C65 and the heater supply of four volts. The heater wire connection was green and poor. Moving the joint with my meter on it gave a very intermittent high and low resistance. I've replaced the heater wires and she fired up and was singing away. A good repair to remember to check the voltages under load . It read 4 volts all day long with the valve removed till I tested it last night under load and this caused the voltage drop at c65. Next job to try to repair the other 6P65 valve and start to reassemble this set for Christmas delivery.
Many thanks for the encouragement and pushing me to find the fault and also for insisting I repair the uu6 heater pins.
Dave.

[Boater Sam]

Great, it had to be something simple. Sorry to nag but I felt that you may be discouraged.
I love these 2 chassis sets, bags of working space.

[Nickthedentist]

Great news, Dave

[Radio Tech]

Just goes to show if your persistent and methodical in tracing faults like this, well done Dave and keep at it.

Ken

[sobell1980]

Will be putting the chassis back in before Christmas delivery. I noticed on the back panel there is a large metal plate with an earth wire for the pick up socket. The plate is not connected to anything else and I've never seen anything like it before? Can anyone share any light on it? Please see photos.
Dave.