Murphy A574

[telstar]

Hi. I have a Murphy A574 valve radio that has a hum problem, and I have tried bridging the main electrolytic capacitors, but with no improvements. The voltage on the first electrolytic is just over 100 volts, and of course it should be about 280 volts. Also, I tried replacing the EZ80 valve, but that did not improve things either.
I checked the mains transformer windings for resistance values, one part of the secondary is a bit larger in resistance to the other.
Wonder if there is an open circuit somewhere in the h.t line (maybe the large resistor @1350 ohms?)
Cheers
Mike

[snowman_al]

Start by checking the AC volts on the transformer, the circuit gives 268 volts AC with respect to the chassis / earth line. They should be very nearly equal. (The circuit gives a resistance of 210 and 230 ohms.)

Measure the DC volts on C73 again to the chassis, circuit says 280 volts.
Then try at C72 expect 220 volts, and what do you get?

You can measure the resistance of the 1350 ohm resistor in circuit, what do you get?

Do the voltages go up if you remove the 6P15?

[telstar]

Originally Posted by snowman_al

Start by checking the AC volts on the transformer, the circuit gives 268 volts AC with respect to the chassis / earth line. They should be very nearly equal. (The circuit gives a resistance of 210 and 230 ohms.)

Measure the DC volts on C73 again to the chassis, circuit says 280 volts.
Then try at C72 expect 220 volts, and what do you get?

You can measure the resistance of the 1350 ohm resistor in circuit, what do you get?

Do the voltages go up if you remove the 6P15?

Hi many thanks for your reply.
I will start to remove the chassis from the cabinet (gosh what a job) and will take measurements and let you know.
The resistances of the secondaries of the mains transformer windings are 203 and 220 ohms respectively on my radio.
Cheers
Mike

[telstar]

I have discovered that the radio does receive stations on long, medium, and vhf bands very well indeed and at good volume. Unfortunately, it is spoiled by the hum.
The 1350 ohm resistor unit reads 1580 ohms.
Wonder if the 32 ohm choke on the o/p transformer primary is not functioning in filtering out the hum?
The voltage at the first electrolytic capacitor has risen to nearly 200 volts.
Will inform more later
Cheers
Mike

[cathoderay57]

Hi Mike, it sounds as if the 1sr electrolytic has reformed a bit, but has probably lost capacity. I would try replacing it before you do much else. Cheers, Jerry

[snowman_al]

The 32Ω 'choke' is working fine. If it had failed you would not get any HT on C72 and therefore nothing from the set.

Have you replaced 'that cap' C63 on the Murphy diagram (C55 on the Trader sheet)? If the output valve is drawing too much current it will pull the HT down. Does the HT improve to near service data values if you remove the 6P15?
If not then the smoothing caps are likely leaky, are they getting warm after a few minutes being on?

[telstar]

Hi. Most of the wax paper and metal foil capacitors have been replaced. I will look for the capacitor C55 to see if this has been replaced.
All the valves marked with a prefix 6 have been substituted by earlier restorers as E valves (EABC80 for instance)
There can not really be much wrong on the radio if it is receiving stations on all bands at quite a volume.
Cheers
Mike

[telstar]

Hi I am sure that I replaced C63 capacitor myself. It is one of the first components that I replace (i.e. "that capacitor).
I have the Murphy A574 service booklet

[Silicon]

The hum reducing abilities of the output transformer rely on the relative size of the V6 anode current and the current through R44.

If the circuit has been modified, it may be enough to disturb the current balance and increase the hum.

I would also start by changing C73.

If that makes no difference you could try adding an extra RC filter stage between C73 and the output transformer. Something like 200 Ohms and 47uF.

V4c is vulnerable to hum pick-up if the control grid circuit and cabling has been modified.

[snowman_al]

I suspect that once the HT is at the correct voltage, it will all fall into place?

[telstar]

Many thanks everyone. I will try and replace R44 (1350 ohms). See wanted components.
Cheers
Mike

[telstar]

Hi. Now that I have got more time to sort out my radios, I thought that I would have another look at my Murphy A574.
The EZ80 rectifier (not faulty) was getting too hot, and the voltage at its cathode measures 155 volts. The voltage at the second electrolytic is 112 volts.
Also, someone has kindly given me a w/w resistor 1.3kohm. Those original ones certainly do want replacing as they were 20% out of tolerance and should only be 5%.
Hopefully someone can supply me with some axial electrolytic capacitors, as I believe that C73 is to blame on this model for the hum on this radio.
Kind regards
Mike

[Freya]

Cricklewood Electronics list 47uF 350 volt axial capacitors, would suggest you replace C72 & C73 and at the same time.

[telstar]

Hi. I managed to obtain a 47uF @ 350 volt capacitor from a friend. Installing the new one cured the main hum fault, but there was some background hum still. I also replaced the 1.35k ohm resistors (which read 1.6k ohm) with a 1.3k ohm resistor.
The old capacitor can I have left in place in the chassis with just the second capacitor used in that can.
Hopefully it will be soon on the mend