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Old 27th Nov 2023, 7:38 pm   #1
Radio Tech
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Default Murphy A90

Hi folks

Got hold of a Murphy A90 and this one had been tampered with by someone who did not know what they were doing, the o/p valve base loose away from the rivets of which I fixed by using nuts and bolts, the mains lead cut off, now replaced, there was a smoothing choke floating around in the cabinet, it was a shear wonder the valves did not get smashed and for some reason the original LS removed and a pm type put in place, the o/p transformer had wires just dabbed on and the switch that allows the internal LS to be turned off was broken. I have got to get the cabinet sanded down but will most likely have to remove the Bakelite frontage.
The drive from the main tuning to the small disc missing I think this might have been a chain driven but no information on the drive cord layout or how this chain links up on the service info

Ken
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Old 1st Dec 2023, 8:23 pm   #2
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Default Re: Murphy A90

Hi Folks

Got the cabinet done and LS with field coil fitted, problem is two things, the Pen 45 o/p valve has the spigot broken off but the main concern is the o/p transformer, the primary resistance according to the trader sheet says the primary resistance is 290 Ohms Approximately, measuring the primary winding reads 22.5K so the question is this , is the trader sheet wrong or is the actual reading I am getting (measured with digital meter) wrong.

Best wishes

Ken
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Old 2nd Dec 2023, 12:06 am   #3
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Default Re: Murphy A90

It's possible that your DMM can't cope with measuring very inductive resistances. Have you an analog meter you could try?

Alternatively, connect the TX primary in series with a 270R resistor and connect a 1.5V cell across the series string. Measure the voltage across the TX primary- it should be roughly half of the cell voltage. If it's nearly the cell voltage, that 22.5k reading, unfortunately, is likely to be a toasted TX primary.
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Old 2nd Dec 2023, 11:20 am   #4
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Default Re: Murphy A90

Watch your fingers making the second suggested test. The emf generated from the stored energy in the transformer core when the circuit is broken will bite.

Leon.
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Old 2nd Dec 2023, 3:45 pm   #5
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Default Re: Murphy A90

Hi Folks

As far as I can make out, the primary of the o/p tx is around 22.5k after measuring the voltage as suggested but what still puzzles me is the trader sheet says the primary has a resistance of 270 Ohms bearing in mind the anode load resistance of the pen 45 is 5.5k.

So still not certain if the o/p tx is at fault

Ken
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Old 2nd Dec 2023, 3:51 pm   #6
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Default Re: Murphy A90

The DC resistance of a transformer's windings has nothing whatever to do with its turns ratio which is the important thing. The impedance ratio (not resistance ratio) is the square of the turns ratio.

The resistance of windings is quoted in service sheets to aid testing.
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Old 2nd Dec 2023, 6:40 pm   #7
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Default Re: Murphy A90

I happen to have some spare bits for the Murphy A90. Even a spare chain I think. You're welcome to some of the bits if you can be specific in your requirements and an wait for a couple of weeks when I go back to the workshop.
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Old 5th Dec 2023, 2:44 pm   #8
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Default Re: Murphy A90

Hi Ian

will send pm to you regarding to spares..

And thanks Graham for the info, comes in handy, as for the transformer I replaced this but will see how it performs when I eventually complete the set.
Pictures to follow

Ken
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Old 12th Dec 2023, 12:28 pm   #9
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Default Re: Murphy A90

Hi folks

Powered the set up but I am baffled as to why no sound, the HT is present at all valves and on the o/p tx so it looks like a long haul to find out why. I have hidden the replacement caps under neath the chassis as I could not get the three of them in one can.

Ken
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Old 12th Dec 2023, 12:30 pm   #10
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Default Re: Murphy A90

No sound at all? Not even hum or hiss?

What's the voltage across the cathode resistor of the output valve?
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Old 12th Dec 2023, 2:29 pm   #11
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Default Re: Murphy A90

Quote:
Originally Posted by Ian - G4JQT View Post
I happen to have some spare bits for the Murphy A90. Even a spare chain I think. You're welcome to some of the bits if you can be specific in your requirements and an wait for a couple of weeks when I go back to the workshop.
Ian, I have a nice example of a A90 but it's missing its Card Back. Obviously if you have one and Ken wants it I understand but if not I would be very interested.

Thanks, Clive.
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Old 12th Dec 2023, 5:55 pm   #12
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Default Re: Murphy A90

Hi Graham

No hum or hiss from the set at all, the voltage is around 3V but will check this again and get back.

Ken
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Old 12th Dec 2023, 9:15 pm   #13
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Default Re: Murphy A90

Hi Graham

The voltage across the o/p valve cathode bypass resistor is 7.8V, Anode 238V . The voltages seem a bit high to me and according to the service sheet so I wonder now if the o/p valve Pen 45 is not as it should be.

Ken
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Old 12th Dec 2023, 9:55 pm   #14
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Default Re: Murphy A90

What's the screen grid voltage looking like?

I assume there's no positive voltage on the control grid?

7.8V between control grid and cathode?

C49 OK? if it's leaky and R28 is at min it'll short anode to cathode.

Speaker Field Winding energised?

Is S5 internal/external speaker in the correct position and making good contact?

Extract from manufacturer's service sheet attached.
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Old 12th Dec 2023, 10:50 pm   #15
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Default Re: Murphy A90

Good evening,
By the look of your first post, it looks as if the output stage has been messed about with in the past. Has this all been correctly connected up. If there was a separate HT choke and a PM loudspeaker, the original speaker must have failed and this is why somebody in the past has fitted them. Has the speaker switch been bypassed.

I would carefully trace through the output stage wire by wire to ensure that there isn't another change that is stopping any audio getting through. If you have 7V across the cathode bypass resistor, there must be a current flowing through the valve. Presumably there is a fairly high voltage on the output valves anode and the cathode current not being supplied from the screen grid?

Christopher Capener
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Old 13th Dec 2023, 1:16 pm   #16
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Default Re: Murphy A90

The screen voltage on the o/p valve has almost the same voltage on it as the Anode, no positive volts on control grid, will re check C49 and R28 and will trace through wire by wire as Chris suggests, the speaker switch I have checked and functions OK along with its wiring.

Ken
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Old 13th Dec 2023, 1:48 pm   #17
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Default Re: Murphy A90

Quote:
Originally Posted by Radio Tech View Post
The screen voltage on the o/p valve has almost the same voltage on it as the Anode

Ken
Hardly surprising. There's no dropper resistor for the screen grid so its voltage will be higher than that on the anode which has a volt drop through the output transformer primary winding.
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Old 13th Dec 2023, 2:03 pm   #18
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Default Re: Murphy A90

I assume you have tried connecting another loudspeaker directly to the secondary of the O/P transformer to check it's not the existing existing loudspeaker?
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Old 14th Dec 2023, 6:37 pm   #19
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Default Re: Murphy A90

Have not tried another LS direct connected to o/p tx, however, the two resistors that are currently unsoldered measure very high, I believe from the trader sheet these are R16 and R17 which should be 47 R and 27 R. R16 measures .5meg and R17 measures 790 Ohms, I have replaced C22 as a matter of course. The component layout of the underneath is not very clear .

I have attached a photo

Ken
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Old 14th Dec 2023, 7:23 pm   #20
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Default Re: Murphy A90

By your component designations I assume you're working from the Trader Sheet?

Nothing you do further back in the circuit will alter the fact that you have no output whatsoever from the speaker.
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