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Old 7th Jan 2021, 1:37 pm   #1
robreddog
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Default Murphy A674. EL84 output valve capacitor.

HI, this guy saw my radios in the shed and said "can you fix them" well only as a pastime i said but next day he rolled up with a sick Murphy A674, wish I had kept it shut. Many caps and two valves later got music but not perfect.

Is 80mV DC too high for the output valve pin2? I have not changed C64 it is ceramic 10pf, but i would like to see if it is leaking or the new C62 that is leaking. If I know that 80mV is too high, can I safely lift the connection between them to check which one is causing the problem if indeed it is. Cannot see that the leakage can be from anywhere else. Or if not a problem I will look elsewhere.

Thanks, keep safe.
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Old 7th Jan 2021, 2:01 pm   #2
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Default Re: Murphy A674. EL84 output valve capacitor.

If it’s a stable 80mv and doesn’t rise uncontrollably after 10-15 mins then it should be ok. Check the cathode components are within spec and that the output valve is not drawing too much current.
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Old 7th Jan 2021, 2:06 pm   #3
snowman_al
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Default Re: Murphy A674. EL84 output valve capacitor.

Check the voltage across R44 (180ohms).
Is the EL84 / 6P15 ok?
You are fine to dis-connect the caps as a 'try out'.
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Old 7th Jan 2021, 2:11 pm   #4
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Default Re: Murphy A674. EL84 output valve capacitor.

I wouldn't worry about 80mV. Check the circuit of the Murphy and see what the EL84 cathode volts should be. If they are within around 10% or so then everything is probably OK. Measuring the cathode volts and then working out the current using ohms law (E/R...the voltage on the cathode divided by the value of the cathode bias resistor will give you the current through the valve in amps) will tell you if the EL84 is working within its limits.
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Old 7th Jan 2021, 2:15 pm   #5
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Default Re: Murphy A674. EL84 output valve capacitor.

Hi Rob,

Usually the ceramic caps of those sort of value do not give rise for concern and yes like you are advised about cathode resistors. Also look for those waxie caps that couple one stage to another these have a tendency to leak, a set of this age may want the electrolytic caps changed as well.

Best wishes

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Old 7th Jan 2021, 2:38 pm   #6
robreddog
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Default Re: Murphy A674. EL84 output valve capacitor.

All the hunts caps, coupling caps and the 2 smoothing caps replaced, one resistor r14 also.
I have got 6.7 volts across r44.
Will look at the valve data sheet.
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Old 7th Jan 2021, 2:41 pm   #7
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Default Re: Murphy A674. EL84 output valve capacitor.

After 10 muns 132 mv on pin 2.
The El84 is a temp replacement out of working set.
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Old 7th Jan 2021, 2:58 pm   #8
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Default Re: Murphy A674. EL84 output valve capacitor.

The mv are arriving via the ceramic cap.
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Old 14th Jan 2021, 1:37 pm   #9
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Default Re: Murphy A674. EL84 output valve capacitor.

the 10pf cap finally arrived yesterday,fitted it and mv dropped to 50,guess this is ok, but still not 100% good sound imho.
so carried on looking around and found c68 hunts .04uf hidden behind the o/p transformer !!,(hidden to me anyway)removed but only had a 47nf cap,sounds ok so is it ok to leave it in or should i get 39nf.
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Old 14th Jan 2021, 1:49 pm   #10
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Default Re: Murphy A674. EL84 output valve capacitor.

Quote:
Originally Posted by robreddog View Post
t only had a 47nf cap,sounds ok so is it ok to leave it in or should i get 39nf.

It's fine to leave that.
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Old 16th Jan 2021, 10:37 am   #11
robreddog
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Default Re: Murphy A674. EL84 output valve capacitor.

hi , sorry all but more questions, advice/help/required please ,did some final checks around the radio which then seemed to work ok but found 120 v on pin 1 v2 (ech81) this is well above stated value on the service sheet which is 78v,is this too high.
i changed caps and one resistor in that part of the cct but that made no change
is this high voltage a sign of something amiss
i have 255v on the top rail , reason i ask is that after 3-4 hrs running yesterday evening the reception started cutting out although the vhf push button could be dodgy.
i replaced the original 6c12 for the ech81 as well.
rob.
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Old 16th Jan 2021, 11:03 am   #12
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Default Re: Murphy A674. EL84 output valve capacitor.

Is the high g2g4 voltage with no signal input?

Lawrence.
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Old 16th Jan 2021, 11:28 am   #13
robreddog
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Default Re: Murphy A674. EL84 output valve capacitor.

Radio on vhf
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Old 16th Jan 2021, 11:36 am   #14
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Angry Re: Murphy A674. EL84 output valve capacitor.

Sorry start again the 120v is on pin 8 v3
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Old 16th Jan 2021, 11:50 am   #15
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Default Re: Murphy A674. EL84 output valve capacitor.

If you haven't already done so check R16, R17 and C41. (manufacturer's sheet).

It's likely that the high reading is down to the fact that you're using a DMM which draws less current from the circuit than a moving coil meter would.
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Old 16th Jan 2021, 12:01 pm   #16
ms660
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Default Re: Murphy A674. EL84 output valve capacitor.

The measurements on the schematic were taken with a 20k OPV meter, the difference with a DMM shouldn't be that much given the circuit resistances of the g2 circuit.

Lawrence.
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Old 16th Jan 2021, 12:15 pm   #17
robreddog
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Default Re: Murphy A674. EL84 output valve capacitor.

High changed c41, c42, and r17,, r16 is dropping 18v,
R17 dropping 45v
Voltage on pin 8 now up to 175??
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Old 16th Jan 2021, 12:36 pm   #18
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Default Re: Murphy A674. EL84 output valve capacitor.

Check the V3 cathode resistor and cathode current are reasonable. What voltage is on the grid? (AGC problem?)

Could just be tired V3.
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Old 16th Jan 2021, 12:37 pm   #19
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Default Re: Murphy A674. EL84 output valve capacitor.

Is that with no signal?

Don't forget that the voltage on g2 can vary according to the signal received because the valve is under AGC control.

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Old 16th Jan 2021, 12:54 pm   #20
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Default Re: Murphy A674. EL84 output valve capacitor.

Switch it to Medium Wave and recheck the voltage.
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