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Old 24th Nov 2004, 1:34 am   #1
johnmarshmanathome
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Default Lispy Murphy A30

chaps
i could swear the murphy a30 is rather lispy - letters s and t especially don't come through properly, even with the tone right up or disconnected. there is a sense of mild distortion which seems to affect higher frequencies more - but this may be subjective. i am aware that the set will not sound right at really low volume, so that is not it. i have compared the grid and cathode voltages of the pen/dd output valve and the grid is about 5v lower than the cathode which is around 11v. is it worth changing the cathode bypass resistor ? - i dislike changing capacitors willy-nilly cos i learn nothing from it and it costs money. or is this a question of the demodulator or even of frequency response in an earlier stage ?
cheers
john
 
Old 24th Nov 2004, 6:55 pm   #2
ukcol
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Default Re: lispy murphy

The output valve’s cathode resistor in the A30 consists of 2 resistors in series, a 140-ohm (the upper one) and a 320-ohm. The bottom end of the valve’s grid resistor is connected to their junction. The voltages you give sound within tolerance but you can check the resistors with an ohmmeter.

If you take a signal from the volume control and feed a separate amplifier you can decide if the fault is in the audio stage or not. It may be an alignment problem. The IF in this set is 119kHz*



*Ref. Valve Radio & Audio Repair Handbook by Chas Miller. P178


Regards
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Old 25th Nov 2004, 12:11 am   #3
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Default Re: lispy murphy

colin
will do as you suggest.
looking at the signal at the volume control with a scope, it seems that it is not symmetric - ie : the excursions of the af signal below the " zero " (scope is on a.c.) are bigger than those above. it is hard to tell if the signal at the anode of the i.f. valve is similarly odd, cos it's harder to " read " . meanwhile, although strong stations do not sound too bad (albeit lispy) weaker stations in between are just whistles - is this an i.f. problem ?
cheers
john
 
Old 25th Nov 2004, 12:32 am   #4
Leon_Crampin
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Default Re: lispy murphy

I have done an extensive restoration on an A30C, and would suggest that either the 'speaker is not fully free, due to stiff cone suspension or rubbing (connect another to the Ext. LS sockets for a trial, but leave the field of the internal LS connected), or that possibly there is an alignment problem.

The IF transformers should peak easily at 119 kHz on this set, but the bandpass pre-selector circuits require careful setting if sideband cutting is to be avoided. I found that excessive capacity in the screened lead to the AC/TP caused by perished insulation initially prevented correct alignment. Don't forget to set the image rejection filter (plastic screw under hole in can) - this is best done by ear, but you need an accurate generator to get the frequency right.

These sets perform really well, despite their 3 valve configuration. The 'speaker on the console has a massive magnet with a coil as big as a toilet roll - mine was open circuit.

If you have any alignment problems, I'll get my notes out, but I don't recall any major difficulties other than the bandpass front end.

Leon.
 
Old 25th Nov 2004, 7:47 pm   #5
ukcol
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Default Re: lispy murphy

Hi John

It certainly sounds like the problem is IF or detector. Since you say you can see that the audio is asymmetrical at the volume control I think alignment is less likely to be the culprit. The fact that the set tends to whistle on week stations suggests instability. You may have to bit you lip and change the decoupling in the receiver section. Leon is right about performance, although I don’t have an A30 all the 30’s and 40’s Murphys I have heard sound superb, so the poor performance will not be normal.
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Old 26th Nov 2004, 12:24 am   #6
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Default Re: lispy murphy

chaps

i think there are maybe 2 problems here - in truth the extreme whistling now seems intermittent after all - and depends upon fiddling with the anode (cap) of the i.f. vv.

i took an a.f. feed off the volume control to another amp, and it sounded just as lispy.

not sure i get the point about decouplers in the receiver section - which one is that ? - i have not toyed with a radio of this type before and can see that there are a fair few extra components locked away inside the cans - there seem to be two cans infront of the freq chgr (is this a bandpass config ?), then one for the local osc' and then one each side of the i.f. vv....

cheers john
 
Old 27th Nov 2004, 10:37 am   #7
ukcol
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Default Re: lispy murphy

Quote:
Originally Posted by johnmarshmanathome
not sure i get the point about decouplers in the receiver section
Hi John

I'd be inclined to replace the following capacitors in the mixer & IF stages:- C8, C9, C13, C19, C20, C21 & C28.

Last edited by Paul Stenning; 26th Dec 2004 at 3:44 pm.
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