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Old 19th Jan 2021, 1:20 am   #1
IanCrowe
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Default Murphy A170 advice and wisdom for beginner

Hi. I recently purchased a Murphy Model A170 from my local auction. It looks to be in reasonable condition (lots of dust) no sign of water or woodworm damage. It looks as if the radio has been stored in the warm and dry. A quick look in the back seems to indicate that it's pretty original (wax paper caps are visible). However two of the screws holding the back on are missing so it's possible it's been "seen to" at some stage of it's life.

I managed to find decent documentation over at www.murphy-radio.co.uk including circuit diagram and parts list. I've ordered a set of replacement caps (polyester and electrolytic). I won't be powering it on as some of the insulation on the wiring is starting to break down (see the photograph of the transformer, red wire leading to the voltage selector).

My current plan of attack is to pull the chassis, clean all the dust out then pull the valves and store them away safely. Once this is done I'll work my way along the chassis replacing caps and dubious wiring and testing resistor values. As far as I can see from the circuit I should be able to test all resistors in-situ without any issues. Once that is done I'll clean and exercise the switches then think about putting the valves back in and powering the unit up.

Once I've got the chassis working then I'll turn my attention to tidying up the case. None of the scratches look very deep and all the veneer is sound so just rubbing down and refinishing should be all it needs.

Any advice/wisdom as to anything that can trip me up or bite me? Does anyone know what type of screws are used to hold the back on? They look like 4BA cheese heads with washers but I'm not sure.

I've attached some photos from my initial examination:-

Thanks in advance
Ian
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Old 19th Jan 2021, 9:26 am   #2
PJC58-Hythe
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Default Re: Murphy A170 advice and wisdom for beginner

Welcome Ian,

Sounds like your plan of attack is sound, but I suggest when you power it up you use a 'lamp limiter', especially if you don't have the means to test the transformer. If you want to see the rebuild I did on one back in 2011 see this thread https://www.vintage-radio.net/forum/...ad.php?t=69167

Good luck and post the results in the Success Stories so we can all see the final outcome.

Regards
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Old 19th Jan 2021, 11:13 am   #3
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Default Re: Murphy A170 advice and wisdom for beginner

Hi Ian and welcome, may I suggest you take several pictures, in addition to ones you have already taken. It is surprising how helpful they can be when you are restoring old receivers.
John
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Old 19th Jan 2021, 3:04 pm   #4
Leon Crampin
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Default Re: Murphy A170 advice and wisdom for beginner

Welcome, Ian. The A170 is a better than average 4 valve + rectifier superhet as it has variable selectivity, ganged with the tone switch. This means that you can choose the best fidelity on a strong station which has no adjacent interfering others, or get the best selectivity when the converse applies.

The Mazda valves are long lived and should be ok apart from the tuning indicator which will be dim. The small valves can be difficult to get out - put a little penetrating oil around the base collar and whilst pulling a valve, push on the central spigot from below with a wooden dowel. The pilot lamps will probably need a separate earth lead to work reliably. The IF transformer core shanks are bakelite and very fragile. I would leave these alone unless adjustment is needed, using a signal generator and output meter. If it has not been got at, the alignment should be ok.

This is a very good set to introduce you to the wonders and sometimes surprises found in restoration. It should work very well once finished.

Leon.

One other "catch". The wavechange switch cuts off the HT to the front end of the set to silence the radio on "gram". The contacts used are prone to arcing over to the earthy adjacent ones and this can cause extensive damage. My set produced a good firework display here. I would strongly recommend disconnecting the wires to these contacts and joining them together. This isolates the damage-prone contacts and enables the set to operate normally. After doing this, my set showed no signs of radio breakthrough on "gram". I think Murphy were just playing safe with this arrangement - switch failures occurred 70 years later.

Last edited by Leon Crampin; 19th Jan 2021 at 3:24 pm.
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Old 19th Jan 2021, 5:19 pm   #5
IanCrowe
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Default Re: Murphy A170 advice and wisdom for beginner

Thanks for you quick and welcoming replies.

Peter: It was your thread in Successes that prompted me to buy the A170 as a starter project. If I can get mine half as good as your finished article I'll be a happy man. One question arose from it. Did you do the alignment procedure or was the set good enough without? I don't intend to strip my unit back to the bare chassis, the RF coil assembly looks like a nightmare for someone as ham fisted as I can be. I hope I can ask more questions in this thread as things progress.

John: Trust me there'll be dozens if not hundreds of pictures!

Leon: By tuning indicator I assume you mean the "Magic Eye" tube on the front panel? The technical notes indicate the A170 was originally fitted with a Mazda 6M1 valve but some units were shipped with Mullard EM34 valve. My unit seems to have the EM34 modifications (extra 1M resistor R29 fitted to the valve base). Does your comment about it being dim only apply to the 6M1 or is it more general one about "Magic Eye" indicators? I've see discussion of the switch in other threads. Do you mean the two connections 22 and 23 on S1.g? (see attached portion of circuit)?

Thanks in advance

Ian
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Old 19th Jan 2021, 11:12 pm   #6
Leon Crampin
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Default Re: Murphy A170 advice and wisdom for beginner

Ian, I'm afraid magic eye valves in general will end up being very dim whilst there is plenty of life left in the other valves. A good EM34 is expensive now.

The part of the circuit you have highlighted is indeed where the switch needs to be isolated. Just join up the HT circuit and leave the 2 switch tags isolated.

Good luck with your restoration - there is plenty of good advice available in the Forum.

Leon.
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Old 20th Jan 2021, 12:02 am   #7
rjmusto
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Default Re: Murphy A170 advice and wisdom for beginner

Hi Ian,

If you haven't got a copy already, I would highly recommend Phil Lawson's book:
Restoring Old Radio Sets

Lots of clear advice - including the recipe for the magic restorer mixture for wooded cabinets Produces amazing results in my experience.

Good luck with the set.

Ralph
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Old 9th Feb 2021, 3:39 pm   #8
IanCrowe
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Default Re: Murphy A170 advice and wisdom for beginner

Hi, Back again.

Good news and bad news..

Good news:-
Only a couple of wires round the transformer were cracking up all the others seem fine as do all the other rubber components on the chassis.
All the components have arrived and I've replaced all the suspect/out of tolerance components (8 waxed paper caps, 5 electrolytics and 11 resistors.
The chassis powers up with the dim bulb properly (bright then dim).
Switch to full power and we have noise from the speaker this goes up and down with the volume pot.
The tone/selectivity switch seems to affect the tone of the noise at the speaker.
The tuning scale bulbs both work.

Bad news:-
It doesn't look as if the EM34 magic eye is not working not even very dimly :-( (must check the wiring to the floating socket for V3).
Plug in 18ft of wire as an aerial and boy do we get hum :-(
Select Medium wave and slowly run up and down the tuning range one definite station and a couple of possibles but they are buried deep in the noise.
Switch to long wave and nothing but static and hum. Didn't bother with SW as I've no idea what to listen for.

I've measured the voltages as per the circuit and only a couple of them seem out (see attached A170 Voltages.pdf) only two seem particularly off.
1) The measurement at V1.a(t) (test point 18) which should be 64V and measures 83.3V.
2) V3.a(1) should be 22V and measures 60.1 (see above).

I'm wondering if there is something wrong with V1 (6C9) as the only component affecting DC conditions is R5 (27k) which measures as 28.9k well within the 10% spec.

I've a feeling that this point is part of the local oscillator which could be causing my problem (see V1 circuit.jpg attached).

Any guidance would be most helpful?

I've a feeling I'm getting close but don't know how to take the final couple of steps. Maybe I shouldn't expect very much from this set with only a shortish length of wire as an aerial? Am I going to need a physical earth or can I expect grounding via the earth pin of the plug to be ok?

Thanks in advance for any help

Ian
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Old 9th Feb 2021, 4:46 pm   #9
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Default Re: Murphy A170 advice and wisdom for beginner

The voltages on the manufactures schematic were taken with a 500 Ohms per Volt meter you should take that into consideration when measuring some of the voltages with a modern meter.

Lawrence.
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Old 10th Feb 2021, 8:48 pm   #10
music-centre
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Default Re: Murphy A170 advice and wisdom for beginner

Re. the hum/buzz you are getting is it just the interference that a lot of people get nowadays radiated from many modern household items?
Would be worth going round turning off items in the house to see if it makes a difference, although it could be from outside of course.

I have got one of these and they should work well with the 18ft of wire you have as an aerial.

If you search for AM interference or similar in the Box at the top of the page there are quite a few posts.
Steve.
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Old 11th Feb 2021, 11:47 am   #11
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Default Re: Murphy A170 advice and wisdom for beginner

Hi Ian

Welcome to our forum, and I admire your enthusiastic way and methods of dealing with the set. If by chance the envelope of the output valve is or has become loose from its base just dribble some super glue around and that should hold it. Yes, you are going in the right direction for what will be a lovely set to admire. Good luck with the progress.

Best wishes

Ken
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Old 11th Feb 2021, 1:01 pm   #12
Edward Huggins
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Default Re: Murphy A170 advice and wisdom for beginner

Be cery careful of that plastic grille. It will be very brittle by now and is nearly impossible to make a good repair to.
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Old 15th Feb 2021, 11:04 am   #13
vinrads
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Default Re: Murphy A170 advice and wisdom for beginner

That reminds me when I repaired the grill on a record player , luckily it had a small vent grill on the rear ,so I pinched that fitted something in its place, very carefully cut a square out and super glued it in , a quick spray of gold paint ,job done. Mick.
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