Murphy A34

[ms660]

So far as I know the IF is 119kHz.

Lawrence.

[LyntonP]

Originally Posted by ms660

So far as I know the IF is 119kHz.

Lawrence.

Agreed Lawrence my service information says the same.
Do you think the IF could be way off?
As I said it was not re aligned during the restoration. I have read the instructions and they don't seem too difficult but call for a "Dummy Aerial" which as far as I can find out is constructed from low value capacitors and an inductor. Is there an alternative way of making the connection or is it vital to the correct alignment?
Lynton

[ms660]

Originally Posted by LyntonP

Agreed Lawrence my service information says the same.
Do you think the IF could be way off?
As I said it was not re aligned during the restoration. I have read the instructions and they don't seem too difficult but call for a "Dummy Aerial" which as far as I can find out is constructed from low value capacitors and an inductor. Is there an alternative way of making the connection or is it vital to the correct alignment?
Lynton

You won't need a dummy antenna for the IF alignment, it's only used for the RF alignment.

With no antenna connected, connect the RF output from the signal generator to the top cap of V1 via a capacitor and manually sweep the generator through the range that covers 119kHz and note at what frequency the output from the receiver peaks to a maximum, it should be at or near to 119kHz (within reason) There should be one peak, if there are two peaks then the IFT's need aligning.

If you decide to do the IF alignment keep the signal output from the generator at a low enough level to avoid AGC action otherwise you might find adjusting the peak difficult, typical symptoms for AGC coming into play when aligning are plenty of output from the receiver but when adjusting for maximum output the peak isn't very well defined.

Also be aware before adjusting the IFT's that two of the IFT's adjustment trimmers will be at HT potential, the one that tunes L15 and the one that tunes L17.

Lawrence.

[LyntonP]

Great instructions Lawrence thank you.
I will carry out the first part of your instructions to see if two peaks are present and report back before continuing.
Lynton

[ms660]

Just to say those instructions were taken from a service sheet not the manufactures manual which might be different.

Lawrence.

[LyntonP]

Originally Posted by ms660

Just to say those instructions were taken from a service sheet not the manufactures manual which might be different.

Lawrence.

Hi Lawrence
Connected the sig gen as advised after setting the frequency to 119KHz. Nothing appeared but a little retuning showed the current IF to be 108KHz (checked frequency with my sw radio). So it is a bit off. Could only find one peak however. Looked for a voltage at the speaker but nothing moved on the AVO set to 2.5 volts.
Yes the instructions are a little different. I says to connect the oscillator to the control grid of V2 via a dummy aerial, adjust trimmers C17 and C18 for max signal and repeat. Next move oscillator to control grid of pentode on V1 and then adjust C15 and C16 for maximum effect.
Next check by varying SG approx 15 KHz either side of 119 so see if output drops evenly either side.
Then says to reset the LO and RF circuits.

Cheers
Lynton.

[Heatercathodeshort]

I don't know the I.F. frequency of the A34. Is it possible that one of the caps across the primary or secondary windings of the I.F. transformers has gone O/C?

Is the tuning sharp when adjusting the I.F.s ? It certainly is an odd one. John.

[ms660]

If you are monitoring at the loudspeaker with a meter it's best to disconnect the loudspeaker and connect a dummy load of a suitable value in it's place and measure the AC voltage across that with the volume turned up as far as needed, for a dummy load a resistor with a couple of watts power rating and a value that's the same or close to the impedance of the loudspeaker will do, if you don't know the loudspeakers impedance you can get a good enough idea by measuring the loudspeakers resistance, if using an analog meter short the meter probes together and set the needle to zero ohms first then measure the loudspeakers resistance, if using a DMM again short the meter probes together and note any resistance reading then measure the resistance of the loudspeaker and subtract the previous reading from that, once you've established what the loudspeakers resistance is add 20% to the result for a near enough value for the impedance of the loudspeaker.

I would align the IFT's to the specified frequency of 119kHz, you don't really need a dummy antenna for that.

Lawrence.

[cathoderay57]

It sounds like the IF alignment is definitely out. However, what is puzzling is that the low volume symptom seems to have appeared suddenly. I have sort-of known this happen once before caused by oxidation on the plates of the IF trimmers. One touch with the screwdriver and the signal level popped right back up. The layer of oxide was preventing the trimmer screw from contacting the top foil of the trimmer properly, and therefore changing its capacitance. Even if that isn't the case here, it will be worth doing the realignment but whether it totally fixes the original fault remains to be seen. Let's hope so! Good luck, Jerry

[LyntonP]

During the investigation I have touched all the IF an RF caps in turn though not turned them. None of them has changed any volume output. What I did notice was when C15 (I think) was touched by the screwdriver there was a loud "pop/scratching sound until firm contact was made but not when touching C17. Does this suggest a lack of HT? or am I mistaken?
If the set had been in a repair shop it would have been scrapped by now LOL.
BTW got the signal tracer working again!
Lynton

[cathoderay57]

Hi Lynton, firstly if you have still got +HT on the pentode anode of the AC/TP then you must have the same voltage on the adjuster screw of C15. Similarly, if there is +HT on the AC/VP2 anode it will be the same on the screw of C17. If the access hole through the IF can is small then it is easy to short the screwhead potential onto the can via the screwdriver blade. To prevent this I have in the past wound insulating tape round the screwdriver shaft leaving only about a millimetre exposed at the end to fit into the screwhead. The popping noise might have been caused by a short circuit similar to that. You mentioned earlier that you were using a SW receiver to check the frequency setting of the sig gen. Do you have such a receiver that covers 108kHz? Otherwise, how can you be sure what the sig gen frequency actually is? I'm just concerned that the difference between 108kHz and the required 119kHz is small enough to mean the sig gen could be out of calibration. That said, you could still get away with realigning the set and if the selected frequency isn't exactly 119kHz it isn't going to be that significant. The real benefit will come from aligning all 4 IF trimmers to the same frequency, which will give you maximum gain, regardless of whether they are still all off 119kHz by a small margin. Looking forward to hearing how you get on. Cheers, Jerry

[LyntonP]

Hi Jerry
Spent most of the day in the shed and am reading your post late afternoon. I didn’t check if there was HT on both of the caps but in hindsight it is easy to do, I will check later.
Yes my Trio R600 covers 0 to 30MHz so setting the precise frequency from the generator is quite simple. To be fair the scale on the SG was very close.
I built a dummy load from some high wattage resistors, bit of a codge but it seemed to work. Problem is with the AVO on it’s lowest scale (2.5v), and the generator turned down so as not to engage the AGC the needle only moves a tiny bit!
I went through the alignment procedure several times using an insulated screwdriver so no big bangs or flashes. No real acute peaks found but I hope I got the best result. When finished the instructions say to move the frequency of the SG a little side to side for equal fall off and it looked ok.
I also resoldered the connections to the tuning cap, they looked ok but I did them anyway.
Also went over the RF caps but to be fair they didn’t need any adjustments
Connected speaker back up and...........The volume on LW is quite loud now, enough to comfortably fill a large room, and even RTE1 comes in strong. MW is also better not as loud, but good enough to listen to.
I also re fitted the original capacitor back in the first IF can. Didn’t change any of the results.
I think I may pause for breath now and get some resistors ordered!
I still have the impression that some little thing isn’t quite right but let’s see if the resistors help.
Once again thanks for all the help, from Lawrence and your good self. I could not have done it without you guys.
Many thanks
Lynton

[cathoderay57]

Well done Lynton, nice job. Cheers, Jerry

[Herald1360]

Originally Posted by cathoderay57

......

I'm just concerned that the difference between 108kHz and the required 119kHz is small enough to mean the sig gen could be out of calibration. That said, you could still get away with realigning the set and if the selected frequency isn't exactly 119kHz it isn't going to be that significant. The real benefit will come from aligning all 4 IF trimmers to the same frequency, which will give you maximum gain, regardless of whether they are still all off 119kHz by a small margin. Looking forward to hearing how you get on. Cheers, Jerry

At the low IF involved here, the frequency offset is nearly 10%. Whilst this won't affect IF gain by much if any, what it might do is upset the tracking between the RF and LO tuning which could reduce overall sensitivity in places as you tune across a band.

[LyntonP]

Spent another couple of days replacing resistors on The Murphy.
Slow going as most of you well know due to the way that the component leads are threaded through holes in the Paxolin boards, then through holes in the tag strips and finally wound around the terminal before soldering. Often more than one component lead are fixed simultaneously making them even more difficult to remove.
All was well until board number 3 where there seemed to be more resistors than shown on the diagrams and one resistor that didn’t seem to conform to any colour code. A yellow body with two black lines around it. I’m sure that many of you more experienced members will say ‘ Oh that’s .......’ ohms, but i couldn’t find anything on the Internet. So I removed it and measured it - 39 ohms. Yellow black black 40 ohms! Simple really, however no resistance of this value was listed on any of the circuit diagrams. Much head scratching, circuit checking, doubting what I had already done etc etc.
Turns out that a specified 140 ohm resistor had been made from two separate resistors each with wires leading off into the set!
The two resistors in situ measured very close to their original value so I have left them in place for now.
Radio tested every few component changes and is still functioning but with no improvement to volume level.
One more board to do then have to venture inside the “canned” parts of the radio.
Photos show board mentioned and snippets from circuit and layout diagrams.
Cheers
Lynton

[LyntonP]

I decided to run through the alignment process again. Everything went well but again no increase in MW volume.
I suppose the next step would be to send the valves for testing, but I thought I would try aligning one more time just in case.
Part of the method involves changing the band mid process. The wave change switch is quite a sizeable mechanical affair with open contacts. With the radio up ended in a frame I operated the switch and a large blob of solder fell out!
Unsurprisingly the volume on MW had increased to a much better level!
I am rather embarrassed that I had not spotted this earlier but the solder must have been there since the first refurbishment only to be displaced by up-ending the radio during the latest work.
Apologies to all the people who tried to solve the problem I had with their suggestions. It was all my fault.
I did complete a full alignment and the radio responded well and is now back to full operation.
Attached is a photo of the wave change switch and the offending blob of solder.
Cheers
Lynton

[Gabe001]

Somebody once wrote on the forum that the waveband switch is the root of all evil.

[AC/HL]

Tape recorder aficionados may well disagree.