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Old 18th Jan 2022, 3:10 pm   #1
agardiner
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Default Murphy A242 VHF trouble

Hi,

Hopefully someone can assist with an issue on a Murphy A242 set. Quick overview - I have replaced the selenium bridge with a silicone one, adding a 100R resistor in series. Electrolytics have all been replaced along with any waxies. The AM part of the set is working well.

VHF is not working at all, and I struggled to align the IF strip, basically because following the instructions, you should connect a voltmeter to the AGC test point, and after peaking the final IF coil, you should adjust the signal generator such that the voltmeter reads 5V, before continuing. My meter read 8V even without the signal generator connected.

After deep diving into the VHF front end, I found C20 (22p) was open circuit, C19 (15p) was reading approx 80p and C8 (500p) was open. NB, C8 is marked 620p on the attachment, but was later modified to 500p and this is a modified set. The offending caps were replaced along with C16 and R2 the 3.9M AGC resistor, which was reading very high. All other resistors test fine.

Once the above was complete, I was able to realign the VHF IF strip properly, according to the manual. However, still no VHF tuning. The voltages on the valves test fine; V1 anode, 146V and V2 anode 142V. I can only read V2 with my basic (20MHz) scope, as my multimeter reads 600V and then says overload. This confirms that the oscillator is at least running, although don't know the frequency. Grounding V2 grid flat lines the scope, and the multimeter will then read, so this also confirms oscillation. I do note that the frequency appears to not change as I tune; as said I only have a 20Mhz scope but I cannot see any change happening.

Any ideas what I have missed? If I inject a modulated 10.7MHz signal, it is clear and loud. Just no VHF tuning. I attach just a snapshot of the circuit covering the area, so as not to break copyright.

Thanks all,

Adrian.
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Old 18th Jan 2022, 4:13 pm   #2
dazzlevision
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Default Re: Murphy A242 VHF trouble

C16, 0.005uF (? image is a bit fuzzy) might be one of those Dubilier "paper dielectric in disguise" capacitors, rather than a ceramic. If it is, I'd recommend its replacement.
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Old 18th Jan 2022, 4:15 pm   #3
agardiner
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Default Re: Murphy A242 VHF trouble

C16 has already been replaced. The original was a waxy.
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Old 18th Jan 2022, 6:12 pm   #4
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Default Re: Murphy A242 VHF trouble

Have you placed another VHF radio next to the Murphy to see if V2 is oscillating and variable?

What is the AGC DC voltage when the set is working on other wavebands?

Have you checked the continuity of the RF coils?

Last edited by Silicon; 18th Jan 2022 at 6:17 pm. Reason: More info
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Old 18th Jan 2022, 6:16 pm   #5
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Default Re: Murphy A242 VHF trouble

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Originally Posted by Silicon View Post
Have you placed another VHF radio next to the Murphy to see if V2 is oscillating and variable?
No. Must do that, but didn't think that trick worked on FM?

V2 is oscillating, but not sure it is variable.
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Old 18th Jan 2022, 6:48 pm   #6
Mr 1936
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Default Re: Murphy A242 VHF trouble

Hi

This is one of the earlier VHF models. It would appear likely that the LO is oscillating. The fact that you have had to replace capacitors associated with LO tuning makes me wonder whether the tuning range has been inadvertently shifted off by too much. I don't know if the LO on this model is supposed to be lower or higher than the RF, both conventions have been used but low-side was more common in the UK for that era.

If so, an RF range of 87 to 100 MHz would have an LO range of 76.3 to 89.3 MHz. You might be able to detect the very top of the tuning range using a nearby second FM set tuned to an empty frequency around say 88 MHz.

Given that the entire LO tuning range is only 17%, I think you might struggle to see the frequency changing on a 20 MHz 'scope.
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Old 18th Jan 2022, 7:07 pm   #7
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Default Re: Murphy A242 VHF trouble

Thank you for that. All makes sense and I will try another set nearby.

I am totally sure the oscillator is running as I have said, my multimeter reads over 600V on the anode due to the RF. Grounding the grid of V2 kills the oscillator and the meter settles correctly to the DC HV. Also I can see oscillations on the scope.

The LO is indeed lower on this set than the tuning range. The service manual clearly shows the differences and I have confirmed this. I think the reason I could not originally carry out a proper alignment was due to C8 being open, which caused random oscillations and instability.

So what remains is why now that it all appears stable, that I have no tuning? I have no reason to suspect any coils; nothing appears to be tampered with and the the set was original.
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Old 18th Jan 2022, 10:19 pm   #8
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Default Re: Murphy A242 VHF trouble

Any background hiss on FM? When you injected the 10.7MHz IF signal was it frequency or amplitude modulated? Other thoughts are V1 anode voltage is a bit low - circuit snip is blurry but looks like it should be 160v therefore V1 is drawing more current than it should or one of the dropper resistors has gone a bit high. Have you tried a replacement 6L34 / EC91? Jerry

Last edited by cathoderay57; 18th Jan 2022 at 10:42 pm.
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Old 18th Jan 2022, 10:34 pm   #9
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Default Re: Murphy A242 VHF trouble

Hi Jerry,

No background hiss. For alignment the 10.7Mhz signal was not modulated as per the service manual. However, when I inject it as an FM modulated signal, it goes through the IF as expected and is successfully demodulated. Feeing an 1KHz tone modulated as FM sounds correct and non distorted.

Any clue?
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Old 18th Jan 2022, 10:55 pm   #10
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Default Re: Murphy A242 VHF trouble

Have you tried injecting the 10.7MHz FM signal at the aerial? Other thought is the trimmers often give trouble. Have a tweak at C21 and C7 and see if bursts into life. Jerry
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Old 18th Jan 2022, 11:19 pm   #11
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Default Re: Murphy A242 VHF trouble

I had thought about the trimmers. I have tried tweaking them with no effect.

Yes, I have tried injecting the modulated signal at the aerial. It gets through albeit at a much lower level.
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Old 19th Jan 2022, 11:09 am   #12
Leon Crampin
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Default Re: Murphy A242 VHF trouble

Take a very critical look at C16 (5nF). It bootstraps the mixer and causes very low gain if faulty. I had this problem on my A272C.

Leon.
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Old 19th Jan 2022, 2:11 pm   #13
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Default Re: Murphy A242 VHF trouble

Quote:
Originally Posted by Leon Crampin View Post
Take a very critical look at C16 (5nF). It bootstraps the mixer and causes very low gain if faulty. I had this problem on my A272C.

Leon.
Thanks Leon. Already replaced and double checked.
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Old 19th Jan 2022, 3:27 pm   #14
cathoderay57
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Default Re: Murphy A242 VHF trouble

Hi Adrian, reading the Murphy service manual it appears that several of the components within the FM section are fairly critical for the stage to work properly. I recommended in Post #8 that you should try a replacement for V1. These seem plentiful and not expensive. Other than that, if you have an FM sig gen capable of producing a signal in the 87-100MHz range try injecting a signal from that and peaking for max signal strength. If not, did you have any success using another local receiver as suggested in Posts #4 and #6? Jerry
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Old 19th Jan 2022, 3:45 pm   #15
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Default Re: Murphy A242 VHF trouble

Hi Jerry,

Thanks for your continued assistance. So I have not had a chance yet to try another receiver; will do so at the weekend.

I have tried replacements for both V1 and V2. My signal generator tops out at 60Mhz, so I don't have something to inject an FM signal, but Radio 2 is normally strong where I am.

I know some of the parts are fairly critical and I may have to resort to rebuilding the FM front end with tight tolerance parts. This morning I did inspect and test the trimmers, and replaced C22. No change of course.

Experience tells me that VHF front ends tend to either just work, (subject to good valves) or cause a lot of headaches!
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Old 19th Jan 2022, 3:47 pm   #16
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Default Re: Murphy A242 VHF trouble

Also, I presume if I disconnect the AGC resistor R2, then the gain will be at maximum? Something I was planning on trying.
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Old 19th Jan 2022, 4:16 pm   #17
cathoderay57
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Default Re: Murphy A242 VHF trouble

Quote:
Originally Posted by agardiner View Post
Also, I presume if I disconnect the AGC resistor R2, then the gain will be at maximum? Something I was planning on trying.
I would think so, yes. I restored a Model A252 some years ago and only had to change out the Hunts caps. It had a different front end though, a 6F12 (EF91). Maybe worth giving murphymad a shout; if anybody knows the answer Mike probably will..... Cheers, Jerry
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Old 19th Jan 2022, 9:36 pm   #18
agardiner
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Default Re: Murphy A242 VHF trouble

Right, so I have obtained a donor set. I intend first to take some comparisons, and then transplant the VHF front end. That should sort the problem and then I can spend more time on the faulty front end as time permits. It will also mean I can safely mess with the coils without worrying about making things worse!

Thanks to everyone for their suggestions and help.

Cheers,

Adrian.
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