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Old 30th Apr 2017, 8:27 am   #41
Phil G4SPZ
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Default Re: MURPHY A272C Restoration.

Quote:
Originally Posted by sobell1980 View Post
Would 1/4 watt resistors be suitable for R3 and R4... R3 and R4 are not very big in physical size.
Don't worry about calculating the wattage rating. Just pick a modern resistor of the same value and physical size. Modern components tend to be smaller than vintage ones, for a given power or voltage rating, so if the new resistors are the same physical size as the original ones, you should be safe.

I have got thousands of resistors in stock. If you let me know the resistance values and the physical size, I'll send you a few FOC.
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Old 30th Apr 2017, 9:49 am   #42
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Default Re: MURPHY A272C Restoration.

Very kind Phil. I will do that later today. I have my morning chores to do first which will hopefully allow some time this afternoon to sit down with the set and go through this. Very much appreciated.
Lawrence and Julie. Thanks for the replies too. I am familiar with ohms law as I work in the motor industry. I've only ever worked out Watts using the V×I= power. I was not aware of working it out without measuring the amps . So thanks for sharing that Lawrence and Julie for making a comparison of that back to ohms law. Useful for everyone to be refreshed of that. That's why I was trying to measure the current as I was trying to use ohms law to calculate power / watts. But appreciate both your replies on how to achieve this with just using voltage and resistance. This will be noted down in my book I have for reference so you won't need to be asked again. I will hopefully get a couple of hours on the set later. I've got my head around the maths and understand the switching of FM, R3 and R4 in my head so I can go in armed knowing what I'm looking for and expected results.
Many thanks.
Dave.
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Old 30th Apr 2017, 10:34 am   #43
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Default Re: MURPHY A272C Restoration.

No problem Dave, but be aware that W = I*V is only good if I is only flowing through a resistive element, if there's reactance present then W = I*V would be the apparent power not the real power.

Lawrence.
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Old 30th Apr 2017, 10:40 am   #44
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Default Re: MURPHY A272C Restoration.

This is where my automotive electrical skills causes confusion and I end up becoming a little confused with the world of wireless. All learning though.
Cheers Lawrence.
Dave.
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Old 30th Apr 2017, 10:44 am   #45
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Default Re: MURPHY A272C Restoration.

Also appreciate what Phil is saying too in his post #41.
Modern equivalents are indeed smaller. It's just good to know the basic theory and a bit of maths behind it all so school boy errors are not made on my part. Appreciate everyone's input.
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Old 30th Apr 2017, 8:46 pm   #46
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Default Re: MURPHY A272C Restoration.

I've only had 30 mins or so this eve. I have changed R3 and R4 then checked voltages.
Here is what I found after changing the resistors.
V1 a anode
Switched to FM 142 volts.
Switched to MW 137 volts.

V1b anode
Switched to MW 29.62 volts
Switched to FM 201.00 volts.

There is now no MW heard on FM band and a definite FM buzz can be heard and a difference on that buzz can be heard when tuning but no stations. I've tried plugging in the FM frame aerial but still nothing. I don't think my scope will measure the high FM frequency to see what it is oscillating at . It's nice to see now though that the anode voltages are changing when switched from MW to FM. The heaters in v1 can both be seen to be working.
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Old 1st May 2017, 9:45 am   #47
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Default Re: MURPHY A272C Restoration.

I have found another crumbling Hunts cap hiding from me which is C5. Shall get that changed too today.
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Old 1st May 2017, 11:16 am   #48
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Default Re: MURPHY A272C Restoration.

C5 forms part of the tuned circuit of L5 and V1a's output to V1b. Hope I'm making some sense here.
Many thanks.
Dave.
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Old 1st May 2017, 8:43 pm   #49
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Default Re: MURPHY A272C Restoration.

Replacement of R3 and R4 restored the working voltages to V1. Replacement of C5 got FM back up and running loud and clear. So all is good. Will have it on test on the bench till the weekend. Receiving loads of stations on FM loud and clear.
Many thanks.
Dave.
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Old 1st May 2017, 9:12 pm   #50
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Default Re: MURPHY A272C Restoration.

Congratulations, Dave! It's nice when a plan comes together. I suppose much of vintage radio fault diagnosis boils down to (a) check the voltages and (b) replace the duff waxies. Then there's (c) replace any Hunts on sight...!
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Old 1st May 2017, 9:29 pm   #51
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Default Re: MURPHY A272C Restoration.

Yes, well done!

I've been following this thread, but your skills exceed mine now, so I had nothing to add
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Old 8th May 2017, 8:40 pm   #52
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Default Re: MURPHY A272C Restoration.

Good evening. Thought I would run through the valve voltages according to the trader sheet before I finish this set. I have come a cropper with v2 screen voltage. Before I make a school boy error, can i confirm with you on here that R5 is the feed to the screen grid? Or have I got completely confused with the pin diagram of the valve base? I'm getting 1.5 volts on the valve side of R5!
Many thanks
Dave.
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Old 8th May 2017, 9:26 pm   #53
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Default Re: MURPHY A272C Restoration.

You're correct Dave, R5 is the screen feed resistor for V2b, should be an easy fix if you've measured the correct valve pin.

Lawrence.

Last edited by ms660; 8th May 2017 at 9:30 pm. Reason: addition
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Old 8th May 2017, 9:26 pm   #54
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Default Re: MURPHY A272C Restoration.

Dave, should be pin 5 on V2 that your R5 is connected to.

Andrew
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Old 9th May 2017, 7:16 am   #55
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Default Re: MURPHY A272C Restoration.

Thought I'd sussed it last night. Obviously first check R5. Measured its resistance as 39k Ohms out of circuit, it should be 27k Ohms. I replaced it but still the same. Ive also snipped out c22. It has been replaced but this one was from a pack of caps of which some were faulty but tested ok. So I get the same voltage to screen grid with c22 disconnected. It can't be R29 as this would also pull down all the other voltages across the board. Although it did measure high at 1.57k ohms instead of 1.35k ohms. So that's about 17% out of tolerance so could do with replacement regardless. Remove the valve 2 and either side of R5 measures approximately the same.
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Old 9th May 2017, 10:38 am   #56
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Default Re: MURPHY A272C Restoration.

That sounds like excessive screen current being drawn by the valve, low anode voltage (pin 2) could cause that, worth checking?

Lawrence.
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Old 9th May 2017, 8:46 pm   #57
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Default Re: MURPHY A272C Restoration.

V2b anode I should have mentioned before but measures at approx 200 volts. Sorry, I should have said but I would have said if it measured low compared to the trader sheet. All valve anodes read high but I put this down to the DMM I'm using.
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Old 9th May 2017, 8:53 pm   #58
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Default Re: MURPHY A272C Restoration.

What's the voltage on the screen grid tag (tag 5) on the valve holder?

Lawrence.
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Old 9th May 2017, 8:53 pm   #59
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Default Re: MURPHY A272C Restoration.

I can only see R5 causing this issue but has since been replaced . C22 is still disconnected but i can't see that making a huge amount of difference while I made some quick voltage measurements.
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Old 9th May 2017, 8:55 pm   #60
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Default Re: MURPHY A272C Restoration.

Approx 1.5 volts Lawrence.
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