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Old 3rd Jul 2009, 2:51 pm   #1
benjamin77
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Default Murphy U472 NO FM !

I have got this set working well on Mw/Lw but it fails on FM . I have connected the arial and checked its wiring as well as swapped in a known good UCC85.

All I get is a high pitched ear piercing wissle Is it likely to be capacitors in the FM head ?
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Old 3rd Jul 2009, 3:09 pm   #2
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Default Re: Murphy U742 NO FM !

do you mean the U472? - I can't find a Murphy U742
Cheers
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Old 3rd Jul 2009, 3:13 pm   #3
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Default Re: Murphy U742 NO FM !

Ooops yes I do .....I should concentrate more when typing...
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Old 3rd Jul 2009, 3:17 pm   #4
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Default Re: Murphy U742 NO FM !

pretty sure when I last did one of these, I had the same problem - turned out to be the caps around the UCC85 - sorry - don't have the service sheet in front of me - I'm sure I ended up swapping out all the caps in the end! (Hunts?)
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Old 3rd Jul 2009, 3:56 pm   #5
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Default Re: Murphy U742 NO FM !

Hi Andy
I am guessing then that they will be in the tuner head itself
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Old 3rd Jul 2009, 4:58 pm   #6
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Default Re: Murphy U742 NO FM !

pretty certain I never opened the vhf can! I only replaced the caps under the chassis
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Old 3rd Jul 2009, 7:54 pm   #7
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Default Re: Murphy U472 NO FM !

It's unlikely to be caps in the tuner head, as these will probably all be low value mica types.

Have a look at the wavechange switching arrangements. I'm pretty sure the HT to the UCC85 is disconnected when switched to AM, so possibly it isn't being reconnected as it should. Check the circuit voltages.

Paul
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Old 4th Jul 2009, 3:23 pm   #8
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Default Re: Murphy U472 NO FM !

Well I have taken some readings. I am not to good at it so at the expense of making myself look like a fool here goes

V6
1 59
2 216
3 4.4
4 132
5 171
6 4.6
7 216
8 4.8
9 216

V5
PIN V
1 0
2 0
3 0
4 92
5 132
6 0
7 3.7
8 3.9
9 0

V4
PIN V
1 0
2 0
3 1.2
4 30.5
5 1
6 0
7 1
8 0
9 0

V3
Pins 1,2,3,6,8 and 9 all 0V Pin 4 77v Pin 5 92v and Pin 7 1.3v

V2
PIN V
1 0
2 0
3 0
4 57
5 77
6 0
7 1
8 0
9 0

What are your thoughts ......
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Old 4th Jul 2009, 4:22 pm   #9
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Default Re: Murphy U472 NO FM !

Well, was it switched to MW or FM. And what was the meter switched to?

Cheers,

Steve P.
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Old 4th Jul 2009, 4:24 pm   #10
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Default Re: Murphy U472 NO FM !

It was switched to MW but I guess it should have been on FM ? and the DMM is AC.
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Old 4th Jul 2009, 4:25 pm   #11
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Default Re: Murphy U472 NO FM !

Switch to FM, Measure DC and also do V1.

Cheers,

Steve P.
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Old 4th Jul 2009, 4:47 pm   #12
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Default Re: Murphy U472 NO FM !

Steve...Second go

V1
Pin V
1 0
2 0
3 0
4 0
5 0
6 133
7 0.4
8 0
9 0

V2
Pin V
1 74
2 0.1
3 0
4 1.7
5 4
6 159
7 0
8 1.1
9 0

V3
Pin V
1 1.4
2 0
3 1.4
4 4.3
5 4.8
6 0
7 162
8 75
9 0

V4
Pin V
1 1.1
2 6.7
3 4
5 0
6 0
7 0
8 1
9 58

V5
Pin V
1 0
2 0
3 15.3
4 4.8
5 0
6 181
7 187
8 185
9 180

V6
Pin V
1 0.3
2 19.8
3 0
4 0
5 0
6 0.2
7 19.2
8 0.1
9 19.6

DMM set to V DC and wave band on FM . I also get a loud whistling...

Cheers.....Ben
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Old 4th Jul 2009, 4:49 pm   #13
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Default Re: Murphy U472 NO FM !

Nothing at all on V1 Pin 1? Sure about that?

Cheers,

Steve P.
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Old 4th Jul 2009, 4:54 pm   #14
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Default Re: Murphy U472 NO FM !

Sorry its actually 97v DC ......what a fool..!
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Old 4th Jul 2009, 4:59 pm   #15
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Default Re: Murphy U472 NO FM !

Pity. Anyway, clean and reclean all sections of the wavechange switch. Also disconnect the Earthed Cable from the head and see if it stops howling.

Cheers,

Steve P.
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Old 4th Jul 2009, 5:43 pm   #16
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Default Re: Murphy U472 NO FM !

I have re cleaned the wave change switch but where on the FM head do I find the earth cable ? is it braided copper ?
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Old 4th Jul 2009, 8:58 pm   #17
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Default Re: Murphy U472 NO FM !

There should be a lead coming out of the VHF head going to the wavechange control. The outer goes to earth, the inner goes to the wavechange pot.

Cheers,

Steve P.
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Old 4th Jul 2009, 9:06 pm   #18
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Default Re: Murphy U472 NO FM !

C14 (4700 pF) in the tuner head is likely a Hunts. I'd be tempted to change that if it was. C4 (470 pF) might be Hunts too (it was in a Murphy A242 I recently did), but it is critical for proper operation, so don't change it unless you're down to your last straw. Too high a value wll kill the gain, while too low will cause it to oscillate (but not a frequencies you're likely to hear, unlikely it would be the source of your squeal). Pin 1 voltage is a little high, should be 70 vdc. You might want to try another V4. Valve bases have also given me considerable problems.
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Old 5th Jul 2009, 7:39 am   #19
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Default Re: Murphy U472 NO FM !

I had this fault on my Murphy U502, does it only screech when you are on a station?
I traced my fault to duff caps on the screens of the 10F18's. I notice your set employs one of these. Go for that cap on the screen of the 10C14 first and see what happens. It's cap C23 0.003uf. Also the other suspect if that didn't work would be cap C39 0.005uf on the screen of the 10F18.

Good luck, cheers.
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Old 5th Jul 2009, 1:35 pm   #20
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Default Re: Murphy U472 NO FM !

I have tried disconecting the earth lead to the chassis from the FM head and it made no difference. It screeches when you change the wave band switch to FM and does not pick up any stations.

I Will try replacing those caps you sugest over the next few days....cheers
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