Murphy U472 NO FM ! - UK Vintage Radio Repair and Restoration Discussion Forum

benjamin77 · 3rd Jul 2009, 2:51 pm

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 ?

ajs37 · 3rd Jul 2009, 3:09 pm

do you mean the U472? - I can't find a Murphy U742
Cheers
Andy

benjamin77 · 3rd Jul 2009, 3:13 pm

Ooops yes I do .....I should concentrate more when typing...

ajs37 · 3rd Jul 2009, 3:17 pm

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?)
Cheers
Andy

benjamin77 · 3rd Jul 2009, 3:56 pm

Hi Andy
I am guessing then that they will be in the tuner head itself

ajs37 · 3rd Jul 2009, 4:58 pm

pretty certain I never opened the vhf can! I only replaced the caps under the chassis
Cheers
Andy

paulsherwin · 3rd Jul 2009, 7:54 pm

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

benjamin77 · 4th Jul 2009, 3:23 pm

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 ......

Steve_P · 4th Jul 2009, 4:22 pm

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

Cheers,

Steve P.

benjamin77 · 4th Jul 2009, 4:24 pm

It was switched to MW but I guess it should have been on FM ? and the DMM is AC.

Steve_P · 4th Jul 2009, 4:25 pm

Switch to FM, Measure DC and also do V1.

Cheers,

Steve P.

benjamin77 · 4th Jul 2009, 4:47 pm

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

Steve_P · 4th Jul 2009, 4:49 pm

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

Cheers,

Steve P.

benjamin77 · 4th Jul 2009, 4:54 pm

Sorry its actually 97v DC ......what a fool..!

Steve_P · 4th Jul 2009, 4:59 pm

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.

benjamin77 · 4th Jul 2009, 5:43 pm

I have re cleaned the wave change switch but where on the FM head do I find the earth cable ? is it braided copper ?

Steve_P · 4th Jul 2009, 8:58 pm

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.

JimMac53 · 4th Jul 2009, 9:06 pm

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.

Hunts smoothing bomb · 5th Jul 2009, 7:39 am

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.

benjamin77 · 5th Jul 2009, 1:35 pm

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

3rd Jul 2009, 2:51 pm	#1
benjamin77 Retired Dormant Member Join Date: May 2009 Location: High Wycombe, Buckinghamshire, UK. Posts: 326	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 ?

3rd Jul 2009, 3:09 pm	#2
ajs37 Heptode Join Date: Jun 2003 Location: Bristol, UK. Posts: 900	Re: Murphy U742 NO FM ! do you mean the U472? - I can't find a Murphy U742 Cheers Andy

3rd Jul 2009, 3:13 pm	#3
benjamin77 Retired Dormant Member Join Date: May 2009 Location: High Wycombe, Buckinghamshire, UK. Posts: 326	Re: Murphy U742 NO FM ! Ooops yes I do .....I should concentrate more when typing...

3rd Jul 2009, 3:17 pm	#4
ajs37 Heptode Join Date: Jun 2003 Location: Bristol, UK. Posts: 900	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?) Cheers Andy

3rd Jul 2009, 3:56 pm	#5
benjamin77 Retired Dormant Member Join Date: May 2009 Location: High Wycombe, Buckinghamshire, UK. Posts: 326	Re: Murphy U742 NO FM ! Hi Andy I am guessing then that they will be in the tuner head itself

3rd Jul 2009, 4:58 pm	#6
ajs37 Heptode Join Date: Jun 2003 Location: Bristol, UK. Posts: 900	Re: Murphy U742 NO FM ! pretty certain I never opened the vhf can! I only replaced the caps under the chassis Cheers Andy

3rd Jul 2009, 7:54 pm	#7
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,944	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

4th Jul 2009, 3:23 pm	#8
benjamin77 Retired Dormant Member Join Date: May 2009 Location: High Wycombe, Buckinghamshire, UK. Posts: 326	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 ......

4th Jul 2009, 4:22 pm	#9
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Murphy U472 NO FM ! Well, was it switched to MW or FM. And what was the meter switched to? Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

4th Jul 2009, 4:24 pm	#10
benjamin77 Retired Dormant Member Join Date: May 2009 Location: High Wycombe, Buckinghamshire, UK. Posts: 326	Re: Murphy U472 NO FM ! It was switched to MW but I guess it should have been on FM ? and the DMM is AC.

4th Jul 2009, 4:25 pm	#11
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Murphy U472 NO FM ! Switch to FM, Measure DC and also do V1. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

4th Jul 2009, 4:47 pm	#12
benjamin77 Retired Dormant Member Join Date: May 2009 Location: High Wycombe, Buckinghamshire, UK. Posts: 326	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

4th Jul 2009, 4:49 pm	#13
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Murphy U472 NO FM ! Nothing at all on V1 Pin 1? Sure about that? Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

4th Jul 2009, 4:54 pm	#14
benjamin77 Retired Dormant Member Join Date: May 2009 Location: High Wycombe, Buckinghamshire, UK. Posts: 326	Re: Murphy U472 NO FM ! Sorry its actually 97v DC ......what a fool..!

4th Jul 2009, 4:59 pm	#15
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	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. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

4th Jul 2009, 5:43 pm	#16
benjamin77 Retired Dormant Member Join Date: May 2009 Location: High Wycombe, Buckinghamshire, UK. Posts: 326	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 ?

4th Jul 2009, 8:58 pm	#17
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	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. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

4th Jul 2009, 9:06 pm	#18
JimMac53 Hexode Join Date: Apr 2009 Location: Naples, Florida, USA Posts: 255	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. __________________ Jim Mac

5th Jul 2009, 7:39 am	#19
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	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. __________________ Lee

5th Jul 2009, 1:35 pm	#20
benjamin77 Retired Dormant Member Join Date: May 2009 Location: High Wycombe, Buckinghamshire, UK. Posts: 326	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