Murphy A262R VHF alignment - UK Vintage Radio Repair and Restoration Discussion Forum

hargeruk · 26th Sep 2019, 2:51 pm

I came across a Murphy A262R from two daughters doing a house clearance of the father’s house. The unit is in excellent condition and came with the original receipt and warranty documents and Operating Instructions. I am mostly interested in restoring the gramophone functionality but would like to get the entire unit in tip-top shape.

The AM functions fine. It took a slight alignment but receives well and sounds good. The turntable is not turning around but I will tend to that last. The a.f. amplifier seems to function fine.

The VHF is a different matter. With volume turned up to max, it makes sounds as the tuner is moved up and down. There are definite differences in the sounds as the dial turns, mostly whistles. I can just make out some voices and music in the middle of the dial. I tuned another radio to BBC Wiltshire at 95.5 and the faint sounds match, so something is getting through.

I have aligned and restored several medium wave units, but not yet tried to align or restore any VHF units. This is my first VHF.

The first thing I did in the VHF stage was replace C67, C68, C80. C90, along with all the other electrolytes. The VHF function is the same as before replacement.

When I attach a meter around the discriminator electrolyte (C67) the reading is 0 volt. There is still noise coming out of the speaker (full volume). I can still hear BBC Wiltshire faintly at 95.5. The V4 pin 2 anode is 0 volt. The V4 pin 1 anode is about 1 volt with no signal applied to the antenna.

I have checked all of the voltages indicated in the schematic. They are all more or less in-spec, except V1 anodes. Both pin1 & 7 are 8 volt on medium wave and 184 volt on VHF. They should be 7V and 64V. Around V1, R1 is a bit low at 1.8 k-ohms, instead of spec of 2.2 k-ohms. L6 & L7 are both near spec at 1.7 ohms.

When I apply 10.7 Mhz signal to V1 pin 8, there is 0 volt across C67.
I have a new V1 (ECC85) and a new V4 (EABC80) on order. Anybody have any ideas of things I can investigate now?

paulsherwin · 26th Sep 2019, 3:05 pm

An FM realignment should only be done as a last resort, or if there is clear evidence of twiddling. It's difficult to do a good job, and it's extremely unlikely that the factory alignment will have drifted enough to produce the symptoms you describe.

A careful clean of the switch contacts is always the first thing to try. Radios often spent decades without the wavechange switch being operated, and the contacts oxidise badly. This is made worse if there are smokers around.

A bad ECC85 is a distinct possibility. Radios were frequently left switched to the AM bands, leaving the ECC85 heated but without HT. This gradually poisons the cathodes.

ms660 · 26th Sep 2019, 3:13 pm

I think V1's anode voltages for VHF/FM given in the schematic are wrong.

Measure the voltage across R1 when switched to VHF/FM.

Lawrence.

hargeruk · 26th Sep 2019, 3:57 pm

ms660 - probably right. I wonder if the schematic is supposed to say 164 instead of 64. Voltage across R1 is about 20V.

Leon Crampin · 26th Sep 2019, 6:00 pm

To add to Paul's comments, I have worked on several of these chassis and their derivatives. C13 is well worth a check and will give surprisingly low gain if faulty. Check all Hunts decouplers in the IF stages, and their associated feed resistors for value.

The ECC85 is a prime suspect, but poor FM alignment is often caused by a mismatch of the two high-perveance diodes in the EABC80, used for FM detection.

Fit a new EABC80 and burn it in for an hour or so. Then adjust the secondary of the discriminator transformer using Murphy's instructions for a null between equal value load resistors and the tertiary winding. This can be done on signal if you're careful, but first tune the set for maximum voltage on the stabilising electrolytic, then adjust the secondary for a null. Some iteration is usually required.

These sets work well on FM when properly aligned, but if a full FM alignment is needed, use a signal generator. In my experience, it's usually only the ratio detector balance which needs setting. The give-away is that the optimum tuning point for signal output does not match the point of lowest distortion when the discriminator is misaligned. Tuning drift is not a problem with Murphy sets of this vintage.

Leon.

hargeruk · 19th Feb 2020, 5:50 pm

Thanks guys. I could really use your help here.
My VHF is almost non-existent. When I connect my 20-metre MW antenna I hear some of the stronger VHF stations if volume is at max. Nothing if I use a normal VHF antenna dipole antenna. I have replaced all components connected to the 6C9 and EABC80 valves. When I align the 3rd, 2nd, and 1st IFT circuits I can get L21, L19, L18, L6, and L7 to respond as expected. There is a peak in response on the DC meter across C67. However, when I adjust L22 the DC meter between C68 and the 100Kohm taps does not respond at all. I can adjust the coil core from well in to almost falling out and there is no response on the DC Meter. The value read about 1 volt. All components in the discriminator have been replaced, V4, R18, C68, C67, R19, R20, C66.
Any comments on how I can get the discriminator properly adjusted?
I don’t think getting the discriminator adjusted will solve my VHF sensitivity issue, but I am focused on this at the moment.

26th Sep 2019, 2:51 pm	#1
hargeruk Diode Join Date: Oct 2016 Location: Chippenham, Wiltshire, UK. Posts: 5	Murphy A262R VHF alignment I came across a Murphy A262R from two daughters doing a house clearance of the father’s house. The unit is in excellent condition and came with the original receipt and warranty documents and Operating Instructions. I am mostly interested in restoring the gramophone functionality but would like to get the entire unit in tip-top shape. The AM functions fine. It took a slight alignment but receives well and sounds good. The turntable is not turning around but I will tend to that last. The a.f. amplifier seems to function fine. The VHF is a different matter. With volume turned up to max, it makes sounds as the tuner is moved up and down. There are definite differences in the sounds as the dial turns, mostly whistles. I can just make out some voices and music in the middle of the dial. I tuned another radio to BBC Wiltshire at 95.5 and the faint sounds match, so something is getting through. I have aligned and restored several medium wave units, but not yet tried to align or restore any VHF units. This is my first VHF. The first thing I did in the VHF stage was replace C67, C68, C80. C90, along with all the other electrolytes. The VHF function is the same as before replacement. When I attach a meter around the discriminator electrolyte (C67) the reading is 0 volt. There is still noise coming out of the speaker (full volume). I can still hear BBC Wiltshire faintly at 95.5. The V4 pin 2 anode is 0 volt. The V4 pin 1 anode is about 1 volt with no signal applied to the antenna. I have checked all of the voltages indicated in the schematic. They are all more or less in-spec, except V1 anodes. Both pin1 & 7 are 8 volt on medium wave and 184 volt on VHF. They should be 7V and 64V. Around V1, R1 is a bit low at 1.8 k-ohms, instead of spec of 2.2 k-ohms. L6 & L7 are both near spec at 1.7 ohms. When I apply 10.7 Mhz signal to V1 pin 8, there is 0 volt across C67. I have a new V1 (ECC85) and a new V4 (EABC80) on order. Anybody have any ideas of things I can investigate now? Attached Thumbnails

26th Sep 2019, 3:05 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,927	Re: Murphy A262R VHF alignment An FM realignment should only be done as a last resort, or if there is clear evidence of twiddling. It's difficult to do a good job, and it's extremely unlikely that the factory alignment will have drifted enough to produce the symptoms you describe. A careful clean of the switch contacts is always the first thing to try. Radios often spent decades without the wavechange switch being operated, and the contacts oxidise badly. This is made worse if there are smokers around. A bad ECC85 is a distinct possibility. Radios were frequently left switched to the AM bands, leaving the ECC85 heated but without HT. This gradually poisons the cathodes.

26th Sep 2019, 3:13 pm	#3
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Murphy A262R VHF alignment I think V1's anode voltages for VHF/FM given in the schematic are wrong. Measure the voltage across R1 when switched to VHF/FM. Lawrence. Last edited by ms660; 26th Sep 2019 at 3:28 pm. Reason: addition

26th Sep 2019, 3:57 pm	#4
hargeruk Diode Join Date: Oct 2016 Location: Chippenham, Wiltshire, UK. Posts: 5	Re: Murphy A262R VHF alignment ms660 - probably right. I wonder if the schematic is supposed to say 164 instead of 64. Voltage across R1 is about 20V.

26th Sep 2019, 6:00 pm	#5
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: Murphy A262R VHF alignment To add to Paul's comments, I have worked on several of these chassis and their derivatives. C13 is well worth a check and will give surprisingly low gain if faulty. Check all Hunts decouplers in the IF stages, and their associated feed resistors for value. The ECC85 is a prime suspect, but poor FM alignment is often caused by a mismatch of the two high-perveance diodes in the EABC80, used for FM detection. Fit a new EABC80 and burn it in for an hour or so. Then adjust the secondary of the discriminator transformer using Murphy's instructions for a null between equal value load resistors and the tertiary winding. This can be done on signal if you're careful, but first tune the set for maximum voltage on the stabilising electrolytic, then adjust the secondary for a null. Some iteration is usually required. These sets work well on FM when properly aligned, but if a full FM alignment is needed, use a signal generator. In my experience, it's usually only the ratio detector balance which needs setting. The give-away is that the optimum tuning point for signal output does not match the point of lowest distortion when the discriminator is misaligned. Tuning drift is not a problem with Murphy sets of this vintage. Leon.

19th Feb 2020, 5:50 pm	#6
hargeruk Diode Join Date: Oct 2016 Location: Chippenham, Wiltshire, UK. Posts: 5	Re: Murphy A262R VHF alignment Thanks guys. I could really use your help here. My VHF is almost non-existent. When I connect my 20-metre MW antenna I hear some of the stronger VHF stations if volume is at max. Nothing if I use a normal VHF antenna dipole antenna. I have replaced all components connected to the 6C9 and EABC80 valves. When I align the 3rd, 2nd, and 1st IFT circuits I can get L21, L19, L18, L6, and L7 to respond as expected. There is a peak in response on the DC meter across C67. However, when I adjust L22 the DC meter between C68 and the 100Kohm taps does not respond at all. I can adjust the coil core from well in to almost falling out and there is no response on the DC Meter. The value read about 1 volt. All components in the discriminator have been replaced, V4, R18, C68, C67, R19, R20, C66. Any comments on how I can get the discriminator properly adjusted? I don’t think getting the discriminator adjusted will solve my VHF sensitivity issue, but I am focused on this at the moment.