Murphy SAD94s Alignment problem - UK Vintage Radio Repair and Restoration Discussion Forum

adeftereos · 25th Apr 2017, 8:57 am

Morning all.

I was hoping to tap into the collective wisdom of the group on this one. It's an Ebay purchase that has obviously been laid up for some time. The cabinet needs a good polish but is otherwise ok. The dial cord had snapped and I feared the dial pointer might have been lost. Fortunately it was found loose inside the cabinet.

On the electrical side it required a complete rewire. That's something I generally try to avoid although there was no escape this time as the insulation on all the wires was crumbling. Naturally it's been recapped although all but one of the micas are good. Many of the carbon resistors were out of spec so all were replaced.

I'm having trouble with the alignment. I've gone through the procedure on the Trader sheet (530) numerous times and am getting a peculiar result on both MW and SW. The IF stages peak nicely but the Osc adjustment is very, very twitchy on both bands. Even when I think I have it right the set sounds ok when it's right on a strong station but there are loud "birdie" sounds immediately on either side of the station frequency.

I'm fairly new to the alignment world so I'd appreciate any thoughts as to where I'm going wrong!

Angelos

vinrads · 25th Apr 2017, 9:16 am

Hi Angelos no doubt you have checked all your previous work reg capacitor replacement , try de tuning the IF'S slightly or try a resistor across the IF coil say start off with a 1meg.

Nuvistor · 25th Apr 2017, 9:27 am

I would check for instability in the IF range, check both mixer, I.F amp and decoupling in the detector. If you have had to rebuild the set check the wiring in the RF/IF circuits follow the original path and you have not made and mistakes with a blanket replacement.
Frank

ms660 · 25th Apr 2017, 9:31 am

Also the valves metalized coating and connections.

Lawrence.

adeftereos · 25th Apr 2017, 9:34 am

Hi Vinrads. I have checked over the new caps indeed but don't mind going over it one more time. It's happened before that I've double checked something only to find a mistake when going over it a third or even fourth time. I'll try your IF detuning suggestion. With respect to the resistor across the IF coil, pardon the dumb question but should the resistor go across the primary pr secondary of one of the IF transformers? Thanks! Angelos

'LIVEWIRE?' · 25th Apr 2017, 10:20 am

If Vinrads means 'damping' the coil with a resistor when realigning, the resistor is temporarily wired across the coli not being adjusted-i.e., when peaking the primary, damp the secondary with a parallel resistor and vice-versa. This applies to Double Tuned IFTs.

adeftereos · 25th Apr 2017, 10:55 am

Thanks everyone for all the advice so far. Much appreciated! I'll give it another go this evening and report back.

adeftereos · 2nd May 2017, 9:28 am

I just wanted to give everyone a quick update on this. Unfortunately it's taken a few days before I had the chance to get back to this. I'm pleased to report that real progress has been made! After all the helpful feedback I methodically went over the RF, mixer and IF stages checking the connections and retesting all components, particularly the capacitors.

Unsurprisingly, it turns out to have been a wiring error on my part! While everything from the antenna to the oscillator checked out well it turns out that instead of earthing the variable capacitor to the circuit ground (point 5 on the diagram) on this ac/dc set I had instead connected it to chassis. This had created the instability and howling. On MW the set is now commendably quiet between stations and alignment was a breeze.

On the downside I seem to have somehow lost half the SW band.... Feeding in a 6.2 Megacyle signal at 49m on the dial, as required on the alignment instructions on the Trader Sheet, only yields a response at a much higher frequency. I'm assuming that's just a harmonic and the rest of the band is quiet due to some issue with the local oscillator. Will keep at it and hopefully also get the SW working!

25th Apr 2017, 8:57 am	#1
adeftereos Tetrode Join Date: Jan 2005 Location: London, UK. Posts: 63	Murphy SAD94s Alignment problem Morning all. I was hoping to tap into the collective wisdom of the group on this one. It's an Ebay purchase that has obviously been laid up for some time. The cabinet needs a good polish but is otherwise ok. The dial cord had snapped and I feared the dial pointer might have been lost. Fortunately it was found loose inside the cabinet. On the electrical side it required a complete rewire. That's something I generally try to avoid although there was no escape this time as the insulation on all the wires was crumbling. Naturally it's been recapped although all but one of the micas are good. Many of the carbon resistors were out of spec so all were replaced. I'm having trouble with the alignment. I've gone through the procedure on the Trader sheet (530) numerous times and am getting a peculiar result on both MW and SW. The IF stages peak nicely but the Osc adjustment is very, very twitchy on both bands. Even when I think I have it right the set sounds ok when it's right on a strong station but there are loud "birdie" sounds immediately on either side of the station frequency. I'm fairly new to the alignment world so I'd appreciate any thoughts as to where I'm going wrong! Angelos Last edited by AC/HL; 25th Apr 2017 at 1:48 pm. Reason: Forum rule B8

25th Apr 2017, 9:16 am	#2
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,754	Re: Murphy SAD94s Alignment problem Hi Angelos no doubt you have checked all your previous work reg capacitor replacement , try de tuning the IF'S slightly or try a resistor across the IF coil say start off with a 1meg.

25th Apr 2017, 9:27 am	#3
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,431	Re: Murphy SAD94s Alignment problem I would check for instability in the IF range, check both mixer, I.F amp and decoupling in the detector. If you have had to rebuild the set check the wiring in the RF/IF circuits follow the original path and you have not made and mistakes with a blanket replacement. Frank

25th Apr 2017, 9:31 am	#4
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Murphy SAD94s Alignment problem Also the valves metalized coating and connections. Lawrence.

25th Apr 2017, 9:34 am	#5
adeftereos Tetrode Join Date: Jan 2005 Location: London, UK. Posts: 63	Re: Murphy SAD94s Alignment problem Hi Vinrads. I have checked over the new caps indeed but don't mind going over it one more time. It's happened before that I've double checked something only to find a mistake when going over it a third or even fourth time. I'll try your IF detuning suggestion. With respect to the resistor across the IF coil, pardon the dumb question but should the resistor go across the primary pr secondary of one of the IF transformers? Thanks! Angelos

25th Apr 2017, 10:20 am	#6
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Murphy SAD94s Alignment problem If Vinrads means 'damping' the coil with a resistor when realigning, the resistor is temporarily wired across the coli not being adjusted-i.e., when peaking the primary, damp the secondary with a parallel resistor and vice-versa. This applies to Double Tuned IFTs.

25th Apr 2017, 10:55 am	#7
adeftereos Tetrode Join Date: Jan 2005 Location: London, UK. Posts: 63	Re: Murphy SAD94s Alignment problem Thanks everyone for all the advice so far. Much appreciated! I'll give it another go this evening and report back.

2nd May 2017, 9:28 am	#8
adeftereos Tetrode Join Date: Jan 2005 Location: London, UK. Posts: 63	Re: Murphy SAD94s Alignment problem I just wanted to give everyone a quick update on this. Unfortunately it's taken a few days before I had the chance to get back to this. I'm pleased to report that real progress has been made! After all the helpful feedback I methodically went over the RF, mixer and IF stages checking the connections and retesting all components, particularly the capacitors. Unsurprisingly, it turns out to have been a wiring error on my part! While everything from the antenna to the oscillator checked out well it turns out that instead of earthing the variable capacitor to the circuit ground (point 5 on the diagram) on this ac/dc set I had instead connected it to chassis. This had created the instability and howling. On MW the set is now commendably quiet between stations and alignment was a breeze. On the downside I seem to have somehow lost half the SW band.... Feeding in a 6.2 Megacyle signal at 49m on the dial, as required on the alignment instructions on the Trader Sheet, only yields a response at a much higher frequency. I'm assuming that's just a harmonic and the rest of the band is quiet due to some issue with the local oscillator. Will keep at it and hopefully also get the SW working!