Wayne-Kerr MS 144 - UK Vintage Radio Repair and Restoration Discussion Forum

Hermit6345 · 8th Apr 2012, 12:59 am

This came my way recently. I thought it might be useful in the workshop for a source of audio and it is portable as well. It's obviously meant for line circuit measurements by BT or the Post Office.

It was given a good clean to remove years of gunge over the case. Someone had already fitted new PP9 batteries. Switching to battery test gave a very low reading and the batteries got decidedly warm.

A look at the schematic showed an oscillatory 30 volt DC-DC power supply using a pair of BC107s as the active components.This PSU was giving no output. One of the BC107s was U/S. To avoid wrecking the batteries any further, the Test Set was supplied with 18 volts from the workshop PSU. The DC to DC PSU board was removed and the duff BC107 was replaced. On switching on once more, the PSU immediately went to current limit again. After checking a few components, it became clear that C42, a 10uF electrolytic across the +/- 18 volt rail was completely S/C. Being a tubular Tantalum I suppose it was to be expected. Replaced with a nice aluminium 10uF 25 volt unit. Now we were getting somewhere. The PSU no longer went into current limit and the off load output of the DC to DC converter was about 40 volts.

The DC to DC Converter Board was bolted back in position and the output pins connected back to the Oscillator Board. The voltage on switch on fell to 29 volts. Result.

A pair of headphones across the AF output terminals revealed a nice tone which would suddenly drop away to almost nothing and then re-appear when the output attenuator knob was turned. Switches were all cleaned but the fault persisted. Eventually the fault was isolated to the area around C37, a 1000uF cap in the Oscillator Board. Pressing the component caused the fault to come and go. Careful examination showed that the leads of the cap were moving back and forth through the soldered "joint" on the foil side. To get at the foil side meant removing the whole PCB with about 17 wires from tags, including one to the foil side of the board. The solder joints on the cap were re-made as were a lot more that looked a bit dodgy to me.

The board was bolted back in position, wires re-soldered and the unit checked. This time the output was solid at any setting of the output attenuators.

The measuring section was set up after a kind member on this forum provided me with the correct jack plugs. (Thanks llama).

After setting up the receive and transmit levels, the frequency accuracy were checked on all four bands from 20Hz to 120kHz. All were in spec. Output substantially flat over the entire range.

It makes a nice source of low distortion audio, and can measure up to +26dBm.

Ian

vinrads · 8th Apr 2012, 6:33 pm

That's a nice bit of kit there Ian well done finding the dry joint .Mick.

julie_m · 9th Apr 2012, 4:47 am

Yes, well done for persevering with it -- intermittent connections like that can be real b

s to find. Looks like a useful piece of test equipment, too -- the sort of thing you end up wondering how you ever managed without.

8th Apr 2012, 12:59 am	#1
Hermit6345 Rest in Peace Join Date: Oct 2006 Location: North Wiltshire, UK. Posts: 638	Wayne-Kerr MS 144 This came my way recently. I thought it might be useful in the workshop for a source of audio and it is portable as well. It's obviously meant for line circuit measurements by BT or the Post Office. It was given a good clean to remove years of gunge over the case. Someone had already fitted new PP9 batteries. Switching to battery test gave a very low reading and the batteries got decidedly warm. A look at the schematic showed an oscillatory 30 volt DC-DC power supply using a pair of BC107s as the active components.This PSU was giving no output. One of the BC107s was U/S. To avoid wrecking the batteries any further, the Test Set was supplied with 18 volts from the workshop PSU. The DC to DC PSU board was removed and the duff BC107 was replaced. On switching on once more, the PSU immediately went to current limit again. After checking a few components, it became clear that C42, a 10uF electrolytic across the +/- 18 volt rail was completely S/C. Being a tubular Tantalum I suppose it was to be expected. Replaced with a nice aluminium 10uF 25 volt unit. Now we were getting somewhere. The PSU no longer went into current limit and the off load output of the DC to DC converter was about 40 volts. The DC to DC Converter Board was bolted back in position and the output pins connected back to the Oscillator Board. The voltage on switch on fell to 29 volts. Result. A pair of headphones across the AF output terminals revealed a nice tone which would suddenly drop away to almost nothing and then re-appear when the output attenuator knob was turned. Switches were all cleaned but the fault persisted. Eventually the fault was isolated to the area around C37, a 1000uF cap in the Oscillator Board. Pressing the component caused the fault to come and go. Careful examination showed that the leads of the cap were moving back and forth through the soldered "joint" on the foil side. To get at the foil side meant removing the whole PCB with about 17 wires from tags, including one to the foil side of the board. The solder joints on the cap were re-made as were a lot more that looked a bit dodgy to me. The board was bolted back in position, wires re-soldered and the unit checked. This time the output was solid at any setting of the output attenuators. The measuring section was set up after a kind member on this forum provided me with the correct jack plugs. (Thanks llama). After setting up the receive and transmit levels, the frequency accuracy were checked on all four bands from 20Hz to 120kHz. All were in spec. Output substantially flat over the entire range. It makes a nice source of low distortion audio, and can measure up to +26dBm. Ian Attached Thumbnails

8th Apr 2012, 6:33 pm	#2
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,762	Re: Wayne-Kerr MS 144 That's a nice bit of kit there Ian well done finding the dry joint .Mick.

9th Apr 2012, 4:47 am	#3
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Wayne-Kerr MS 144 Yes, well done for persevering with it -- intermittent connections like that can be real bs to find. Looks like a useful piece of test equipment, too -- the sort of thing you end up wondering how you ever managed without. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.