UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Rather grim Thorn 1400

1100 man · 23rd Nov 2022, 7:03 pm

I've been doing a bit more to this set over the last week or so. The board is very sensitive to movement or taps with the blunt end of a screwdriver. Some of this is due to filthy valve pins & bases and some due to the systems switch.

Very noticeable though was the amount of hum distorting the raster. A large band would pass up or down the screen pulling the verticals sideways. The main smoothing block seemed a good place to start. This has two 150mfd and three 100mfd caps in the same can. I removed it from the board and two of its legs fell off!

I would normally, before applying power for the first time, remove the main smoothing cap and reform each section using the variac (the only use I've ever found for it!), rectifier, series resistor and AVO to monitor the leakage current. I do one section per evening, gradually increasing the voltage over several hours. A leakage current of 200 - 400 microamps seems fairly typical for a 100mfd cap running at 300V.

So rather belatedly, as I'd already been running the set for a short while, I went through the reforming process anyway. The end result was quite good with all sections reading close to their stated capacitance on my cheap tester and leakage currents below 400 microamps.

After repairing the failed legs and reinstalling it in the board, the hum problem was no better. I assume it is caused by either mains hum or 50Hz from the frame stage, or a combination of both, getting into the line oscillator/ sync separator causing a change in phase of the line pulses? I will have to investigate further. Does anyone have any good ideas as to what might be causing it?

Another issue is slightly low width, with the control at maximum. I'm guessing, like the 1500, the 1400 had its component values changed during production? The circuit in the 'red books' shows the width control to be 1Meg with 330K in series. My copy of the 'schools' version (which would have been a large screen), shows the control as a 2.2Meg with 680K in series. Does anyone have the correct information?

I'll have another play with it tonight and see what I can find.

Cheers
Nick

23rd Nov 2022, 7:03 pm	#9
1100 man Octode Join Date: Apr 2016 Location: Ventnor, Isle of Wight, & Great Dunmow, Essex, UK. Posts: 1,377	Re: Rather grim Thorn 1400 - Marconiphone 4649 I've been doing a bit more to this set over the last week or so. The board is very sensitive to movement or taps with the blunt end of a screwdriver. Some of this is due to filthy valve pins & bases and some due to the systems switch. Very noticeable though was the amount of hum distorting the raster. A large band would pass up or down the screen pulling the verticals sideways. The main smoothing block seemed a good place to start. This has two 150mfd and three 100mfd caps in the same can. I removed it from the board and two of its legs fell off! I would normally, before applying power for the first time, remove the main smoothing cap and reform each section using the variac (the only use I've ever found for it!), rectifier, series resistor and AVO to monitor the leakage current. I do one section per evening, gradually increasing the voltage over several hours. A leakage current of 200 - 400 microamps seems fairly typical for a 100mfd cap running at 300V. So rather belatedly, as I'd already been running the set for a short while, I went through the reforming process anyway. The end result was quite good with all sections reading close to their stated capacitance on my cheap tester and leakage currents below 400 microamps. After repairing the failed legs and reinstalling it in the board, the hum problem was no better. I assume it is caused by either mains hum or 50Hz from the frame stage, or a combination of both, getting into the line oscillator/ sync separator causing a change in phase of the line pulses? I will have to investigate further. Does anyone have any good ideas as to what might be causing it? Another issue is slightly low width, with the control at maximum. I'm guessing, like the 1500, the 1400 had its component values changed during production? The circuit in the 'red books' shows the width control to be 1Meg with 330K in series. My copy of the 'schools' version (which would have been a large screen), shows the control as a 2.2Meg with 680K in series. Does anyone have the correct information? I'll have another play with it tonight and see what I can find. Cheers Nick