UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Solartron CD 1400 Oscilloscope Refurbishment

Chris55000 · 2nd Jul 2020, 12:47 am

Hi!

I think I can explain it!

The centre–connection of the h.t. voltage–doubler at the lower end of the 155V secondary of the mains transformer/junction C201/C202 is a lower voltage h.t. point about +180V, and the two deleted components R108 & C109 were intended to provide an auxiliary h.t. supply of about +160V for other purposes, and at the same time, Solartron wanted to set the minimum B.A., I.P.S. and Astigmatism control min. potential a little more accurately, so they returned the lower end of the control network to the junction of R304 and R312, forming a potential–divider fed from this spare +160V line, giving an approx. unloaded voltage at this point of:–

160 × (27,000/45,000) = 95V approximately.

One point to note is that your unit uses a P11 blue phosphor CRT, and it's very possible that this needed higher voltages on the B.A., I.P.S. and the Astig. Anodes A3L and A3U than the standard green phosphor model CRT, so almost certainly your modification applies to very early models and those with non–standard CRT's only.

If R304 (18k) & R312 (27k) are connected across a nominal 160V supply, they would draw a current of (160/45,000 = 0.00355A or 3.55mA) and this current through a 18k ¼W resistor is close enough to it's rating to cause it to start wandering up in value gradually over many years use, until it rises in ohmic–value to the point where even a milliamp or two will burn it out!

At some point fairly early on in the production run, Solartron obviously found out (or were advised by the CRT manufacturers, 20thC. Electronics) that about 95V was too high for the minimum values of the B.A./I.P.S. and Astigmatism control potentials, so by deleting R304 from Board 3, and R108/C109 from Board 1 (as this supply wasn't needed for any other purposes) this reduced the min. potential of this electrode group to the value either recommended by the CRT makers, or simply found by trial–and–error to be correct, so they simply instructed their Draughtsman to omit these components and associated connections, board artwork lines, etc., from the circuit and PCB layout diagrams!

I will include them with an explanatory note in my diagrams now being draughted!

Chris Williams

2nd Jul 2020, 12:47 am	#22
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,874	Re: Solartron CD 1400 Oscilloscope Refurbishment Hi! I think I can explain it! The centre–connection of the h.t. voltage–doubler at the lower end of the 155V secondary of the mains transformer/junction C201/C202 is a lower voltage h.t. point about +180V, and the two deleted components R108 & C109 were intended to provide an auxiliary h.t. supply of about +160V for other purposes, and at the same time, Solartron wanted to set the minimum B.A., I.P.S. and Astigmatism control min. potential a little more accurately, so they returned the lower end of the control network to the junction of R304 and R312, forming a potential–divider fed from this spare +160V line, giving an approx. unloaded voltage at this point of:– 160 × (27,000/45,000) = 95V approximately. One point to note is that your unit uses a P11 blue phosphor CRT, and it's very possible that this needed higher voltages on the B.A., I.P.S. and the Astig. Anodes A3L and A3U than the standard green phosphor model CRT, so almost certainly your modification applies to very early models and those with non–standard CRT's only. If R304 (18k) & R312 (27k) are connected across a nominal 160V supply, they would draw a current of (160/45,000 = 0.00355A or 3.55mA) and this current through a 18k ¼W resistor is close enough to it's rating to cause it to start wandering up in value gradually over many years use, until it rises in ohmic–value to the point where even a milliamp or two will burn it out! At some point fairly early on in the production run, Solartron obviously found out (or were advised by the CRT manufacturers, 20thC. Electronics) that about 95V was too high for the minimum values of the B.A./I.P.S. and Astigmatism control potentials, so by deleting R304 from Board 3, and R108/C109 from Board 1 (as this supply wasn't needed for any other purposes) this reduced the min. potential of this electrode group to the value either recommended by the CRT makers, or simply found by trial–and–error to be correct, so they simply instructed their Draughtsman to omit these components and associated connections, board artwork lines, etc., from the circuit and PCB layout diagrams! I will include them with an explanatory note in my diagrams now being draughted! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed! Last edited by Chris55000; 2nd Jul 2020 at 1:09 am.