Telequipment Serviscope Minor X Input - UK Vintage Radio Repair and Restoration Discussion Forum

loadedtoad · 26th Apr 2019, 6:48 pm

Hello everyone
New to posting on the forum and to vintage valve equipment generally.
I am looking at purchasing a Serviscope Minor with the intention of housing a curve tracer/component analyser inside. (A Mr Carlsons lab project).
There appears to be a couple of versions of this scope one with a X Input on the rear and one without.
The one i'm looking at hasn't got the external X input
I have a service manual but the circuit diagram also does not illustrate an External X input.
I have attached the circuit diagram.

The timebase can be switched off so is it simply a matter of sending a signal via a capacitor through the existing resistor R42 onto the Grid of Valve V5
The timebase switch also reduces the Anode voltage when turned off so i am assuming i will need this part of the switch left on.

Any help greatly appreciated

Argus25 · 26th Apr 2019, 7:13 pm

Probably it won't be too good trying to get V5 to behave as a linear amplifier as it is set up as a Miller-transitron oscillator by the look of it, even when it is disabled/off for that mode. You could simply lift the earthy leg of C8 and couple directly into one of the CRT's X deflection plates where the X position control is. You might need to increase C8's uF value depending on the frequency range you plan to inject there if it is low. I would perhaps replace C8 with a good quality high voltage poly cap. You might need to make an amplifier to get enough deflection for your task. If you do this don't forget a resistor to ground on C8's input side to set the input resistance and zero the voltage on that leg of C8 as there could be some small leakage in C8, with the original capacitor at least.

loadedtoad · 27th Apr 2019, 11:17 am

Thanks very much for the reply.
I will purchase the scope and have a go almost certainly need a recap on the Electrolytics anyhow.
I did find a photo with the covers off of the model with the X Inputs.
Not a great shot but it does look like the X inputs One High and a One low do both go to the tube base ,the higher one with an additional resistor before the capacitor i'm guessing.
Would it be better to leave C8 as is and inject via another capacitor or would this filter out the 1 Khz signal from the component analyser.
The component analyser has an unused opamp in one of the chips so no problem if a bit more signal is required.

Found another photo again not great on detail but does show capacitor from X socket on the rear to base of the tube.

26th Apr 2019, 7:13 pm	#2
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Telequipment Serviscope Minor X Input Probably it won't be too good trying to get V5 to behave as a linear amplifier as it is set up as a Miller-transitron oscillator by the look of it, even when it is disabled/off for that mode. You could simply lift the earthy leg of C8 and couple directly into one of the CRT's X deflection plates where the X position control is. You might need to increase C8's uF value depending on the frequency range you plan to inject there if it is low. I would perhaps replace C8 with a good quality high voltage poly cap. You might need to make an amplifier to get enough deflection for your task. If you do this don't forget a resistor to ground on C8's input side to set the input resistance and zero the voltage on that leg of C8 as there could be some small leakage in C8, with the original capacitor at least.

27th Apr 2019, 11:17 am	#3
loadedtoad Retired Dormant Member Join Date: Feb 2019 Location: Derby, Derbyshire, UK. Posts: 2	Re: Telequipment Serviscope Minor X Input Thanks very much for the reply. I will purchase the scope and have a go almost certainly need a recap on the Electrolytics anyhow. I did find a photo with the covers off of the model with the X Inputs. Not a great shot but it does look like the X inputs One High and a One low do both go to the tube base ,the higher one with an additional resistor before the capacitor i'm guessing. Would it be better to leave C8 as is and inject via another capacitor or would this filter out the 1 Khz signal from the component analyser. The component analyser has an unused opamp in one of the chips so no problem if a bit more signal is required. Found another photo again not great on detail but does show capacitor from X socket on the rear to base of the tube. Attached Thumbnails