Home–Built Laboratory Oscilloscope!

[Chris55000]

. . .Circuit and Veroboard Layout of Board No. 4, the X Final Amplifier - I haven't been able to identify the functions of leads annotated 'A', 'B' and 'C' yet!

Chris Williams

[Chris55000]

. . .X Final Amplifier, Board 4

Chris Williams

[Chris55000]

. . .By the way, I didn't attempt to power up the Oscilloscope when I received it as there was so many disconnected/unidentifiable leads that there was no telling what damage would have been done to the C.R.T. or other components in it!

Chris Williams

[Chris55000]

. . .Corrected Drawings of Board 4 with additional view of board less heatsinks to show all component/link wires and track cuts!

Chris Williams

[Chris55000]

. . .I've now drawn out the circuit of the smallest and simplest module, the Blanking Amplifier Board No. 5, but because there were so many disconnected and unidentifiable wires in the area of this Module, I can't work out how the Blanking was connected in the Grid/Cathode circuit of the C.R.T.!

. . .The two most common un–blanking methods are to use a "floating" grid/cathode E.H.T. supply whose positive return is connected to the blanking amplifier and the negative supply direct to the grid, usually with an "intensity range" preset between floating E.H.T.– and the grid of the tube!

. . .The blanking then "bounces" the whole floating E.H.T. supply up and down over a range of about 30–50 V with the "intensity range" preset setting the C.R.T. grid just at cut–off in the absence of an "unblanking" pulse from the Trigger or T.B. circuits!

. . .The second method, used in many Gould and T.Q. Oscilloscopes, and the W.W. 1975 50 MHz design, is to use a special C.R.T. with a purpose–made "blanking anode" which can be directly connected to the "un–blanking" amplifier – if the blanking and first anodes are at similar potentials, usually chassis in most cases, the electron beam is un–hindered and can pass through the anodes towards the deflection plates and screen, but when a positive voltage pulse of about +50 V is applied to the blanking anode the electrons are deflected into the anode structures of the gun and the C.R.T. is blanked out!

. . . Unfortunately this home–built Oscilloscope has neither a separate "floating" E.H.T. supply, nor a C.R.T. with a special blanking anode, so I'd welcome Other Members' opinions on how to arrange the C.R.T grid/cathode circuits!

Chris Williams

[Radio Wrangler]

There is a scheme, used in some Tek and HP scopes with a single HT winding that makes a second eht by capacitive coupling and something like a DC restorer circuit. Inevitably the DC restorer can do DC blanking, but has to be slow because of the capacitance it has to drive. So this is backed up by a capacitor coupled fast blanking path.

Fiddly, but it works.

David

[Chris55000]

. . .Any particular Models come to mind David?

. . .I've had a look through a number of Tek and H.P. Oscilloscope Manuals but all the ones I've seen have the "Floating 2nd E.H.T. Supply" technique!

Chris Williams

[Radio Wrangler]

No model numbers in my head (I'm away from home at the moment and not sure I've any such in the manuals I've kept.)

Scopes done this way are out there. It's tedious keeping looking in hope. Maybe one of the low budget instruments, Scopex or an old CDU150?

The HP one I'd check first is a lowih bandwidth cheapo special with a number like 12??. It has a plastc chassis and no PDA, clean the screen and static charge seflects te trace off the screen!

David

[Chris55000]

. . .Now added the Bright-Up Amplifier, Board 5!

. . .I wouldn't be able to say for definite until I've drawn out in full and tested the Trigger/Timebase/Delay Module Board 3, but I have a feeling that the Original Builder used a negative-going "Un-blanking Pulse" connected to the C.R.T. cathode circuit, rather than a positive-going one connected to the grid circuit - as far as the C.R.T. is concerned, it would make no difference as to which of the two electrodes is used for "Un-blanking" as long as the polarity and amplitude of the pulse is correct!

. . .It isn't possible at this stage to work out where leads 'A' and 'B' connected to pins 501 and 502 of Board 5 go to as of yet!

Chris Williams

[Chris55000]

. . .Further proof that this Oscilloscope Design might have a commercial manufacturer's parentage is given by the Vertical Amplifier Input Attenuator – any home–made design would have had the simplest possible attenuator sections with components, usually without frequency compensation, wired directly to the Vertical Gain switch, but this one has Attenuator Sections, fully Frequency Compensated, switched in by Plessey "PLY7xxx" series relays, four in total, welded to a Special Bracket screwed to a specially made right–angle aluminium bracket!

. . .There's part of a factory stamp, etc., on the relay bracket, but it's impossible to make out what it says!

Chris Williams

[Chris55000]

. . .More pics!

Chris Williams

[Chris55000]

. . .The switch with the cylindrical green capacitor on it is the D.C. blocking capacitor usually fitted to the "A.C./D.C./Gnd." Switch, which is a three position rotary wafer switch on this Oscilloscope!

Chris Williams

[Chris55000]

. . .Close–Up view of relay pins and contacts identification!

Chris Williams

[Chris55000]

. . .The Relays are marked "26.5 V 1250 ohm" in Radio Constructor Magazine Diagram Lettering, therefore implying that the most likely power supply voltages in this Oscilloscope, apart from the C.R.T. negative E.H.T./P.D.A. supply and the heaters, plus the H.T. for the Deflection Amplifiers, are +12 V and +24 V, most likely with additional auxiliary components on the Timebase Module for the +5 V D.T.L. and T.T.L. Supply, but I've not got that far yet!

Chris Williams

[See_Mos]

Those military grade hermetically sealed relays were also available from RS. I saw quite a few when repairing gear for the oil rigs. Interesting how they can be fitted into a PCB either way round and still work.......... except that some had different coloured glass around one of the coil pins to indicate the most sensitive polarity and if my memory is correct there was a variant with one of the coil pins soldered to the case.

[Chris55000]

Morning Trevor!

. . .The relays I have don't appear to have any recommended polarity indication nor have a pin connected to the screening!

. . .I'm now trying to fathom out the Trigger, T.B. and Delay Panel, Board 3 – I've counted about 28 short bits of identical short black wires from the Timebase Chassis switches and controls to all the pins on the Veroboard – there's 30 Terminal Pins on it!

Chris Williams

[Chris55000]

. . .I have made a Wiring/Connection Schedule of all the black wiring and tinned copper wire connections made to the multiple rotary switch wafers (appear to be of the R.S. "Maka–Switch" variety – as I mentioned previously, practically EVERYTHING in this Oscilloscope is directly out of the R.S. catalogue!

. . .So far I have identified 102 separate wiring connections between the Trigger/T.B./Delay Veroboard 3, and all the chassis mounted controls and switches, adding all the various disconnected bits of wiring brings the total to 105 separate connections to be identified and listed – substantially greater (by about four times!) than anything I have seen in all the Construction Plans, including the latest R.C. 1973/1975 ones, found so far!

Chris Williams

[Chris55000]

. . .Just a quickie one!

. . .This Home–Built Oscilloscope had a toggle–switch fitted to the front panel annotated "ALT", which (to the best of my memory!) had three anonymous black wires going to three of the 38 Terminal Pins on the Trigger/T.B./Sweep Delay Veroboard No. 3., would I be correct in assuming that in one position, the "ALT" switch switches the "X Amplifier" input direct to the Timebase Ramp, and in the other position, the "X Amplifier" Input connects to the Delayed Sweep Signal after what is (presumably!) a Delay Monostable?

. . .Please note that this is only a single Vertical Amplifier Channel Oscilloscope, so there would be no "CH1/CH2/CHOP/ALT" switching in it!

. . .I haven't drawn out all the Circuits on Veroboard No. 3 for the Trigger/T.B./Sweep Delay yet, so if anyone has come across an Oscilloscope Circuit with a circuit that works like this I would welcome any insights or tips – if I know exactly what function that the "ALT" switch does, it will help me identify the correct wiring for it!

Chris Williams

[Chris55000]

. . .I'm now finding that with the Trigger/Timebase/Sweep Delay Veroboard No. 3 in particular – it's very, very difficult to determine precisely what the interconnections between sections of circuitry and all the original front panel controls and switches is proving a particularly difficult headache – the reason for this is that I'm coming across groups of two/three individual transistors whose base/emitter connections simply terminate in terminal pins and absolutely no idea of what they're for or connect to, and it's already become apparent to me that I've made some monumental "howlers" in my written notes – according to them I ended up with the decade steps T.B. sweep timing capacitors simply switched across a +5 V supply rail and chassis–earth, which can't possibly be correct!

. . .The original builder built this Oscilloscope with at least three dozen lengths of identical pieces of thin black 7/0.2 p.v.c. connecting wire between the Veroboard and front panel controls, and the other Veroboards were wired with whatever gash bits of odd colour/striped colour wire he had to hand and worse, there were odd bits of different colour wire joined end–to–end to make one connection!

. . .The biggest difficulty is that I can't find a single example of anything published, whether home–constructor plans or a commercial Oscilloscope that might have been copied, even in part, to make this – for example, the low voltage stabilised power supply circuits use two ten–lead TO-99 LM305H regulators, which I've not found a single commercial example of one in use anywhere in any type of gear, then there's one of those "square base" three pin metal case radar type transistors that I believe the BRC2000 used in earliest production chassis!

. . .The original builder was either a genius or a sadist of the first order!

. . .Has anybody any more tips or am I on my own here?

Chris Williams

. . .I also have, in the Shed, a Digital Storage Adaptor of unknown technical specification, built in an identical manner to this Oscilloscope in the same range of cases with identical white Bulgin Knobs and front panel labelling, lettered "JTL 4/01" – would the "JTL 4/01" offer any clues or not? – I've not dared look in this yet in case it's even worse!

. .

[frsimen]

JTL - John Thornton-Lawrence, perhaps. He had quite a few test gear designs published in the magazines in the 1970s. One which comes to mind was the digital frequency counter published in Practical Wireless in 1971.

Paula