Home–Built Laboratory Oscilloscope!

[Chris55000]

. . .Circuit & Veroboard Layout Diagram of Power Supply, Veroboard No. 6!

. . .N.B!

. . .The Voltage Readings will be added later when the two small 150 W Cased Electronic Loads I've ordered arrive for me to be able to test the P.S.U. at its rated currents of approx. 210 mA per rail!

Chris Williams

[Chris55000]

. . .Corrected Veroboard 6 Diagram - I annotated the transistors in the H.T. regulator circuit 6TR1 & 6TR2 incorrectly!

Chris Williams

[mhennessy]

It's late in the day, but 6TR2 is confusing me - I can't see how it can work unless you swap base and emitter?

As it stands, the base-emitter junction is reverse-biased to the tune of nearly 40V, which will have it "zenering" with 6R4 limiting the current to around 1.2mA.

But even with that swap made, I'm not sure how it regulates given there's no feedback from the HT rail. I almost wonder if 6R1 might be part of that, rather than just a bleed resistor.

Given that it agrees with that part of the layout, I wonder if the original constructor made a mistake?

That aside, it's another beautiful diagram

[Chris55000]

Morning Mark!

. . .It does look very peculiar to me as well Mark!

. . .So far, I can see that 0TR1 and 6TR1 form a conventional "Darlington" transistor series regulator, but the function of 6TR2 (BC117) is somewhat uncertain – I can understand it might have been the intention of the Original Designer to have intended the H.T. to be stabilised by one of the L.T. lines, and it's also possible that it's intended to be some sort of H.T. line short–circuit protection, but I'm waiting for two small 150 W electronic loads to arrive so I can can run some practical tests on the circuit!

. . .The idea behind connecting the tap between the 27k and 100k 2 W resistors in 6TR2 base circuit to the –14.5 V power supply output is also a mystery to me as well!

. . .Thank you for your comments Mark, I really appreciate other members giving me their time to comment on this design as the seller I bought it from said "the original constructor passed away years ago!" & a request to him to enquire if he came across any folders, notes, diagrams, etc., that might have related to this Oscilloscope, (and the three other small pieces of equipment I bought from this seller that appear to have been built by the same constructor!) went unanswered!

. . .The drawings were drawn using "Abacom's sPlan 8" with circuit symbols designed by me!

Chris Williams

. . .PS!

. . .I checked over my drawing work and the lead connections to all of the transistors used in the circuit, and I'm certain I've not drawn anything incorrectly!

[mhennessy]

I've just been having a careful look at the picture of the PSU board back in post #3, and I think I've spotted something...

It looks like there's an error with 6R1. Rather than going to -LT1, I think it goes to the base of 6TR2. That would make sense, as -LT1 becomes the reference for the HT rail (technically +LT1 is part of that, as that's where the emitter returns to).

That makes more sense than what I posted last night, well after my bedtime

Agreed about the Darlington pair

[Chris55000]

. . .I'll have another check of my work Mark, it certainly is very possible I've made an error round there!

. . .(Long experience drawing things out has proved in the past that if my drawing doesn't make sense, there's been a mistake occured somewhere in the tracing–out process!)

Chris Williams

[mhennessy]

If it helps, this image shows what I think could be happening - see the position of the bottom end of the 100k resistor.

(I do a fair amount of reverse-engineering, and it's definitely not easy. I make mistakes all the time!)

[Chris55000]

. . .Thank you for spotting the error Mark, and you're spot–on – I'd drawn the bottom end of 6R1 (100 k 2 W) as going to hole AM4 instead of hole AI4 , and as I drew the Circuit Diagram directly from the layout plan, the circuit didn't make sense to you or me either – with 6R1 connected to holes AA4 and AI4, 6R1 and 6R4 form a standard stabilised H.T. feedback potential divider between the stabilised H.T.1 line and the -14.5 V output, with the emitter of 6TR2 connected to the +23.5V line as a reference point – if (assumption here as I've not carried out a practical test yet!) the stabilised H.T. is +170 V, the open–circuit voltage at the junction of the two resistors is (27/127 × 184.5) V –approx. 39 V above the –14.5 V line, or about +25.5 V relative to earth, so subtracting the emitter voltage of 6TR2 from this gives an open–circuit forward bias on 6TR2 of about a volt, therefore I surmise that the stabilised H.T. was designed to be 160–170 V or thereabouts!

. . .I'll post a corrected diagram tomorrow!

Chris Williams

[Chris55000]

. . .Corrected Circuit and Layout Diagram for the Power Supply, Veroboard No. 6!

. . .Voltage readings will be added when my 150 W Electronic Loads arrive and I can carry out a practical test at nominal output currents of 200 mA from the LM305H regulators!

Chris Williams

[Chris55000]

. . .I have now completed two further Drawing Sheets for this Home-Built Laboratory Oscilloscope - these are the Sheets for the Vertical Attenuator and Volts/Div. Switch!

. . .Would I be correct in saying that if I replaced the Y Pre-Amplifier Board 1 (circuit for this will be posted in due course) with a 1M resistor representing the Amplifier Input, then each Attenuator Section, when switched into circuit by the relays, becomes a "pi" section Network?

Chris Williams

[Chris55000]

. . .So far, my Documentation for the "John Thornton Lawrence" Home-Built Laboratory Oscilloscope runs to 32 Sheets, (less Chassis and Case metalwork!) of which the following are complete :-

1. Front and Rear Views ;
2. Overall Block Diagram ;
3. Vertical Attenuator ;
4. Volts/Div. Switch ;
7. Board 2 Y Final Amplifier ;
9. Trigger Amp, Timebase & Sweep Delay (Interim) ;
10. Trigger Amp, Timebase & Sweep Delay (Part Complete) ;
11. Board 3 Trigger Amp. Timebase & Delay Layout Diagram (Part Complete Less Heatsink) ;
12. Board 3 Trigger Amp. Timebase & Delay Layout Diagram (Part Complete With Heatsink) ;
13. Time/Div. Switch Wiring ;
18. Sweep Delay Switch Wiring ;
19. Board 4 Final X Amplifier ;
20. Board 4 Final X Amplifier Layout Diagram (With Heatsinks) ;
21. Board 4 Final X Amplifier Layout Diagram (Less Heatsinks) ;
22. Board 5 Bright-Up Amplifier ;
23. Board 5 Bright-Up Amplifier Layout Diagram ;
24. Board 6 Power Supplies ;
25. Board 6 Power Supply Layout Diagram ;
26. E.H.T. Rectifier Board 7 & C.R.T. (Original) ;
27. E.H.T. Rectifier Board 7 & C.R.T. (Revised) ;
28. E.H.T. Rectifier Board 7 Layout Diagram (Original) ;
29. Board 8 10 V Calibrator ;
30. Board 8 10 V Calibrator Layout Diagram.

. . .Approximately ten sheets covering the remainder of the circuits and wiring layouts remain to be completed, then I will begin on the Chassis and Metalwork Drawings!

. . .I have attached the latest complete sheets to this post, please note, I haven't determined all the front control potentiometer, toggle and rotary switch connections for the "Trigger, T.B. and Sweep Delay" Veroboard 3 yet, as quite a number of these were disconnected when I got the instrument, and as previously mentioned, the original builder wired all this Veroboard up with three dozen short lengths of identical black wire!

. . .Any input to suggest how I can complete the missing wiring will be greatly appreciated!

Chris Williams

. . .N.B.

. . .The wavy borderless sides for the orange blocks for the Trigger/T.B./Sweep Delay Veroboard No. 3 indicate that particular circuit block is only a part of the Veroboard!

[Chris55000]

. . .First Sheet of my Document Set for this Oscilloscope!

. . .I had permission from the seller to reuse the pictures!

Chris Williams

[Chris55000]

. . .I have worked out that some of the drawing blocks for Veroboard No. 3 (Trigger, Timebase And Sweep Delay) weren't drawn in the most optimal positions to provide short and direct signal routings, so I'm currently in the process of part re–drawing this sheet!

. . . Unfortunately I'm still a bit stumped with all the connections between Veroboard Nos. 3 and Veroboard No. 4 (Final X Amplifier) and the external switches for "5x EXP", "ALT" and the "X SHIFT" control at the moment!

Chris Williams