A lovely find at a car boot

[top204]

I went to a car boot sale on Sunday with my wife and found a wonderful object that I never ever thought I would find anywhere, never mind a car boot sale. And I only payed £1 for it, which is just as well, because I cannot afford silly high prices that some of the sellers want. But one pound! It is very similar in design to the one dad made back in the early 1960s that I used to play with as a boy because of its knobs and toggle switches and its lovely little glass screen, and this scope also looks as though it is from the late 1950s to the mid 1960s, but its coloured sockets look like those from the late 1960s onwards?

I have attached some photos of it, and I'm sure some of you will also appreciate its charm and appeal. It looks similar to the DIY Heathkit scopes of its day, but it does not have any make on the front, so it may have been from a learner kit or a college project.

Its valves light up and so does the CRT, but there is nothing on the screen, so when I get my workshop up and running, this will be on my bench and I'll check its high tension voltages and its capacitors. It has an EF80 Pentode valve and, what looks like a V7xx valve, but I've never used the V type values before so I'm not quite sure what type it is because the writing on it is very faint and mostly wiped off. It could be a double triode. But I'll get there, and make it work and clean it so it can sit on my shelf and look good. I have not looked for the CRT part number because it is under its holding strap, but it is a 2" type, so it will be something like a lovely little 2BP1. I still cannot believe I found it!

I have also attached a picture of my future workshop. It is the garage attached to our house, so I am lining it and insulating it when I can afford the wood, and it will be both my office and my workshop, so I can get my life back together. It needs a lot of work, and I am working on it when my health allows. I have no place to put my bits and pieces while I convert it, so I have to keep moving them around. I cannot fit any more of my stuff up in the loft. :-)

[See_Mos]

Nice find, and for only £1

From what I see the heater glow looks fairly central in both valves and the internal screen makes them both look like RF pentodes.

It might be worth posting the pictures and asking for information here https://www.vintage-radio.net/forum/

The site is very interesting for those of us who grew up servicing stuff when valves were still common.

[kcsl]

Wow, that's the original scope that the British National Radio and Electronics School did as a home study kit.
Still got mine... and it still works.
I've attached the circuit diagram for it.

As far as I remember, both valves should be EF80's.

They also did a solid state MK2 version that I also have but there's not much documentation for that available that I've found.

Regards,
Joe

[ken_k]

A very good find. Here is a spice model for an EF80.


**** EF80 ******************************************
* Created on 05/28/2022 00:14 using paint_kip.jar
* www.dmitrynizh.com/tubeparams_image.htm
* Plate Curves image file: ef80.png
* Data source link: <plate curves URL>
*-------------------------------------------------------------posted---------------------
.SUBCKT EF80 P G2 G K ; LTSpice tetrode.asy pinout
* .SUBCKT EF80 P G K G2 ; Koren Pentode Pspice pinout
+ PARAMS: MU=51.5 KG1=737.47 KP=186.79 KVB=1085.66 VCT=0.2779 EX=1.558 KG2=724.57 KNEE=963.53 KVC=1.795
+ KLAMG=1.102E-4  KD=118.44 KC=571.64 KR1=0.00625 KR2=0.1745 KVBG=0.008104 KB1=0.908 KB2=0.01702 KB3=0.00625 KB4=3.442 KVBGI=0.04391 KNK=0.05906 KNG=1.122E-4 KNPL=0.3353 KNSL=0.07806 KNPR=621.29 KNSR=5.842E6
+ CCG=7.5P CGP=3.3P CCP=0.012P VGOFF=-0.6 IGA=3E-10 IGB=0.02556 IGC=20 IGEX=1.413
* Vp_MAX=350 Ip_MAX=35 Vg_step=1 Vg_start=0 Vg_count=10
* X_MIN=58 Y_MIN=202 X_SIZE=746 Y_SIZE=406 FSZ_X=1296 FSZ_Y=736 XYGrid=false
* Rp=1600 Vg_ac=23.5 P_max=2.5 Vg_qui=-23.4 Vp_qui=240
* showLoadLine=n showIp=y isDHP=n isPP=n isAsymPP=n isUL=n showDissipLimit=y
* showIg1=y isInputSnapped=n addLocalNFB=n
* XYProjections=n harmonicPlot=y dissipPlot=n
* UL=0.43 EG2=170 gridLevel2=y addKink=y isTanhKnee=n advSigmoid=y
*----------------------------------------------------------------------------------
RE1  7 0  1G    ; DUMMY SO NODE 7 HAS 2 CONNECTIONS
E1   7 0  VALUE=  ; E1 BREAKS UP LONG EQUATION FOR G1.
+{V(G2,K)/KP*LOG(1+EXP((1/MU+(VCT+V(G,K))/SQRT(KVB+V(G2,K)*V(G2,K)))*KP))}
RE2  6 0  1G    ; DUMMY SO NODE 6 HAS 2 CONNECTIONS
E2  6 0  VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))} ; Kg1 times KIT current
E4   8 0  VALUE={V(P,K)/KNEE/(KVBGI+V(6)*KVBG)}
E5  81 0  VALUE={PWR(V(,KB1)}
E6  82 0  VALUE={PWR(V(,KB2)}
E7  83 0  VALUE={PWR(V(,KB3)}
E8   9 0  VALUE={PWR(1-EXP(-V(81)*(KC+KR1*V(82))/(KD+KR2*V(83))),KB4)*1.5708}
RE4  8 0  1
RE5 81 0  1
RE6 82 0  1
RE7 83 0  1
RE8  9 0  1
RE21 21 0 1
E21  21 0 VALUE={V(6)/KG1*V(9)} ; Ip with knee but no slope and no kink
RE22 22 0 1 ; E22: kink curr deviation for plate
E22  22 0 VALUE={V(21)*LIMIT(KNK-V(G,K)*KNG,0,0.3)*(-ATAN((V(P,K)-KNPL)/KNSL)+ATAN((V(P,K)-KNPR)/KNSR))}
G1   P K  VALUE={V(21)*(1+KLAMG*V(P,K)) + V(22)}
G2   G2 K  VALUE={V(6)/KG2*(KVC-V(9))/(1+KLAMG*V(P,K)) - V(22)}
RCP  P K  1G     ; FOR CONVERGENCE
C1   K G  {CCG}  ; CATHODE-GRID 1
C2   G P  {CGP}  ; GRID 1-PLATE
C3   K P  {CCP}  ; CATHODE-PLATE
RE23 G 0 1G
GG G K VALUE={(IGA+IGB/(IGC+V(P,K)))*(MU/KG1)*
+(PWR(V(G,K)-VGOFF,IGEX)+PWRS(V(G,K)-VGOFF,IGEX))}
.ENDS
*$

Ooops it changed some of the formula to smileys so a text copy is included.

[top204]

Wow! Thanks Joe (kcsl).

I thought I had seen it before somewhere, and that advert clicked everything into place. :-) In the Practical Wireless magazines I used to read as a young-un! That is also why it is such an old design, but built in the 1970s probably. Practical wireless, and a lot of electronic companies here in the UK, still used valves up until the mid-1970s! And the PW magazine used to have projects still using valves that could have easily been replaced with a few BC109 or 2N2222 transistors and a 9 volt battery, but the older writers for the mags, and editors of the mags did not like modern transistors much. LOL

Many, many thanks for the circuit diagram and the document. They will be used to get the unit working again. The right hand side valve has been replaced with a VX something or other 7, the writing on it is very faint and almost gone. But that also showed you were correct Trevor. It was another Pentode. It seems strange for it not to have a triode pre-amp for the Y-input, and my first thoughts were that it may be a double triode or a triode/pentode valve, but a single pentode it is.

It has a 500 volt transformer in it! Whenever I used the smaller CRTs all of those years ago in our back shed, I always used a 250 volt transformer and voltage doubled or tripled or quadroupled it, so I will need to be careful with the voltage. Looking at the circuit, I have a suspicion of the BXY10 rectifier or the 4.7K resistor in series with it. At such high voltages, these can go open with the slightest overload. It's not very efficient using the same higher voltage to supply the CRT, then reduce it so much for the two valves via resistors? But I so look forward to getting into it. :-)

Crickey we are such nerds, but I am very proud of being a Nerd. :-)

[See_Mos]

The V7xx is probably a W719 AKA EF85 which is close enough for this application.