New project - 1967 oscilloscope restoration

I picked up this from a for sale group (these were the photos on the site)



I was immediately attracted to the cut off twin flex!

The guy who sold it to me told me it has been sitting in a shelf for years since his sister bought it from a school sale. So no real idea of the history of the scope, but at least it hasn't been "tested".

The first thing I did was test for continuity through the transformer and shorts to the case. There were no obvious problems, so that's a good start.

Google couldn't tell me anything about the Maxtern model 539 oscilloscope (if anyone does, then please pass it on). It looks like a fairly simple "budget" scope.

Opening it is simply a case of removing 2 screws, in on the underside, and another on the back.



After that the whole case slides off.

But what is that control between the two trimmers? It doesn't want to move. That's of because all the other controls seem really smooth.



Aside from some dust, it's in pretty good condition inside



Now I can see that control at the back is a switch, and with enough force it turns (I thought it was a pot). But something is weird. There's wires hanging there. Are they cut?



There's not a lot of room to see what's in there



It seems to be a puzzle to solve later...

The tubes are all Toshiba brand and the Caps also appear to be "good" makes. I removed all the valves and got some compressed air to clean out the dust. Much better!



Oh, I forgot to show you underneath



This was before cleaning. Very little dust.

And the high voltage filter caps give us a date



The transformer has the windings marked and seems to be in great condition



There's 6 valves in this scope.

1. 6X4 full wave rectifier
2. 1X2B 1/2 wave EHT rectifier
3. 6AQ8 double triode
   
4. 6AU6 pentode
   
5. 6DJ8 dual high transconductance triode
6. 12AU7 dual triode

And only 5 electrolytic caps.

And 2 suspect looking paper caps, a 0.2uF 400V oil/paper caps, and a 0.2uf? 600V "oil" cap.

I think the dual 0.2uF 1500V cap looks like a film capacitor.

 

The electrolytic capacitors all read well over their marked value, that's a pretty good indication they're leaky.

If anyone can confirm whether that dual 0.2uF 1500V cap is likely to be electrolytic I'd be very appreciative.

 

Sorry, I don't get you?

 

I'm not sure if that was to save space or to minimize noise. I kind of suspect the latter, but it seems a lot of effort for what isn't a particularly high end piece of equipment.

Indeed, but quite cute in its own way.

 

Yep, I suspect that's the case.

 

Seems to be a reasonable deduction.



Certainly 11 pin.

Heater on pins 1&11, 5&6, and 8&9 appear to be deflection plates.

There seems to be a separate heater winding for the crt. I guess this is due to the cathode being highly negative compared to the other cathodes.



The 2 lugs are the mains winding. The orange pair heading up and to the right go to the heater. That other red/brown pair in a sleeve go to the heater/cathode of the 1X2B.

 

First replacements are the 20uF 350V and 20uF 450V. Both replaced with 22uF 450V.

The negative side of the original caps was connected to the chassis. There middle lug of the tag strip was unused (and connected to the chassis) so I used that as a common point. I added a wire from the chassis ground point to the center lug because I can't be sure how good the contact is.

 

Wow, 5Hz to 400kHz response (3db).

No mention of that third control on the back on a quick reading of the manual...

Thanks @bertus

 

As the new capacitors arrive one by one...

0.1uF 100V replaced with 0.1uF 400V. This one was probably good, but it got replaced anyway.

And the 50uF 15V cap replaced with a 47uF 63V cap I had in my junkbox. That one measured 107uF out of circuit, so it's definitely leaky.

 

There is a different model described on this page.

It's hard to say whether this is an older or newer model. On the one hand it contains some solid state devices, and newer looking resistors, but it has an earlier (lower) model number and is estimated to be from c1959. On the same site there is information on a model 537, looking much more modern and being (other than the crt) completely sold state (and dated 1970)

 

My next day deliveries are taking longer than usual. I wonder if they've been souvenired by some passer by?

Anyway, the 10uF 150V cap is now replaced with a 10uF 450V cap.

The original was an actual cap connected from somewhere to ground.



The replacement radial capacitor is connected to ground at a different point.

 

Let's take a look at that capacitor on the input terminal.

From the bottom



And from the side



And when removed...



Like most of the others it reads about 50% above it's nominal value, so it's probably leaky.

A shame I left the replacement at home...