Sprague Tel-Ohmike TO-6A

[TomCarlos]

Friends...

I picked up a Sprague Tel-Ohmike TO-6A from eBay. I thought it might be a "fun" item to have around. I have seen posts here were we talk about the Tel-Ohmike but none about the unit itself.

Initial testing says the unit is working- "accuracy" might be another thing.

And of course, the first thing I do when I buy something old is to open it up and have a look around.

The tubes test ok. The "Eye" glows brightly. But some of the Electrolytic caps appear to be cracking and bubbling at the leads.

Electrolytics include:

12uf, 450 vdc
5uf, 25 vdc
20uf, 350 vdc
50uf, 3 vdc

I think I can probably go with 10uf, 4.7uf, Should be able to find a 20uf, and a 47uf caps.

C10, which goes to the Power Factor control, calls for a 2uf, 200 vdc, polyester film. There are (2) 1uf caps in parallel for C10 (just like the manual says there could be). There are other polyester film caps in this unit.

So the electrolytics are easy to get to and replace. But I am curious about the polyester caps. Did they use polyester back in the day because they were cheap and available or was there some other "property" or electro-consideration that went into the design choice?

Thanks in advance... Tom

[nickb]

Not sure what your are asking. Polyester in preference to what?

I haven't seen the schematic but there is a good chance that these caps will affect the accuracy in same way. Polyester is something of a compromise dielectric having a decent dissipation factor but not costing too much. There are many things to consider, especially in a measuring instrument, voltage, tolerance, DF, thermal co-efficient, voltage co-efficient, long term stability, physical size and not lease cost all have to be balanced.

Scroll down to "Characteristics of plastic film materials for film capacitors"

[TomCarlos]

Hi Nick...

I guess one of my questions is .... If you need a 2uf, 200vdc cap (back then), were electrolytics available (for that value and voltage rating) or is this a reason why the poly caps were used - (because of value and voltage rating availability)?

[nickb]

Hi Nick...

I guess one of my questions is .... If you need a 2uf, 200vdc cap (back then), were electrolytics available (for that value and voltage rating) or is this a reason why the poly caps were used - (because of value and voltage rating availability)?

Yes, they were. I see that they have e.g. 20uF 350V ones in there. But they are not suitable for where you need any accuracy. Back then the tolerance was -20+80% to mention just one issue. Polarity is another.

Here is the manual with schematics with nice explanations. I miss manuals like this ( sigh )

EDIT. I didn't realise you ave already linked to manual. Now I feel pretty dumb.

[Tom Phillips]

Tom,
There is a TO-5 / TO-6 discussion at Antique Radio Forums ? View topic - Sprague Tel-Ohmike TO-5 calibration .
Although the discussion subject title is "Sprague Tel-Ohmike TO-5 calibration", there is a lot of TO-6 related content that gives incite into the circuit operation and calibration tips.
Cheers,
Tom

[TomCarlos]

Thanks Tom....

[Enzo]

Oh those are great, cool old things.

[drewl]

AGH! Flashback!

I remember working on those in the '80's.