heath o'scope

[billcreller]

 I just traded an old marine binnacle compass for a Heath 10-18 o'scope.  And it works!!
I have a manual for learning scopes that was evidently furnished with kits, but the scope schematic in it seems to be generic, although somewhat like what I have.  I've been on the 'net looking for a shematic for the 10-18 model, but on luck yet.

[tubesornothing]

http://www.vintage-radio.info/heathkit/

??

[billcreller]

Thanks  Nothing on that one :(
 Anyway, it seems to work, sort of.  It's a tube-type, with Mullard tubes, which appear original, and a bit dusty.  The manual I have shows how to check it out.

[RicharD]

The Heath directory appears to be GONE from BAMA.   :-\
I did find a trace of the "ham" who had provided BAMA with a schemo for the io-18
K. Brigode, WB8DBF
Everybody else wants to sell copies.  I've always wondered how that could be legal?

[PRR]

> Heath 10-18 o'scope

I would try assuming that a 10-18 is an improved 10-10, which IS on that http://www.vintage-radio.info/heathkit/  site. There's a gimmick about entering a number to get the file but that seems to just be a robot-stopper, not anything dangerous.

The 10-10 is a very fine machine for your uses. It has an actual sweep-generator, not a gas-tube like my Eico 427. Both channels H and V are identical and DC-coupled, very deluxe for a low-price tube scope. The bandwidth is, napkin-estimate, over 200KHz, which is all you can use in tube audio amplifier work. And there's no goofy peaking coils.

It IS a Heath. There's clever/cheap details. The DC Bal trim is sure to drift. If you are setting very small traces to not-quite zero, you have to re-check your zero often. Fortunately tube-work rarely needs a nearly-zero reading with any accuracy.

The 10-18 has terminals and trap-door on the back? I don't see that on the 10-10 schematic. It was popular to have Intensity "Z" input, and direct access to the deflection plates. I think you can ignore these frills.

One vintage oddity: the input impedance seems to be 3.6Megs, whereas modern custom is 1Meg. That seems "better", but modern 10X probes assume 1Meg load. Maybe this is where the 10-18 differs. You can test this. Power-off. Scope input on DC X100. Put your ohm-meter across the scope input. It reads the actual input resistance. 1Meg is now-normal. 3+Meg is unusual today but fine for low-volt work.

If you have an ohm-meter that stops at 1.999Meg, put a good 1Meg across the input and read the parallel combination. 3.6Meg||1Meg is 782K.

To dummy-up a 3.6Meg input for 1Meg standard load, add 1.385Meg across the input.

Personally the only time I'd really want a divider probe, I'd want 1/100 and I would not need huge input impedance or perfect accuracy. 1Meg+10K gives 1/100, and that 10K end does not care if loaded in 1Meg or 3.6Meg. Now you can poke 550V DC supply rails and watch them ripple and sag.

Be sure the two gas-tubes light-up. They go bad with age. (There is a spec of radioactive in there to jazz the gas, it has a half-life of decades, it has been decades since these tubes were in production.) I'm sure you can get NOS gas-tubes. But in future decades we will probably have to Zener these machines.

Is the trace small and clean? It should focus to less than a millimeter. Fuzz can be power supply ripple. The low-volt stuff is guitar-amp practice (tho the -75V is grounded oddly so that a missing gas-tube will shut-down the bias). The KiloVolt supply is routine. The rectifier is "backward" because we want big negative. Those caps do need to be HIGH voltage, and stacked electrolytics may leak too much; try Ham suppliers.

[billcreller]

 Thanks for all that info PRR.  The trace on the screen is quite precise and clear when focus is adjusted.
I did notice that there are a bunch of the high voltage caps in there. I'll try to absorb and use what you have listed for input.

[billcreller]

 PRR: you mentioned a gas tube. There's a 1V2 in there that shows no light when power is on. Is it supposed to glow below the plate/shroud in there?

[sluckey]

1V2 is your high voltage rectifier. Since you have a trace, I'd say it's OK. I doubt you can see a filament glowing.

[billcreller]

OK !!
 I see that the 1V2 has a very low heater voltage, like 5/8 of a volt.  Maybe not unusual for a special purpose.

[PRR]

As Slucky says: high-volt rectifier. Also LOW current. Your big-amplifier needs 200mA of DC current, uses a 5U3 which eats 15 Watts heater power and glows good. The 'scope HV supply is 1mA or less, uses much lower heater power. It must glow, but inside the HV plate and any safety-shields, you may not be able to see it without getting your nose shocked.

Do NOT get too close to that 1V2!

> you have a trace, I'd say it's OK.

Right.

Outside the HV area you may have a couple ~~100V gas tubes. These glow violet or orange "cloud" inside a plate. They should light-off after the 300V rectifier comes up, a few seconds after power-on.

In my even older General Radio ACVM, the gas tubes do not want to start, and the unregulated B+ gives errors in readings. For now, they will light-off if I let them warm and then touch them. But I fear the 1930s technology will eventually need a 1977 transplant (a Zener-string).