HP 200CD too much output

[Randall]

I have a military HP 200CD signal generator. The amplitude control on it has long been over sensitive on the low side. It is a dual ganged 600 ohm 2 watt AB two legged attenuator in sealed bodies. I have always been unable to get any DeOxit into it, and thus it is very jumpy and annoying when turned down to almost off, where I have to use it since it blows out way too much signal for my puposes otherwise.

Today I bought a new 1/16" titanium bit and bored a little hole in each one to spray some cleaner in there. It worked pretty good, but it's not perfect, and it still has way too much output for me.

So I want to replace the pots with something that will knock down the output a fair bit. Problem is, the parts list shows R39 A & B as being dual 600 ohm 2 watt attenuators, and they measure as such. But the schematic shows one as 25K and the other as 250K ohm.If we expect both to be the same, then we can chalk it up to a typo. But, then why so far off from 600 ohms?

What I really would like to know is, what value dual ganged pot can I put in there that will knock the output down, yet still give me some room for adjustment on the CCW side? And A or B taper?

[nosaj]

You know what I've done, I've gotten a pomoma adapter that goes to BNC and I use an oscilloscope probe a 1x-10x to knock output down some.
nosaj

[Enzo]

I suspect you are not going to be driving 600 ohm balanced lines anyway, so why not ditch the thing and mount a plain old volume control in its place.?

[The Dude]

Dual pots with different values are harder to come by. I'd use a dual 250k audio taper pot. The audio taper will give you better control on the low end.

[Randall]

So, how do we wire up this dual A250K pot?

[The Dude]

Like the schematic shows. That said, I like Enzo's idea better. Ditch it and wire in a plain old volume pot.

[g1]

From what someone said elsewhere, putting a 600 ohm termination cuts the output level by about half.
Some generators have an int/ext switch for the 600 ohm termination, why not just hardwire it in? That should get you into a more usable range of the pot.

[Randall]

Is this simply a 600 ohm resistor across the output connectors?

[g1]

Is this simply a 600 ohm resistor across the output connectors?

Yes. 560 ohm would be the common part value.

[Randall]

I ended up with a 150 ohm, not sure why, but it works for me.

[Helmholtz]

From what someone said elsewhere, putting a 600 ohm termination cuts the output level by about half.
Some generators have an int/ext switch for the 600 ohm termination, why not just hardwire it in? That should get you into a more usable range of the pot.

I'm no expert, but to me it looks like a variable bridged T attenuator, intended to provide constant input and output impedance at different attenuation.
With my signal generator I just plug into an external vol. pot consisting of a 5k multi-turn pot mounted in a small housing.

BTW, as there is an output HF transformer, what is the lowest signal generator frequency?

[vintagekiki]

But the schematic shows one as 25K and the other as 250K ohm

25K / 250K in the attenuator is OK. The different values are due to the constant output resistance of the attenuator.
You can reduce the output voltage only by using an external 600 Ohm attenuator.
All improvisations degrade the HP 200CD and turn it from a professional device to frivolous construction.

https://www.manualslib.com/manual/1250634/Hp-200cd.html
HP 200CD Operating And Servicing Manual

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Tee Attenuator Calculator

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https://www.allaboutcircuits.com/tools/t-pad-attenuator-calculator/ ¹⁾

[mozz]

Yes, just use a attenuator or resistor across the output. Actually i have a few, i buy from hamfests for $5 and build 18w marshalls in the case. Certain models you can find A mullard gz34 in there. I have the pot if you want it i'll mail it out, didn't measure it yet.

[g1]

I'm no expert,

Oh sure, tell us another one.

[dbdoc]

The HP 204C I repaired put out 5Hz to 1.2 MHz and was very flat to over 100KHz. I certainly wouldn't replace the output attenuator; keep it original. They are cool looking. The 204 was stable down to 0dBu/.775 V. If you need lower, use an external pot like suggested below. Any pot 5-10K or even more, single turn is ok, log taper is good for audio, in a project box with whatever connectors you want works great.