Beyond bizarre mystery HP 204c ailment

I am in the process of refurbing a HP 204C audio oscillator with its NiCad power supply option.

Amazingly, the ancient NiCads seem to still work. Or at least pretend they do. They DIRECTLY form the +6 -6 supply for the circuit proper.

All the power line supposedly does is deliver one of two chosen series current source values to maintain charge.

The symptom is this: The output audio amplitude works just fine with the line cord unplugged, but DROPS IN HALF when plugged in. Distortion remains negligible pure sinewave.

I see no obvious, rational, or sane explanation for this.

I have yet to check the supply voltages. Again, the output DROPS when the line cord is active!

Schematics at

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and elsewhere.

What do you think is happening?

--
Many thanks, 

Don Lancaster                          voice phone: (928)428-4073 
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Reply to
don lancaster
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Looks like the output is AC coupled while the AGC loop is a DC path, so if one half of the power supply is dead, it might still "work" from the point of view of the output terminal but with inadequate (or no) control range.

I'll bet the distortion gets a lot better when you plug it in... and are you checking the amplitude into a 600-ohm load, or into a scope probe?

-- john, KE5FX

Reply to
John Miles, KE5FX

scope probe only so far.

--
Many thanks, 

Don Lancaster                          voice phone: (928)428-4073 
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Reply to
don lancaster

Try applying that scope probe to the +6V and -6V supply (not at the same time), and see what they look like.

A dead-ish battery may show higher impedance than the designer expected, or a dead-ish to dead electrolytic in the charger circuit WILL show higher impedance than the designer expected. Either one would cause that AC ripple that John mentioned.

--
Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

That would be my guess. However, it is two pairs of 6 volt nicads. Not shown correctly on the schematic. Also, the batteries provide some of the regulation for that optioned unit.

Tom

Reply to
Tom Miller

Here's a discussion on the topic.

Looking at the schematic on Pg 7-7/7-8 (Fig 7-3), there are a total of

4 batteries involved at +13.5v and -13.5v. I think you might be misreading the +6 -6 supply which actually reads B+ and B- on the unreadable copy of the manual.

Probably at least one dead NiCd battery but most probably all of them are dead, dying, ready to die, or otherwise defective. Also check the contacts and solder connections for corrosion. In my never humble opinion and extensive experience with old test equipment and batteries, the batteries should be considered dead, even if the look good, or the former owner claims that they were "recently" replaced. I just replace two NiCd batteries in my Tek 213 oscilloscope. They lasted about 7 years.

What I like to do is substitute LiIon for NiCd and NiMH batteries. Before you panic about the incompatible charging, I usually add a series diode to insure that the battery is only discharged by the instrument, and not charged by the instrument. For your 13.5v battery, methinks that the common 14.8v (4S) LiIon battery should work. In addition, I would get a battery pack normally used by the RC (radio control) crowd (and NOT use something borrowed from a laptop). For charger, these RC batteries use a system called "balance charging" which had done wonders for the lifetime of my assorted LiIon batteries. No more reverse charging one weak cell, or tossing the battery pack because one cell is "weak".

I'm not sure if it will fit, but two of these should do the trick:

For charging, I have one of these: It works well, but took me a frustrating evening to decode the instructions. Note that it will charge (and discharge) most chemistries.

You'll also probably need two XT60 connectors and/or adapter cables:

I've also built up battery packs using 18350/18650/18450/16340 LiIon batteries, and attaching my own JST balance charger cables for charging:

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

Oh-oh. I just noticed that Fig 7.3 says "mercury battery" which is not rechargeable and an ecological hazard. You won't find an exact battery replacement. If you want it to be portable, think of a different chemistry battery with an external charger. Also, please recycle the old mercury batteries properly.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

There are ( or were ) three flavors of 204c power supply: ac only, mercury, and ac+nicad.

This is an ac+nicad unit which itself is available in two flavors, regular and mil.

Almost certainly at least one nicad is partially defective.

The dilemma is that nicad replacement or ps rebuild puts the item out of competitive eBay price range.

Yet it DOES work, sort of. And is otherwise quite clean.

--
Many thanks, 

Don Lancaster                          voice phone: (928)428-4073 
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Reply to
don lancaster

There were three options for the 204C- AC power only, AC + rechargeable NiCd, and AC + Hg non-rechargeable.

The AC only supply has series pass voltage regulators.

The NiCd unit has a current regulated charger with two rates.

The Hg unit was a use it portable until the batteries died then replace same.

tm

Reply to
Tom Miller

You may be able to find some 5 cell packs cheap on ebay. The original cells were only 225 mAh.

Charging is either 6 mA or 20 mA, switch selectable.

Reply to
Tom Miller

The batteries and charger I recommended total about $80 with shipping and taxes, which is probably more than the 204c is worth. A cheaper charger will drop that to about $50. The demand for battery operated portable audio generators is quite small, but I suspect anyone that really needs one will pay your price for it. Otherwise, converting the unit to AC only would probably result in the quickest sale. The AC powered HP204C goes for about $50 to $80. The battery powered variety for $340. If you go stock and just replace the NiCd packs, they can be had for about $15 each. A $30 investment to gain a $290 price markup, seems like a good investment (if it sells). Well, maybe not. This one sold for a total of $226.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

Possibly the amplitude stabilization is nonfunctional until a supply voltage larger than +/-6V shows up.

RL

Reply to
legg

I strongly suggest your measure supply/bias voltages on batteries/line before continuing because someone else may have made some adjustments after one or more battery died. Then, when you plug it in and get full voltage, it does what you see.

Or, just plug it in and make the symmetry adjustments and see what it does on battery.

Good luck.

Reply to
John S

For battery replacements, download my chart at

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Reply to
Robert Baer

Maybe, maybe not: first cut, I see this circuit relies on the NiCd to regulate the voltage? Then, a pair of zeners is a reasonable battery substitute (let the 'charge/operate' current source run the gizmo on AC). Add capacitors in addition to zeners...

With switch S4 in the 'slow' position, current source is 12 mA. In 'fast' position, it's 25 mA.

Second cut, can a modern battery be substituted, maybe some lithium cells with a provision to prevent overcharging, for the NiCd stack? It'll be +/- 7.2V instead of 6.25, but maybe that's OK.

Reply to
whit3rd

The zener idea sounds like a cheap fix. Maybe use a pair of 6.8 volt 1 watt zeners in series for each rail. A pair of 25 volt junk drawer electros would last forever.

It is two 6.25 volt batteries in series for each rail. They are only rated for 225 mAh so the unit does not use much power since it will run for a day on batteries.

Reply to
Tom Miller

You're probably not going to like this...

I don't like that thing. Take a look at a Wavetek 111 and you'll toss the t hing in the trash. But that is not the point. The point is getting it strai ghtened out.

I would pull the batteries and just refit the regulator a bit. That is the stupidest power supply I have seen in a long time. Of course sometimes I ha ve to remind myself I was nine years old when it was built.

Could I do better ? I sure as hell could on that power supply. How hard is it to take a couple of diodes and make it so it will run with dead batterie s. And man, ;when the batteries are good, itt won't. Look at it. the regula tor is a single ended current source and the thing always runs off the batt eries. If it runs with dead batteries they are bad. Look at the circuit, co nstant current into a dead battery should only put low voltage across it.

You have several options. I think the best of them is to just stick a coupl e of Zeners in there. The supply shows -33 and obviously +12.5. That is 45.

5 volts. All the current comes through that 523 ohm resistor. That comes ou t to a hair less than 87 mA. That, times 12.5 volts is 1.09 W.

So take two 12 volt Zener diodes and wire them where the batteries used to be. Get five watt units to be on the safe side. If you want to get picky an d really must have 12.5 volts instead of 12, which SHOULD not matter, stick a 1N4007 in series with each Zener. Observe polarity, when you do stuff li ke that it can be easy to make a mistake and not have anything there. It go es the opposite way. But figuring about 0.7 V at 87 mA, (if that) you don't have to worry about their disspation. No matter what, I would say get the diodes from the same batch so the voltages are close. They are the ONLY thi ng providing the split supply. I am thoroughly underwhelmed by the design o f that thing.

Of course I am spoilt. I had a Tektronix scope by the time I was 13. I've h ad a Wavetek generator for over twenty years I just had to throw in a filte r, but it is fine. You should see how THEY make a sine wave. That impressd me somewhat. They use a three stage slope modifier to a precision triangle wave which is totally frequency independent. There is no AGC nor is any nee ded.

Anyway, all you need is a couple of Zeners and probably some 22 uF caps to go across them. If you want batteries, there are plenty of ways to charge t hem. Plus they are bound to be smaller and lighter.

Bottom line, it is probably working correctly when plugged in. It probably has more harmonic distortion when not, but that is not for sure because it is a Wein bridge oscillator. It may be very little and you need a distortio n analyser to tell. It takes quite a bit of distortion before you cna reall y se it on the scope.

I was reading an article about that. If you can print the scope trace and h ave datum points, you can use a ruler to determine how good a sine wave is. I didn't read the whole thing but it was kinda interesting. Maybe I'll get back into it. If you want it I'll Dropbox it.

Anyway, two Zeners and two lytics. for now.

Reply to
jurb6006

I got 87 mA, where did I go wrong ?

Reply to
jurb6006

'fast'

You have to show your work to get an answer to that question.

?-)

Reply to
josephkk

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