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Wavetek 175 ARB: on power-on display flashes, then goes dark

Hi there,

I have bought a used "Wavetek 175" arbitrary waveform generator on eBay, which was said to have a defective power switch -- now it turned out that the power switch is perfectly fine, but there are other problems with it:

- When turning the device on, starts humming silently, showing that there is power applied to the power supply. Thus, I find the switch to be okay. This low humming is the same for "bad" and "good" behaviour.

- In about 4/5 of all power-on attempts, the display lights up random data shortly, then turns dark again. The service manual says that this is caused either by the "+5V logic supply in overvoltage protection mode" or the "+15V supply in current overload mode".

- After more switching on and off, the device finally turns on, and works normally from then on -- until being turned off.

I live in Germany with nominal 230V mains voltage, so I jumpered the device to the 240V setting, specified fr 216-252VAC. When it arrived, it was set to the 220V jumper position, specified for 198-231V -- probably because many years ago, the mains voltage standard here was changed from 220 to 230V, and this device is older than that change. As the power switch was described faulty by the former owner, I suppose the problem observed has been present with the former setting, too. I only tested that 220V setting once, and the device did show the same symptoms as with the 240V setting.

Now my question, with the hope for an answer of other users of such aged equipment: Is there any prime suspect for this type of failure? Something in the Wavetek 175 that uses to fail after some years?

I did send mail to "Willtek", a company that once bought Wavetek, but they "sold the product group to Fluke" and do not have any more information on the thing. Fluke, on the other hand, did not reply to my inquiry. So, any help would be very appreciated.

Thankyou, and best regards, Thiemo

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Reply to
Thiemo Nordenholz
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Sounds like one or more of the power supplies in the unit are haywire. Have you checked the power supplies? Electrolytic capacitors in the power supplies are always prime suspects. Have you scoped the power supplies to see the ripple/noise? It should be quite low; typically less than 0.100 Vpp on each supply if the supplies are switchers, less than 0.05 Vpp if linear supplies. Watch the behavior of each supply output when you turn it on. Best done with a scope, since this will show more detail than a simple voltmeter.

If the voltage sources check out OK, then what other diagnostics have you done?

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Dave M
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Reply to
DaveM

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I second the 'bad capacitors' suggestion. Don't worry about which ones, they're all old-ish, just get replacements for the dozen largest electrolytic capacitors and when the solder's cool, retest the unit.

None of the capacitor values will matter to the calibration, and the values aren't critical.

Smaller aluminum electrolytic capacitors, distributed around the logic circuitry, are also subject to age, and if you feel inclined to do so, it won't hurt to replace them all.

Reply to
whit3rd

I had a look at ripple on the supplies: On the +/- 15V lines it is around

500mV p-p, both in normal and failed state. As the output signals are clean, I suppose that's all right. +5V logic supply looks filtered quite well.

However, I have now found out that it's not overcurrent, but an overvoltage protection circuit turning the +5 logic power off -- interestingly, the largest capacitor in the device is used for stabilizing exactly that supply, among a handful smaller ones. So I'll be heading the replacement way now, and report back on success when it's done.

Thank you for the suggestions! Thiemo

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Reply to
Thiemo Nordenholz

Hi once more,

I have now replaced the largest capacitor in there (52'000 uF), and the problems are gone -- the thing powers on every time it is switched on.

So, thanks for the suggestions!

Best wishes, Thiemo

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Query a PGP key server (e.g. http://www.pgp.net/) for my public key 41068629.
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Reply to
Thiemo Nordenholz

Good work, Thiemo. I assume that the capacitor that you replaced is the 'reservoir' capacitor in the power supply. Always good to check the voltage, ripple and noise on the power supplies before doing any further diagnosis. Without proper power, you can certainly get led down the proverbial 'primrose path' by chasing misleading symptoms.

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Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

Life is like a roll of toilet paper; the closer it gets to the end, the faster 
it goes.
Reply to
DaveM

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