Waking up gear that's not been used for a while - what is the best practice?

[RichardHowells]

My HP 175A scope had lain dormant for a while. Last night I applied power and it failed the smoke test almost instantly.

Whilst I have it in pieces replacing an electrolytic in the power supply I am wondering if there is a better approach then just "turn it on and hope".

Thinking about the replacement cap led me to an article (https://www.nmr.mgh.harvard.edu/~reese/electrolytics/) on how electrolytics need to be charged regularly to stay in shape. When left for a long time (not sure what duration is long in this context) they can be 'reformed' by applying a slowly rising voltage and watching the leakage current and temperature. The article mentions hours/days as the timescale for this.

I am too lazy to isolate the dozens of electrolytics in this item and attempt to reform them individually.

I do have a variac. Is it wise/irrelevant/plain stupid to apply power more gently, when I try to wake it up after the repair?

Are there any general principles here for waking up old, especially valve based, kit?

[paulsherwin]

It's a good idea to power up using a lamp limiter as this may limit catastrophic damage and give feedback if the equipment is overcurrent.

[RichardHowells]

Lamp limiter is new to me. I guessed that the clue was in the name and a quick google helped. However the page I discovered spoke of a 100w lamp as being a good choice for devices with a draw of about 1-200mA. The spec's suggest that this CRO draws up to 2A ('depending on plugins'). Simple extrapolation (is that correct) suggests that the suitable lamp might be about 1KW.

Have I completely missed the point?

[G6Tanuki]

Originally Posted by paulsherwin

It's a good idea to power up using a lamp limiter as this may limit catastrophic damage and give feedback if the equipment is overcurrent.

Be careful though if the equipment uses a SMPS: some can go into strange and potentially traumatic oscillation-modes or spurious lockups if supplied from a power-source with significant impedance.

[HamishBoxer]

As said SMPS go into mega failure mode if ran up slowly in a lot of cases.Sony comes to mind for one.

Check the caps for good esr and give them the full 240AC.

Valve gear normally benefits from a slow awakening .


All though as I have said before Gerry Wells used to give all stuff the full 240V AC and no wake up!More a sudden shock.

[Hartley118]

I've never seen any benefit in 'waking up' valve heaters gently - they warm up slowly anyway. HT smoothing electrolytics, however, may well benefit from being given a chance to reform before receiving full voltage.

If I'm feeling cautious with a piece of valve equipment, I'll connect my variable HT power supply to the HT line and bring up the volts gradually whilst monitoring the current drawn, possibly over a period of hours, whilst the electrolytics reform.

I would say though that I think the main benefit of the approach is the avoidance of spectacular white smoke and a potentially nasty sub-chassis chemical mess. I suspect that an electrolytic that chooses to release its smoke would have been unlikely to reform anyway whereas a good one can survive the brutality of a sudden wake-up and subsequently reform over the next few hours.

Martin

[paulsherwin]

Originally Posted by RichardHowells

Lamp limiter is new to me. I guessed that the clue was in the name and a quick google helped. However the page I discovered spoke of a 100w lamp as being a good choice for devices with a draw of about 1-200mA. The spec's suggest that this CRO draws up to 2A ('depending on plugins'). Simple extrapolation (is that correct) suggests that the suitable lamp might be about 1KW.

Have I completely missed the point?

2A is an awful lot of current for a piece of test equipment to draw, and it's difficult to imagine where all that power is going. I suspect there is some confusion about this. The 175A is a big old beast but I would expect a lot less than 1A to be drawn in normal operation, as with other large valve scopes. You may need to use a 150W bulb, or two 100W bulbs in parallel.

Equipment isn't expected to operate normally with a limiter in place and there will be a significant voltage drop which will cause the bulb to glow. With experience you will be able to distinguish between normal and abnormal levels of brightness. If nothing has gone bang after a few minutes of operation then you can switch the bulb out.

[G6Tanuki]

2A may be the current draw when set to 110/120V....

[RichardHowells]

I may well be confused.

The book says Power: 425 watts max (depends on plugins used). I rounded that up to 2A which is about 500 watts on our mains.

I don't have much expertise here. I assumed that the number documented would be a max continuous draw. It doesn't say anything to hint that it might be a startup surge. There are a lot of valves in this thing - about 50 plus the CRT. I assumed that powering all those heaters would be a significant fraction of it.

[paulsherwin]

50? Good grief, what are they all doing?

In that case I'm afraid you're on your own. I've never dealt with a valve scope with even half that number of valves. You can use a lamp limiter with high current devices, but you need lots of bulbs in parallel so it's somewhat impractical.

[RichardHowells]

I assume it was built in the 1960's, probably designed in the late fifties. The book is copyrighted 1963. Even so it's a combo valve/transistor design.

On a closer look it's not as many as 50. I'd scanned down the parts list and missed that there were some gaps in the numbering.

We have:

10 - vertical input and output assys
7 - sweep genererator assy
9 - horizontal amp
8 - HV power supply

Total closer to 35.

To bulk it up a bit I'm only looking at the book for the main chassis. So that excludes any plugins. Most of the valves are double triodes.

[factory]

The HP 175A is featured in the April 1962 Journal here;
http://www.hpl.hp.com/hpjournal/pdfs...Fs/1962-04.pdf

There are actually only 30 valves in the mainframe (ignore the neon tubes listed with them in the parts list) and up to 19 more with the early plug-ins, later ones contains fewer valves & more transistors. The power supply section in the mainframe is fully transistorised apart from the neon regulator. There are a few extra fuses inside for the various DC outputs from the power supply.

I usually only use a lamp limiter when fault finding an over-current fault.

So far the only item of HP test equipment in my collection that has smoked on power up was a 606B signal generator and it was only a small amount from an inductor being overloaded, I only powered it briefly so the inductor survived, the fault in this case was an air filed valve and two open circuit resistors.

I have also had another smaller item that arrived with a blown mains fuse, one of the rectifier diodes had been replaced with one that had a too low voltage rating in the past and ended up shorted. It was a HP numbered part too, so took a while to track down.

David

[McMurdo]

My old Tek valve scope used to heat the room so much that vest & pants were the order of the day for a difficult fault!

[hannahs radios]

Yes with valve gear and even high power transistor gear run it up on a variac start at about 80 volts then over a couple of hours bring it up to 240 volts this will ensure that components get used to having volts on them