Variac old scope?

[wengr]

Hello. I have a Tek 2235 inbound. Main use will be for tube guitar amps. I have to assume that this scope has not been in regular use in many years. Should I bring it up slowly on a variac as I would a tube amp?
Also, Same question for an old Viz audio generator?

[Kwakerman]

If I remember correctly the 2235 has a RIFA capacitor (possibly more than one) located somewhere under the IEC inlet socket, that should be replaced before you power it up (just search for RIFA on here to see what they look like and the damage they can cause).  Its a bit of a pain to get to as its inside the PSU cage but doable if you take your time. 

Bring it up on a variac over say 10 seconds, some switch mode PSUs don't being kept at low voltages outside their operating voltage range.  A dim-bulb' tester may be better than a variac as  sometimes PSUs need a kick to get then running rather than bringing them up slowly. 

Just remember that most variacs dont isolate you from the mains, you should have an isolation transformer too, especially if working on valve amps.

[wengr]

If I remember correctly the 2235 has a RIFA capacitor (possibly more than one) located somewhere under the IEC inlet socket, that should be replaced before you power it up (just search for RIFA on here to see what they look like and the damage they can cause).  Its a bit of a pain to get to as its inside the PSU cage but doable if you take your time. 

Bring it up on a variac over say 10 seconds, some switch mode PSUs don't being kept at low voltages outside their operating voltage range.  A dim-bulb' tester may be better than a variac as  sometimes PSUs need a kick to get then running rather than bringing them up slowly. 

Just remember that most variacs dont isolate you from the mains, you should have an isolation transformer too, especially if working on valve amps.

Thank you for this reply. I do have the dim bulb tester, and I plan to get a hospital grade transformer and mod it to be fully isolated.

[AVGresponding]

No!  A device with a switching power supply should never be brought up on a variac.  The power supply is not designed for that.

Indeed. If it has an SMPS, it will suck huge amounts of current at low voltages, in trying to compensate.

[wengr]

Ok thanks for the replies. So no variac for the scope. What about old audio generators with a tuning cap?

[Filippo52]

A few years ago, I got hold of a TEK 2236A. When I was young, it was my dream, and I couldn't even imagine buying one.
It worked right out of the box, and it worked well. After three days, one of these capacitors exploded, making such a smell that it took days to get rid of it in my room.
I replaced it with a similar one I had at home; it won't last another 50 years, but I don't need it to last another 50 years; I won't be here.
I wanted to do the famous recapping operation, but I saw that there are schools of thought that disagree.
Honestly, if I had found a ready-made kit, either from Tektronix or some reputable lab somewhere in the world, I would have done it with patience and pleasure, but searching for all the spare parts one by one from different online sellers, with the risk of buying some unsuitable components, doesn't seem like a good idea.
So far, it's been working for two years. I turn it on periodically for 10 minutes at least once a month and look at it with satisfaction.

Good advice is to use the variac with a great care. But here, as with the "recap," there are opposing schools of thought, and some, not having an expensive variac, resort to the trick of using an old incandescent light bulb with enough wattage to operate the device but capable of limiting the damage caused by any failure of an old component.
After blowing the input fuse a couple of times, if you can find one, I'd use an incandescent light bulb with a wattage of, say, 2-3 times the oscilloscope's power draw. If the light bulb doesn't light up and the oscilloscope does, you're in luck.
If the light bulb lights up, turn everything off immediately and perhaps you'll have time to see the component that was starting to burn out without any disastrous damage.

[wengr]

What about old audio generators with a tuning cap?

I don't think there is any Viz equipment that is old enough that it would have capacitors that need reforming.  What model is it?

Viz WA-504B/44D

[BradC]

Good advice is to use the variac with a great care. But here, as with the "recap," there are opposing schools of thought, and some, not having an expensive variac, resort to the trick of using an old incandescent light bulb with enough wattage to operate the device but capable of limiting the damage caused by any failure of an old component.

No. A VARIAC and dim bulb tester have completely different purposes. It's not a case of "resort to the trick", it's a case of knowing the difference and applying the correct technique.

[TERRA Operative]

I have a video on this model of scope, it'll tell you basically all you need to know.
If you have any questions, I'm happy to help.

https://youtu.be/lrXCVg6T-ek?si=767YOECFu9Jl97dx


The main points are:
- Replace Rifa capacitors
- Replace electrolytic capacitors in the PSU section
- Replace the R888 - R892, R894 with new Metal film Vishay HVR series 1/2W 510Kohm.

Also very important:
- The PSU service modification (to make sure the PSU stays operating rock solid)
- The CRT filament voltage service modification to avoid overvolting the CRT filament.

These two service modifications may already be done, but you'll have to open the scope and take a look to see.

A fan is optional, but I put one in to keep the PSU happy.


It's a really nice scope. I really liked mine and it was hard for me to put it up for sale. It is a nice simple scope to use that works well once the above housekeeping is taken care of.

[Filippo52]

BradC, you're right.
It's all about knowing the subject.
I won't boast about my credentials because that's always stupid on social media.
However, anyone interested can point out that there are highly qualified YouTubers who use both VARIACs and incandescent bulbs specifically to power devices with switching power supplies, and as I said, with due care, for example, immediately after a repair or when several fuses have already blown.
I personally prefer the bulb because, if calculated with a minimum margin, it allows for the inrush current needed for switching devices to reach full capacity. However, if the current tends to exceed the assumed threshold, it switches on immediately, preventing serious damage.

[AVGresponding]

It's nothing to do with inrush current, it's operating current. The SMPS in these tries to supply constant power, so if you run it at a lower voltage, it will draw more current to compensate.

[Filippo52]

AVG Responding
But I don't think Wengr intends to use it permanently with a Variac. I understand that since it hasn't been used for several years, they're worried about a destructive initial failure and therefore want to protect this old device in some way when it first starts up.
I think an incandescent light bulb is a good method; some people use Variacs, but I've explained why I prefer incandescent bulbs.
What protection would you use to avoid a serious initial failure that could destroy some PCB tracks?

Filippo

[WattsThat]

…
What protection would you use to avoid a serious initial failure that could destroy some PCB tracks?

The exact manufacturers specified fuse. Anything beyond that that would required study of the device and an appropriate method of protection to address your specific concerns. Applying broad brush sweeping “protections” will no doubt result in failures not considered. If it’s a dead common replaceable item, let it rip. Irreplaceable? Vintage one off? Stop and think. Choose wisely.

Ps: Replace the Rifa caps without question but you’ll find most of the electrolytic caps in the PS are not commonly found due to their physical configurations. Don’t go ripping them out of them without confirmation they actually need to be replaced, the vast majority of them will be fine unless the scope has been baked by overheating with the fan slowed by dirt on the blades and or crud in the bearings. The commonly accepted “Recapping” of older devices, especially higher end test equipment can introduce more problems than it solves

[Filippo52]

Whatts, thanks for the reply, but we'd already assumed that. If the fuse doesn't blow when you turn it on, then we can continue.
In my first my message, I specified that the following strategies apply only...
"After blowing the input fuse a couple of times."
Of course, it would be helpful to do as you say:
"Anything beyond that would require a thorough study of the device."
But this isn't always possible, and it's not always worth doing, even if it's possible.
I've therefore outlined the techniques I've seen most used by qualified YouTubers and also expressed my preference for the incandescent bulb, explaining the reasons.

Filippo

[KungFuJosh]

If I remember correctly the 2235 has a RIFA capacitor (possibly more than one) located somewhere under the IEC inlet socket, that should be replaced before you power it up (just search for RIFA on here to see what they look like and the damage they can cause).  Its a bit of a pain to get to as its inside the PSU cage but doable if you take your time. 

Bring it up on a variac over say 10 seconds, some switch mode PSUs don't being kept at low voltages outside their operating voltage range.  A dim-bulb' tester may be better than a variac as  sometimes PSUs need a kick to get then running rather than bringing them up slowly. 

Just remember that most variacs dont isolate you from the mains, you should have an isolation transformer too, especially if working on valve amps.

Thank you for this reply. I do have the dim bulb tester, and I plan to get a hospital grade transformer and mod it to be fully isolated.

No! Absolutely not, this is terrible advice. Do not isolate from mains! Everything should be properly grounded. If you need to probe without standard grounding a probe, use a differential probe.

[AVGresponding]

…
What protection would you use to avoid a serious initial failure that could destroy some PCB tracks?

The exact manufacturers specified fuse. Anything beyond that that would required study of the device and an appropriate method of protection to address your specific concerns. Applying broad brush sweeping “protections” will no doubt result in failures not considered. If it’s a dead common replaceable item, let it rip. Irreplaceable? Vintage one off? Stop and think. Choose wisely.

Ps: Replace the Rifa caps without question but you’ll find most of the electrolytic caps in the PS are not commonly found due to their physical configurations. Don’t go ripping them out of them without confirmation they actually need to be replaced, the vast majority of them will be fine unless the scope has been baked by overheating with the fan slowed by dirt on the blades and or crud in the bearings. The commonly accepted “Recapping” of older devices, especially higher end test equipment can introduce more problems than it solves

This ^


If you use an incandescent lamp as a current limiter on an SMPS, you will most likely cause a glitch state that looks like a fault, but isn't (it'll pulse on and off). This method is only useful on linear power supplies.

If you are really concerned about damage, disconnect the PSU from the rest of the scope, and connect some nominal dummy loads across the power rails.



And as KungFuJosh says, DO NOT use an isolation transformer. This defeats the protection you have from your GFCI/RCD

[Kwakerman]

As with most things 'it depends'.  Isolation transformers come in two types, 'with ground' and 'without' depending on your particular needs.  The main point of isolation transformers is to provide an isolated 115v (or 240v) supply (at a 1:1 ratio) that is not directly connected to your domestic supply.  Some old valve TVs and some amps can have the chassis at 1/2 mains potential and you cant ground them.

[Kwakerman]

If you use an isolation transformer then you wont get a shock unless you touch the line and neutral within the equipment you are working on as there is no return path to the domestic ground.  You can touch an 'isolated' live and any 'domestic' grounded item (such as a scope) and you wont get a shock as there is no  return path for the current to flow.

[AVGresponding]

As with most things 'it depends'.  Isolation transformers come in two types, 'with ground' and 'without' depending on your particular needs.  The main point of isolation transformers is to provide an isolated 115v (or 240v) supply (at a 1:1 ratio) that is not directly connected to your domestic supply.  Some old valve TVs and some amps can have the chassis at 1/2 mains potential and you cant ground them.

I can promise you that a GFCI/RCD will NOT trip, with a galvanically isolated supply. You could short either end of the secondary to earth, and it won't trip, it'll rely on whatever overcurrent device it has fitted.

[AVGresponding]

If you use an isolation transformer then you wont get a shock unless you touch the line and neutral within the equipment you are working on as there is no return path to the domestic ground.  You can touch an 'isolated' live and any 'domestic' grounded item (such as a scope) and you wont get a shock as there is no  return path for the current to flow.

Touch two different potentials within the scope simultaneously, and you'll definitely be needing to comb your hair back flat (assuming you have any hair, and assuming you survive the shock, especially if it's from hand to hand, across the heart).

[Kwakerman]

I know it won't trip as there is no direct connection to ground.  Can you explain how you can get a shock from an isolated supply when there is no return path?

[Kwakerman]

If you use an isolation transformer then you wont get a shock unless you touch the line and neutral within the equipment you are working on as there is no return path to the domestic ground.  You can touch an 'isolated' live and any 'domestic' grounded item (such as a scope) and you wont get a shock as there is no  return path for the current to flow.

Touch two different potentials within the scope simultaneously, and you'll definitely be needing to comb your hair back flat (assuming you have any hair, and assuming you survive the shock, especially if it's from hand to hand, across the heart).

I know, read post 17 again.

[Filippo52]

AVGresponding
You won't cause any glitches if the incandescent bulb's wattage is the values ​​I indicated.

Disconnect the PSU from the rest of the scope???

Do you have any idea how difficult this could be?

I don't mean to be rude, but I think I'll stop with this answers. I gave my advice; everyone is free to follow it or others.

Now I've seen another dispute arise over another controversial issue: the insulation of the measuring instrument from ground.
Given the current situation, I'm leaving and leaving you the link



where all the strategies for avoiding a blast are examined carefully and competently: from cutting the ground wire to using an isolation transformer to a differential probe; each with its own advantages, disadvantages, and dangers.

Filippo

[m k]

Troubleshooting tips on 2200 PSU
https://w140.com/tekwiki/images/b/b3/Troubleshooting_tips_on_2200_ps.pdf

[KungFuJosh]

I know it won't trip as there is no direct connection to ground.  Can you explain how you can get a shock from an isolated supply when there is no return path?

YOU can become the path to ground. Your scope can also become the path to ground, but that's less lethal.