Protek P-3502C 'Scope - Broken?

[jjasilli]

Just rec'd delivery of this scope bought used on eBay.  THis is my fistr scope experience. Issues:  

*  self test -- instructrions say trace LINES should appear.  Only a moving dot appears for ea channel
*  self test -- cannot get a self-generated square wave, only a vertical bar appears.

Probe:  I have only a simple BNC probe with alligator clips.  Cannot get a sine wave from my signal generator.

So, is it broken?

[tubesornothing]

It sounds like your timebase is really slow, like 1sec/division.

[tubesornothing]

You might be able to get a sine wave from a tuner or metronome that has a 440Hz signal on it.

[sluckey]

*  self test -- instructrions say trace LINES should appear.  Only a moving dot appears for ea channel

That's the way a scope works. The horizontal sweep moves a single dot from left to right. Increase the horizontal sweep speed (AKA timebase time/division) until the dot moves so fast that the phosphor persistance of the CRT face and your eyes can think it's a solid line.

*  self test -- cannot get a self-generated square wave, only a vertical bar appears.

That's probably a trigger problem. Set the vertical attenuator to give about 3 or 4 divisions of vertical deflection. Then set the timebase to internal trigger on the vertical channel you're applying the signal to. Adjust the trigger level for a stable display. If the cal out is a 1KHz squarewave, set the timebase speed to 1ms/div or 500µs/div.

EDIT... If you can provide a link to an illustrated operator's manual, I can be specific.

[jjasilli]

OK, I clearly have no idea what I'm doing; and the instructions are not helpful to me.  Buy YOU guys are!  

1.  Sine Wave OK!  Based on Tubesornothing's posts and playing with the time/div knob, I now get a clean sine wave from my signal generator.  And can get it to stand still, by setting the coupling SW to TV.  

2.  Square wave NOT a Wave!  With the signal generator putting-out a square wave, the 'scope is not displaying a wave per se, but rather 2 sets of horizontal bars; offset like a squarewave, but with no connecting vertical lines.  Is that normal?

3.  Internal Test?  I cannot get that to happen.

4.  To fashion a makeshift a 10:1 probe, can I just put a 10M resistor in series with the +alligator clip for now?

The manual is available here online, under "P3502C": http://www.protektest.com/ProdDnLd.asp?type=MANUAL

[jjasilli]

Update:  OK, I had the chance to try again.  I'm getting a self-generated test wave now.  Not exactly square; more of a distorted sine wave; very clear, a bit shaky. 

Guess I'm making progress.  Thanks again to Tubesornothing and Sluckey. 

[sluckey]

2.  Square wave NOT a Wave!  With the signal generator putting-out a square wave, the 'scope is not displaying a wave per se, but rather 2 sets of horizontal bars; offset like a squarewave, but with no connecting vertical lines.  Is that normal?

It's still out of sync, ie, trigger problem. Never use the tv sync/trigger unless you're looking at a tv video signal. Set your trigger source to internal and sync on the signal you have connected to the vertical input.

[tubesornothing]

2.  Square wave NOT a Wave!  With the signal generator putting-out a square wave, the 'scope is not displaying a wave per se, but rather 2 sets of horizontal bars; offset like a squarewave, but with no connecting vertical lines.  Is that normal?

Sounds like a good square to me.  If you crank up the intensity you'll see very faint vertical bars.

[PRR]

> squarewave, but with no connecting vertical lines

That would be correct for a Perfect squarewave with infinitely fast rise/fall times on an equally  perfect 'scope with infinitely tiny dot.

The verticals should be faint, but in most practical cases you can find them.

At this point, stop looking for flaws in the seller and start giving the buyer LOTS of experience and practice.

[tubesornothing]

I think first order of business is get your favorite amp, crank it to your favorite distortion.  Put a nice 200mV 500Hz sine wave on the input. Clip the scope to various sections (after V1, after V2, after PI, at speaker) and see how the signal changes.  Play with the tone stack, see how things change. Play with the frequency of the signal gen and see how things change.

Post lots of pictures.  I love pictures.

[jjasilli]

OK. Thanks guys!  Looks like the scope works.  Now to learn how to use it!

Special thanks to tubesornothing for getting me to make the plunge; and sluckey and PRR for the past threads and references.   :3some:

But without this help, I would never have got this darn thing up and running.  Did I hear anyone say "learning curve"?

(Meanwhile, one Radio Shack DVM died; both my Heathkit IM-18 VTVM's are down and it's not tubes; a "new" Fluke 8000A DMM just arrived malfunctioning; a friend's amp is sitting exposed on my bench; and I just came back from a jam and my timing's gone all to hell!  At least my metronome still works.  Happy New Year!)

[tubesornothing]

and I just came back from a jam and my timing's gone all to hell! 

now that really hurts...

[HotBluePlates]

...both my Heathkit IM-18 VTVM's are down and it's not tubes

What's wrong with them?

I ask because I'd bought an RCA WV-77 VoltOhmyst a while back (first VTVM) that was dead on arrival. I checked it out, and with power applied, the tubes had filaments light up, but there was no high voltage. While the selenium half-wave rectifier could have been bad, or the single small filter cap (both replaced on general principle), the real fault was no high voltage from the power transformer. I figured I had nothing to lose, so I peeled back the insulating paper on the PT and found a small break in the high voltage winding where it was soldered to the lead running out of the transformer (broke right at the solder joint). Some peeled paper, a resoldered hair-like wire, and some insulating tape restored it to operation.

I've also got an embarassing number of old Hewlett-Packard VTVMs and oscillators (don't ask how many... it's embarrassing). The long term goal is to clean them up and calibrate them (I've also got the HP calibrating gear, minus 1 specialized oscillator that I've got my eye on now), and probably resell at a fair price. Some of this stuff has its limits (do you really need a VTVM that does a.c. only?) but is great for what it is.

[jjasilli]

What's wrong with them? I don't know yet.  The Heathkits (most circuits are the same in all these old units, according to the Tone Lounge) run on a 6AL5 and 12AU7. Always suspectuing tubes first I bought a sleeve of 6AL5's and tested the VTVM's when the tubes arrived.  But it looks like it's not a tube problem.

Next suspects are the filter cap 16uF 150V, and the diode rectifier.   My guess is to just replace those; then take voltage readings per the schematic.

I assume the old ceramic caps will be OK; but the old wax/paper tubular cap .047 1600V shoould be assumed to be leaky.  See:  http://www.mouser.com/ProductDetail/Cornell-Dubilier/940C20S47K-F/?qs=oz45iF9aKufHwZtwXmdzGQ%3d%3d. 

Looks like it's about time for a parts order.  But any parts that Doug stocks, I'll get from el34 World.

[HotBluePlates]

The Heathkits (most circuits are the same in all these old units, according to the Tone Lounge) run on a 6AL5 and 12AU7.

And the Tone Lounge is right. The RCA WV-77 circuit is a 6AL5 and 12AU7. The 12AU7 is set up in a differential pair/bridge circuit.

You didn't identify what they were/weren't doing. And I'm assuming you already have a schematic. If not, google BAMA (boatanchor manual archive). It will very likely be there.

[jjasilli]

One of my Heathkit IM-18's came with the entire manual:  build instructions, calibration, schematic, etc., etc.  Last I looked a recent new owner of the Heathkit name has purged the internet of free Heathkit schematics and is now selling them exclusively.  The prices aren't bad; but I have a copy of my manual and, not to run afoul of the copyright laws, if you or anyone else wold like to borrow it, I'll  be happy to lend it to you if you send me a snail-mail address.

Anyway, with no probe plugged-in the pointer in each of the meters jumps to the end of the scale and stays there when the selector SW is set to read +DC or Ohms. 

The last IM-18 that died was, if I remember, set to read AC on the 1500V scale - testing pre-rectified AC on the PT secondary of a vintage amp I was rebuilding.  While troubleshooting the amp, I watched the meter slowly fade to zero, along with my heart, until I realized that it was the meter and not the vintage PT that failed. 

[PRR]

> do you really need a VTVM that does a.c. only?

Yes.

(Besides the fact that the Boonton sensitive AC voltmeter came before the all-round VTVM we are familiar with.)

The all-purpose VTVM has a simple input rectifier, usually a 1.5V lowest range, with the low end fairly non-linear. While they are usually marked Sine RMS, if you deal with non-sines, it can be hard to guess what the actual response is. Frequency response is loosely specified and different on the high ranges.

The Boonton (and H-P ACVMs) are high-gain feedback amplifiers with the rectifier and meter movement inside the loop. First advantage is that it reads accurately to very low level, 0.01V full-scale on my Boonton. They are classically averaging meters; if otherwise, that's clearly spelled out in the manual. The frequency response is clearly specified. The H-P add 400Hz hi-pass and 30KHz lo-pass switches to discount hum/buzz and supersonics.

If you deal with sub-Volt signals and/or want precision, you need a good ACVM.

Anyway you need more than one meter for many tests. Power versus frequency, you want a good AC meter on the output, and it may be nice to have a good DC meter inside the amp to see if it is sagging or cross-conducting (transistor amps at >20KHz may flow more current rail to rail than they do to the load, and cook their guts). Once you need more than one meter, some of them may as well be specialized beyond what you usually get in an all-purpose meter. For audio, the ACVM is a logical 3rd or 4th meter.

And most DVMs really suck on AC above 400Hz.
> Always suspectuing tubes first I bought a sleeve of 6AL5's

In the general-purpose VTVM, the 6AL6 is -only- for ACV. You can measure resistors and batteries without it. (And even if ACV is the only dead mode, replace the ACV input cap first, and be generous.)

> pointer ...jumps to the end of the scale ....when the selector SW is set to read +DC or Ohms.

And where does it lay for -DC?

Have you turned the Zero Adjust? It may be a 15-turn pot, you may have to turn and turn.

Next I would swap the 12AU7. You can use AX or AT for diagnosis; they won't calibrate right but if an AT/AX appears to work OK then your 12AU7 was bad. However two dead 12AU7s in two VTVMs in the same shop the same week defies probability.

Open it, put DVM black to case, DVM set to DCV, poke both ends of the main filter cap (the only mondo electrolytic in there). One is dozens of volts positive, the other is dozens of volts negative (these tubes work halfway between the extremes). And the sum of the DC voltages should be a bit higher than the PT secondary ACV. If it is same or lower, replace the filter cap. Anything 10uFd-100uFd 150V-500V is fine.

The 1600V cap is only mandatory for ACV and only if you poke high-volt tube circuits. If you live in an under-250V world, 400V cap is ample. However in THIS forum, you would want to find something HIGH voltage, certainly 600V, more if you recklessly poke OTs.

[HotBluePlates]

Last I looked a recent new owner of the Heathkit name has purged the internet of free Heathkit schematics and is now selling them exclusively.

People with real power have tried to purge the internet of more nefarious things, and unsuccessfully.

Anyway, I coulda swore BAMA had a Heath page, and maybe they did prior to voluntarily taking it down. But a quick search yielded a number of other places to download these manuals for free. You won't get evey model, and you might not get the best quality image, but they're out there.

[DummyLoad]

I've also got an embarassing number of old Hewlett-Packard VTVMs and oscillators

would you happen to have a HP 410C you'd be willing to part with?   

[HotBluePlates]

would you happen to have a HP 410C you'd be willing to part with?

No, probably because it's a Fet-voltmeter (FVM?). But I do have a pair of 410B's...

[DummyLoad]

would you happen to have a HP 410C you'd be willing to part with?

No, probably because it's a Fet-voltmeter (FVM?). But I do have a pair of 410B's...

410b would be nice too... care to part with one?

[HotBluePlates]

...it's a Fet-voltmeter (FVM?).

I did a little checking, and I think the 410C has at least 1 tube inside. Oh well...

410b would be nice too... care to part with one?

I don't think that would be a problem. When I get back stateside, I'll need to check them over to make sure eveything is working like it should. I have most of gear needed to calibrate this meter, and I recall some various issues to address like crumbling cords that need to be replaced.

[jjasilli]

Thanks PRR: got 1 VTVM up, running & calibrated.  I think it was a bad 12AU7 tube, and intermittent continuity in the probe.  Now fixed.  Will work on the other VTVM in the coming week. 

Hotblue & others:  A free, decent Heathkit site is backl!:  http://www.vintage-radio.info/heathkit/

[HotBluePlates]

Congrats!

Barring extreme abuse, the only thing to do to get one of these types going is replace the selenium rectifier with a 1N4007, replace the 1 filter cap, check for presence of all voltages. Occasionally, there are unusal faults like a broken resistor or separated lead, maybe a couple of leaky caps. But on the whole, there's not a lot that could fail and not be very obviously burned.

[DummyLoad]

I don't think that would be a problem. When I get back stateside...

HBP, thank you. looking forward to getting one without the fleabay worries. anything in decent shape seems to have dried up at the ham fests...

[jjasilli]

fleabay worries:  Yes, I just got delivery of a Fluke 8000A described to be in good working condition.  But it reads 90 some odd ohms at turn on; cannot take accuratwe readings; and push-bottons stick to boot!  Anyway, it was a reputable seller who promptly made a full refund; and doesn't want the Item back.

Anyone here who wants a project can have it for shipping costs, figure $10 to CONUS.

[HotBluePlates]

Anyone here who wants a project can have it for shipping costs...

So as I said, embarrassing number of tube test equipment items. The #1 reason for getting them, in spite of advanced age, possible problems, heat, weight and somewhat poorer specs? If they're not working, I can get in and fix the problem, and without some very specialized tools needed to remove IC's and such.

In general, the circuits are simpler as well, although documentation can sometimes be time-consuming to find.

Anyway, yeah... Once I get back, I'll have to post some pictures and maybe offer up a few items for sale. There's probably 8 old tube-based Tek scopes, and bunches of HP meters, oscillators, distortion analyzers, power supplies, etc.

[tubesornothing]

Anyone here who wants a project can have it for shipping costs, figure $10 to CONUS.

PM sent...

[jjasilli]

Progress:  12AU7 tube confirmed, though it tests OK in my Sencore Mighty Mite, for what that's worth. 

VTVM #2:  bad filter cap confirmed by measuring way-off + and (-) voltages on either end of the cap.

Per Hotblue: I agree to the KISS approach and will replace only the diode & filter cap in ea VTVM.

Thanks again for all the help & support!

[jjasilli]

More Heathkit:  http://www.nostalgickitscentral.com/heath/products/test.html

[tubesornothing]

OK, I dug up my BK VTVM, all ready to put it on the selling block. Then after reading this, I am wondering is there a reason I should keep it?

I already have a Simpson 260 I purchased for my analog needs, which is just great - so can the VTVM really help me anywhere with amp building?

jj - what are you going to use yours for?

[HotBluePlates]

The Simpson meters are usually 20k ohms/volt, unless they've updated and raised that (which they might have with an input Fet). That means a 1M input impedance, if your on the 50v range.

If you needed to measure grid voltages, that resistance can be significant when you have a 200k-1M grid reference resistor. Most consumer VTVMs are a 10M input impedance, plus 1M in the probe, for 11M overall. That means little meter loading up to a 1M impedance that you're measuring, and little deviation in measured voltage vs actual voltage.

With most meters, you can't hope to probe a long-tail grid or bootstrapped cathode follower and get anything close to a meaningful measurement. My HP 412A meter might only be good for DC, but there's a 122M input impedance, and higher on some of the upper ranges if I remember right. Good technique means I probably still wouldn't measure grid to ground on those special circuits (their real input impedance is much, much higher than the circuit resistances would have you think), but for most anything else the deviation between reality and what the meter shows is small enough that I can't see a difference on the analog scale.

That's the real reason for any electronic meter compared to the old-school Volt-Ohm meters that did not have electronic buffering to raise the effective impedance.

But then again, sometimes they're better than expensive DMMs. I have a Fluke that would set you back $400 when it was new, and it has a lot of useful functions. AC, DC ohms, current, continuity, diodes, cap value, peak-value, RMS, freq counter, etc. But if you get a too-low AC frequency, the meter gets confused and won't give a stable reading. If you go too high, the meter indication falls off (too high is often ~100-200kHz). The frequency counter has a fairly limited range: audio and a bit more (maybe to the same 100-200kHz, don't have the manual handy).

So there's plenty of specialized stuff out there that won't do everything, but what it does, it does better than anything else out there.

I'm eager to test, calibrate and try out the clip-on current meter I've got. Yeah, we can slap 1-ohm resistors in stuff, but It's also cool to have a tool to use on many circuits now and without unsoldering a wire and putting the meter in series.

Anywhoo...

[PRR]

> I probably still wouldn't measure grid to ground on those special circuits

Measure voltage at bottom of cathode resistor; that won't sag. Then use a battery DVM to measure -across- each 1Meg grid resistor. If there's grid current, the meter will cause a 10% change in the 1Meg's voltage, but still give pretty-near the true grid voltage. And because ideally we have NO grid current, ANY voltage across the 1Meg is just wrong, and the exact amount wrong is not of much interest.

Another trick for a rainy day: set up a lowish-impedance voltage divider pot to give a spread of voltages near the expected grid voltage. Bridge the battery DVM across from grid to tap. Use another meter to measure voltage from tap to common. Adjust tap so DVM shows zero volts. Now divider and grid are at the same voltage, and your other meter tells that voltage.

Generally though, if these circuits show reasonable cathode and plate voltages, you may assume the grid is doing OK. And if voltages are wrong, you can try snipping the (leaky?) grid-cap and shorting the 1Meg. And of course change the tube.

> clip-on current meter

Will it measure DC? I don't think mine does.

[jjasilli]

jj - what are you going to use yours [VTVM] for?

I got it origianlly as an inexpensive way to get a meter that would read 600VDC volts and more.  I needed that rebuild some vintage PA amps, but all I had then were cheap Radio Shack $20 handhelds, which maxed out at 500V.

Price.  These old VTVM's are very cost effective, and easy to repair.  I don't know how to repair SS stuff and am turned off by the cramped circuit boards.  Redundancy. I think it's good to have muiltiple meters in case of a problem, or just doubt about the accuracy of piece of test equipment.  Multiple Readings. Sometimes I like to steadily meter one circuit, say plate voltage, while using another meter to test other parts of the circuit.

I guess for my purposes a DMM is just as good.  But there's always nostalgia.

[HotBluePlates]

> clip-on current meter

Will it measure DC? I don't think mine does.

It's an HP 428B, which is a clip-on dc milliammeter. There's another probe and current amplifier that they made for measuring ac and allowing wider range. The clip-on ac probe that I'm talking about generally interfaced with an o'scope.

[PRR]

> HP 428B

Cool! And it's as big as a H-P 200AB oscillator! It has tubes, yet is very space-age!

I never looked into what a Flux Gate is. Interesting. The conventional thought is that an inductor will only pick up a change in current. A DC current thumps at turn-on and then nothing to the inductor. We can "chop" the DC current and pick up its magnitude, but that's intrusive. However if we "chop the inductor", each time it comes off zero inductance we get a spike proportional to DC current. We can chop with a rotary slug but that's awkward. Around 1960 guys were messing with saturable inductors, which can "go away" with current level.

The box sells for $10-$700, but I'm not seeing one offered WITH probe. Without the probe it's pretty useless except as a small power supply with box and a meter.

[HotBluePlates]

The box sells for $10-$700, but I'm not seeing one offered WITH probe. Without the probe it's pretty useless except as a small power supply with box and a meter.

Exactly. Most of the fleabay ones are minus their probe. I don't know why everyone sells them like that, except maybe the folks that buy this stuff at auctions/estate sales, acquired it without a probe, and didn't know one was needed.

Again, I'll have to check when I get back home, but I think I have a pair of 428B's and a pair of probes. If I recall correctly, the price was right and I figured any unobtanium part that's broken in one would probably be serviceable in the other.

[sluckey]

Jjasilli, how goes the scope? Getting used to it yet? Find anything interesting to look at? Got probes and BNC cables/adapters?

[jjasilli]

I was hoping to get a 'scope for Christmas, but missed that.  My wife got this for me for my birthday which is this weekend.  When the scope arrived I checked that it works -- thanks to help from you guys -- and packed it up again (so I can be surprised!)

PROBES:  Not a probe per se:  All I have lying around is a 1X BNC cable with alligator clip leads (I don't even know where it came from!).  It makes a clear trace from my signal genrator.  If I put the +lead in series with a 10M resistor, I get a clear square wave from the output lug of the scope's internal generator. 

I also have some spare DMM probes.  I could solder a 10M resistor in seriers wth a red DMM probe; cover the joint with heat shrink tubing; and clip the +end of the BNC cable to that for a makeshift 10X probe.  Even so, I still need a probe for Channel B.

What do you suggest?

[sluckey]

I like the Tektronix P6105 type probes for bench work. 6' length is good. And I sometimes use a small flexible BNC cable with an adapter that has 8" test leads with grabber style clips. I keep a .1uF/600vdc cap handy to put in series with a probe whenever I want to look at a plate that has over 400vdc on it.

[tubesornothing]

Jjasilli, if you PM me with your address, I will send you a scope probe.  Not quite an HP, but will get the job done.

[jjasilli]

Well it's almost April.  I've read-up online on this stuff and am still pretty much at a loss.  I'm about to start with the online simulator program in the hopes of getting a clue.  I can also put a known waveform on the screen from my signal generator; then see if I can work backwards from the readings on the screen asa reality check.

I'm in the middle of the rebuild of a Traynor Bassmate.  No rush.  I hope to learn to use the scope to measure:  B+ ripple; fixed bias circuit ripple; signal waveforms of various preamp mods; and at a dummy load.   

Heck if it gets bad enough I might even play my guitar!

[tubesornothing]

I think the easiest to do is put a signal in the input and measure it after each coupling cap of each gain stage and watch how it changes.    You can use the scope to set the input signal to about 200mV.  As you measure each progressing stage adjust the amplitude sensitivity on the scope so that the signal can be viewed on the screen.  For the larger signals, you will probably have to slip the "10x" switch on the probe, as the signal might be too much for the scope itself.

[jjasilli]

This is my theory of operation, which I hope to try-out, but I'm just an amateur here and real life keeps happening!  The screen shows 3 important things (to my limited understanding):  voltage (vertical); period (horizontal); and shape.  The reciprocal of period = frequency.

So:  put a known wave into the scope:  i.e. frequency & voltage.  Adjust the scope controls to size the wave form on the screen: a) vertical -- bottom @ 0% to top at 100% on the graticule (can't wait till my next scrabble game); b) horizontal: equal increments across one full waveform.

Then, the voltage controls on the scope should match my volt meter connected to the signal generator.  The reciprocal of the period on the screen should match the frequency on my signal generator.  If not, I have some vodka!  If yes, the world :tongue8: will soon be mine.

[sluckey]

Then, the voltage controls on the scope should match my volt meter connected to the signal generator.

Not quite. Ie, if your sig gen is set for sinewave output, your meter will probably read the voltage as an RMS value. The scope displays peak to peak values. V_rms = 2.818V_pp. So if your meter reads 1V_rms the scope will display a 2.8V_pp sine waveform.

[jjasilli]

OK, thanks, Sluckey.  That's very helpful.  At least it seems I am generally on the right track, but that "fudge factor" would have really thrown me.  Who knows if I would have ever recognized 2X √2 !?!

BTW my scope (BNC Inputs and the manual) warn 300V_pk Max.  I want to measure AC ripple where B+ voltage is over 400VDC. If I set the scope to AC (block DC) is this OK? 

[sluckey]

BTW my scope (BNC Inputs and the manual) warn 300Vpk Max.  I want to measure AC ripple where B+ voltage is over 400VDC. If I set the scope to AC (block DC) is this OK?  

Don't do it. This is an area where you really need to know your scope. Same with measuring small AC voltages on a tube plate with high DC voltage. In cases where you want to measure an AC signal that rides on a DC level that may exceed the max input level of your scope, just use a .1µF dc blocking cap in series with your probe. Your absolute voltage measurment may be skewed but you can still get a relative reading.

And remember this... 300VAC_pk is equal to 212VAC_rms. So, be careful.

[jjasilli]

Glad I asked. Thanks again!  (My preamp voltages in this amp are over 212VDC which I foolishly thought were safe.)

[sluckey]

Your scope will safely handle up to 300VDC (same as the max peak voltage). So, just don't probe any place in the amp that has more than 300VDC present. Use the series cap if in doubt or measure with your DVM to verify it's safe to connect your scope. BTW, the old tube type scopes (like TEK 545) had a much higher max input level, but back then a tech might routinely be using a scope in a tube circuit. Today's scopes are made for solid state work with much lower voltage levels.

Measuring AC voltages is where you may get into trouble. For example, maybe you want to measure the voltage on a 275-0-275vac PT secondary. Sounds safe enough, but that 275VAC_rms is really 389VAC_pk. (VAC_pk = 1.414VAC_rms)You would be putting your scope at risk.

My Tektronix 465B is good for 400V_pk. I have used it to look at the peak unfiltered dc pulses out of a 5Y3 rectifier. The peak voltage was 420. I was nervous about doing this because I exceeded the mfgr's max warning. I had calculated the peak voltage and knew it would slightly exceed my scopes rating, but I was interested in measuring the difference in pulse amplitudes for each half of the 5Y3 rectified waveform. I was using a 10x probe and vertical was set to 50V/div. My scope only has 8 vertical divisions so it can only measure up to 400v. Sure enough, the waveform was slightly bigger than 8 divisions so I had to use the vertical position to move the display such that I could see the positive peaks.

I was lucky that my scope was not damaged. I'd never recommend someone do this with their scope. But actually looking at ripple on the B+ rail is a useful check. Just use the series cap trick. However, your DVM is very useful for measuring ac ripple on the B+ rail too. And you shouldn't have to worry about exceeding a max input rating unless you're working on some super power amp.

[tubesornothing]

So to measure B+ ripple on the B+ rail, just set the meter to ACV, one lead on the B+, and the other on ground?  I presume meters have a blocking cap built-in somewhere...