Heathkit general purpose oscilloscope model 10-21

[punkykatt]

Hello all,  My drummer came to the gig last night and said, look what I found, can you use this. Does anyone have any experience with the Heathkit model 10-21 general purpose oscilloscope. Are these antiques worth fixing up to work on tube amps? I did a quick search on the net and couldnt find much info(manual/schematic) on this model.  I know very little about oscilloscopes, but would like to learn and add one to my shop tools.  I do love making useless junk useful. Thanks for any help or advise you can give me.  Punky

[tubesornothing]

Well sure why not!

If you like jumping right in, plug it in and see what problems - does it have a trace line on the screen?  Is it a dot?  Is there nothing?  report back.

If you like to clean first, treat it like a tube amp: cap job, test the tubes, check solder joints, switches, etc.

[punkykatt]

Thanks Tubesornothing,  So your saying it will be a good piece of equipment to have if fixed up and operating properly?   Its going to need a cap job for sure. It looks like its been sitting for years. I may just do a cleaning and try firing it up with a currant limiter and vari vac.

This unit came without any probes.  Will this be a problem?

[PRR]

It's a fine 'scope for working in 1950s tube gear (which is all tube guitar amps).

Remember there is 300V a lot of places and 1,000V in a few places. Watch your fingers. FWIW: if everything is connected, it won't "hold a charge" like a TV CRT will after you unplug it.

Don't start re-capping. Plug it in, short the inputs, set INT to max, focus midway, and fool with V-POS and H-POS, see if you can get a line or a dot on screen. If you do, turn the INT down! so it glows but does not GLOW (burns the screen). Fiddle focus and astig for clarity.

If the caps are truly shot, you won't be able to get a fine focus. But there are a LOT of caps, also VR tubes, so it will probably show an OK trace even if it is out of spec.

The 0A2 and 0C2 must light-up, purple or orange. On my older Boonton, the VR tubes did not like to start. Waiting a while and then touching the glass usually got them going.

It does not have "normal" probe connectors, not the "normal" input impedance. You may be able to stick a banana plug in the center, and ground to a screw, be able to see signals. An ungrounded finger should give 60Hz wiggle. You will need an attenuator probe to work with tube voltages, use 1Meg + 120K for approximate 10:1 divider.

It appears to be uncalibrated. That's fine, you have DVMs for precision numbers, a 'scope is for seeing shapes. You can rough-calibrate by reading 1.5V and 9V batteries.

[punkykatt]

Thanks PRR for the heads up on the high voltages.   This unit does not have a 0A2 and a 0C2 voltage regulators?  What it has for tubes is 1-6X4 rectifier, 1- 1V2 high voltage flyback rectifyier, 1- 12AX7, and 4- 12AU7`s.
Please pardon my ignorance. Im not sure how to connect the inputs on this. It has both red and black jacks for horizontal and vertical inputs. (set the INT to max) Im not sure where that is on this? Can I use the test leads from my multi meter to do this test?

[tubesornothing]

I wonder - does that thing even have a trigger?  How do you get it to sync?

[PRR]

Oh, man, that's not the 'scope I thought it was. I think it is less fancy than my EICO 427.

No, I don't see the INTensity control either. Nor focus or astigmatism. It is posssible they just fixed intensity at maximum. Focus may be internal; don't fool with it now unless you are sure it has been tampered. And it is quite possible it lacks an astig control. All of which leads to a dot which is not very bright or sharp, but Heath didn't sell junk so it will be OK.

Hmmm. It has to be AC-coupled. That spoils a few neat DC tricks, but in audio amps you mostly work AC coupled anyway. The main "problem" is that you can't calibrate against a battery (the trace will jerk and quickly return to "zero"). But it basically isn't calibrated at all! Like a gitar amp, you turn the "amplitude" knob until the output is "good", and enjoy the sound/shape. If you want some rough idea of actual voltage, compare to a 6V AC winding, or turn-down 6VAC with a pot and get numbers from your DVM.

> Can I use the test leads from my multi meter

The old-style VOM leads should be perfect. New-style "safety" leads won't fit.

You can also get alligator clips to clamp the inside of the jack, though they'll want to pop off.

It also works like a good hi-fi/PA speaker connector. Unscrew, and there is a hole in the side to shove a bare wire in, screw down to clamp. So just peel some good stranded wire, put a gator clip on the end. One red one black.

There's two jacks and a switch on the back. You connect these to the plates of your transmitter to adjust the modulation (direct connection to CRT); otherwise you NEED this switch set to the internal amplifier or you won't get nothing. You can verify this by eye. The switch has 6 lugs. Center lugs go to CRT. One end-pair goes to a 12AU7. The other end-pair goes to the rear jacks.

> does that thing even have a trigger?  How do you get it to sync?

It's gotta have a sync. The EICO 427 had a gas sweep, a glorified neon oscillator. It ran all the time. By leaking signal to it, it kinda-sorta tended to sync, if you twiddle the "freq" knobs very carefully. I would assume, by the tube lineup (if it is correct), that this has a 12AU7 multivibrator instead of the gas oscillator. Might be more stable. And with a synced oscillator, you always see "something"; with fancy triggering, it is easy to get set wrong and the dot never moves.

[punkykatt]

Well guys, Im batting zero on finding a schematic/manual for this dinasore. I was over at the tone lizard site, he kinda made it sound like these old units are better off used as a conversation piece for "remember when" topics.  Me not having any experience with oscilloscopes and how to use them takes the wind out of my sails on this project for now.  Thanks for all your time trying to help.  Punky

[PRR]

Do you have ANY line-dot at all?

There's something in your picture but I can't tell if it may be a reflection.

Tell me how far you get and we'll figure this out.

Do the Centering knobs move the line/dot?

Put the Amplitude knobs to max and the bottom center knob all the way left (looks like external input). You should have a dot. Grab one end of a 100k resistor, and stick the other end at the inside of each red connector. The dot should expand to a line, either vertical or horizontal depending which connector. This shows it responds to the buzz on your body (same as touching a guitar cord tip to see if the amp is alive). The Amplitude knobs should reduce the size of the line. The signal is weaker while you touch the case (same as the gitar cord buzzes less if you are touching the shell).

Turn the bottom center knob to Line Sweep. I bet this is synced to the 60Hz wall outlet. You should have a horizontal line. Now when you poke the Vertical input, you should get a stationary sine-like pattern. This is body-buzz referenced against wall-hum. Since your body-buzz IS the wall-hum field in the room, it will sync.

Turn the lower center knob one more click, the slowest sweep range. Now your body-buzz won't sync, but turning Freq Vernier should pull it into stationary sync.

Adapt a guitar plug to the V input, ground to black. Play. Adjust V Amplitude for a good fit on screen. It won't sync except by coincidence, but you can see some waveshapes. Playing a single note and diddling the center knobs, you should get sorta-solid sync. Play an octave or a fifth from the first note, it should sorta-sync. The octave should show twice as many waves on screen; the fifth should be 1.5 times. Intervals like augmented fourth probably won't sync well... the 'scope can show you why some intervals are simple/easy and some are less euphonic.

> these old units are better off used as a conversation piece

Disagree. 99% of anything you want to scope in a guitar amp, pedal, etc, can be scoped with this guy, or possibly this plus a little booster (such as a boost-pedal).

> batting zero on finding a schematic

Not very important. It's so simple the key parts are "obvious". You found the 6X4, you know there is a cap, a resistor, another cap, and probably around 300V there to keep the 12AU7s fed. The input jack probably goes direct to the V Amplitude pot, and from there it is roughly a "guitar amp": one stage and a longtail PI. The CRT probably needs a push-pull drive just like a push-pull guitar amp output stage. Differences are lower impedances (for extended treble) and not tone controls.

What type-number is the CRT? How many pins on it?

[tubesornothing]

Me not having any experience with oscilloscopes and how to use them takes the wind out of my sails on this project for now. 

You are so.... close.... don't give up, PRR will take you through it.  And you will learn very useful stuff (believe me!).

Reach for your favorite Edison quote and give 'er another try.

[quayhog]

I have one of those too.  I have the assembly manual (if I can find it).  I'm on the road and won't have access to my files until after christmas.  Search for IO-21.   Not 10-21. 

[punkykatt]

WOW!!!!  you guys are the greatest!!!  Thanks for bring me back to life on this project.  I just printed all this fabulous info so I can bring it out back to the shop. That dot on the screen is a reflection. I have to do a general cleanup yet on it  before I attempt to fire this thing up. I have a pretty busy day scheduled for today, so it may be a day or so before I report back.  Thanks again  Punky

[punkykatt]

Dang it all.  the power tranny is shot.  the HT leads to the plates of the 6X4 rectifier are open. I removed the tranny , pulled the end bells and unwraped the paper hoping to find a broken wire.  The problem is internal.  I will see if I can find a replacement at a reasonable cost.  Punky

[tubesornothing]

As Edison says:  "there is opportunity in every disappointment"  (I doubt he said it, but it sounds like he would).

Once you put it all aside, lets start looking at ebay - perhaps we can find a deal...  maybe even a broken Heathkit that has a good tranny, but a busted CRT.

[punkykatt]

Thanks for the encouragement Tubesornothing.  Flee baaay looks to be the only sorce of ever finding a replacement.

[PRR]

> the HT leads to the plates of the 6X4 rectifier are open.

Is that all?

Can a second PT fit in there?

Any little Champ PT (or less) can make all the +300V DC that this scope needs. Keep the original for the +700V, and the heaters since they are already wired.

[punkykatt]

Is that all?     The other windings  all have continuity. I checked for shorts between primary and secondary windings, none found. Before these test, when I tried to fire the unit up using a currant limiter and vari vac, the 0.5A line fuse blew before the bulb even made a flicker.  Tomorrow I will tape the leads to the bench and apply voltage to the primarys(fused) and take some voltage readings if the fuse dosent blow. OH YES!!! the center tap wire on the tranny has NO continuity with any of the secondaries??? Is that a problem for the  1V2 rectifier or  the CRT????? 

[punkykatt]

Hmmmmmm!  I may have jumped the gun on condeming this tranny. I did some voltage tests, and found out that my continuity light  tests miss led me. I also did some resistance readings to try to figuire out why the test light test threw me. I drew up a schematic showing voltage readings and another schematic showing resistance readings. Please check them out to see if Im on the right track.   Im thinking the tranny is good. Yes? No?   My scanned file is too big to post here, so I took a picture of it.  I hope it comes through readable.  Thanks  Punky 

[PRR]

You've misunderstood the PT winding. The "0.9 ohms" does not make sense across two 2.77K windings. But in fact it is the right number, just drawn wrong.

And BTW: here the 1V2 is not used as a "flyback rectifier", it is a plain old wall-power rectifier. Half-wave, which is good enough for the very small current needed.

It needs 1,000V for the HV, and 1V (actually 0.625V) for its heater. It is used -negative- output, so the cathode connects to the HV AC. Then it is expeditious to just wind another 1V on the end of the 1,000V, and get both HV and heater on two leads. And while the 1,000V winding is several K ohms, that 0.6V winding is around 1 ohm.

> my continuity light tests misled me

What is "continuity"? If you are checking car headlights, the wires must be much less than one ohm. But the bulb may be 5 ohms. If you are checking a long guitar cable, 100 ohms is not a real problem. So where do you draw the line?

Also depends what you use to measure. If you have a 1.5V lamp and battery, sub-Ohm wires will light the lamp, but 2K wires won't dribble enough juice to get a glow. A 117V line and neon lamp will light on these 2K windings, but will read "good" on a car trailer connector which is more damp mud than copper contact; and can be dangerous.

Sometimes a go/no-go tool is not good enough.

[tubesornothing]

OK, back to our regular channel... pictures! pictures!

If I didn't like guitar amps so much, I would become "the oscilloscope guy"...

[punkykatt]

Thanks PRR for the continuity lesson.  I put the end bells back on the transformer, and started to look for any possible shorts down stream of the rectifiers. The PS cap can, a Mallory 40uf/450v, 40uf/450v, 30uf/250v,and 30uf/250v appears to have what looks like hardened black puss that had started to ooz out of the top of the can at one time. I cant find a replacemant can anywhere. There is very little room on this unit for a cap farm. Im thinking of gutting the original can and installing some small radial caps inside. How close do I have stay to the original uf values? Can I use 33uf for all four caps with out and problems? If I go with 47uf(the next size up)for the two 40uf`s, they all wont fit in the can.

[DummyLoad]

antique electronic supply has something close....

http://www.tubesandmore.com/

cap section - FP type  - p/n C-EC40-40-30  kinda buck-ish though @  ~~$32

hang a 33uF 350V inside... a single illinois 33@350V maybe will fit?  AES p/n C-ET33-350  ~~$2

[PRR]

any possible shorts down stream of the rectifiers

Why? Have you smoke-tested and found no B+?

Set the 6X4 aside. Put a 1N4007 in series with 10K 10W and tack it on the 6X4 socket plate-cathode, diode stripe pointing to first cap. If something is dead-short, you will have NO B+ and a hot resistor. If stuff just leaks real bad, you will find a low B+. If everything is actually OK, it will come to about 100V-200V. If the first cap node is low and a downstream cap node is zero, you found your short.

You should do this by clip-leads on the meter and quick blip of the power switch, to minimize strain on parts feeding possible shorts.

I expect 33uFd everywhere would work.

[tubesornothing]

If you haven't powered it up yet, and if you are a bit worried that smoke testing it will cause a fire, just build a lamp limiter or get a stock of fuses for it.  Plug it in, hook up the meters like PRR describes and blip it on.


...take the plunge...

[punkykatt]

This is from one of my previous posts in this thread   ("when I tried to fire the unit up using a currant limiter and vari vac, the 0.5A line fuse blew before the bulb even made a flicker.")    I still have to put the transformer back in.  Maybe it was a fluke the line fuse blew?? I didnt have any 0.5 fuses on hand at the time, thats when I pulled the tubes and started continuity tests.

[tubesornothing]

Oops, sorry I missed that.  Yeah, I'd be doing the same thing.  Continuity tests.

[PRR]

> the 0.5A line fuse blew

A) You are sure it was un-blown before plug-in?

B) 0.5A 117V seems small for a 'scope. Is that marked on the case, or just what was in it?

[punkykatt]

Yes, fuse was good. I always pull and check fuses on every piece of equipment I work on as a first thing rule. No marking on chassie/case. I thought 0.5A was small too, so I checked a Heathkit model 10-10 schematic that had a 6X4 and a 1V2 tubes. It showed a line fuse of 0.5A so I figuired it was correct. 

[punkykatt]

I picked up some 0.5A slo-blo fuses and installed the transformer.
The slo-blo fuses last about 20sec. I hooked  my meter up to the cathode of the 6X4 and the highest the voltage would get is about 50 vdc before the fuse blows. I guess its time to order those 33uf caps and pull the guts out of that cap can while the caps are on order.  PRR, you asked about the CRT number and the number of pins. No number found and there are 10 pins on it.

[punkykatt]

I just did some additional schematic searching and found one with the same tube setup  except one of the dual triodes is a pentode/triode. Low and behold it has a line fuse of 1.5A.  I cant wait to see what happens tomorrow.

[tubesornothing]

Any chance you can point us to a schematic or post it?  It would help.

[punkykatt]

Lets try this.

[PRR]

> except one of the dual triodes is a pentode/triode.

Look at that one's V and H channels.

Pad-switch, cathode follower, gain pot, gain-stage, long-tail phase splitter, CRT.

The OM-2's H input is all triode. The V input changes the second stage to a pentode.

In general you do that to get a little more gain and bandwidth. But at higher cost. So when you need a bottom-price model, you go back to a triode, save a dime and offer a lower spec so you know why/why-not you might want the next-up model.

It is traditional to have the V channel a little more sensitive and wider bandwidth. This is best for normal sweep-display work.

The H-channel on the OM-2 (and probably both channels on yours) has about 500KHz bandwidth per stage. With two gain stages plus some incidental losses, they may have rated it as 250KHz. This is entirely fine. The input sensitivity should be better than 1V per cm (grid line), and maybe 1V for 3 or 4 grid-lines, which is ample to see pickup signals.

On the OM-2 (and probably on yours), the 12AX7 is an astable flip-flop. The right cathode charges the R and C on the Freq switch and pot, then snaps back: your horizontal sweep. The left grid gets a sample of the V-channel signal (or 60cps or jack), which tends to pull the sweep into sync with the V-signal (or other sync source). Not triggered, but very functional for our needs.

Yah know, there isn't much could pop the fuse instantly. Unless there is an internal PT short, or a heater circuit short, pretty much just the first filter-cap in each DC line. Try with both rectifiers out. Try one at a time.

[tubesornothing]

Yah know, there isn't much could pop the fuse instantly. Unless there is an internal PT short, or a heater circuit short, pretty much just the first filter-cap in each DC line. Try with both rectifiers out. Try one at a time.

He beat me to it - darn that man!

[punkykatt]

The 1.5A fuse is holding fine.  I took voltage readings of  the 6x4  cathode and the plate of the 1V2. I turned the unit on and off about a dozen times. Each time the 6x4 k would increase in voltage, first time it only went to 140vdc, last time it was up to 190vdc. The 1V2 plate started out with  -170vdc and decreaced each time, and the last time it was at   -123vdc . I stopped because the plate of the 1V2 was starting to glow red (with the lights off).  I did not see anything on the CRT screen??  Is this bad news ? 

[tubesornothing]

I'd approach this like the first power up of a new amp.  Pull all the tubes except the recto tubes.  Pull the socket off the crt.  Measure voltages at the recto and of the lower voltage tube sockets.  I'd be too chicken to measure the voltages of the CRT socket,  I'd wait for PRR's suggestion on how to look at those voltages.

[PRR]

The +300VDC supply has a very leaky cap. It "might" heal, but I think you should re-cap.

> The 1V2 plate started out with  -170vdc and decreaced each time, and the last time it was at -123vdc . I stopped because the plate of the 1V2 was starting to glow red

You probably should not be metering this point. If it worked, and had power, the voltage is higher than most digital multi-meters are rated for. But also this is a very weak supply, and even a DMM may be loading it more than it can stand.

Still, I don't get red-plate. Unless the HV caps are oddly ill. Or...

Put the whole 'scope face-down. Use the handle of a rubber-handle screwdriver to tap all around the neck of the tube, an inch or two down from the socket. The idea is to dislodge any flakes of metal which may be inside the neck, get them to fall down to the screen, where they may lay harmlessly. Also check the CRT socket for damp dust or cockroach corpses.

Next find your HV cap or caps. Discharge them (insulated screwdriver to chassis). Carefully snip the hot end as shown below.



You do NOT have Intensity or Focus knobs? Internal trimmers?

In any case: that HV diode plate goes to a cap and a string of resistors/pots to ground, the voltage divider to set the grid, cathode, and "screen" voltages. Find where that comes to ground. Lift the ground end of that resistor and insert 15K (or 10K or 22K) to ground. The stock string was something like 1.5Meg, 1,500K. Now it is 1,515K with a 15K or 1% tap. Meter across THIS resistor. The voltage will be 8V to 15V, no risk to meter. The worst-case loading (shorted meter) only puts the resistance back to stock.

From your AC voltage readings, the tap voltage should be 8V or 9V. If your divider string is not like the OM-2, it may be a little off, but I doubt it can be very different. OK?

Now tack the HV caps back in one at a time. If they are good, the tap point should be 8V-15V. If it is much less than the no-cap voltage, that cap is for-sure bad.

I fear this could be true. And film-caps won't heal. These need to be at least 0.01uFd maybe 0.2uFd (note what yours are marked) and at LEAST 1,500V. Not gitar-amp caps! These will be test-gear or radio-ham caps. gonna have to hunt. Try Nebraska Surplus, try Apex Jr. Stay within 2:1 (half to double) the original values. If the first cap is more than double, the rectifier could over-load on start; second cap more than double is probably a budget-buster, and less than half what Heath used will probably be inadequate and give a fuzzy dot.

[punkykatt]

I didnt have much time to work on this project today. I did find a number on the CRT, its 3RP1. I drew a picture of how the CRT is wired up. I hope its readable.  I did find replacement caps 0.1uf.1500v at Nabraska Surplus. Thanks.

[quayhog]

Do you still need the schematic and assembly instructions?  I found them.  See my post on this topic dated 12/7/09.   I'm leaving town soon on business so let me know soon.  It's 37 pages.

[punkykatt]

Yes quayhog. I would like all the info you can get me.  I finally got all the electrolytic and the super high voltage caps installed today. Success!! Its up and running. All the controls seem to be working. When I hooked up the guitar as PRR instructed I did get sine waves. Are there any calibration procedures in your info? Thanks a million.  Punky

[tubesornothing]

Congrats!  I wouldn't worry so much about calibration, as PRR says, a DMM is much more useful for precision measurement, scopes are for seeing shapes.

Now to get that baby on some amps and see what things really look like!

Does your probe have a 10x attenuator?

[quayhog]

What I have is the complete Heathkit documentation for the kit.  It has several pages for adjustments and calibrations.  I sent you a message yesterday with my contact information, check your mailbox.  email me at that address.  It needs to be xeroxed and mailed to you.

[punkykatt]

quayhog, I sent you an e-mail.  tubesornothing, this oscilloscope came without any probes. Im going to have to make or buy some probes. I havent a clue what to look for or even how they are constructed. Are they anything like multi meter leads? Are all probes alike?  Punky

[quayhog]

the probes are regular multi-meter probes with banana clips on one end and meter point probes on the other.  the manual shows meter leads but with alligator clips with no insulation. 

[punkykatt]

Thank you. those probes should be easy enough to make. Do you know what length they are? Does it matter?,  and do I need to make four of them?

[tubesornothing]

It's be really nice if you could make them out of coax.  Some of the guitar signals are small (like on the input, or after a reverb dropping resistor) and will pick up noise of the lead length is too long, or is draped over other large signal sections.

Here is a picture of a modern scope probe with a BNC connector: http://en.wikipedia.org/wiki/Test_probe#Passive_scope_probes

[punkykatt]

Thanks tubesonnothing, for that info. Is it necessary to have the 10x attenuator for this old unit?   That short wire on the tip of the newer probes, is that the shield? and gets connected to a convient ground on the unit under test, is this correct?  Please excuse my total lack of knowledge working with oscilloscopes.

[PRR]

> nice if you could make them out of coax.

IMHO: not worth the trouble.

This is NOT a high-sensitivity 'scope. It has only two gain stages, both triodes. Assuming 100V at the deflection plates, the maximum full-scale sensitivity is not much under 1V. I ran 1V signals around classrooms in unshelded wire.

Let's get on with basic use-a-scope fun. Lampcord, bell-wire, whatever. Clips are good so you don't slip. Even when I had probes, I mostly connected with gator-leads. Only when you get deep into a small-hum problem do you "need" shielded probes. And even then, grounding and induction cause more phantom hum than unshielded connections.

Lash black to chassis. Connect red to a grid (NOT a plate!). At the first grid you see about the same as direct guitar. In a simple amp, the second grid shows a MUCH bigger signal, which can be reduced with the Volume knob. When you get past any Tone control, you see that treble-boost gives more narrow spikes, bass-boost gives more broad hills. At the output tube grid (please do this on a self-bias stage; fix-bias can be upset) you will see when you play REAL loud the wave goes flat on top. Also peep the speaker connection.

[PRR]

I don't see a strong need for an X10 probe.

We use them for two reasons:

1) to reduce capacitive load on the circuit being tested

2) to read higher voltages

If you don't shield, the added capacitance is small. Anyway this is a 500KHz 'scope on a 50KHz geetar amp and if you get to 5KHz "nearly flat" you are golden. 30pFd of stray capacitance on a guitar-amp may shift the rolloff from 10KHz to 7KHz, but so? You will be looking for real-wrong stuff, or comparing pretty-good stuff, not doing precicion measurements.

A hi-sensitivity 'scope often can't take over 50V at the jack and stay clean. Higher voltages may damage it. BUT this crude bastard just won't care. On the X100 setting and full-up it should take 50V and stay clean, then turned-down you can go 1,000V and still be clean. At 1,000V the input resistors will be hot, and they are not really good for over 500V. That's ample for everything except the power stage plate and OT primary. Which is really too high for common X10 probes (I've arced a few that way), and more suited for a custom probe like 1Meg/2W:10K.

[tubesornothing]

Well there ya go, enough of my fancy probe stuff!  Give 'er a shot post some pics!  (lets see, two hand on the guitar, two hands on the camera, and another hand on the scope...)