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I just acquired a Type 284 that looked in decent shape, and the square and sine waves look good (as far as I can see on a 475 and 2465A; the latter barely shows the 1 ns sine). DC voltages were spot on, but pretty noisy until I replaced C226 and C266.

The pulse is not working, of course, so naturally I took apart the air line to see if I even had a tunnel diode, since one of the clamp nuts shows plier marks from some savage or other.

With the TD out and the back termination installed, I measured 641 ohms from the TD end of the center conductor to the rear lead of the termination where it's visible in the hole, and 695 ohms between the rear lead of the termination and the tube. From this, I conclude R184A is 54 ohms, as specified, but R184B and R184C have drifted high. From the archives, I assume this is why I have no pulse, so I'll have to fix this.

When I removed the head end, the plastic TD holder was jammed on the end of the center conductor that goes inside the tube, which I think would defeat the purpose of the spring tip in making contact with the external center conductor.

Does this sound/look right? I've posted a couple of photos to Dropbox:





The arrow is up direction on the outboard side of the tube, so that's just how it came out. The TD leads are bent towards the back of the instrument, and captured in the slots of the plastic holder. It looks like the holder slots are indexed to the resistor in the head, so I'm really confused by the photo on TekWiki, which looks impossible.



The manual mentions a "Dot on top lead" of the TD (Figure 4-4b), but I'm also confused by what "top" means in this context. Any ideas on how to tell if this thing was assembled correctly? I reassembled it just as it came out, and it did not magically start working.

thanks,
Adam

Hi Adam,

For tunnel diode orientation, have you looked at the second video by Zenwizard in the links of 284 Tekwiki?

I measured 641 ohms from the TD end of the center conductor to the rear lead of the termination where it's visible in the hole, and 695 ohms between the rear lead of the termination and the tube. From this, I conclude R184A is 54 ohms, as specified, but R184B and R184C have drifted high.

? 641 is less than the nominal 390 + 270.

Have you verified the pulse waveforms at J135 and J145?
Albert

Hi Albert,

On Tue, Dec 12, 2023 at 10:01 AM, Albert Otten wrote:

For tunnel diode orientation, have you looked at the second video by Zenwizard
in the links of 284 Tekwiki?

Yeah, that's the other thing that had me wondering, as it seemed like the
plastic holder and diode stayed in the head when he took the air line apart.
My holder has a loose fit on the center inside the head, and a snug fit on
the spring tip (although actually it's a fit over the center conductor in the
air line; maybe that center tube isn't seated correctly in the rear).



Somehow I missed his entire discussion of the TD orientation in the holder,
so thanks for that! This should answer most of my questions.

I measured 641 ohms from the TD end of the center conductor to the rear lead

of the termination where it's visible in the hole, and 695 ohms between the
rear lead of the termination and the tube. From this, I conclude R184A is 54
ohms, as specified, but R184B and R184C have drifted high.

? 641 is less than the nominal 390 + 270.

Indeed it is, and I should not have been trying to do simple addition and read
a schematic that close to midnight. Ouch.

Have you verified the pulse waveforms at J135 and J145?

No, and I was wondering about the best way to probe those. Pull the coax
connector on the board and read it directly on the board? I couldn't tell what
he was probing for the tripper pulses in the video.

thanks,
Adam

I have had a few things happen with these. All TD's have a polarity but they will conduct in BOTH direction. They are not like a normal diode especially D180 in a 284. Of the units I have rebuild I had 1 bad diode, one diode that some how wound up in the holder backwards, and one diode that was good but did not have enough drive current to fire. Caused by resister drift in R184. In ALL 3 units I needed to rebuild R184 B and C. I have not had one yet that did not drift high but I also only have a sample size of 3.

The diodes are crashed in to the diode from both sides with a wave form and are slammed over the trip edge with a tripper pulse that is just fired in to the air line. So to get the diode to go you have 4 different wave forms coming from 4 different directions at 2 different polarities.

Check the spring at the back of the air line I did need to stretch 2 of the units out a bit to get the diode to have good contact in the air line. Spring is located at the rear of the air line. When everything is together correctly the diode is suspended in the air line loosely held by the plastic holder but smashed (gently) on the face of the air line.

Care should be used when checking the D180 diode as some meter will have to much forward current and can toast the diode . A curve tracer is definitely the way to go. If I remember the spec correctly D180 should go at about 20ma don’t quote me on that double check. Some meters especially older ones I know my 410B is this way they will source 100ma on the lowest ohms range for resistance testing and are NOT suitable for testing a TD of this sensitivity.

I do have a video planned for calibration on a 284 but I need to get some 3.5mm (RF) cables I have a sampling scope of an appropriate rise time to do the adjustments and calibrations, but I need to interface the GR connectors with the 11801C jacks. It should also be noted that if the pulse looks a little slow everything might be working and cabling might be the issue. <70ps Edges is a 5Ghz wave form and this exceeds the speed of even the best BNC cable at 4Ghz so there will be some speed and amplitude attenuation in the cable this will be exaggerated with cable length.

Zen

On Tue, Dec 12, 2023 at 08:55 PM, Adam R. Maxwell wrote:

Have you verified the pulse waveforms at J135 and J145?

No, and I was wondering about the best way to probe those. Pull the coax
connector on the board and read it directly on the board? I couldn't tell what
he was probing for the tripper pulses in the video.

Hi Adam,

(I saw a reaction by Zen arrived while typing but nevertheless post my message.)
It was in 2007 that I dismantled the pulser assembly in my 284. So regrettably I don't remember all the details.
The waveforms at J135 and J145 can simply taken from the resistors R139 and R149, where they are shown in the schematic.
I haven't looked at trigger pulses in the video yet. I measured the trigger pulses by disconnecting J165 (or J166) and connecting a suitable coax cable to the input of a sampling scope. The pulses were about 300 ps wide at 50% amplitude. Peak at less than 11 mV (the screen photo shows 11 div peak, but readout says <2mV/div, so uncalibrated).

You might check the TD in situ via current supply into the output connector, P180 and P184 removed. I did such using the 576 with collector voltage range 15 V and internal resistance 650R. Of course this shows the behaviour of the combination TD + 54R, but switching of the TD is easily demonstrated this way. Of course you can do something similar with a DC current current supply and a voltmeter at the input; increase the current until you see the jump in the voltage.
Ip of the TD may have drifted up somewhat away from 20 mA. For testing purposes you might increase the +20V adjustment somewhat. That also applies to drifted up airline resistors.

Albert

(continued) In hindsight I doubt that 11 mV I mention can't be correct (way to low I think). Please forget this value. Albert

Adam,

Good to see someone working on the Tek 284! I had watched Zen's videos on the 284 and decided to tackle mine. At the time the sine and square wave outputs were working, but there was no pulse output.

I have some images here which may help you:

Shaun M.

Albert, Zen, Shaun:

Thanks for all the helpful suggestions. I probed R163 and R167 for the tripper pulses, which I think is correct from the schematic? Both were present and one was inverted, which is what I noticed in Zen's video.

From Shaun's excellent macro photo that showed the dot, I think my TD was 180? off. Sadly, rotating the holder 180? and reassembling the airline did not give me a pulse, so I'm assuming the TD is toast. Adjusting the fine or coarse bias moves the trace vertically, but no pulse. R184B+R184C measures 641 ohms, and R180 measures 758 ohms, which I think is in spec.

Here I have the trigger output plugged into channel 2 and the pulse output in channel 1, both with 50 ohm thru terminators. There's a step far to the right of the trigger pulse, and adjusting bias stretches it vertically, but it's not the pulse we're looking for.



Going all the way down to 2mV/div and looking at the signal near the trigger pulse, it looks like something is happening, and the wiggly area shifts left and right a bit if I change the delay setting. I'm guessing this is there the TD should be firing.



No curve tracer here, and I'm a bit wary of putting any current through the TD with a DC supply, as I'm not confident that I know what I'm doing. Maybe I'll watch Alan Wolke's TD video again and think about that one.

thanks,
Adam

On Wed, Dec 13, 2023 at 07:20 AM, Adam R. Maxwell wrote:

Here I have the trigger output plugged into channel 2 and the pulse output in
channel 1, both with 50 ohm thru terminators. There's a step far to the right
of the trigger pulse, and adjusting bias stretches it vertically, but it's not
the pulse we're looking for.

Hi Adam,

I guess you set horizontally 0.2 us/div and that what you see as "step" is actually the *end* of the total 2 us Q135/Q145 *on* period. Clearly the TD never fires.
You can better recognize that 2 us period if you set the time base slower to display the next period also. I guess you will see that the trigger pulse shows up about midway within the on period, not at the start.
The output looks negative going during the on period. The Bias balance has effect on this.
In guess you can't crank up the bias current far enough or the TD is defective. It's not due to a problem in the pair of trigger pulses to the the TD.
BTW I can add to Zen's findings that R184B+C in my 284 had also drifted up in resistance. The fact that you measured about the nominal value might indicate that these two resistors were renewed in the past.

Albert

On Dec 13, 2023, at 03:38 , Albert Otten <aodiversen@...> wrote:

I guess you set horizontally 0.2 us/div and that what you see as "step" is actually the *end* of the total 2 us Q135/Q145 *on* period. Clearly the TD never fires.
You can better recognize that 2 us period if you set the time base slower to display the next period also. I guess you will see that the trigger pulse shows up about midway within the on period, not at the start.

Yes, that is exactly where it shows up! I'd originally thought this was the pulse when I first fired it up, but it was about as slow and ugly as me.

The output looks negative going during the on period. The Bias balance has effect on this.

Correct, it is negative. Adjusting the bias balance on the pulse board allows it to go more negative, so that step looks taller, or positive until the on period is slightly above the off. No hint of TD firing during any of this.

Just in case someone had replaced the TD upside down (it has a dot on both faces), I tried removing it from the holder and flipping it over this morning, and get the same behavior as observed before.

thanks,
Adam

Just in case someone had replaced the TD upside down (it has a dot on both
faces), I tried removing it from the holder and flipping it over this morning,
and get the same behavior as observed before.

Hi Adam,

All this dismantling of the airline and diode seems more dangerous to me than testing the diode with DC current via the output connector.
Anyway, the following procedure is save. Use ordinary 10X probes and a 50R terminator at the output. Look at the differential voltage between R138 and R148. The "on" pulse then shows the total voltage across the airline.
The total resistance is 1410 ohm nominally. The current to/from GND will be almost negligibly as long as the TD doesn't fire, and the voltage across the TD very low.Thus slightly over 28 V is needed to fire the 21 mA TD. Can you attain that "on" voltage and preferably a little higher?
My 284 is rather temperature sensitive in that respect.

Albert

Hi Albert,

On Wed, Dec 13, 2023 at 08:03 AM, Albert Otten wrote:

Just in case someone had replaced the TD upside down (it has a dot on both
faces), I tried removing it from the holder and flipping it over this

morning,

and get the same behavior as observed before.

Hi Adam,

All this dismantling of the airline and diode seems more dangerous to me than
testing the diode with DC current via the output connector.

You're probably right, but I burned out a fuel gauge in my '68 Jeep when I was 16,
testing it with a model train transformer. Ever since then, I've been cautious about
applying currents to unobtainium parts that I don't fully understand. it took me 30
years to find a replacement gauge.

I'm new to this, so trying to learn as I go. I needed a scope to fix a radio, and then
a scope to fix that scope, which led to calibration gear to see which scope is lying…

Anyway, the following procedure is save. Use ordinary 10X probes and a 50R
terminator at the output. Look at the differential voltage between R138 and
R148. The "on" pulse then shows the total voltage across the airline.
The total resistance is 1410 ohm nominally. The current to/from GND will be
almost negligibly as long as the TD doesn't fire, and the voltage across the
TD very low.

My next thought was that I should probably start checking for appropriate input
signal level to the TD instead of lazily checking presence/absence. It looks like the
schematic shows expected +15 and -15 to ground between those points, so
about 30V between them, which I think matches what you wrote below. I guess
you came up with that voltage based on 21 mA across the TD? Sorry if that's
obvious, but I'm trying to understand how to approach a problem like this this.

I'll check this when I get home from work tonight. I have the 10X probes and a
50 ohm termination.

Thus slightly over 28 V is needed to fire the 21 mA TD. Can you
attain that "on" voltage and preferably a little higher?

I have an HP bench supply that can provide that voltage, but I think you're asking
if the internal supply is providing 28V, correct?

thanks,
Adam

i Adam,

Yes I understand your hesitation to measure something without knowing exactly what you are doing and might destroy. You won't be the first one here among the many of us who have made terrible mistakes (that includes myself).

My next thought was that I should probably start checking for appropriate input
signal level to the TD instead of lazily checking presence/absence. It looks like the
schematic shows expected +15 and -15 to ground between those points, so
about 30V between them, which I think matches what you wrote below. I guess
you came up with that voltage based on 21 mA across the TD? Sorry if that's
obvious, but I'm trying to understand how to approach a problem like this
this.

Hi Adam,

You can measure those + and - 15 V (approx.) one at a time with the same probe and then do the math. Ignoring the 54 and 50 ohm (in and external) to GND the current caused by the voltage difference has to pass through 750 ohm, TD and 660 ohm. The voltage drop across the TD in low state is perhaps 0.15 V or so, to be neglected. Then you have 30 V (approx.) across 140 ohm, resulting in more than 21 mA.
In the balanced case no current flows to GND and the TD is effectively paralleled by 100 ohm (approx.). About 1.5 mA or so is bypassing the TD and has to be delivered by higher voltage across the airline., as if it were a 22.5 mA TD, requiring 31.7 V. Hmm, that's more than I expected but I don't know the correct value for the TD peak point voltage (assumed 0.15 V but might be 0.1 V or lower as well).
I'm curious to see what you can get with Bias controls rotated for maximum, perhaps after warming-up of the 284..

Albert

Albert

In my previous message I made a mistake in writing:

In the balanced case no current flows to GND and the TD is effectively paralleled by 100 ohm (approx.).

That's not the balanced case. In the balanced case the output is 0 V (before the TD fires) and the TD is effectively paralleled by R184B+C. That makes it worse for the required voltage across the airline. (I hope I made no new mistake here!)

Albert

Hi Albert,

On Dec 13, 2023, at 12:32 , Albert Otten <aodiversen@...> wrote:

In my previous message I made a mistake in writing:

In the balanced case no current flows to GND and the TD is effectively paralleled by 100 ohm (approx.).

That's not the balanced case. In the balanced case the output is 0 V (before the TD fires) and the TD is effectively paralleled by R184B+C. That makes it worse for the required voltage across the airline. (I hope I made no new mistake here!)

I set GND to be the centerline and measured those +/- 15V at the same time, and came up with a differential of <26V. If I understand correctly, the TD isn't even close to firing, and adjusting bias balance does not make up for this. This time on the 2465A so the photos are more helpful.



I also measured at bases of Q140 and Q130 (R122 and R124), which should be at least +15 and -15 as well, according to the schematic, and this time used the 10V/div scale. This is also low by about the same amount.



DC supply voltages at the test points are right on 20/-20, and signals at the base of Q110 and at TP111 look just as they do on the schematic, so the oscillator is doing its job.

You and others have mentioned temperature sensitivity, and I'd assume the temperature in my shop (~3.5C) isn't helping anything warm up, but it's pretty stable. BTW, this unit is S/N B050419, and has the gold PCBs like my 475. It was dusty, but seems in good shape.



thanks,
Adam

.....
I set GND to be the centerline and measured those +/- 15V at the same time,
and came up with a differential of <26V. If I understand correctly, the TD
isn't even close to firing, and adjusting bias balance does not make up for
this.

TD BIAS (R132), not the bias balance, should control the swing.
Ozan

On Dec 13, 2023, at 22:57 , Ozan <ozan_g@...> wrote:

.....
I set GND to be the centerline and measured those +/- 15V at the same time,
and came up with a differential of <26V. If I understand correctly, the TD
isn't even close to firing, and adjusting bias balance does not make up for
this.

TD BIAS (R132), not the bias balance, should control the swing.

I probably misinterpreted something in the archives that I thought referred to
that as a coarse bias. Maximum bias voltage with the pot on the front of the
unit is 27V; I had it set about midway for my photos.

thanks,
Adam

Differential 27V across J135 and J145 looks a little low for maximum TD BIAS setting. What voltage do you see at the bases of Q130 and Q140 for maximum setting? Assuming Q120 and Q125 are driven on/off, we expect ~ 20V*(45k//18.6k)/(2k2+45k//18.6k)=17V peak-to-peak at each base (34V differential). I expect maximum setting to produce >30Vpp at J135/J145.

If we assume voltage is close to zero volts across load and R184A before trigger, diode needs 21mA*(390+270+750) =~ 30V to be at the edge of triggering, then C185 and C186 gives additional kick to trigger it.

I don't have this unit, all my comments are based on the schematic.
Ozan

On Dec 13, 2023, at 23:56 , Ozan <ozan_g@...> wrote:

Differential 27V across J135 and J145 looks a little low for maximum TD BIAS setting. What voltage do you see at the bases of Q130 and Q140 for maximum setting?

I get about 27.2V differential at bases of Q130 and Q140, and about the same between positive and negative peaks at J135 and J145. Well short of the ~30V needed.

thanks,
Adam

Hi Adam

My 284 isn't cooperative today, refused to produce an output pulse. Two days ago I think it started (after heating up somewhat with a f?hn) but then the scope broke down. (Extreme bright display, likely a grid DC restorer problem.) Anyway, I measured at maximum about 28 or 28.5 V across the air;line. Even with the +20V set at maximum (20.7 V) no firing TD at all.
Perhaps time for me to recheck the TD via the output connector.
If you want to check the TD that way, it's save in my opinion. Suppose you have a variable DC power supply 0-15 V.Then with a 560 ohm resistor the supply can source some more than 21 mA to the TD, but not excessive. First reliably connect DC minus to 284 GND. Then a lead to DC plus and at the end of the lead the resistor. Now you can touch the inner connector of the GR output with the free resistor end. But grep the resistor only at the DC end, not the GR end.
To see what happens you can put an ammeter in series with the lead and a voltmeter across the GR connector. Crank up the DC voltage slowly until the TD fires.
When orientation of the TD seems wrong then repeat things with DC plus and DC minus interchanged. And don't forget to remove the internal supplies from Q135/Q145.

Albert