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I’d be grateful for comments on my component choices below, which are for an upgrade of my 2230 PSU as per Tek’s Product Modification 060-2239-03 (‘Q935…Q9070 Replacement’). Although my scope works, I’m wary given some of the cautionary tales told in this group and the early SMPS design concepts used and improved on by Tek over the years.

The Tek modification involves replacing most of the critical semiconductors in the pre-regulator, crowbar and inverter, so correct component choice is vital. Your comments and the reasons behind the advice would be welcome. NB In the UK the mains can be up to 245Vac, which affects component voltage ratings in the pre-regulator.

Circuit Ref, ‘Tek description’, originally specified (as fitted in italics) part, my suggested replacement, followed by my reasons for selection:

Q935, ‘Thyristor, SCR 8A, 200V sens gate, TO-220’, C10682X283 (GE C10682), ST Micro TS820-600T – An old GE datasheet for the C106 series lists the ‘On’ condition with 1mA of gate current and the TS820-600T has Ig and Vg as 0.2mA and 0.8V respectively, together with higher voltage and current ratings. NB R935 will shunt the first 6mA of any leakage in VR935 and CR948 before Vg reaches 0.8V.
Ref. -

Q946 and Q947, ‘Transistor NPN, 50V, 150mA, 200mW, inverter’, SJE389, On Semi MJE15032G – The manual shows 80Vp-p swings at Q947c and the later TIP31C fitted was rated at 100V and 3A. The MJE1503G is rated at 250V for extra margin and has a greater SOR (safe operating area) than the TIP31C (some obvious replacements don’t). The hFE is a bit higher at 100 rather than 40 for the TIP31C but as close as I could find while having the higher voltage and SOR rating. It’s not obvious how or if a higher hFE would affect operation of the inverter and I can’t model it, hence my caution. Ft is higher at 30MHz but hopefully that won’t be a problem.
Ref. -

Q9070, ‘MOSFET, N-channel, TO-220’, STP3000 (Motorola 8630), Toshiba TK10E60W – The Toshiba TK10E60W has higher voltage and current ratings, while keeping the Rds On, gate capacitance and gate charge below that of the IRF730 Tek suggest as a replacement. The gate threshold voltage is within range and modelling in LTSpice suggests it should work well.
Ref. -

VR935, ‘Zener, 51V, 5%’, SZG35009K7 (1N978B), Nexperia BZX79-C51, 143 - It is not obvious why the original is being replaced, but the mod sheet says all listed components should be replaced. A low power 500mW Zener is indicated to ensure that the full 51V develops when a few mA passes through it. My PSU already has a 1N978B fitted, so I’m strongly tempted to leave it in, given that it shouldn’t be under stress and the available replacements don’t appear to offer an advantage in performance or reliability.
Ref. -

CR907 (and CR906), ‘Rectifier diode, silicon, 400V, 1.5A, 50ns’, DSR3400X (BDY73), Vishay MUR460 – The Tek modification sheet recommends 2 x BYD73G in parallel, which makes sense given their rating. LTSpice modelling shows currents of around 1.7A ave and 1.8A rms in CR907. I have a single BDY73G fitted which sounds unhealthy, so the MUR460 looks better with its 4A and 600V ratings, together with a short 50ns recovery time.
Ref. -

Finally, I’m reluctantly replacing all the electrolytics in the PSU for safety. While the existing capacitors all read okay in terms of value, leakage and ESR, I’d rather not take the risk given the early nature of the design and known component stress in places. I’m also replacing C907 (1uF non-polarised) as modelling shows that this takes a lot of punishment – 1.2A during the ‘on’ period with 245Vac mains – and failure would break a lot of expensive parts!

Everything else is being kept original. I also have a parts list for Mouser is this is of use to anyone.

Sorry for the long post and thanks in advance for your comments.

Alastair Knights

I recently went thru hell fixing a 4-module Allen-Bradley mod pot in an HP counter. I ended up repairing some fractured plastic with epoxy and moto-tooling it to final shape. It took me weeks to get a workable unit again. In the meantime, I tracked down the current owner of the Allen-Bradley mod pot line, but they weren't interested in selling me just the pushbutton momentary switch module I needed. They made it clear that whatever I needed would have to be replaced in whole and would cost an arm and a leg.

The Allen-Bradley mod pots are black, and I understand that the Bourns are blue.

So, did Bourns duplicate the Allen-Bradley line, so that this might be a second source for those pots?

On Wed, Mar 24, 2021 at 02:06 AM, M Yachad wrote:

I do have kits available, which I custom assemble, to suit your EXACT machine.
I'll need CLEAR photos of your A1 A2 A3 A4 A5 A9 A14 PCB's

Saves you the time and hassle of sorting and sourcing, and wondering if you
made the correct choices.

Here's the ebay ad


Forum members who buy via my contact directly here (NOT via ebay), get a 15%
discount.

That's a VERY well thought-out, well-written ebay ad. Anyone here who's trying to recap one of those scopes should just take advantage of the 15% discount for TekScopes members and buy one of these kits directly.

One question: The caps in the grey and red rectangular boxes, I thought those were film caps. Why would they need to be replaced? Do those have a history of failint? I have some boards from the '60s with Wima film caps like those. Those Wima caps are al still good.

Or did Tek use something less reliable than Wima?

New pictures available for those interested in my modest restoration of this unit.
You’ll notice that this units, unlike some that I’ve seen, doesn’t have a fixed line cord.
Probably intended for export so different models can be used. Also has a switchable voltage switch 115-220V.
Also, although the plug is a 3-prong, no earth connection is made inside.
I’ve installed a piece of old Fender amp grill cloth in place of the decayed filter, in front of the fan.
It has worked very well for me on other projects (RTM506 for example), and I know it will not decay anytime soon.

New pictures here:
/g/TekScopes/album?id=261997&p=Created,,,50,2,0,0

Spelling errors as well. Sorry for my terrible iPhone... ???

On Wed, Mar 24, 2021 at 03:03 AM, Eric wrote:

Stephen,

The gr connectores will be the hard part. The 5ns cable I belive you can
use and "good quality" 50 ohm cable. The importent part is the impedance
match. 75 ohm and 50 ohm cable and jacks are easly confused and
substituded. I find my self using tek, amphional, and belkin cables for
genral lab use unless I need something more exotic based on speed or
frequancy. The speed of the cable is a property of the dielectric insulator
around the inner conductor. If memory serves air is best but difficult to
work with, next down the line is teflon.

Sorry if there is any spelling errors typed out on a phone.

Eric

Eric,
Thanks.

I have a GR to BNC brand new adapter coming in. But it might take a while from the US. I wouldn’t mind having a couple of extra ones and stuff.

I’m in the middle of recapping the whole thing at the moment.
Mine used GaAs diodes. Does yours too?

The strobe is adjustable in several ways (avalanche volts, snap-off current, bridge volts). But wouldn't insufficient drive show up as an incorrect sample time, lack of sensitivity, or both?

As I mentioned, it works properly with my pulser (4 ns flattop pulse with 650 ps risetime). The sampler risetime is at least in the ballpark too (would expect to display sqrt(650^2+350^2) = 738 and that looked about right when I checked).

The problem I'm seeing is exactly consistent with the photos in the manual (setting transient response at 1 MHz). There is too much feedthrough aka blowby and the inverted signal through Q13 does not have enough adjustment range to null it out. I thought that double diodes would help this. How would going back to single diodes help, which will double the feedthrough capacitance? Please explain further, I'm learning as fast as I can.

Also it will be mechanically difficult for me to jumper one diode without damaging or destroying anything ;)

Stephen,

The gr connectores will be the hard part. The 5ns cable I belive you can
use and "good quality" 50 ohm cable. The importent part is the impedance
match. 75 ohm and 50 ohm cable and jacks are easly confused and
substituded. I find my self using tek, amphional, and belkin cables for
genral lab use unless I need something more exotic based on speed or
frequancy. The speed of the cable is a property of the dielectric insulator
around the inner conductor. If memory serves air is best but difficult to
work with, next down the line is teflon.

Sorry if there is any spelling errors typed out on a phone.

Eric

Anyone has GR874 to BNC adapters, T’s and 50ohm through terminations they’re willing to part with?
5ns cable as well?

Thanks

Hello folks,
I enjoy reading here very much. Indeed everyone can learn something.
One of my past job duties was to teach young professionals a basic understanding of aerodynamics dealing with mining ventilation systems. Trying to do so without starting with lots of maths put me often into trouble to find easy examples. I have burnt much midnight oil but frequently ended spreading calculations all over the board as there was no better proof of theory than figures.
I had to tell the guys: "You must stand this now ..."
Regards, Joe

Hi Tom,

Thanks for your gracious reply. I do see your point now and it makes sense to me. Also, I agree, there are too many problems with the example I gave. Also, somehow I missed your "thermostat" example in my read of the thread. That is clearly a great example, and when I read it I regretted even more even posting mine. Should have just said "Thermostat example is the one!" :-) Also, my friend Matt's example of automobile steering seems like a useful deeper dive into the idea, and seems to hold up well - at least to me.

Wish I could delete my post about the kids...guess I'll have to figure out how to do that - but for future readers, if you still see it, skip it and read Tom and Matt's examples.

Love this forum. Not just people shouting nonsense back and forth, but actual dialogue and learning. (at least I'm learning :-)

Cheers to all,

Keith
CBG

Thank you Raymond. I just looked briefly at internet too see whether or not it could do random sampling.
Charles can forget my inappropriate warning.
Albert

I do have kits available, which I custom assemble, to suit your EXACT machine.
I'll need CLEAR photos of your A1 A2 A3 A4 A5 A9 A14 PCB's

Saves you the time and hassle of sorting and sourcing, and wondering if you made the correct choices.

Here's the ebay ad



Forum members who buy via my contact directly here (NOT via ebay), get a 15% discount.

Yachadm AT gmail DOT com
CondorAudio

Using double diodes changes a lot of the conditions. Try going back to single diodes instead of series doubles, and see how it works. You can temporarily short one of the diodes in each pair for experimenting. There may not be enough strobe drive - it may be adjustable and fixed that way. Also, check the other adjustments and study their purposes and effects.

Ed

You may recall that I have an S-1 sampling head with blown diodes, which I replaced with M/Acom MA4E1339 Schottky diodes in SOT-323 packages. I used both diodes in the package in series, max 1.2 pf each, to keep the capacitance down. They are mounted on small pieces of copper-clad and the package spans a gap in the copper. Hopefully low capacitance.

Anyhow, it works fine on fast pulses, and at DC too. But I ran a 1 MHz square wave through it (something I would not normally use a sampling scope for) and found that the transient response is waaaaay undercompensated. There's a bad overshoot, then undershoot, finally recovery. Per the S-1 manual that is the input to use while adjusting the transient response. But there isn't enough range on the pot (R13) to get it near flat, although with it all the way "up" it's better. I'll post display pics tomorrow.

I conclude that either my mount has excessive capacitance causing blowby in that frequency range, or possibly Q13 is bad. Although the trigger pickoff does function (Q13 drives Q17 which sends the trigger to the 3S2). It will be difficult to make a mount with much less C... the diodes themselves, at 0.6 pf, should not be a problem.

So I could use some suggestions. If I have to modify the circuit to allow for more out-of-phase signal to bypass the sampling bridge, what would be easiest to change with minimal side effects? Thanks.

Both marked 820 ohms (grey-red-brown); initial measure in circuit R155 real
value 1053 ohms (same value when removed), R290 real value 832 ohms and they
look like 0.5W (if not more). Now the two have places switched, but, as I
said, no difference in behavior of the stage

It is better to place the proper value, 820-ohm nominal, in both places.

Voltages look OK ballpark. Perhaps some of the transistor gains decreased over time and now positive feedback coming back through C247 is not strong enough. I assume your serial number doesn't have the R247 resistor parallel to C247 given R155/R290 were 820-ohm nominal. You could try adding another 2.7pF in parallel with C247 to see if it helps. Note that C247 was selected so its value is critical to proper operation.

At the higher frequency settings (higher charge/discharge currents) slight mismatches in source and sink current may be flowing into R155/Q292 junction and place the operating point in the bad stable condition.

Looking at the min/max voltages at Q292 collector in normal operation in those high frequency settings may give other clues.

Ozan

Tomorrow I'm going to make a table with gate times both A and B at each
setting.

I observed today the gate width was same as 1ns on 10ns and 100ns, there
must be some logical explanation for that?

Can't think of an easy explanation. When you are making the table could you also mark down both A gate width and B gate width while you are measuring B sweep timing? Also please observe if you really see a fast sweep when gate is shorter than expected or if the gate is cut short. If you apply a signal you can tell from the screen whether sweep is complete but fast or if it is truncated.

Ozan

On Tue, Mar 23, 2021 at 11:44 PM, Albert Otten wrote:

The PM3340 only does sequential sampling and requires signal delay..

My PM3340 has a signal delay line... Trigger pickoff right at the N-socket and a nice blue Suhner coax cable after that for each channel.
Nice but very heavy beast.

Raymond

Hi Max,

It's nasty that the PG506 trigger output is not usable as pretrigger. Also (e.g. in my PG506) there is too much between-trigger jitter to view the fast edge of the next pulse. That's why a 7M11 delay line is used in the calibration section. But the 7M11 is not ideal. Hence the PG506 signal output waveform is compared with a faster TD pulser waveform. The PM3340 only does sequential sampling and requires signal delay..
BTW I found that the fast-rise rise time depends a lot on the amplitude, something like 0.75 ns for low to 1 ns for max amplitude.

Albert

Hi Albert,

you made me doubt about the PG506 now :-)
I would not like to have calibrated the 7A16A trying to compensate for the aberrations of the PG506 itself.
In the weekend I'll check the fast rise output of my PG506 using the 3S11/3T77A and also with a 2 GHz Philips PM3340 that I received a month ago (fully functional, like new).
I'll post some photos as soon as possible.

Max