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Thanks! Feel free to keep sharing your knowledge ;) I have never played with varactors so that C-V curve didn't even occur to me... so what happens to the junction capacitance under forward bias?

But the data sheet does not show a curve- the only entry is:
Total Capacitance (Ct ) Vr = 0 V, F = 1 MHz, 1.2 pF max.

That leads me to believe that my 0.6 pF conjecture is not so far off, if the capacitance is maximum at 0 (as you said, and where the data sheet figure was measured). I'll take a closer look when I get a chance, and revisit the strobe adjustments too. Possibly I adjusted it into a corner where everything is working except the transient response?

Charles, the reverse-biased capacitance of two diodes in series is not necessarily one-half of the nominal C (from the spec) of either one. The capacitance is nonlinear, and is maximum near zero bias. So, putting two in series not only changes the DC bridge voltage and balance and strobe requirement, but may also (counter-intuitively) actually increase the capacitance. There's not all that much reverse bias on the bridge in the first place, so the diodes operate possibly in the steep part of the C-V curve. Since you are using a known part that hopefully has specs available, check the C-V curves in the data sheet, and estimate the C you would get for one diode only, from the actual bias conditions on the bridge. Now consider how much C each diode would have at half the voltage, assuming they each share the total reverse bias equally, if you have two in series. Then divide that C by two for the series C estimate for the pair. Depending on the curves, you may find one diode is better than two. Ideally, you want the bias to land it in the flatter part of the C-V curve, where the C is low, and fairly constant. This is optimized at the maximum possible reverse, but of course, that means more strobe is needed. It's all in the trade-offs. Also note that all the adjustments tend to interact, and the diode C-V characteristic is a big part.

The easiest thing conceptually, is to just go back to single diodes and see what happens, where the system was intended to operate. Yes, I understand it may be physically difficult to make the mod, but consider that you may end up doing it anyway, after studying the situation.

Ed

I had already considered the option of Condor Audio a few months ago, when looking directly at the site, the cost was lower than the current offer but we know that the cost of living goes up continuously.

But I also need other parts for other jobs, which alone would not justify the € 20 shipping cost from Mouser. In this way I exceed 50 € of goods and the shipping is free.

I am retired, sure that for those who do not want to waste some time, Yachad's offer is interesting.

Giorgio

I ??didn¡¯t?? pay attention...

On Wed, Mar 24, 2021 at 05:30 AM, Albert Otten wrote:

The earth pin is screwed to the mounting plate, which is chassis GND. Or did
you install that pin?
Albert

Good catch Albert! I did pay enough attention.
Thanks for correcting me.

Pricing info would be interesting. They say no minimum order and 2 week lead time on custom assemblies.

--
Paul Amaranth, GCIH | Manchester MI, USA
Aurora Group of Michigan, LLC | Security, Systems & Software
paul@... | Unix/Linux - We don't do windows

On Wed, Mar 24, 2021 at 03:40 PM, Stephen wrote:

Also, although the plug is a 3-prong, no earth connection is made inside.

The earth pin is screwed to the mounting plate, which is chassis GND. Or did you install that pin?
Albert

The current owner of the Allen-Bradley mod pot line is State Electronics in East Hanover, NJ. I found their design guide pdf file to the mod pot line and just uploaded it to the files section.

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 ¨C 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 ¨C 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 ¨C 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 ¨C 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 ¨C 1.2A during the ¡®on¡¯ period with 245Vac mains ¨C 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