Test Equipment Anonymous (TEA) group therapy thread

[W6EL]

Concerning the 8640B: would anyone have an idea where I could find 3D drawings of those gears? My son could produce them in some resin.
stl., gif.... ipj..

It's not quite a drawing, but there is a guy from India selling brass replacements on ebay.

https://www.ebay.com/itm/154054911606?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=OdhhNC_bQTK&sssrc=2047675&ssuid=fwddx0t1ry6&widget_ver=artemis&media=COPY

I have not tried his product but they do look quite good. Since he's down to the last one, maybe buy a set and ask him for the design drawings?

[TERRA Operative]

My newest fleabay toy has been delivered: a Tek THS720A scope. Isolated inputs, 2 channel, 100MHz 500MS/s, with an almost complete selection of accessories. It works nicely except that the NiCd battery is dead. Not really surprising given the age.

Condition was "used" without mentioning the battery. Price was very decent but not very low.

Replacement batteries (NiMH not NiCd) available for £34.

I have some old (i.e. unused expired) sub-c NiCds, and could probably disassemble the existing battery for its strange "collar" connector.

Options:

• do nothing, mains only operation. (Don't actually need the scope, let alone battery operation) - but that's par for the course here!
• buy near replacement
• hack my own
• complain to vendor

I'd go for make my own, batteries in something 20+ years old are a consumable.
As for complaining to the seller, how likely are you to consider buying from them again? As they could potentially add you to the blocked buyer/bidder list.

Standard sized C-cells are used in the battery packs in these scopes. With a bit of extra wire and a contact bent out of nickel, you can make the scope work on 4x individual cells.

Also, if you are up for a few SMD resistors, you can convert to a THS720P for free. (No recalibration/adjustment needed), also if you are good with soldering SMD chips
Also you can remove the Flash chip and upgrade the firmware to the latest v1.16 with a chip programmer (If not already there). I used my TL866+ programmer, and then solder the chip back in place, but a recal/adjustment is needed.

And I have made a design (Which I'm planning a second revision) to convert the backlight to LED. Double the brightness for half the power use. (I can send you gerbers, but best to wait for the new version I think)

Only a few resistors labelled on the board, do the others downgrade it to a THS710A?

David

I rebuilt my batteries with tabbed Ni-MH C cells.
I carefully pulled the ring and strap off the old pack for the side contact after cutting the black heatshrink away.
I then soldered together the 4 C cells and taped them solid with kapton tape. After adding more kapton tape to where the contact ring sits for a gently firm fit, the ring and strap were soldered to the battery too.
After that I just covered in some new black battery pack heat shrink from ebay and then applied a label I made and printed out on printable sticker paper (I used thin packing tape to laminate the sticker to make it shiny and protect it).
I've made like 3 packs this way, they all worked out well with various amounts of swearing.

tggzzz, if you let me know the capacity of your final battery, chemistry and month/year of construction, I can spit out some labels in a PDF file for you to print your own labels if you like.



For those resistors, if you install all of them, you'll get a THS710A. Not sure why you'd want the downgrade though.
 
Note! There is no worthwhile way to upgrade a THS710STD or 720STD to THS720A or THS720P as the loadout on the main board (and maybe the main board itself) is different (Mainly stuff for triggering from the DMM jacks, but some other bits too IIRC).
Also, while you can tell your scope it is a THS730A with the resistors (removing both R204 and R205), you won't get the increased bandwidth without finding some replacement input hybrids and a few other bits, in which case you'll be pulling parts from a THS730A anyway, so the point is moot...

Oh dear, that thread looks like another interesting time sink

But thanks

The different battery clip is an interesting concept; I'll consider that. The NiCds I have are tabbed an in an 8-cell battery, so some "adjustment" would be required in any case. The advantage of having one contact on the side of the battery pack is that if the pack is inserted the wrong way round no damage is done. When I first looked glanced at the battery I wondered about inserting it backwards.

I'm prepared to consider adding a few SMD resistors, not desoldering a flash chip.

I have found the details of the official process for upgrading the firmware that needs no desoldering or adjustment and uploaded to Tekwiki, but it needs an obsolete and practically unobtanium dev board... (a Motorola M68332BCCDI 'DIBUG' debug module) The Tek motherboard for the programming pod is otherwise pretty simple to construct, if it weren't for that apparently rare dev board.
I think maybe it could all be emulated or reverse engineered to a raspberry pi, except I have no experience with such software things, and all my THS scopes are updated anyway... so.... meh?

If someone with the correct skillset could get their hands on the correct dev board and/or if the software could be examined, I'm sure that Raspberry Pi could be pressed into service without toooo much difficulty...?

[EggertEnjoyer123]

Well, I bought some more stuff at the swap meet this month, like this noise generator which cost me $25. It didn't work initially, and I determined that the filament wasn't being powered. After testing all the semiconductors on the board, I found a zener diode which has failed open. After replacing the diode, everything worked correctly.

The noise output seems to be flat up to just under 1 GHz. The noise generator uses a vacuum diode tube to generate the noise. I believe the tube used is the Sylvania 5722. There is 200 volts on the board (guess who shocked themselves with that), and the filament voltage goes from 2 - 5 volts.

The advantage of using a tube is that no calibration is needed, unlike diode based noise generators. The vacuum tube generates shot noise, and the current noise density is 2 * e * I, where e is the charge of an electron and I is the current going through the tube. The voltage noise density is 2*e*I*R^2. For a 50 ohm resistor, the voltage noise density is 4 * k * T * R, and if we find the ratio, we get (2*e)/(4*k*T) * I * R. At T = 290K, 2e/4kT is extremely close to 20, so the noise figure is just 20 * I * R (you will find this formula on the datasheet for the 5722). Notice that you just need to measure the current to get the noise figure, unlike with solid state noise generators. (Of course, this instrument is 56 years old judging by the date codes, so the current sensing resistors probably have drifted a lot).

The disadvantage is the power consumption, and the fact that high voltages are needed.

I tested the return loss and the noise output with a spectrum analyzer. The meter reading seems to be off by 2 dB (at 15dB noise figure, we would expect -114+15 = -99, not -97). This is most likely because the resistors inside have drifted a lot. I'll probably have to replace all the carbon comp resistors someday. Also, measuring the current through the tube seems to be nontrivial, given the entire mess of wires.

[wolfy007]

Finally found a Fluke 8800A for a reasonable price, and after minor repairs (though someone has been in there before me), its working and reading pretty much the right voltage. 
Still needs performance check and maybe calibration as I think resistance is reading a touch out of spec as may be AC voltage.
PS: also a bit more cleaning, I only wiped it down a bit.
PPS: That 4000uF 10V cap is reading 20uF, the two 220uF 10V caps read ~400uF, one of the 220uF 40V reads 300uF, the 50uF 50V read a not as horrible ~60uF, but ESR on all is not great, especially the 4000uF.

[tggzzz]

I rebuilt my [THS7xx] batteries with tabbed Ni-MH C cells.
I carefully pulled the ring and strap off the old pack for the side contact after cutting the black heatshrink away.
I then soldered together the 4 C cells and taped them solid with kapton tape. After adding more kapton tape to where the contact ring sits for a gently firm fit, the ring and strap were soldered to the battery too.
After that I just covered in some new black battery pack heat shrink from ebay and then applied a label I made and printed out on printable sticker paper (I used thin packing tape to laminate the sticker to make it shiny and protect it).
I've made like 3 packs this way, they all worked out well with various amounts of swearing.

Neat I like the kapton tape idea for holding them before adding the heatshrink.

My current thinking <insert timestamp here> is to leave the original battery untouched, but then make a new battery from stuff in my junkbox treasure chest:

• use my existing sub-C cells (NiCd, 2Ah, tabbed, NOS but expired 4 years ago) which look like being the right diameter but a few mm short
• hopefully just bend the existing three tabs between 4 cells through 180degrees, so the cells are above each other rather than next to each other
• use some sheet phosphor to create the ring contact and "wire" to the top end
• use insulating spacers and/or metal washers to extend the battery to the correct length
• cover the lot in heatshrink

There are a few details that I'll work out as I go.

It will then be perfectly obvious that this is an, ahem, aftermarket battery, and can be compared with the original.

That's what I've done when remaking Tek1502 batteries, which are a pain by comparison (always leaking, sometimes the battery box is broken).

tggzzz, if you let me know the capacity of your final battery, chemistry and month/year of construction, I can spit out some labels in a PDF file for you to print your own labels if you like.

Thanks, but I'll just put a handwritten paper label on it.

For those resistors, if you install all of them, you'll get a THS710A. Not sure why you'd want the downgrade though.

Oh.... pffffft. That's what happens when I don't read things correctly before replying.

I'm not too worried about removing SMD resistors; I had to do that on a Tek 2445A where the electrolytic had wreaked its traditional havoc.
 

Also, while you can tell your scope it is a THS730A with the resistors (removing both R204 and R205), you won't get the increased bandwidth without finding some replacement input hybrids and a few other bits, in which case you'll be pulling parts from a THS730A anyway, so the point is moot...

Yeah, that would be a step too far, and I have other scopes with better bandwidth (Tek 485, Philips PM3400).

I have found the details of the official process for upgrading the firmware that needs no desoldering or adjustment and uploaded to Tekwiki, but it needs an obsolete and practically unobtanium dev board... (a Motorola M68332BCCDI 'DIBUG' debug module) The Tek motherboard for the programming pod is otherwise pretty simple to construct, if it weren't for that apparently rare dev board.
I think maybe it could all be emulated or reverse engineered to a raspberry pi, except I have no experience with such software things, and all my THS scopes are updated anyway... so.... meh?

Sounds like too much trouble for too little benefit.

It has the 1.14 firmware, and has apparently only been powered up 480 times. Until I find otherwise, that will be sufficient.

[TERRA Operative]

Yeah, I reckon do the THS720P hack, make a battery and build a USB cable as I detailed earler for convenience if you want to connect to a PC.
If you have the correct probes, then you are pretty much set.

PPS: That 4000uF 10V cap is reading 20uF, the two 220uF 10V caps read ~400uF, one of the 220uF 40V reads 300uF, the 50uF 50V read a not as horrible ~60uF, but ESR on all is not great, especially the 4000uF.

Check the dissipation of the caps that read high.
If they are leaky, they can throw off the capacitance tester as the dissipation/leakage means the cap takes longer to charge etc which can play games with the charge/discharge-over-time calculations done by the tester and lead to inflated numbers.

[TERRA Operative]

And on the note of THS scopes, I just won an auction on a P5102 probe with all accessories.

I'm well on my way to having a complete set of accruments for my THS720P.

[tggzzz]

If you have the correct probes, then you are pretty much set.

Two P6117 (200MHz, rubber, 300V), so close Has accessories but only one witches hat.

[TERRA Operative]

The P6117 are 200MHz probes and are included with the THS730A, THS720A, and THS710A so not bad at all. The P5102 is the high voltage probe, but if you aren't exceeding the capability of the P6117 then you are set.

[factory]

Concerning the 8640B: would anyone have an idea where I could find 3D drawings of those gears? My son could produce them in some resin.
stl., gif.... ipj..

It's not quite a drawing, but there is a guy from India selling brass replacements on ebay.

https://www.ebay.com/itm/154054911606?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=OdhhNC_bQTK&sssrc=2047675&ssuid=fwddx0t1ry6&widget_ver=artemis&media=COPY

I have not tried his product but they do look quite good. Since he's down to the last one, maybe buy a set and ask him for the design drawings?

Doubt they would give out their drawings, reading the description they only list a few at a time, currently shows 5 sets of the three gears that fail, I'm probably going to need another set for the 8640A that I got a month or two ago, but won't order any until next year, shipping too busy & delays are likely this time of year.

David

[factory]

PPS: That 4000uF 10V cap is reading 20uF, the two 220uF 10V caps read ~400uF, one of the 220uF 40V reads 300uF, the 50uF 50V read a not as horrible ~60uF, but ESR on all is not great, especially the 4000uF.

Check the dissipation of the caps that read high.
If they are leaky, they can throw off the capacitance tester as the dissipation/leakage means the cap takes longer to charge etc which can play games with the charge/discharge-over-time calculations done by the tester and lead to inflated numbers.

Those old caps were often specced at -10%, +50%, +75% or sometimes +100% tolerance, as can be seen in the parts lists (for HP TE anyway), dead ones are usually reading very low, of course ESR isn't great for older caps and leakage can be a problem too (both electrical & liquid leakage). Even new old stock caps show this loss of capacitance, usually all from that batch are bad, others of similar age can measure fine, YMMV.

Modern ones are made precisely, to minimize material(s) usage and maximize profit  , most seem to measure near the lower end of the tolerance.

David

[Wolfgang]

Nice thing ! Never seen one of those in the wild.

Look here for tube based noise generators:

https://electronicprojectsforfun.wordpress.com/making-noise/noise-sources-i-have-built/a-tube-noise-source-using-the-2d3b-tube/
https://electronicprojectsforfun.wordpress.com/making-noise/upgrading-a-vintage-rohdeschwarz-sktu-noise-source/

[Robert763]

I weakened and bought another RF Spectrum Analyser. I'm back up to 5. It's fairly modern handheld, colour touch screen 1GHz max, markers etc. It also has a Selective Level Meter and QAM analyser. Complete with battery that works and soft case
At least it didn't break the bank, £24.99 includng post 

[Robert763]

And I also succumbed to my current clamp fetish.
A nice 1000A AC/DC clamp in red complete with carry case and instructions. Looks unused, works and only cost £7.93 including postage.
Not a big sender this week....

[TERRA Operative]

Well, the HP 8648C arrived early, literally landed in the genkan just now. 

And, so far, so good! The output looks ok on the frequency counter, and the output level is pretty close too (Pics taken with no warmup time).

I'll need to give it a spit polish etc. You can see the front panel is smashed on the left (Requires replacement of the entire front panel chassis part) there is a missing rear foot (very 3D printable if I can't find a replacement) and I might have to disassemble the LCD to clean the zebra strips to find some missing pixels.

But so far, I'm happy with it.


Net step, make a shopping list. 

[TERRA Operative]

And first item on the agenda, replace the Rifa capacitors before they...

ZAP!

F**k that stinks! 


I just popped my Rifa cherry! I'm officially part of the club. 

[wolfy007]

PPS: That 4000uF 10V cap is reading 20uF, the two 220uF 10V caps read ~400uF, one of the 220uF 40V reads 300uF, the 50uF 50V read a not as horrible ~60uF, but ESR on all is not great, especially the 4000uF.

Check the dissipation of the caps that read high.
If they are leaky, they can throw off the capacitance tester as the dissipation/leakage means the cap takes longer to charge etc which can play games with the charge/discharge-over-time calculations done by the tester and lead to inflated numbers.

Those old caps were often specced at -10%, +50%, +75% or sometimes +100% tolerance, as can be seen in the parts lists (for HP TE anyway), dead ones are usually reading very low, of course ESR isn't great for older caps and leakage can be a problem too (both electrical & liquid leakage). Even new old stock caps show this loss of capacitance, usually all from that batch are bad, others of similar age can measure fine, YMMV.

Modern ones are made precisely, to minimize material(s) usage and maximize profit  , most seem to measure near the lower end of the tolerance.

David

 Went back and checked. Dissipation factor on the 4000uF was just rubbish, and one of the 220uf 40V was bad but the other seemed marginal (this would reflect the out of spec supply voltages and the meter appearing locked up all 0's on display). The two 50uF 50V (read as 60uF) seem OK on dissipation factor, the 220uF 10V caps one was OK the other seemed marginal. The electrolytic parts for the 8800A were listed as +50or75% & -10%.
In any case I replaced them all, so Im not putting any back  , will keep the caps that seem OK in my used box parts.

[Robert763]

And first item on the agenda, replace the Rifa capacitors before they...

ZAP!

F**k that stinks! 


I just popped my Rifa cherry! I'm officially part of the club. 

Oh no,
That is NOT a normal RIFA use case. It's not an EMI filter. It's a snubber across the SMPSU MOSFET.  Often when they fail in this design of PSU they take out multiple components. This can include the feedback transformer (just visible in your photo) which is a special part.
If the unit was still working post RIFA failure don't run it until the snubber is replaced. Unlike the EMI filter caps running with this one failed can cause more damage. The same circuit is used in the HP 8920A/B radio test set and I've repaired a couple of them and they are a PITA to repair.
Robert.

[TERRA Operative]

Ooh, I'll check that! Thanks for the heads up!

I've already replaced the cap (and the other Rifa cap on the mains input) with an equivalent modern part, I wonder if there are schematics for this PSU module around somewhere?

[Neomys Sapiens]

I just popped my Rifa cherry! I'm officially part of the club. 

Which club are you referring to? Or should I ask: Striped or spotted?...
 

[Robert763]

Ooh, I'll check that! Thanks for the heads up!

I've already replaced the cap (and the other Rifa cap on the mains input) with an equivalent modern part, I wonder if there are schematics for this PSU module around somewhere?

The power supply is based on the Computer Products / Artesyn NFS series. Circuit diagrams are hard to find. One unusual feature is the feedback loop and isolation. It is based on a small toroidial transformer with a iron dust or ferrite core. The core forms part of the drive circuit osciator and the feedback. It appears to use saturation of the core caused by current in the low voltage side winding to control the pulse width. Very clever design but  right PITA to troubleshoot I've had a couple, from HP8920A/B radio test sets with blown snubber RIFAs. This results in the MOSFET going short to the gate. That then literally blows up two or three resistors, a oscillator transistor and one of the windings of the feedback transformer. Re-winding the little transformer is possible but a bit of a pain.

Robert.

[W6EL]

Ooh, I'll check that! Thanks for the heads up!

I've already replaced the cap (and the other Rifa cap on the mains input) with an equivalent modern part, I wonder if there are schematics for this PSU module around somewhere?

The power supply is based on the Computer Products / Artesyn NFS series. Circuit diagrams are hard to find. One unusual feature is the feedback loop and isolation. It is based on a small toroidial transformer with a iron dust or ferrite core. The core forms part of the drive circuit osciator and the feedback. It appears to use saturation of the core caused by current in the low voltage side winding to control the pulse width. Very clever design but  right PITA to troubleshoot I've had a couple, from HP8920A/B radio test sets with blown snubber RIFAs. This results in the MOSFET going short to the gate. That then literally blows up two or three resistors, a oscillator transistor and one of the windings of the feedback transformer. Re-winding the little transformer is possible but a bit of a pain.

Robert.

Robert,

When I opened mine (8647A) up to replace the RIFAs (and there are many, small and large, look carefully, @Terra Operative, I saw the word "BAD" written on the back of it, and realized someone had gone in there and fixed it up some prior to my ownership.

If anyone ever finds a schematic, I am sure the DIY community would be most grateful.

[W6EL]

I fixed my Fluke 8505A pair last night.

First, as I mentioned before, I moved the working parts to one unit, which basically got that one going just fine.

The second unit now had a bad ohm board and another issue which was easily conflated with the first. The second issue was that the 4W switch only jumped one of the two required terminals across. This has the effect of issuing the "Error H" warning, which means it is complaining about an invalid connection.

The first issue though, was that the ohm board latching relay (K1) would latch on but not latch off. This causes the ohm source lines to always be active, and in 2W mode, it drags down the voltage reading significantly.

I took out the relay, but it tested just fine. So then I put wires on the coil footprint pads and read them as the ohm board was put in and out of function. Tip, place the thing in manual trigger mode when you do this so that it isn't trying to do anything else at the same time. The "on" pulse looked good, solid 5 volts. The "off" pulse was dumpy, not lasting very long and with a strange decay shape. Under load of the relay, the off pulse was basically not happening, hardly a blip. I first replaced the tantalum pulse-shaping capacitor, C17, but this had no effect. (I am always suspicious of these little "drop" tantalums, I've seen so many go bad.) Then I went to Q3. This transistor tested at 100uA with a beta of.... 22. Yeah, it was bad. Put a new one in with a beta of +200 and now it flicks that relay with a nice satisfying snap.

As for the 4W switch, I read the service manual carefully and realized the board on the side of the unit can be extracted through the rear. Pretty cool, and beats my idea of taking the entire chassis apart. I shot some cleaner through the 4W switch and worked it some, and it seems to be working just fine now. Cleaned the guard switch while I was there. I did not clean the front/rear button as I will never press it, and I doubt it has been worked much before either. Better to just leave it be since it's working. Tempting to solder over it and gain a few more PPMs...

So yeah, a working pair of Fluke 8505A digital meters with ohms and AC (one has averaging, one has RMS). The RMS seems very good too, I must say, keeping up with my scope's RMS calculation for even the most atrocious waveforms.

Calibration is next. I am working my way through this LM299 board from China. So far it seems like this board needs a *long* warmup period before it's stable. Like, an hour or so. I sent the seller an email, I want to make sure I'm using the jumpers correctly and all that. I plan to surround it with ESD-safe black foam and place it inside a metal oatmeal can. 15V regulator on top. Then I can bring it around to calibrated meters and hopefully use that calibration at home. We will see. It will need to demonstrate that it comes up the same way every day first.

[Robert763]

<SNIP>

Calibration is next. I am working my way through this LM299 board from China. So far it seems like this board needs a *long* warmup period before it's stable. Like, an hour or so. I sent the seller an email, I want to make sure I'm using the jumpers correctly and all that. I plan to surround it with ESD-safe black foam and place it inside a metal oatmeal can. 15V regulator on top. Then I can bring it around to calibrated meters and hopefully use that calibration at home. We will see. It will need to demonstrate that it comes up the same way every day first.

Don't use the black conductive foam for thermal (or other) insulation. It will cause leakage. ESD should not be too muh of an issue for fixed insulation on an installed device. Use standard non-conductive foam for insulation.

Robert.

[W6EL]

<SNIP>

Calibration is next. I am working my way through this LM299 board from China. So far it seems like this board needs a *long* warmup period before it's stable. Like, an hour or so. I sent the seller an email, I want to make sure I'm using the jumpers correctly and all that. I plan to surround it with ESD-safe black foam and place it inside a metal oatmeal can. 15V regulator on top. Then I can bring it around to calibrated meters and hopefully use that calibration at home. We will see. It will need to demonstrate that it comes up the same way every day first.

Don't use the black conductive foam for thermal (or other) insulation. It will cause leakage. ESD should not be too muh of an issue for fixed insulation on an installed device. Use standard non-conductive foam for insulation.

Robert.

Robert,

You make a good point. Nothing even mildly conductive will help touching this thing! I will re-think the thermal insulation plan. Could always put some standoffs and an air gap in there...

--E