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Now all is successfully re-assembled and essentially operational.... but that does not mean correctly!! All PSU voltages are set correctly (a low +55V due to a faulty U712 had been upsetting all the other rails).

So now I have FOUR traces on screen and the vertical mode switch does nothing. Disconnecting P306 on the vertical board makes no difference and it appears that all vertical channel functions are being enabled including ADD and ALT (running the timebase slowly proves that ALT is enabled rather than CHOP). ADD is obviously active since there is an interaction between CH1/2 when using either shift control. Both CH1&2 are operational since I can get a waveform on the screen plugging a (sinewave) into either input jack.

There are dynamic waveforms at each of the diode switches. Trying to apply a +5V (via a 1K ohm) to any of the ROM inputs via P306 makes no difference to the display. The outputs of the ROM are all active, that is switching waveforms appear at all Q outputs.

If I disconnect the Vert Alt Sync coax (J501) from the A15 board I get only CH2 displayed.

Without deep analysis at this point, it would appear that all the horizontal functions are working including A,B and delay etc.

I should say that the DVM board is NOT connected since I already had some other suspicions in that area, so decided not to complicate matters. I prefer to get the analog part working first.

Any ideas?

I am restoring a Tek 475 that took an impact to the front of the scope. It broke some knobs and I know at least one of the pots is damaged. Part Number 311-1701-00, 20k with a switch that is dpdt. I know this is a modular linear switch double gang with a detent for the switching, but I cannot find any other manufacturers that make this. Bourns should have one, but it is not showing up on any searches.
Any help with what others have done to work around this issue would be greatly appreciated.

Thank you.

On Mon, Mar 28, 2022 at 12:46 PM, Greg Muir wrote:

That sure looks like Tek took one of the old General Radio RF oscillators, added a few refinements and blended it into their own product. The control knob, scale and physical appearance was unique to General Radio products.

I've been going through the calibration procedure in the manual, and there is a note at step #10, 65 - 500 MHz FREQUENCY ±2%, that reads "The dial is matched to the oscillator by individual marking by GENERAL RADIO CO." so I'd say that your assessment is spot on. This is, in fact, just a rebadged GR instrument, possibly with some bells and whistles added by Tek.

-- Jeff Dutky

Found a data sheet online for the bridge:
Assuming those specs are accurate (not always the case with Chinese parts!), it looks like the single-half-cycle surge rating is somewhere between 1700 and 2000 amps, so that should be ok.

The Ifrm is not specified, but there is a graph of surge current vs. number of cycles. At 100 cycles/1.6 seconds, the rating is 700 amps (60 cycles/1 sec. is 750 amps) and I'm right about there. I may need a 300 amp bridge to be comfortably within all specs... doesn't look like I should add more filter capacitance without some kind of soft-start circuitry.

Some transient measurements with my balky old 564 storage scope - just added to an album.
The first peak (measured at the transformer secondary = bridge rectifier input) as the capacitor charges is 560 amps. That should be within Ifsm specs for an alleged 200 amp bridge but I don't have any spec sheet (would also like to know the Ifrm).
I am somewhat alarmed to note that when the inverter starts up (420 amps DC inrush), for the first 1.0 second the transducer is maxed out, and it can indicate up to 700 amps... I may have to find a BIG 50 mv shunt and a differential probe if I want to know the true magnitude. But I have started it dozens of times already and so far the bridge is holding up! If it blows up I'll get a bigger one :)
Line current inrush appears to be reasonable for the size of this supply, and is within the capabilities of the circuit breakers.

The following photos have been updated in the Power supply inrush measurements album of the [email protected] group.

• Bridge input current, inverter startup

By: Charles <charlesmorris800@...>

On Tue, Mar 29, 2022 at 01:25 PM, Ed Breya wrote:

I'd suggest doing some experiments to get a good handle on what each part of
the system actually does. The big thing is the dynamotor, which can probably
be soft-started with some simple old-school techniques too. If you have a big
enough variable PS or other means, you can estimate the current (starting
torque) needed just to spin up the motor a little. Then you'll have some idea
of how low it can be limited, and still get going. The next is to consider the
load. Is the dynamotor output driving say, a bunch of tube equipment, with all
those heaters to light up? Actually, I'm wondering what is the form of its
output? I had a small one many years ago, that was only for making tube B+
from 28 VDC, but I think once you have the motor part, the "dyna" part could
be whatever you want.

Also, what is the control sequence? Do you juice up the 28 VDC, then throw a
big switch or contactor to start the motor? Or, does the line input get
switched, and everything goes? Another question is whether you have any taps
on the big transformer that may allow for reduced-voltage starting. Without
knowing more about the pieces, but presuming your 240 branch line wiring
includes the neutral, one alternative is start the whole works on 120 V, then
relay switch to the full 240 after some short delay, once the motor gets
going.

Since you have that nice current sensor, you can check out whatever you need.
It would be good to check the magnetizing current of just the transformer with
no load.

Thanks for the observations, Ed. In reverse order:
I actually checked the magnetizing current when I first got the transformer, and it doesn't even move the needle on my Amprobe clamp-on meter's 6 amp range.

The 240 volt outlet is line, line and ground only. No neutral and it's not code to use the ground wire as one. Besides, I already had the 12-2 (+ ground) Romex. Have you seen the prices of 12-2 lately... let alone 12-3???
No transformer taps, just two 120 volt primaries in series.

The rotary inverter I'm currently (pun intended) experimenting with is a 115 volt, 400 Hz, three-phase 2 KVA (or 1.5 kva single-phase) machine with carbon pile regulators for frequency (RPM) and output voltage. Applying +28v to the Switch On terminal actuates an internal contactor to turn on the motor and also a relay that connects the three-phase to the output terminals. No starter circuitry. As it turns out, the transfer relay does not engage until the DC is 21 volts or higher (probably to protect the generator in the event of low DC bus voltage) so the huge drop during "cranking" keeps it offline until it's spinning nearly full speed anyway. It's not a bug, it's a feature!

So my starting procedure is: Power supply AC breaker on, wait a second or two for inrush; DC breaker on; trigger the relay line and away she goes :) I can actually turn the DC on first with the relay line tied high, and flip the AC breaker, and it doesn't trip. But that's hard on the breaker contacts as well as adding the capacitor inrush AND transformer magnetizing surge to the heavy motor inrush. As I posted, once the inverter is at full speed it's only drawing 35 amps. Then I can turn on whatever 400 Hz load I wish to connect :)

I think this evening I'll do some experimenting with the Ultrastab current sensor and report back later :)

I would throw 'em out. I've seen too many of them short and cause a bunch of collateral damage. In my shop they are dumpster fodder.....

Tom

Tantalum Capacitors don't always fail right away when installed backwards...

I too experienced the exploding Capacitor effect early in my career. I used to repair boards in a Fan-out all day long for a few years. The boards that were failed in system test, which they just pulled, tagged and sent to the Fan-out area for debug. The Fan-out was just a system where all of the boards were fanned-out in front on someone who troubleshot the boards with schematics, Design Verification Routines, a DMM and an oscilloscope. We would debug to the component, have it reworked until it was working. Often a pair of dikes was used to cut IC pins on older RTL/DTL Wired-Or'd Logic busses and circuitry. Ah yes, loved those 16-bit busses with 10 agents. Anyway...

About 20 feet away was bed-of-nail testers for first time power-up and test of the boards. It was common to be disturbed (or awakened for some - grin), by a loud BANG from a reversed Tantalum Capacitor blowing up. In those days they were the Axial Package Sulfuric Acid Tantalum Capacitors. The operators all wore safety glasses and smocks to prevent injury, but they did spray the acid on the board which caused latent problems too. They were supposed to inspect them before test, but you know how some people do their jobs...

The WORST occasion though was with a Yellow T495 SMT Tantalum Capacitor on a Server card in development. It was later as an Engineer with a product in development in a lab. A new Server CPU card in development had some Tantalums on it and one was reversed. It had been in test for a couple of weeks (yes weeks), and was left overnight in Burn-in. During the night, the air conditioning was reduced to save money across all of the buildings. This elevated the temperature in the labs too - bonehead management decision.

One night, one Tantalum that was installed reversed finally failed. It was found the next day. It had burned a hole in the board 2 inches in diameter. It was funny to look at it where all of the FR4 was burned away leaving several copper layers and plane portions visible through the multilayer board. The High Current Server Power Supply had not protected the system as this had appeared as a high resistance short until the adjacent Planes finally shorted together killing the Power Supply Fuse. I guess there wasn't enough smoke to activate the Fire Control System - good thing we had enough trouble just trying to get the 5V 60MHz Frying-pan Pentium chips anyway. The lab was full of systems in development.

This is also why you specify 94V-0 flammability rating from all of your PCB vendors and 94V-2 flammability for all Assemblies.


Ross

Makes sense, Charles.JimSent from my T-Mobile 4G LTE Device

Sounds like you have things covered.

Good job!


Ross

Charles, I totally agree that a good old-school brute force linear PS is best for this. Using standard SMPSs for motor-start gets big and complicated. You can probably find or build ones made for the purpose, but why bother when you have a good setup already. A few years back, I wanted to build a line powered 20 VDC supply for running my 20 V battery tools, especially heavy loads like saws, that quickly deplete and shorten the lives of the batteries. I made an adapter from an old shot battery pack, with a heavy cable to connect the PS. My first thought was to use up a bunch of paralleled old laptop PC power bricks - I have quite a few of same-type ones in the 20 V, 4 A range. After slapping a few together, and some experimenting just trying to run a drill, I concluded that they're just not forgiving enough for real motor operation and work. I pictured needing about a dozen of them, and all the complexity of hooking together, ballasting, and protection. I used an old open-frame linear supply that I had, with about the right stuff, and deleted the regulator parts. It worked beautifully, and simply. The PS unit itself occupies about the same volume as a dozen of the SMPS bricks.

I'd suggest doing some experiments to get a good handle on what each part of the system actually does. The big thing is the dynamotor, which can probably be soft-started with some simple old-school techniques too. If you have a big enough variable PS or other means, you can estimate the current (starting torque) needed just to spin up the motor a little. Then you'll have some idea of how low it can be limited, and still get going. The next is to consider the load. Is the dynamotor output driving say, a bunch of tube equipment, with all those heaters to light up? Actually, I'm wondering what is the form of its output? I had a small one many years ago, that was only for making tube B+ from 28 VDC, but I think once you have the motor part, the "dyna" part could be whatever you want.

Also, what is the control sequence? Do you juice up the 28 VDC, then throw a big switch or contactor to start the motor? Or, does the line input get switched, and everything goes? Another question is whether you have any taps on the big transformer that may allow for reduced-voltage starting. Without knowing more about the pieces, but presuming your 240 branch line wiring includes the neutral, one alternative is start the whole works on 120 V, then relay switch to the full 240 after some short delay, once the motor gets going.

Since you have that nice current sensor, you can check out whatever you need. It would be good to check the magnetizing current of just the transformer with no load.

Ed

On Tue, Mar 29, 2022 at 11:04 AM, Jim Ford wrote:

Um, measuring voltage across a thermistor to determine current through it does
not seem very productive.? By its very nature the resistance will be changing
with the current!? I would go with the current transformer, as others have
recommended.

All I was interested in was the magnitude of the initial peak current in the first quarter-cycle (4 ms). The thermistor resistance initially is known to be 2 ohms. Although they do heat rapidly, it's not instantaneous and I think I got a pretty good estimate of the peak. You are quite right that the decay curve would be nearly meaningless as the resistance drops quickly to around .1 ohms (and .02 at full load) :)

Thanks Cristler, this must have been a lot of work! I like the extensive description in the README.md file. I guess I dis similar things (also in Python) in a far less sophisticated manner and left that path because it's such a terribly slow process as you mentioned also. One thing I didn't know was that Debugger mode could be entered with an RS232 command also.
Perhaps I'll try our program sometime with my CSA803(A), I read that your program can be stopped and restarted later on. Just to see if it works since I have the images.

Albert

On Tue, Mar 29, 2022 at 09:59 AM, Raymond Domp Frank wrote:

Is there a way to determine if these are prone to failure?

This is like asking how to know when a politician is lying. It’s when his lips are moving.
It seems to be its nature for a dipped tantalum to fail. Perhaps stress-testing dipped tantalums is more like testing wooden matches by lighting them.
I recall being in one of the assembly buildings at Tek Beaverton when I was startled by what sounded like a gunshot. The technician next to me said not to worry and that it was just another tantalum exploding!
If you decide to voltage test them, without current limit, prepared for the likely explosion. Consider containment, ventilation and very hot debris. Do not hot swap the part. Maintain your distance and use a remote ignition switch.
This sounds very exciting! I can’t wait to try it myself!

On Tue, Mar 29, 2022 at 05:16 PM, Glenn Little wrote:

I have a number of tantalum capacitors in my spares.
Is there a way to determine if these are prone to failure?

If they are of the hermetic, glass sealed, grey colored axial variety, they are much more reliable than traditional Al caps. Their main failure is a short, which sometimes happens in SMPS's. I'd just check them for capacity.

The colored, epoxy-coated variety are the infamous types.

Raymond

For a first test I would just check that they aren't already shorted. I've had several tantalum caps that were shorted but didn't look damaged externally. Checking for shorts should not require any significant voltage.

-- Jeff Dutky

I have been using Hall effect sensors for moderately high currents in a solar panel management system with great success. The sensors I am using are in the low current end of the available range. They offer complete isolation (2.4 kV) and a reasonably large bandwidth (120 kHz). pololu.com has a fairly good selection up to 50 Amperes mounted on a on easily installed carrier. Mounted cost (Allegro ACS724) is $15.00 and (Broadcom ACHS-712) is $4.95. Insertion resistances are 1.2m_ohm and 0.7 m_ohm respectively. I see no reason not to use the Broadcom unless its bandwidth (80 kHz) is of concern. If you need higher current, there are probably other Hall effect sensors from other suppliers available. The current specs refer to the linear response range. I'm sure they would not be destroyed by much larger short duration currentpulses. Check Digikey. I see they list a few Allegro chips in the 100-400 Ampere range under $10.00 (no stock, though)


Stephen

Um, measuring voltage across a thermistor to determine current through it does not seem very productive.? By its very nature the resistance will be changing with the current!? I would go with the current transformer, as others have recommended.?Jim FordSent from my T-Mobile 4G LTE Device

On 2022-03-29 11:16 AM, Glenn Little wrote:

I am not a design engineer, but, am toying with the idea of building a test jig to place the capacitors in that will apply rated voltage across the capacitor and monitor leakage current.
Would this be a valid test?

FWIW I would never apply full rated voltage to a tantalum. For test I might go to 90%, wear safety glasses, and have it in a well-shielded metal box, and never left unattended. I have seen three tantalums go "pop" over my career spanning several decades, and that was three too many. I rarely design a tantalum cap into anything any more, and I would never use them at anything above half their rated voltage. Ever see a Roman candle go off?

Steve Hendrix