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The ROM inputs may float high. Try grounding a ROM input with P306 disconnected.

--
Bob Haas

I thought you were referring to the Trigger Hold-off control, but that's a 311--1401. Which control do you mean? In any case, the vintageTEK Museum can probably supply the exact replacement part. Contact the Museum via contactus@....

--
Bob Haas

Yes, I understood that, thanks... but the 240 volt, 20 amp outlet (protected by a standard SquareD "QO" breaker) can handle the AC inrush adequately, especially since I have an NTC inrush limiter on the transformer primary. Certainly that huge toroid transformer is not at risk. My main concern is for the bridge rectifier. Silicon junctions are far less tolerant of even momentary overloads, after all.

Anyhow, my original aim was to make the power supply big enough to withstand the inverter's inrush current and it seems to be doing that. I doubt very much if I will ever encounter a piece of mil-surplus gear that has a similar large startup demand ;)

Just for kicks, I looked through one my "trophy case of failures" (cookie tins), and it looks like all the failed Ta caps I've found are the dipped type with color stripes as the only marking.? I did have one of the hermetic Kemet caps in a 184 die years ago, but that must have happened before I started keeping failures.? As a former component engineer, I have no problems with name brand Ta's, provided they are spec'd correctly.? I've had to explain the history of their failures to more than a few designers who had no clue...
-Dave

Interesting suggestion, thanks! I could use a triac with a slow ramp-up of duty cycle, like turning up a light dimmer?
Seems like it would not lower the peak inrush current at the diode though, just chop it up into narrower pulses and more of them?

You might consider a zero-voltage-switching triac module on the transformer AC input, for somewhat more consistent and gentle turn-on events...

Jon,

I will replace the 1mfd and less tantalums because these are typically high in ESR by now. The three 4,7mfd tantalums in the 2400 series power supplies Mr. Y. from Condor Audio says will be open is spot on. I now have 50V film types in there. The ones I pulled were open. The R82 5% type is the one I usually use as replacements.

For your 2/, I will use ULD 47mfd 25V as the decoupling in place of the tantalums. I have had enough bad to question them. Ones that are still good, I leave in until they fail provided the voltage rating of the tantalum is enough above the B+ voltage. The additional capacitance and the specs of the ULD is suitable for the original tantalums. Your statement about these failing is right. One place that has what you describe is the 13V supply in a 485 using 15V decoupling tantalums. Mine uses the above value and voltage.

For your 5/, what 7000 series are you referencing? I have seen the nominal 54V supplies use 60V or 75V axial tantalums off the rectifiers. Those tantalums got rather warm in operation after a couple of minutes. Mine now have 100V UHE types in 47mfd or 56mfd. I know it is well high as replacement. The ripple is very low now. The decoupling from the series pass regulators is also the same value and voltage.

Mark

I will add in the file section some pics how to modify the DS1742 to add an external Li cell.
Lutz, DH7LK

Bonjour,

We have boxes of these, since 1970s.




And worked to fix many TEK TM500 and 7000 and 2465 scopes with failed tants.

1/ The metal can axial hermetic tants of epoch 1960s..1990s ...very low ESR, long life and high quality. we have not seen a failure. Used in MIL, space and telecomm apps.

2/ The susceptible tants are the radial dipped tants in drop shaped packages, used for 5..25 V bypass. Typical failuere is short then open and splitting after emit smoke or small flame,

3/ old Sprague and other USA tant mfg specs and cats I recall that:
Use in current limited circuits only
Subject to microsecond shorts
Derate voltage.

4/ The ones to recap are the radial plastic dipped.

5/ One 7000 mainframe had tants that were very close to the bus voltage. Those failed open.

Happy to unearth the old stock and test some metal hermetic and dipped untis, any suggested vlaues and bus V vs rated V?

Kind Regards


Jon

The following files and folders have been uploaded to the Files area of the [email protected] group.

• /TDS3032 NVRAM.pdf

By: DH7LK <dh7lk@...>

Description:
Repair of DS1742W-120 NVRam/Timekeeper

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