What was Tek thinking? Capacitor C420 in the 181's schematic gets spec'ed as an electrolytic cap of
6.25 uF at 300 volts, -10%-+100% tolerance. It's filtering the -25-volt line, so 10 uF at 450 V should
do nicely. as a replacement.

I can't figure out why a Tek engineer would specify a nominal value of "6.25 uF", considering that electrolytics
aren't that precise to begin with.

Any insights would be welcome.

Thanks, and 73--

Brad  AA1IP

Hi Mark & Others,

I have finally managed to program new EPROMs after a bit of a nightmare in having misplaced my old programmer and also loosing the software for another one.

The bad news is the re-programmed EPROMs don't fix the issue. I was starting to get a bit suspicious now about the processor itself which is a S68A08 and having looked around on the web for a possible replacement I found a few other posts where people had issues with the S68A08 processor in the 2445.

The IRQ waveform is not as expected as per the trouble shooting procedure in the service manual. I have run the NOP and the address lines do seem to have a binary count on them but I am still not convinced yet that the processor is working correctly.

Do you have any recollection of what the faulty processor was you had mentioned previously? This one is an AMI (American Microsystems) S68A08. From what I can tell that specific microprocessor was not very common.

Sorry for the very belated reply.

Hi Chuck,
When we left Yahoo for Groups.io there were so many new issues to deal with that what would happen to the Yahoo TekScopes was the last thing I was thinking about. Once the move was completed and the initial chaos settled down and the world didn't end we all breathed a sigh of relief. By then my entire focus was discovering all the new features and help that Groups.io provides. I never looked back.

I didn't realize I could lock the Yahoo TekScopes down. Yahoo stopped providing help and support long before we left them - that was a significant reason we chose to leave so there was no one at Yahoo who might have suggested it would be wise to lock it down.

If it isn't too late to "lock it down" do you know how to do that? I am still the administrator of the Yahoo TekScopes so I should be able to do if as long as I know what is required.

Dennis Tillman W7pF

Is there something going on at Yahoo that would cause several people
to unsubscribe from the YAHOO (not the Groups.io) TekScopes all at the
same time?
Of course it hasn't been active for over 2 years but that hasn't
stopped a handful of new people from trying to subscribe to it each month.
I still get those requests from Yahoo and, of course, I politely tell
the potential new Yahoo TekScopes member where we moved to.
Dennis Tillman W7pF

Is there something going on at Yahoo that would cause several people to
unsubscribe from the YAHOO (not the Groups.io) TekScopes all at the same
time?
Of course it hasn't been active for over 2 years but that hasn't stopped a
handful of new people from trying to subscribe to it each month.
I still get those requests from Yahoo and, of course, I politely tell the
potential new Yahoo TekScopes member where we moved to.
Dennis Tillman W7pF

Disconnected the GPIB option and the 2465B is back working like a champ.

Thanks for pointing me in the direction of the option, not the logic main
board.

I know for a fact I will never need GPIB on this 'scope, so problem solved.

On Fri, May 1, 2020 at 9:56 AM Andy Warner <andyw@pobox.com> wrote:

Thanks for that, reading the section of the manual around the diagnostic
tests is not very clear to me - I have a hunch that at some point the penny
will drop and then I can see what the text was trying to tell me, but I am
not there yet. I have zero plans to use the GPIB interface on this scope,
so I will plan an initial re-test with the GPIB option disconnected, if
that resolves the issue, I might stop looking at that point.

My de-oxit comment was more about the CH 2 trace missing, since that pot
was noisy, I wondered if it had hard failed.
I found the front panel exerciser test, and it looks like the pot is
working fine, at least as far as the micro is concerned.

Will learn more when I get it on the bench this weekend.

On Fri, May 1, 2020 at 7:51 AM Chuck Harris <cfharris@erols.com> wrote:

U2160 is not the problem. It is a spare 2764 pad left just
in case Tektronix needed more EPROM space for some option.

The options buffer would be located in the first option board.

What options do you have?

Odds are pretty good that your GPIB card is probably not
installed properly.

DeOxit won't hurt anything. If anything it can be messy, but
electrically it is nonconductive.

-Chuck Harris

Andy Warner wrote:

I recently re-capped my 2465B and replaced the DS1225 (copied the

contents

over.)

All seemed to go well, and the unit performed well in my post-rework
testing.

However, when I turn it on now, about 2 weeks later, it reports a
diagnostic failure:

TRIG'D flashes, and the following pattern is displayed:

Ready - Off
+ - Off
- - On
CH 1 - On
CH 2 - On
CH 3 - On
CH 4 - On

Reading Section 6 of the service manual, I think Tables 6-6 thru 6-8

are

telling me the problem is with U2160, Buffer ROM. Am I decoding that
correctly ?

Pressing TRIG A/B moves things along, but I am unable to see a trace

for

Channel 2, all other channels seem to work correctly.

Before I clear space on the bench and tear into the unit again, I

thought I

would check with the group for advice on these issues. Whether they are
likely to be connected or separate and coincidental (I don't usually
believe in coincidences.) I am not even sure what U2160 does, I need to
look that up.

I did not remove or recap the main board, it looked like a whole raft

of

hassle that I did not want to get into. I did recap the PSU, A5, A9 and

the

GPIB option card.

It is possible that some de-oxit got where it shouldn't have, I was
careful, but perhaps not careful enough. While I had the case open, I

gave

a couple of the switches and pots a shot (including the CH2 position

pot),

because they were noisy.

Regards,

--
Andy

Hi, Albert.

I don't know about discrete RF Darlingtons, but Avantek (bought by HP, spun out as
Avago Technologies and then merged with Broadcom) used to have RF amplifiers based on
the Darlington configuration.

Incidentally, I've run across the Motorola/Freescale/NXP MRF transistors elsewhere as
well.  We use the MRF275G 150 Watt UHF devices in some of our radios at Raytheon
Technologies; they are made by MACOM now.  The MACOM transistors seem to be pretty
bulletproof; we also use one PolyFET device, which is not as robust, so I don't
recommend them.

I looked up the MRF511; it looks like several manufacturers are still making it.  I
wasn't able to get a datasheet (I didn't try very hard), but I did see that it was in
the micro-X package.  The 4 leads are probably Base and Collector 180 degrees apart
and 2 Emitter leads to reduce inductance to GND.

HTH.

Jim Ford
Southern California
USA

Thank you Greg, Raymond for your teaching of 7L13 precautions.

It will take me a good while of studying the details of the RF, microwave circuits, attenuators, filters, mixers, oscillators and other more mundane details of the 7L13, including ways to protect the input (maybe a pair of schottky diodes, preceded by bipolar diodes, to limit input to 0 dBm or less), all this stuff BEFORE I ever plug this triple unit into the mainframe.

And better there be no tantalum capacitors in there! I'm not looking forwards to disassemble this monster to look for them.

Ernesto

A note on connecting test equipment (namely spectrum analyzers) to an outdoor antenna.

There is little that can save an instrument from external environmental electrical effects (mostly static) that will wreak damage to the front end unless properly protected. But then damage may still occur so it is a gamble one must make if wanting to do so.

As was mentioned, one need not be near an active lightening event to sustain sizeable static buildup on an external antenna. Clouds characteristically possess static charges as they move through the atmosphere. This creates a hidden potential difference between the cloud and the earth. The electric field is normally not a problem since the other electrode – the earth – is a solid ground point. But when you add an ungrounded antenna (such as a longwire or ungrounded element antenna) significant potentials can build until they find someplace to go to the earth ground.

DC blocks are rated for low voltages. As you may well be aware of, these items are usually very compact devices with low voltage rated internal component(s) that will not withstand overvoltage conditions. The main internal component, usually a capacitor, has a small geometry which means the input-to-output distance can be a very easy “spark gap” to jump when a sizeable static discharge is impressed upon it.

Risetimes of the rogue voltage can also be a problem. Even if one fabricates a high-voltage blocking device, any rapid cloud-to-ground or ground-to-cloud discharges far off in the distance can create a transient that can couple to the antenna. Given the fast risetime of these transients and their higher frequency characteristics this will more effectively couple through the capacitive element used in the blocking device thereby presenting a more significant voltage spike to the input of the connected instrument.

One way to overcome these problems would be to fabricate an inductive coupler between the antenna and instrument. The primary (input) winding can be grounded on the low end thereby passing any static buildup on the incoming coax to ground. But there would still be an issue with any external transients that may couple into the antenna since they are not DC potentials.

Off-the-shelf coaxial line transient suppressors are usually rated for normal lightning suppression purposes and can have a characteristically high breakdown voltage much more that the relatively low maximum voltage permitted by the instrument input. Many instruments may display a warning on the front panel near the input connector sporting maximum voltage levels anywhere from a few upwards to 50 volts or more before damage will occur. These levels should be heeded lest one be faced with costly instrument repair or none at all given the legacy ages of much of the equipment possessed by members on this list.

As the well-known Clint Eastwood once said – “How lucky do you feel?”

Greg