Date   

Re: Resistor in series

 

David Partridge wrote:

sadly they did make that very mistake quite a few times! Witness the 7904
vertical board ... R724 and R725 (499 Ohm 1/2W) always run way too hot
(burnt fingers hot) and are often well off value from overcooking ... the
PCB gets well toasted too! I normally replace them with two 1kOhm 2W metal
film in parallel and they still run pretty warm.
So, while I'm in here, and planning to replace these two resistors, should I increase the wattage? I expect that these are carbon composite resistors (but certainly don't know that for sure) and what I have ordered are supposed to be metal film 1/2 W resistors. Maybe I should order a kit of 1 W metal film resistors as well and use them instead?

Part of me thinks that it won't make much difference: these are underneath the metal shield over the HV section, so they have restricted air-flow under any circumstance. Will larger components really make much difference in their heat dissipation if they are trapped in a hot box?

Also, these resistors have likely been cooking for over 30 years, and they've only drifted by about 20%, so either they're not getting too badly cooked, or they're made of sterner stuff.

-- Jeff Dutky


Re: Anyone willing to make 3D printing of multiplier boxes

Mlynch001
 

Pictures of my enclosures are here:

https://groups.io/g/TekScopes/album?id=244462

Michael Lynch
Dardanelle, AR


Re: Resistor in series

 

sadly they did make that very mistake quite a few times! Witness the 7904
vertical board ... R724 and R725 (499 Ohm 1/2W) always run way too hot
(burnt fingers hot) and are often well off value from overcooking ... the
PCB gets well toasted too! I normally replace them with two 1kOhm 2W metal
film in parallel and they still run pretty warm.

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Tom Lee
Sent: 25 November 2020 23:20
To: TekScopes@groups.io
Subject: Re: [TekScopes] Resistor in series

In a word, no.

Anything is possible, I suppose, but asking us to believe that Tek's design
team carelessly forgot to check power dissipation is a lot to swallow.


Re: Anyone willing to make 3D printing of multiplier boxes

Mlynch001
 

The 465/475 box is an almost exact replica of the original. Modified the mounting to make it easier to install.

--
Michael Lynch
Dardanelle, AR


Re: Anyone willing to make 3D printing of multiplier boxes

Mlynch001
 

I have designed and printed several for the 465-475 etc. I also have printed a stand alone box for 455/475M as well. I can supply an STL for either one.

--
Michael Lynch
Dardanelle, AR


Re: Anyone willing to make 3D printing of multiplier boxes

 

encapsulating the HV multipliers is easily done using a "Potting Shell".
they are available cheaply off-the-shelf from many vendors, here's two examples:

https://www.polycase.com/potting-box?p=2
https://www.okwenclosures.com/en/Products/Potting-boxes.htm

a thin-shelled 3D printed box would be pretty fragile, I think.

all the best,
walter

--
Walter Shawlee 2
Sphere Research Corp. 3394 Sunnyside Rd.
West Kelowna, BC, V1Z 2V4 CANADA
Phone: +1 (250-769-1834 -:- http://www.sphere.bc.ca
+We're all in one boat, no matter how it looks to you. (WS2)
+All you need is love. (John Lennon)
+But, that doesn't mean other things don't come in handy. (WS2)
+Nature is trying very hard to make us succeed, but nature does not depend on us.
We are not the only experiment. (R. Buckminster Fuller)


Re: Anyone willing to make 3D printing of multiplier boxes

Tam Hanna
 

Hello,

how big are they.


Tam

--
With best regards
Tam HANNA

Enjoy electronics? Join 15k7 other followers by visiting the Crazy Electronics Lab at https://www.instagram.com/tam.hanna/


Re: Delay Time Position Vernier

 

Colin Herbert wrote:

Your other problem is setting the Vernier where it is supposed to sit when you put it all
together again! They don't all sit at 0.00, There is data about where they are supposed
to read when the pot is at the extreme anticlockwise. If you have problems, just make
them known here and you should get the right advice. Don't just adjust it to 0.00 - it is
likely to be wrong.
Is this covered in the service manual? I would be interested in knowing how this needs to be set, as I am planning to move the guts of one 475 into the frame of another, and that would obviously involve disassembling and reassembling the delay vernier.

-- Jeff Dutky


Re: Fix or Part Out a Tek 475A

 

David Collier wrote:

Yes the trigger slope shafts were bent as well! Now straightened.
How did you straighten the shafts? I mean, other than bending them, did you do anything special?

I've got two parts scopes that I'd like to make into one working scope and one parts scope, where the main damage is to the knobs and shafts on the front panels of both scopes (and, like yours, the scopes themselves seem to be in almost perfect working order but for the front panel damage), and I would appreciate any pointers you might have in how to repair the trigger slope switch shafts (I may just have to accept broken knobs, since I haven't found any good source for the trigger level knob, which is what appears to be the common victim of this mistreatment).

The 475 seems to be a remarkably durable instrument, even in the face of obvious physical and electrical abuse. I had thought that it was amazing that my father's scope was in such good working order, but it had been treated with kid gloves and stored in a dehumidified basement for most of its life. One of my parts scopes is clogged with black, sooty dust and covered in something like grey paint, but seems to be in even better working order than my father's scope. And the 475A that I'm currently working on clearly had something happen in the HV section, but the rest of the scope appears to be in very good shape (meaning that other functions seem to be working even though the beam intensity amplifier is busted).

I was expecting all kinds of things to be damaged: blown diodes, transistors, and ICs, dried out and shorted caps, resistors that had drifted out of spec, dim or busted CRTs, etc., but, aside from the 475A (which is certainly delivering on some of those expectations) things have been quite functional, and I deliberately bought the cheapest, most likely to be busted scopes I could find on eBay! It's a testament to the engineering prowess of the folks who designed and built these machines.

-- Jeff Dutky


Anyone willing to make 3D printing of multiplier boxes

Mark Vincent
 

I was wondering if anyone was willing to make the boxes for the multipliers
so ones that go bad can be made. The box would need the holes for the wires
and mounting threads as original. The old epoxy will break down like the
transformers do. Anyone willing to rewind/build replacement transformers
would be welcome. Buying new diodes, condensers, etc. to put in the box
then use silicon sealant will give enough insulation. I know the sealant
works on 21kV. I had to put some on the PDA where the white was no longer
sealing. The clear type I used stopped the arcing and the scope works fine.
The diodes that can be used are 2CL2FM. They are 20kV and heavier capacity
than originals. I know the diode and condenser from ground to the current
limiter/sensing is low voltage. The diode should be very fast with low
reverse current at 600-1000V, e.g. 1N4937, and the condenser is ,01mfd. I
see comments about multipliers going bad. This would be a good way to get
the scope back working.

Mark


OS-34/URM-32 and OS-4B/AP schematics wanted

Mark Vincent
 

I am looking for the schematics of the two military scopes. I prefer paper
or print-out. Thanks.

Mark


Re: Delay Time Position Vernier

Dave Peterson
 

Ah, thank you Colin. I was wondering about that. My working scope isn't at 0.0, and this one wasn't either. And I have no recollection of what the Army scopes were set to. I can get close 'cuz I do remember that it was very close to 0 and like 1/4 turn ccw on the fine knob.
Long way to go before I'm back to setting this!
Dave

On Wednesday, November 25, 2020, 03:17:39 PM PST, Colin Herbert via groups.io <colingherbert=blueyonder.co.uk@groups.io> wrote:

Your other problem is setting the Vernier where it is supposed to sit when you put it all together again! They don't all sit at 0.00, There is data about where they are supposed to read when the pot is at the extreme anticlockwise. If you have problems, just make them known here and you should get the right advice. Don't just adjust it to 0.00 - it is likely to be wrong.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Dave Peterson via groups.io
Sent: 25 November 2020 21:13
To: TekScopes@groups.io
Subject: [TekScopes] Delay Time Position Vernier

Anyone know the trick to removing the Delay Time Position Vernier pot?

I have the fine adjustment knob off. Now what!?

Befuddled.


Re: Resistor in series

 

Chuck Harris wrote:

So, I would guess that a smaller resistor was specified when the
board was designed, and it was found to get too hot
but this resistor pair is present in the schematics, not just for the 475A, but also in the early 475 service manual schematics. I know that's not really a refutation of your point, but it sure looks like they meant to do this from really early on.

The on-end resistors are clearly visible in the PCB images. I haven't opened up my oldest 475 to check the physical board, but I was planning to do that, and will report back what I find.

Also, I know that schematics do not necessarily precede the physical objects they represent, so they may not accurately reflect original intent.

I once worked at an engineering company where, as we were packing a large machine to be shipped to the client the lead engineer was taking each part and comparing it to the existing drawings, in order to find parts that had been modified (or completely fabricated) during testing and development. When he would find a part that didn't have a drawing he would quickly gin one up in AutoCAD before the part was packed and shipped. My impression, at the time, was that this was part of our contractual obligations to the client, but it occurs to me now that it may have been entirely internal; so that our people would be able to correctly reassemble the machine at the client site based on the engineering drawings.


Re: Resistor in series

Tom Lee
 

In a word, no.

Anything is possible, I suppose, but asking us to believe that Tek's design team carelessly forgot to check power dissipation is a lot to swallow. This wasn't their first rodeo -- they'd already done the 465, 465B (it was pretty much the same team) and the 475. If you look at the 465, there is a similar series combo in the same general part of the blanking circuit. It is obviously done to reduce capacitance. There is a tiny trimmer of about 1pF across the combo, so clearly they were carefully accounting for fractions of a pF.

I'm sticking with parasitic capacitance concerns as the most likely reason for the choice in the 475A.

The team did make some mistakes (in the 465 in particular; they fixed those in the B version), but forgetting to calculate power dissipation here was not one of them.

Cheers,
Tom

Sent from my iThing, so please forgive the terseness and typos.

On Nov 25, 2020, at 14:54, "Chuck Harris" <cfharris@...> wrote:

I've done this for many, many years, and usually my reason is
I cannot fit a higher wattage resistor in the same space as two
smaller resistors sitting upright like this.

So, I would guess that a smaller resistor was specified when the
board was designed, and it was found to get too hot, so two resistors
of the same wattage, in series were fitted... with no changes in
the circuit board.

-Chuck Harris.

Jeff Dutky wrote:
Chuck Harris wrote:

How do you know tektronix did this? Most of the stuff we have hasn't
been at the factory for 20 or 30 years.
The main reasons that I think Tek did this are 1) that the two resistors appear in the schematic in the service manual, and 2) the resistors look like matches for resistors found elsewhere on the unit. Also, the method of vertically mounting the resistors and soldering their upward pointing leads together is found elsewhere on the unit, specifically on the vertical preamp board in the 3rd stage amplifier.

-- Jeff Dutky









Re: Delay Time Position Vernier

Colin Herbert
 

Your other problem is setting the Vernier where it is supposed to sit when you put it all together again! They don't all sit at 0.00, There is data about where they are supposed to read when the pot is at the extreme anticlockwise. If you have problems, just make them known here and you should get the right advice. Don't just adjust it to 0.00 - it is likely to be wrong.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Dave Peterson via groups.io
Sent: 25 November 2020 21:13
To: TekScopes@groups.io
Subject: [TekScopes] Delay Time Position Vernier

Anyone know the trick to removing the Delay Time Position Vernier pot?

I have the fine adjustment knob off. Now what!?

Befuddled.


Re: Fix or Part Out a Tek 475A

 

Thanks Tom, Raymond & Jeff,
Yes the trigger slope shafts were bent as well! Now straightened. Display is bright, and power supplies are well within spec for residual hum. All electrolytics are original. Fan motor dissected/oiled and now fine: at the back it has a screw (underneath glue) to adjust free play, all behind the PC board which has to be unsoldered first. Over-engineered! Someone had the Y amp board off because the 100MHz/20MHz/trigger view shaft was wrongly adjusted. I notice 2mV presets on it as well as 5mV presets, though there is only 5mV and greater shown on the rotary switches.
This scope appears to be working well in all respects. No sign of shorting tantalum caps. The mains TX has taps for the optional 12V battery circuit, which I might try. This follows restoration of a Fairchild Dumont 765H (very bright and sharp display); Heathkit IO-14 (now with a TEK trigger circuit), and TEK 191 sig.gen.
I still have two TEK 545B's to restore; one obviously with the well-known epoxy potted EHT transformer problem. Along with one early 1A1 and two 1S1 plugins.
The National R1230/FLR wide band receiver awaits future attention to sharpen up its IF response for practical use on short wave.
Regards


Re: Tektronix 492A input attenuator repair ????

Tom Lee
 

If it does turn out to be a blown attenuator, perhaps the best option is to use an HP attenuator instead (they are much more available). A small bit of logic is needed to remap control signals and space is tight, but it can be done. I've fixed two this way after giving up any hope of fixing the original attenuator. Essentially 100% of 492 attenuators found on eBay are blown (keywords are "untested", "as-is" and "from working unit").

There is an extensive set of instructions online for the mod. I don't have a link handy, but a minute of googling should lead you to it.

Good luck!

Cheers,
Tom



Sent from my iThing, so please forgive the terseness and typos.

On Nov 25, 2020, at 14:31, "Bob Koller" <fourdesigns@...> wrote:

Unfortunately, there are no parts, except from another Tek attenuator. Is it blown, or are the contacts "dirty"? These were often damaged by DC or excessive input power. If it is damaged, you need to check the first mixer for damage as well. You may be able to find a good attenuator on eBay, perhaps elsewhere, but these can be difficult to find in good condition.
It is possible to disassemble these and make one good unit out of two. The thin film resistive elements can also be inspected. Usually the damage is readily apparent under magnification.
It would be possible to fit a different attenuator, with the correct steps, and frequency range, altering the drive signals as required.
Be sure to take anti static precautions around the mixer.

Good Luck,





Re: Resistor in series

Chuck Harris
 

I've done this for many, many years, and usually my reason is
I cannot fit a higher wattage resistor in the same space as two
smaller resistors sitting upright like this.

So, I would guess that a smaller resistor was specified when the
board was designed, and it was found to get too hot, so two resistors
of the same wattage, in series were fitted... with no changes in
the circuit board.

-Chuck Harris.

Jeff Dutky wrote:

Chuck Harris wrote:

How do you know tektronix did this? Most of the stuff we have hasn't
been at the factory for 20 or 30 years.
The main reasons that I think Tek did this are 1) that the two resistors appear in the schematic in the service manual, and 2) the resistors look like matches for resistors found elsewhere on the unit. Also, the method of vertically mounting the resistors and soldering their upward pointing leads together is found elsewhere on the unit, specifically on the vertical preamp board in the 3rd stage amplifier.

-- Jeff Dutky






Re: 2710 Spectrum Analyser 'Cannot count VCO, IF' error

Martin Whybrow
 

Thanks everyone; thankfully mine is a later production version, post B020319, so hopefully not the unrelaible version.
I've checked the 100MHz reference and have healthy outputs on all 3 reference outputs (no tracking generator option installed), roughly -8dBm on J110, -10dBm on J130, and -12dBm on J140 (all approximate as I'm pushing the bandwidth of my cheap scope probe at 100MHz).
One thing I have discovered is that what I thought was an input to the board on J110, the Beat Signal input, was in fact common mode noise from my LED lighting! There appears to be no signal at all coming from the 1st LO buffer, so that's what I'm now looking at.


Re: Tektronix 492A input attenuator repair ????

 

Unfortunately, there are no parts, except from another Tek attenuator. Is it blown, or are the contacts "dirty"? These were often damaged by DC or excessive input power. If it is damaged, you need to check the first mixer for damage as well. You may be able to find a good attenuator on eBay, perhaps elsewhere, but these can be difficult to find in good condition.
It is possible to disassemble these and make one good unit out of two. The thin film resistive elements can also be inspected. Usually the damage is readily apparent under magnification.
It would be possible to fit a different attenuator, with the correct steps, and frequency range, altering the drive signals as required.
Be sure to take anti static precautions around the mixer.

Good Luck,