475 and 475A's repair log


Zentronics42@...
 

A few 475's and 475A's have landed on the bench of healing to see if I can breath life in to them again. I am starting with a unit that was owned by IBM and has the DM44 on the top. cosmetically the unit is in amazing shape however the DM44 is out to lunch and latching up imminently after boot up. The scope is running and showing a trace however the -15V rail is low 12.8Vdc. I have checked Q1474 and Q1478 and I have not ruled out U1464 quite yet but the -8 rail also seems to be affected slightly as well. I have ordered the service manuals with schematics from Arktech so I can dig in to this -15Vdc rail. Curious if anyone has any thoughts in the mean time? also does anyone one know common fail modes on the DM44. At the moment I am less worried about the scope then I am the DMM. The display comes on and shows random data but it never updates. Even with valid input to the terminals.

Zen


 

The DM44 has it's own regulator board mounted in the CRT compartment, which provides +12, -12, and +5 V rails. It also seems to take +50, +15, +5, and -8 V from the scope's main power supply. I don't see any mention of the -15 V rail being used, but I've just had a cursory look at the schematics, so I could have missed something. It's not entirely clear to me, from the schematic, what tap from the main LV xfmr is being used to develop the -12 V rail, but maybe it is the same one that is used for the -15 rail in the main PSU. The fact that the -15 V rail is so close to -12 V right now seems suggestive to me.

I would have a close look at the power rails on the DM44, and specifically have a look at the regulator board for failed components. It's too bad that the regulator is so hard to get at.

-- Jeff Dutky


Mark Vincent
 

Zen,

Check the 9V zeners in the scope(s) and the DM44. The ones that are open, use BZX85B9V1. In the scope, a 9V zener is going to the opamp for the 50V supply. Jeff D. is right about checking the supplies for the DM44.

Mark


Zentronics42@...
 

I got the -15 Vdc rail back in action this however did not fix the DMM44 it is still out to lunch. The issue with the -15Vdc rail was R1474 this was a 50 K 0.25% resister that decided it wanted to be a 58K resister. So now that the main power supply is back in action I can keep going on the restoration. For as nice a condition as this scope is in it is out to lunch electrically.

Zen

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Jeff Dutky
Sent: Sunday, May 22, 2022 10:33 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 475 and 475A's repair log

The DM44 has it's own regulator board mounted in the CRT compartment, which provides +12, -12, and +5 V rails. It also seems to take +50, +15, +5, and -8 V from the scope's main power supply. I don't see any mention of the -15 V rail being used, but I've just had a cursory look at the schematics, so I could have missed something. It's not entirely clear to me, from the schematic, what tap from the main LV xfmr is being used to develop the -12 V rail, but maybe it is the same one that is used for the -15 rail in the main PSU. The fact that the -15 V rail is so close to -12 V right now seems suggestive to me.

I would have a close look at the power rails on the DM44, and specifically have a look at the regulator board for failed components. It's too bad that the regulator is so hard to get at.

-- Jeff Dutky


Michael W. Lynch
 

I have repaired a couple of DM43 and DM44 units. Once the power supplies were sorted out, both units had issues with failed Op-Amps and 74 Series Logic IC's..

--
Michael Lynch
Dardanelle, AR


Zentronics42@...
 

That makes sence I might just shotgun test all the IC's, I have a good chip tester in the lab. This unit also has the crappy TI sockets everywhere that might be complicating things. The power rails to the DM44 are in good shape so it is time to look at logic IC's

Zen

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Michael W. Lynch via groups.io
Sent: Tuesday, May 24, 2022 11:39 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 475 and 475A's repair log

I have repaired a couple of DM43 and DM44 units. Once the power supplies were sorted out, both units had issues with failed Op-Amps and 74 Series Logic IC's..

--
Michael Lynch
Dardanelle, AR


Roy Thistle
 

On Tue, May 24, 2022 at 08:38 AM, Michael W. Lynch wrote:


I have repaired a couple of DM43 and DM44 units. Once the power supplies were sorted out, both units had issues with failed Op-Amps and 74 Series Logic
A kind of cascading failure? ... as in power rails fails... and unprotected components suffer the consequences?
It's good? for business. But, it's a thing we try to design against these days. Most 'chips' have some protection, and power supplies fails are designed to try and prevent cascading damage. Or is it, still good? for business.



--
Roy Thistle


Roy Thistle
 

On Sun, May 22, 2022 at 06:43 PM, <Zentronics42@...> wrote:


Curious if anyone has any thoughts in the mean time?
Well... fair warning...my thoughts, as far as they qualify as thus, and as curious as they be... they're only opines.
Just so. Yet, I do recall that if you had a DMxx squatting atop a 465, and rid'n a Scope-mobile, with a long cord... you thought you was the "boss."
IMO, I never liked the top riding eye candy... it spoiled the inimitable classic look of 465 and 475 scopes.
I also recall, a ways back, that Ebayer's thought these were the 'bee's knees', and apparently uncorrelated to function and price (never mind taste), such disfigured units were pricey, and sought after.
Well its deja vu all over again.
As I'm sure, YouTuber's with be convince'n sellers and buyers who can't change their socks, to slap-a-cap, and flip these things on Ebay, at greed level 11.
It's what they do.

--
Roy Thistle


Michael W. Lynch
 

The "shotgun" approach is what I used along with following the schematics and the incorrect signals that resulted. The TI sockets are a real problem and re-seating the components can help, so the shotgun method kills two birds with one stone.

--
Michael Lynch
Dardanelle, AR


 

On Tue, May 24, 2022 at 01:30 PM, Roy Thistle wrote:


I never liked the top riding eye candy... it spoiled the inimitable
classic look of 465 and 475 scopes.
I also recall, a ways back, that Ebayer's thought these were the 'bee's
knees', and apparently uncorrelated to function and price (never mind taste),
such disfigured units were pricey, and sought after.
I'm not sure about spoiling the look of a 465/475, but I in my callow youth (of almost two years ago) thought that they were something special. I was lucky enough to obtain a DM44 as part of a 475A parts unit, and have been planning, ever since, to transplant it into another 475A that I was able to get working.

I understand now that neither the DM44 nor the 475A were such spectacular improvements over a base 475, but I still like the look of the additional LED readout (though doesn't look as nice a 2236 with the 9-digit VFD readout). Now, if the 485 had a CTM option like the 2236 (or the 2465) I would be very happy indeed. It even seems like you could shoehorn the readout and controls into the empty space on the front panel currently taken up by the 485 livery. That would have been a cool scope.

-- Jeff Dutky


Michael W. Lynch
 

I only buy these things if they are a screaming deal. No way I am paying more for a DM43/44 equipped machines. If the instrument is relatively "Cheap" then I will buy, otherwise, no deal. I consider "Cheap" as a unit that has no external appearance issues and front panel damage, shows some sort of CRT activity and is priced at or below $100 including shipping. I have some DM43/44 models as a part of a larger collection of 4xx series scopes. I do prefer the "standard" model and simplicity of the same.

--
Michael Lynch
Dardanelle, AR


Zentronics42@...
 

If I remember correctly thew 485 has a channel 1 output that is a duplicate of the waveform at a set amplitude. This can easily be fed in to a frequency counter to get a frequency readout. Needs a second box but adds the functionality. Now if I could only get one Rb based with enough digits to match a 485.

I have the Rb in the lab I need to digits.

Zen

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Jeff Dutky
Sent: Tuesday, May 24, 2022 1:45 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 475 and 475A's repair log

On Tue, May 24, 2022 at 01:30 PM, Roy Thistle wrote:


I never liked the top riding eye candy... it spoiled the inimitable
classic look of 465 and 475 scopes.
I also recall, a ways back, that Ebayer's thought these were the
'bee's knees', and apparently uncorrelated to function and price
(never mind taste), such disfigured units were pricey, and sought after.
I'm not sure about spoiling the look of a 465/475, but I in my callow youth (of almost two years ago) thought that they were something special. I was lucky enough to obtain a DM44 as part of a 475A parts unit, and have been planning, ever since, to transplant it into another 475A that I was able to get working.

I understand now that neither the DM44 nor the 475A were such spectacular improvements over a base 475, but I still like the look of the additional LED readout (though doesn't look as nice a 2236 with the 9-digit VFD readout). Now, if the 485 had a CTM option like the 2236 (or the 2465) I would be very happy indeed. It even seems like you could shoehorn the readout and controls into the empty space on the front panel currently taken up by the 485 livery. That would have been a cool scope.

-- Jeff Dutky


Zentronics42@...
 

It is good for Video? I am shooting one for the channel. I should clarify that these are not my units I am mearly the "repair" tech on these. Though I will admit owning a 465 and 485's in the lab and being in one with a DM44. I am not sure how much the DM44 brings to the table. Especially with how it is connected and integrated. So I am kind of glad mine don’t have them. This perticuarl example is an IBM branded one MOD PG to which I have zero info for. The power rail issues was in the scope its self not the DMM. The DM44's power rails are fine but there is definitely something wrong in the logic.

Zen

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Roy Thistle
Sent: Tuesday, May 24, 2022 1:07 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 475 and 475A's repair log

On Tue, May 24, 2022 at 08:38 AM, Michael W. Lynch wrote:


I have repaired a couple of DM43 and DM44 units. Once the power supplies were sorted out, both units had issues with failed Op-Amps and 74 Series Logic
A kind of cascading failure? ... as in power rails fails... and unprotected components suffer the consequences?
It's good? for business. But, it's a thing we try to design against these days. Most 'chips' have some protection, and power supplies fails are designed to try and prevent cascading damage. Or is it, still good? for business.



--
Roy Thistle


 

Zen,

I think the 465/475+DM44 is nice to make a video of: lots of eye-candy with the green/blue CRT and the red LEDs, as well as an interesting dual intensified mode for doing timing measurements (at least, that's how I think it's supposed to work from skimming the manuals. Am I wrong? It would be nice to see it in action in your video) that is even cooler looking than the A INTEN BY B mode.

How big is a Rubidium standard? Things are pretty tight inside a 485. I assume that you could fit 9 or 10 digits of readout in the space where the 485 livery is on the front panel (using the tiny "bubble" LED 7-segment displays that would be period "correct"), and maybe you could fit a rotary selector switch for the CTM functions, but finding space for a CTM board (either the CTM board from the 2236 or the DM44 board) would be a challenge.

-- Jeff Dutky


Tom Lee
 

If price is not a concern, there's always a Symmetricom (Microsemi, now) chip-scale Rb standard. It's a remarkable piece of engineering.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 5/24/2022 21:13, Jeff Dutky wrote:
Zen,

I think the 465/475+DM44 is nice to make a video of: lots of eye-candy with the green/blue CRT and the red LEDs, as well as an interesting dual intensified mode for doing timing measurements (at least, that's how I think it's supposed to work from skimming the manuals. Am I wrong? It would be nice to see it in action in your video) that is even cooler looking than the A INTEN BY B mode.

How big is a Rubidium standard? Things are pretty tight inside a 485. I assume that you could fit 9 or 10 digits of readout in the space where the 485 livery is on the front panel (using the tiny "bubble" LED 7-segment displays that would be period "correct"), and maybe you could fit a rotary selector switch for the CTM functions, but finding space for a CTM board (either the CTM board from the 2236 or the DM44 board) would be a challenge.

-- Jeff Dutky