475A Problem


Dick
 

I have a 475A, ex-IBM with DMM Option. It recently developed a problem.
You can see a picture of what it looks like in the Photo's section, under
475A problem.

If you let it run for 10 minutes or so, the problem seems to get better, but not
completely gone.

Probing the DC Voltages while the problem is there shows the -8vdc line has
dropped to -2.1vdc, and the +5vdc line has dropped to +1.0vdc. When the
problem seems to go away, those voltages come back up to normal.

Anyone have any ideas what to look at ?

Thanks for any help,

Dick, W1KSZ


Dave Peterson
 

Looks like your "Low Line" light is on, meaning your +55v supply is low. The +55v drives all the low voltage regulators, so that needs to be fixed first. See the power supply calibration procedure in the service manual.

I would guess you have a problem with a rectifier or power supply cap (or both).

I'm more of a 465 guy, so I may very well be off in the 475A details. Is it +55v in 475A as well guys?

Dave

On Sunday, June 20, 2021, 9:26:46 AM PDT, Dick <w1ksz@outlook.com> wrote:

I have a 475A, ex-IBM with DMM Option. It recently developed a problem.
You can see a picture of what it looks like in the Photo's section, under
475A problem.

If you let it run for 10 minutes or so, the problem seems to get better, but not
completely gone.

Probing the DC Voltages while the problem is there shows the -8vdc line has
dropped to -2.1vdc, and the +5vdc line has dropped to +1.0vdc. When the
problem seems to go away, those voltages come back up to normal.

Anyone have any ideas what to look at ?

Thanks for any help,

Dick, W1KSZ


Dick
 

The "Low Line" light does come on, but goes out when the
problem seems to go away.

I saw a +50vdc line in the Power Supply, which measures
+46.2vdc, which rises to +48.4vdc when the problem seems
to go away.

Dick, W1KSZ
________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Dave Peterson via groups.io <davidpinsf=yahoo.com@groups.io>
Sent: Sunday, June 20, 2021 9:32 AM
To: TekScopes@groups.io <TekScopes@groups.io>
Subject: Re: [TekScopes] 475A Problem

Looks like your "Low Line" light is on, meaning your +55v supply is low. The +55v drives all the low voltage regulators, so that needs to be fixed first. See the power supply calibration procedure in the service manual.

I would guess you have a problem with a rectifier or power supply cap (or both).

I'm more of a 465 guy, so I may very well be off in the 475A details. Is it +55v in 475A as well guys?

Dave


On Sunday, June 20, 2021, 9:26:46 AM PDT, Dick <w1ksz@outlook.com> wrote:

I have a 475A, ex-IBM with DMM Option. It recently developed a problem.
You can see a picture of what it looks like in the Photo's section, under
475A problem.

If you let it run for 10 minutes or so, the problem seems to get better, but not
completely gone.

Probing the DC Voltages while the problem is there shows the -8vdc line has
dropped to -2.1vdc, and the +5vdc line has dropped to +1.0vdc. When the
problem seems to go away, those voltages come back up to normal.

Anyone have any ideas what to look at ?

Thanks for any help,

Dick, W1KSZ


Mark Vincent
 

Dick,

Check VR1416. Mine was open and I replaced it with a BZX85B9V1. The filter cans can also be going bad. Mine now has new high temperature long life ones in it. The output filter, 4,7mfd, of the 50V supply may also be bad. You can put something like 22mfd 100V as a replacement. The additional filtering will not hurt. The drifting voltage as it warms meas something is temperature sensitive. You can check R1416. Mine was dark brown in the body and I replaced the original 1/2 W with a 2W 1% type of 5600 ohms that tested at a few ohms below the original 5620 ohms. Also make sure to set the 50V adjust to 50V output after you find any problems and replace any parts.

Mark


 

The 50 V rail plays the same role on the 475/A as the 55 V rail on the 465. No idea why they are different, but here we are. 46 V is well out of spec for the 50 V rail, even 48 V is suspiciously low. I second Mark’s suggestion to check the rectifier and caps. Once that’s fixed I would verify all the power rails, voltages and ripple currents. If the electrolytic on the 50 V rail have gone dry there may be others that need replacement as well.

— Jeff Dutky


Renée
 

48V is low.....the other issue I noted on my 475 is when the main caps allow too much ripple...the tants start to go due to Peak voltages exceed ratings.
Renée

On 6/20/21 10:47 AM, Jeff Dutky wrote:
The 50 V rail plays the same role on the 475/A as the 55 V rail on the 465. No idea why they are different, but here we are. 46 V is well out of spec for the 50 V rail, even 48 V is suspiciously low. I second Mark’s suggestion to check the rectifier and caps. Once that’s fixed I would verify all the power rails, voltages and ripple currents. If the electrolytic on the 50 V rail have gone dry there may be others that need replacement as well.

— Jeff Dutky




Dick
 

Replacing all 6 of those large, and expensive, caps
is deal breaker. I have another 475A, plus two 485's
and a few of the larger TEK scopes.

I'll put it into the Goodwill pile.

Thanks for all the replies,

Dick, W1KZS
________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Renée <k6fsb.1@gmail.com>
Sent: Sunday, June 20, 2021 11:05 AM
To: TekScopes@groups.io <TekScopes@groups.io>
Subject: Re: [TekScopes] 475A Problem

48V is low.....the other issue I noted on my 475 is when the main caps
allow too much ripple...the tants start to go due to Peak voltages
exceed ratings.
Renée

On 6/20/21 10:47 AM, Jeff Dutky wrote:
The 50 V rail plays the same role on the 475/A as the 55 V rail on the 465. No idea why they are different, but here we are. 46 V is well out of spec for the 50 V rail, even 48 V is suspiciously low. I second Mark’s suggestion to check the rectifier and caps. Once that’s fixed I would verify all the power rails, voltages and ripple currents. If the electrolytic on the 50 V rail have gone dry there may be others that need replacement as well.

— Jeff Dutky





Michael W. Lynch
 

Dick,

I would love to have that scope as a parts unit, if nothing else.

--
Michael Lynch
Dardanelle, AR