Question: R7903 Cap Rectifier Board 130v supply


Joe Sleator <joe@...>
 

Well, some good news and some bad news, long confusion about diodes in this
board.

The good news is that I've isolated the problem in my second 7903 down to
the cap rectifier board in the PSU. The bad news is that it seems there is a
shorted diode in the set of diodes CR1320-1323. Anyone familiar with these
boards and the S/M will know that Tek says these are a matched set. On one
board I have they seem to be 1N4936, and on the other they are Motorola
W400. The 4 may not all be the same part, and the manual doesn't say what
the properties are. Further, I can't tell without pulling out the healthy
ones, it simply says:

SEMICOND DEVICE:SILICON,MATCHED SET with a PN of 153-0052-00

So, I need to make up another matched set, I guess.

Now, as far as diodes go, the only things I could think of to match would be
voltage drop, punch-thru voltage, or perhaps breakdown v or max wattage, or
reverse recovery time, but not sure how they would test most of these from a
batch without destroying them, other than the drop and recovery times.

I'm assuming that that in the 70's and 80's when these were manufactured and
my manual was printed, the QA for diodes was so dodgy for this HV and
recovery time requirement that perhaps Tek needed to select them rather than
rely on manufacturers specifications for the parts.

I also see from looking at the scematic that these form what looks like a
normal bridge rectifier for power, then thru a sort of PI network that forms
the 130V supply to the rest of the world, seemingly unregulated. Hanging off
the secondary on the AC side of the bridge is a half-wave made out of
ordinary 175v 100ma diodes. This goes to the inverter control dividing
network and the master adjustment pot for the supply, R1293.

Anyway, could I just shove a high-voltage bridge in here, or replace with
normal off the shelf 4936 diodes? If anybody has any info on this puzzle,
they could have a direct hand in bringing another 7903 back to the land of
the living, so thanks in advance for any advice!

Cheers,
Joe Sleator


Miroslav Pokorni
 

I checked manuals for 7904, which should have the same power supply as 7903,
just packaged differently.

The manual for serial numbers below 260000 shows CR1320 & CR1321 being
replaceable with MR814 and CR1322 & CR1323 with MR880 for instruments with
S/N B010100 to B039999. That is pretty funny, because Tektronix P/N is
152-0413-00 for all four of them. The MR814 is Motorola's kind of fast
recovery diode, maximum recovery time 750 ns (marketing recovery time
(typical) 350 ns), reverse voltage 400 V. I was not able to find MR880, but
CR 1345 through CR1348 are 'selected from MR880' and part number assigned is
152-0397-00, so MR880 seem to exist.

For instruments with S/N B040000 & up CR1320 through CR1323 became 'matched
set of 4', with P/N 153-0052-00. The manual for S/N B260000 & up shows
CR1320 to CR1323 as P/N 152-0400-00 supplied by Tektronix (no manufacturer
quoted, no matched sets either).

Interestingly, diodes CR1320 & CR1321 were marked as removed after S/N
281169; at the same time the PCB (A12) changed. One would think that
transformer was also changed to a center tapped one, but transformer changed
at S/N B269360 and there is no indication that there was another change.
Anyways, it is hard to say what was configuration after PCB was changed.
Also, one should never forget errors in manuals; I am sorry to say, but
Tektronix did less than a sterling job of proofreading and schematic
checking. There were 'cute' sketches decorating schematics, but there were
no excesses in the checking process.

All those changes seem to show that Tektronix groped with converter design.
I had few brief conversations with a gentleman in his late 60s or early 70s,
who now lives down here in Southern California. He claims that he worked for
Tektronix and that he worked on converters. From his rush conclusions about
other things, I would be not surprised if he took typical values of
components to make a design, what would lead to all those selected
components. I find it mind boggling that transistors for driving lamps are
selected from a standard transistor type, e.g. Q238 in 7904 is selected from
MPS918. To my mind that shows a lack of adult supervision in design
department. Sure, all of us make selections without doing full home work,
but after my boss wiped floor with me few times, the notion that you do not
use typical values for design did get hold.

As for your comment about problems with getting components that confirm to
data sheet, I seem to remember that even in early 80s fast recovery diodes
and high speed power transistors were tested in innovative ways so the
recovery time was quite misleading. There were reports that switching power
supply manufacturers had to establish qualification tests and even incoming
lot testing. That problem was fixed later on, when switching supplies became
widely accepted.

As you can see from changes of diodes, the converter seems to have started
life with 350 ns diodes. I am willing to put some money on the fact that
design was based on typical recovery time. When diodes started to die, test
fixture for recovery time had to be dusted off, so that diodes with 750 ns
recovery did not commit suicide by making a short across transformer. I
would guess that for matched quads of diodes, turn on time (not too fast)
was matched with reverse recovery so that a short circuit is minimized.

It seems to me that if you use 1N4936 (maximum recovery 300 ns), or even
better, if you spring for MUR140 (maximum recovery 100 ns), you would not
need to wary about matching sets.

Also, bear in mind what Denis Cobley said about Motorola's diodes with
markings ZM and ZS (a post titled '2247A Power Supply Troubleshooting' ). I
suspect, those markings designate plants where diodes were manufactured, but
I am not positive. Anyways, Denis said that diodes with thoseoffending
markings go leaky over time.

Hope this helps.

Miroslav Pokorni

----- Original Message -----
From: "Joe Sleator" <joe@adaptit.com.au>
To: <tekscopes@yahoogroups.com>
Sent: Friday, October 31, 2003 1:06 PM
Subject: [TekScopes] Question: R7903 Cap Rectifier Board 130v supply


Well, some good news and some bad news, long confusion about diodes in
this
board.

The good news is that I've isolated the problem in my second 7903 down to
the cap rectifier board in the PSU. The bad news is that it seems there is
a
shorted diode in the set of diodes CR1320-1323. Anyone familiar with these
boards and the S/M will know that Tek says these are a matched set. On one
board I have they seem to be 1N4936, and on the other they are Motorola
W400. The 4 may not all be the same part, and the manual doesn't say what
the properties are. Further, I can't tell without pulling out the healthy
ones, it simply says:

SEMICOND DEVICE:SILICON,MATCHED SET with a PN of 153-0052-00

So, I need to make up another matched set, I guess.

Now, as far as diodes go, the only things I could think of to match would
be
voltage drop, punch-thru voltage, or perhaps breakdown v or max wattage,
or
reverse recovery time, but not sure how they would test most of these from
a
batch without destroying them, other than the drop and recovery times.

I'm assuming that that in the 70's and 80's when these were manufactured
and
my manual was printed, the QA for diodes was so dodgy for this HV and
recovery time requirement that perhaps Tek needed to select them rather
than
rely on manufacturers specifications for the parts.

I also see from looking at the scematic that these form what looks like a
normal bridge rectifier for power, then thru a sort of PI network that
forms
the 130V supply to the rest of the world, seemingly unregulated. Hanging
off
the secondary on the AC side of the bridge is a half-wave made out of
ordinary 175v 100ma diodes. This goes to the inverter control dividing
network and the master adjustment pot for the supply, R1293.

Anyway, could I just shove a high-voltage bridge in here, or replace with
normal off the shelf 4936 diodes? If anybody has any info on this puzzle,
they could have a direct hand in bringing another 7903 back to the land of
the living, so thanks in advance for any advice!

Cheers,
Joe Sleator


Stan & Patricia Griffiths <w7ni@...>
 

Hi Joe,

I found some info on the matched set of diodes for your 7903. A set of 4
diodes with part number 153-0052-00 is composed of 4 each 152-0414-00 diodes
matched for 50mV of forward voltage drop at a forward current of 500mA.

152-0414-00's are known by the following generic numbers:

RGP10D Series
SR2069RL
UTR308
MR812

They are silicon rectifiers rated at 1A and 200 volts. Fast recovery, 750
ns.

I hope this gets it for you, Joe.

Stan
w7ni@easystreet.com

----- Original Message -----
From: "Joe Sleator" <joe@adaptit.com.au>
To: <tekscopes@yahoogroups.com>
Sent: Friday, October 31, 2003 1:06 PM
Subject: [TekScopes] Question: R7903 Cap Rectifier Board 130v supply


Well, some good news and some bad news, long confusion about diodes in
this
board.

The good news is that I've isolated the problem in my second 7903 down to
the cap rectifier board in the PSU. The bad news is that it seems there is
a
shorted diode in the set of diodes CR1320-1323. Anyone familiar with these
boards and the S/M will know that Tek says these are a matched set. On one
board I have they seem to be 1N4936, and on the other they are Motorola
W400. The 4 may not all be the same part, and the manual doesn't say what
the properties are. Further, I can't tell without pulling out the healthy
ones, it simply says:

SEMICOND DEVICE:SILICON,MATCHED SET with a PN of 153-0052-00

So, I need to make up another matched set, I guess.

Now, as far as diodes go, the only things I could think of to match would
be
voltage drop, punch-thru voltage, or perhaps breakdown v or max wattage,
or
reverse recovery time, but not sure how they would test most of these from
a
batch without destroying them, other than the drop and recovery times.

I'm assuming that that in the 70's and 80's when these were manufactured
and
my manual was printed, the QA for diodes was so dodgy for this HV and
recovery time requirement that perhaps Tek needed to select them rather
than
rely on manufacturers specifications for the parts.

I also see from looking at the scematic that these form what looks like a
normal bridge rectifier for power, then thru a sort of PI network that
forms
the 130V supply to the rest of the world, seemingly unregulated. Hanging
off
the secondary on the AC side of the bridge is a half-wave made out of
ordinary 175v 100ma diodes. This goes to the inverter control dividing
network and the master adjustment pot for the supply, R1293.

Anyway, could I just shove a high-voltage bridge in here, or replace with
normal off the shelf 4936 diodes? If anybody has any info on this puzzle,
they could have a direct hand in bringing another 7903 back to the land of
the living, so thanks in advance for any advice!

Cheers,
Joe Sleator


Denton, Adam (Exchange)
 

I find it mind boggling that transistors for driving lamps
are selected from a standard transistor type,
e.g. Q238 in 7904 is selected from MPS918.
Are you serious?!! MPS918 is used as a LAMP DRIVER??!!
My god, MPS918 is a rather delicate RF transistor
(VCEo = 15V, fT = 600MHz, Ic = 50mA). Just about the
LAST part I would ever choose to drive a lamp. Ouch!

To my mind that shows a lack of adult supervision in design
department.
Scary. Did they employ the same lackey in the design of
the now-infamous 24xx U-800? :-O

-Adam

From: Miroslav Pokorni [mailto:mpokorni2000@yahoo.com]
Sent: Friday, October 31, 2003 10:42 PM
To: tekscopes@yahoogroups.com; Joe Sleator
Subject: Re: [TekScopes] Question: R7903 Cap Rectifier Board 130v supply

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Bear Stearns is not responsible for any recommendation, solicitation,
offer or agreement or any information about any transaction, customer
account or account activity contained in this communication.
***********************************************************************


John Miles <jmiles@...>
 

Read as a memo from some PHB at Tek: "We ordered WAY too many of those
darned MPS918s. Find a use for them."

-- jm

I find it mind boggling that transistors for driving lamps
are selected from a standard transistor type,
e.g. Q238 in 7904 is selected from MPS918.
Are you serious?!! MPS918 is used as a LAMP DRIVER??!!
My god, MPS918 is a rather delicate RF transistor
(VCEo = 15V, fT = 600MHz, Ic = 50mA). Just about the
LAST part I would ever choose to drive a lamp. Ouch!

To my mind that shows a lack of adult supervision in design
department.
Scary. Did they employ the same lackey in the design of
the now-infamous 24xx U-800? :-O

-Adam


Miroslav Pokorni
 

No, it was not MPS918, I got reference number wrong and from there part
number was wrong, too. The lamp driver that I started from is Q469, generic
number MPS6521, but this one is not a very sturdy device, either. The
continuos collector current is 100 mA, not very much for an incandescent
lamp cold filament surge. The basic device has gain between 150 and 600. I
wonder whether it was higher gain that was selection criteria.

Regards

Miroslav Pokorni

----- Original Message -----
From: "Denton, Adam (Exchange)" <adenton@bear.com>
To: <tekscopes@yahoogroups.com>
Sent: Wednesday, November 05, 2003 9:09 AM
Subject: RE: [TekScopes] Question: R7903 Cap Rectifier Board 130v supply


I find it mind boggling that transistors for driving lamps
are selected from a standard transistor type,
e.g. Q238 in 7904 is selected from MPS918.
Are you serious?!! MPS918 is used as a LAMP DRIVER??!!
My god, MPS918 is a rather delicate RF transistor
(VCEo = 15V, fT = 600MHz, Ic = 50mA). Just about the
LAST part I would ever choose to drive a lamp. Ouch!

To my mind that shows a lack of adult supervision in design
department.
Scary. Did they employ the same lackey in the design of
the now-infamous 24xx U-800? :-O

-Adam

From: Miroslav Pokorni [mailto:mpokorni2000@yahoo.com]
Sent: Friday, October 31, 2003 10:42 PM
To: tekscopes@yahoogroups.com; Joe Sleator
Subject: Re: [TekScopes] Question: R7903 Cap Rectifier Board 130v supply

***********************************************************************
Bear Stearns is not responsible for any recommendation, solicitation,
offer or agreement or any information about any transaction, customer
account or account activity contained in this communication.
***********************************************************************