Selenium rectifiers in Tek equipment


cmjones01
 

I've just taken delivery of a very tidy example of the Tek 127 Preamplifier
Power Supply. I've often wished that my 535A had more horizontal bays, and
now it can have them.

I note that it has selenium rectifiers. I've seen pictures of it powered
up, and it doesn't smell of rotten cabbage, so I'm hoping they're basically
OK for now.

How worried should I be about their reliability? I'll check the power
supply rails under load, but I don't want an unexpected stink bomb, and I'd
rather not have to spend the time fitting silicon rectifiers right now.

Chris


Morris Odell
 

I have a 127 too and had no compunction in replacing the Se rectifiers with Si. Nobody can tell you how long it will be before the Se rectifiers fail and you will probably always have it in the back of your mind every time you switch on the 127. It's not a diffcult or expensive modification to do and gives you peace of mind.

I used cheap packaged 1000 volt 1 amp Si bridge rectifiers and replaced R601 and R602 with 15 R 3W resistors. I aso inserted a 10R 10W resistor in the negative ground connection of the SR603 replacement.

The Si bridges I used had metal cases so I mounted them on insulating posts, espaciallyu the SR607 replacement which is elevated a few hundred volts above the chassis.

Good luck!

Morris


Dave Wise
 

? Even if the Selenium stacks haven't burned up, by this time their series resistance is out of spec so the power supplies don't have as much mains voltage headroom as they should. I'd replace them without ado unless the piece was meant to be a shelf queen.


Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Morris Odell via groups.io <vilgotch1=gmail.com@groups.io>
Sent: Friday, February 05, 2021 2:29 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

I have a 127 too and had no compunction in replacing the Se rectifiers with Si. Nobody can tell you how long it will be before the Se rectifiers fail and you will probably always have it in the back of your mind every time you switch on the 127. It's not a diffcult or expensive modification to do and gives you peace of mind.

I used cheap packaged 1000 volt 1 amp Si bridge rectifiers and replaced R601 and R602 with 15 R 3W resistors. I aso inserted a 10R 10W resistor in the negative ground connection of the SR603 replacement.

The Si bridges I used had metal cases so I mounted them on insulating posts, espaciallyu the SR607 replacement which is elevated a few hundred volts above the chassis.

Good luck!

Morris


Joe Laffey
 

If you’ve ever smelled a burned selenium rectifier (stinking up your office or house for days) you would replace all of them before even turning them on.

--
Joe Laffey

On Feb 5, 2021, at 5:24 PM, Dave Wise <david_wise@phoenix.com> wrote:

? Even if the Selenium stacks haven't burned up, by this time their series resistance is out of spec so the power supplies don't have as much mains voltage headroom as they should. I'd replace them without ado unless the piece was meant to be a shelf queen.


Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Morris Odell via groups.io <vilgotch1=gmail.com@groups.io>
Sent: Friday, February 05, 2021 2:29 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

I have a 127 too and had no compunction in replacing the Se rectifiers with Si. Nobody can tell you how long it will be before the Se rectifiers fail and you will probably always have it in the back of your mind every time you switch on the 127. It's not a diffcult or expensive modification to do and gives you peace of mind.

I used cheap packaged 1000 volt 1 amp Si bridge rectifiers and replaced R601 and R602 with 15 R 3W resistors. I aso inserted a 10R 10W resistor in the negative ground connection of the SR603 replacement.

The Si bridges I used had metal cases so I mounted them on insulating posts, espaciallyu the SR607 replacement which is elevated a few hundred volts above the chassis.

Good luck!

Morris









Tom Lee
 

And besides the smell, Se is just not a good thing to inhale. Although I am generally opposed to blanket replacements of components, selenium rectifiers are an exception.

Tom

Sent from my iThing, so please forgive typos and brevity.

On Feb 5, 2021, at 3:35 PM, Joe Laffey <joe@thestable.tv> wrote:

If you’ve ever smelled a burned selenium rectifier (stinking up your office or house for days) you would replace all of them before even turning them on.

--
Joe Laffey

On Feb 5, 2021, at 5:24 PM, Dave Wise <david_wise@phoenix.com> wrote:

? Even if the Selenium stacks haven't burned up, by this time their series resistance is out of spec so the power supplies don't have as much mains voltage headroom as they should. I'd replace them without ado unless the piece was meant to be a shelf queen.


Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Morris Odell via groups.io <vilgotch1=gmail.com@groups.io>
Sent: Friday, February 05, 2021 2:29 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

I have a 127 too and had no compunction in replacing the Se rectifiers with Si. Nobody can tell you how long it will be before the Se rectifiers fail and you will probably always have it in the back of your mind every time you switch on the 127. It's not a diffcult or expensive modification to do and gives you peace of mind.

I used cheap packaged 1000 volt 1 amp Si bridge rectifiers and replaced R601 and R602 with 15 R 3W resistors. I aso inserted a 10R 10W resistor in the negative ground connection of the SR603 replacement.

The Si bridges I used had metal cases so I mounted them on insulating posts, espaciallyu the SR607 replacement which is elevated a few hundred volts above the chassis.

Good luck!

Morris













John Williams
 

I have a. 551 that looks like I also need to replace one of the selenium rectifiers. The 225 volt supply has large hum on it and the voltage out of the rectifier bridge is also low. The other supplies test ok. I have checked caps and tubes in the supply and everything is good. I am asking if there is any way to confirm the problem is the rectifier. I haven’t been able to figure out which one is the 225 volt. Any advice will be appreciated.


Ken Eckert
 

Burning selenium is very toxic, really bad for your health

On Friday, February 5, 2021, Joe Laffey <joe@thestable.tv> wrote:

If you’ve ever smelled a burned selenium rectifier (stinking up your
office or house for days) you would replace all of them before even turning
them on.

--
Joe Laffey

On Feb 5, 2021, at 5:24 PM, Dave Wise <david_wise@phoenix.com> wrote:

? Even if the Selenium stacks haven't burned up, by this time their
series resistance is out of spec so the power supplies don't have as much
mains voltage headroom as they should. I'd replace them without ado unless
the piece was meant to be a shelf queen.


Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Morris
Odell via groups.io <vilgotch1=gmail.com@groups.io>
Sent: Friday, February 05, 2021 2:29 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

I have a 127 too and had no compunction in replacing the Se rectifiers
with Si. Nobody can tell you how long it will be before the Se rectifiers
fail and you will probably always have it in the back of your mind every
time you switch on the 127. It's not a diffcult or expensive modification
to do and gives you peace of mind.

I used cheap packaged 1000 volt 1 amp Si bridge rectifiers and replaced
R601 and R602 with 15 R 3W resistors. I aso inserted a 10R 10W resistor in
the negative ground connection of the SR603 replacement.

The Si bridges I used had metal cases so I mounted them on insulating
posts, espaciallyu the SR607 replacement which is elevated a few hundred
volts above the chassis.

Good luck!

Morris














Gary Robert Bosworth
 

Good to know.

On Fri, Feb 5, 2021, 16:42 Ken Eckert <eckertkp@gmail.com> wrote:

Burning selenium is very toxic, really bad for your health

On Friday, February 5, 2021, Joe Laffey <joe@thestable.tv> wrote:

If you’ve ever smelled a burned selenium rectifier (stinking up your
office or house for days) you would replace all of them before even
turning
them on.

--
Joe Laffey

On Feb 5, 2021, at 5:24 PM, Dave Wise <david_wise@phoenix.com> wrote:

? Even if the Selenium stacks haven't burned up, by this time their
series resistance is out of spec so the power supplies don't have as much
mains voltage headroom as they should. I'd replace them without ado
unless
the piece was meant to be a shelf queen.


Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Morris
Odell via groups.io <vilgotch1=gmail.com@groups.io>
Sent: Friday, February 05, 2021 2:29 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

I have a 127 too and had no compunction in replacing the Se rectifiers
with Si. Nobody can tell you how long it will be before the Se rectifiers
fail and you will probably always have it in the back of your mind every
time you switch on the 127. It's not a diffcult or expensive modification
to do and gives you peace of mind.

I used cheap packaged 1000 volt 1 amp Si bridge rectifiers and replaced
R601 and R602 with 15 R 3W resistors. I aso inserted a 10R 10W resistor
in
the negative ground connection of the SR603 replacement.

The Si bridges I used had metal cases so I mounted them on insulating
posts, espaciallyu the SR607 replacement which is elevated a few hundred
volts above the chassis.

Good luck!

Morris


















Harvey White
 

I remember an old story about sheep eating a certain plant, loco weed, and having severe congnitive problems.

Turns out that loco weed is high in selenium.

Harvey

On 2/5/2021 7:52 PM, Gary Robert Bosworth wrote:
Good to know.

On Fri, Feb 5, 2021, 16:42 Ken Eckert <eckertkp@gmail.com> wrote:

Burning selenium is very toxic, really bad for your health

On Friday, February 5, 2021, Joe Laffey <joe@thestable.tv> wrote:

If you’ve ever smelled a burned selenium rectifier (stinking up your
office or house for days) you would replace all of them before even
turning
them on.

--
Joe Laffey

On Feb 5, 2021, at 5:24 PM, Dave Wise <david_wise@phoenix.com> wrote:

? Even if the Selenium stacks haven't burned up, by this time their
series resistance is out of spec so the power supplies don't have as much
mains voltage headroom as they should. I'd replace them without ado
unless
the piece was meant to be a shelf queen.

Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Morris
Odell via groups.io <vilgotch1=gmail.com@groups.io>
Sent: Friday, February 05, 2021 2:29 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

I have a 127 too and had no compunction in replacing the Se rectifiers
with Si. Nobody can tell you how long it will be before the Se rectifiers
fail and you will probably always have it in the back of your mind every
time you switch on the 127. It's not a diffcult or expensive modification
to do and gives you peace of mind.
I used cheap packaged 1000 volt 1 amp Si bridge rectifiers and replaced
R601 and R602 with 15 R 3W resistors. I aso inserted a 10R 10W resistor
in
the negative ground connection of the SR603 replacement.
The Si bridges I used had metal cases so I mounted them on insulating
posts, espaciallyu the SR607 replacement which is elevated a few hundred
volts above the chassis.
Good luck!

Morris


















Jim Ford
 

How could they tell; aren't sheep one of the dumbest animals anyway?!

But seriously, I read somewhere that trace amounts of Se are good for you. Like most things, even water, too much is bad.

Jim Ford

------ Original Message ------
From: "Harvey White" <madyn@dragonworks.info>
To: TekScopes@groups.io
Sent: 2/5/2021 5:20:43 PM
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

I remember an old story about sheep eating a certain plant, loco weed, and having severe congnitive problems.

Turns out that loco weed is high in selenium.

Harvey


On 2/5/2021 7:52 PM, Gary Robert Bosworth wrote:
Good to know.

On Fri, Feb 5, 2021, 16:42 Ken Eckert <eckertkp@gmail.com> wrote:

Burning selenium is very toxic, really bad for your health

On Friday, February 5, 2021, Joe Laffey <joe@thestable.tv> wrote:

If you’ve ever smelled a burned selenium rectifier (stinking up your
office or house for days) you would replace all of them before even
turning
them on.

--
Joe Laffey

On Feb 5, 2021, at 5:24 PM, Dave Wise <david_wise@phoenix.com> wrote:

? Even if the Selenium stacks haven't burned up, by this time their
series resistance is out of spec so the power supplies don't have as much
mains voltage headroom as they should. I'd replace them without ado
unless
the piece was meant to be a shelf queen.

Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Morris
Odell via groups.io <vilgotch1=gmail.com@groups.io>
Sent: Friday, February 05, 2021 2:29 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

I have a 127 too and had no compunction in replacing the Se rectifiers
with Si. Nobody can tell you how long it will be before the Se rectifiers
fail and you will probably always have it in the back of your mind every
time you switch on the 127. It's not a diffcult or expensive modification
to do and gives you peace of mind.
I used cheap packaged 1000 volt 1 amp Si bridge rectifiers and replaced
R601 and R602 with 15 R 3W resistors. I aso inserted a 10R 10W resistor
in
the negative ground connection of the SR603 replacement.
The Si bridges I used had metal cases so I mounted them on insulating
posts, espaciallyu the SR607 replacement which is elevated a few hundred
volts above the chassis.
Good luck!

Morris






















Harvey White
 

Well,  I guess even sheep behave according to the bell curve (or bell wether).  I think you can tell when a sheep is seriously deranged, somehow.....

While trace amounts are good, remember the guy who turned himself blue by overdosing on Selenium?

Harvey

On 2/5/2021 8:44 PM, Jim Ford wrote:
How could they tell; aren't sheep one of the dumbest animals anyway?!

But seriously, I read somewhere that trace amounts of Se are good for you.  Like most things, even water, too much is bad.

Jim Ford

------ Original Message ------
From: "Harvey White" <madyn@dragonworks.info>
To: TekScopes@groups.io
Sent: 2/5/2021 5:20:43 PM
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

I remember an old story about sheep eating a certain plant, loco weed, and having severe congnitive problems.

Turns out that loco weed is high in selenium.

Harvey


On 2/5/2021 7:52 PM, Gary Robert Bosworth wrote:
Good to know.

On Fri, Feb 5, 2021, 16:42 Ken Eckert <eckertkp@gmail.com> wrote:

Burning selenium is very toxic, really bad for your health

On Friday, February 5, 2021, Joe Laffey <joe@thestable.tv> wrote:

If you’ve ever smelled a burned selenium rectifier (stinking up your
office or house for days) you would replace all of them before even
turning
them on.

--
Joe Laffey

On Feb 5, 2021, at 5:24 PM, Dave Wise <david_wise@phoenix.com> wrote:

? Even if the Selenium stacks haven't burned up, by this time their
series resistance is out of spec so the power supplies don't have as much
mains voltage headroom as they should.  I'd replace them without ado
unless
the piece was meant to be a shelf queen.

Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Morris
Odell via groups.io <vilgotch1=gmail.com@groups.io>
Sent: Friday, February 05, 2021 2:29 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

I have a 127 too and had no compunction in replacing the Se rectifiers
with Si. Nobody can tell you how long it will be before the Se rectifiers
fail and you will probably always have it in the back of your mind every
time you switch on the 127. It's not a diffcult or expensive modification
to do and gives you peace of mind.
I used cheap packaged 1000 volt 1 amp Si bridge rectifiers and replaced
R601 and R602 with 15 R 3W resistors. I aso inserted a 10R 10W resistor
in
the negative ground connection of the SR603 replacement.
The Si bridges I used had metal cases so I mounted them on insulating
posts, espaciallyu the SR607 replacement which is elevated a few hundred
volts above the chassis.
Good luck!

Morris


























tgerbic
 

I assume you have the older power supply. The only photo I could find of the supply was the newer one with silicon rectifiers. Could you take a couple of clear pictures of the supply showing the rectifiers up close and of the overall wiring between the tubes and rectifiers. From that it should be possible to point out the rectifier used for the 225V supply.

Alternately I noticed the #3 power pin is an unregulated 330V connected directly to the sel rectifier. I would suggest you find the wire coming from the #3 pin of the power connector and just follow it back to the rectifier you are looking for.

To test, you can jumper a ~400V 100 to 300uf cap from the #3 pin (+) to ground, or hook the cap across C695, and see if the ripple is greatly reduced. If so C695 is probably bad. If it makes little difference the sel rectifier is probably bad. The easiest fix is to just replace the sel rectifier with a silicon bridge (or four IN4005 rectifiers) and be done with it. I would expect that if one of these sel rectifiers goes bad the others many not be too far behind.

Once you have replaced the sel rectifier in the 225V circuit, I would go back and check the 330V unregulated voltage pin 3 and see how much higher the voltage is. I would expect that voltage would go up and might be too high for other circuitry down stream. The 225V regulation circuitry should be able to handle the higher voltage.

Though it is good practice to replace all the selenium rectifiers with silicon, you may end up with an over voltage problem. Look at the two schematics (old rev/new rev) and see what the outputs of the transformers are. The older supply has higher voltage output at the secondary side of the transformer, assuming a higher drop across the selenium rectifiers. Additionally the input voltage assumes 117VAC which is a bit low for most modern outlets. Modern outlets generally provide 120VAC to 125VAC. All transformer voltages will be about 6% to 7% higher. Filament voltages may be closer to 6.7VAC. You should check this to be sure. The output of the bridge rectifiers may be higher due to higher AC and less drop through the rectifiers. I think the assumption in the older supply is for a 10V drop in the sel rectifier. The old supply 330V comes from 127VAC secondary with selenium rectification. The new supply 330V comes from a 117V secondary with silicon rectification. This added 10V coupled with ~6% higher AC would give you around 360V unregulated instead of 330V. This 330V supply goes to the CRT circuit so you need to recheck the CRT high voltage and adjust it with R811.

This is just an opinion based on looking at the two supplies. But I think it deserves some thought. This problem is the same as you encounter when repairing old tube radios using selenium rectifiers. Perhaps someone else on this forum may have some experience dealing with the higher voltages and how they might effect the scope.

Regards
Tony


tgerbic
 

One more thought...

Morris has a good solution with the dropping resistors for the regulated voltages. Still should expect to see higher than normal filament voltages and the HV supply should need adjusting.

Regards
Tony


Renée
 

I should be dead if that is the case.....and that was 45+years ago....stunk up my bedroom and the whole house more than once! ok too many times!!
Renée

On 2/5/21 4:42 PM, Ken Eckert wrote:
Burning selenium is very toxic, really bad for your health

On Friday, February 5, 2021, Joe Laffey <joe@thestable.tv> wrote:

If you’ve ever smelled a burned selenium rectifier (stinking up your
office or house for days) you would replace all of them before even turning
them on.

--
Joe Laffey

On Feb 5, 2021, at 5:24 PM, Dave Wise <david_wise@phoenix.com> wrote:

? Even if the Selenium stacks haven't burned up, by this time their
series resistance is out of spec so the power supplies don't have as much
mains voltage headroom as they should. I'd replace them without ado unless
the piece was meant to be a shelf queen.

Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Morris
Odell via groups.io <vilgotch1=gmail.com@groups.io>
Sent: Friday, February 05, 2021 2:29 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

I have a 127 too and had no compunction in replacing the Se rectifiers
with Si. Nobody can tell you how long it will be before the Se rectifiers
fail and you will probably always have it in the back of your mind every
time you switch on the 127. It's not a diffcult or expensive modification
to do and gives you peace of mind.
I used cheap packaged 1000 volt 1 amp Si bridge rectifiers and replaced
R601 and R602 with 15 R 3W resistors. I aso inserted a 10R 10W resistor in
the negative ground connection of the SR603 replacement.
The Si bridges I used had metal cases so I mounted them on insulating
posts, espaciallyu the SR607 replacement which is elevated a few hundred
volts above the chassis.
Good luck!

Morris














.


John Williams
 

Thanks Tony. Yes the 330v unreg is at 230v. This is what directed me to think part of the rectifier for the 225v could be open. I will post a picture.


stevenhorii
 

Selenium is one of those micro nutrients that you do need. However, it is
also toxic if you get too much. Overheated selenium rectifiers can produce
selenium dioxide which can be toxic if too much is inhaled. If you are
going to power up a piece of equipment with a questionable selenium
rectifier - do it in a place with good ventilation (preferably venting
outside - not to the rest of your workshop or house). This discussion from
the Antique Radio Forums:

https://antiqueradios.com/forums/viewtopic.php?f=6&t=36453

This suggests being cautious, but not obsessive-compulsive cautious. We
probably get exposed to more lead fumes from soldering than selenium from
bad rectifiers.

On Fri, Feb 5, 2021 at 20:44 Jim Ford <james.ford@cox.net> wrote:

How could they tell; aren't sheep one of the dumbest animals anyway?!

But seriously, I read somewhere that trace amounts of Se are good for
you. Like most things, even water, too much is bad.

Jim Ford

------ Original Message ------
From: "Harvey White" <madyn@dragonworks.info>
To: TekScopes@groups.io
Sent: 2/5/2021 5:20:43 PM
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

I remember an old story about sheep eating a certain plant, loco weed,
and having severe congnitive problems.

Turns out that loco weed is high in selenium.

Harvey


On 2/5/2021 7:52 PM, Gary Robert Bosworth wrote:
Good to know.

On Fri, Feb 5, 2021, 16:42 Ken Eckert <eckertkp@gmail.com> wrote:

Burning selenium is very toxic, really bad for your health

On Friday, February 5, 2021, Joe Laffey <joe@thestable.tv> wrote:

If you’ve ever smelled a burned selenium rectifier (stinking up your
office or house for days) you would replace all of them before even
turning
them on.

--
Joe Laffey

On Feb 5, 2021, at 5:24 PM, Dave Wise <david_wise@phoenix.com> wrote:

? Even if the Selenium stacks haven't burned up, by this time their
series resistance is out of spec so the power supplies don't have as
much
mains voltage headroom as they should. I'd replace them without ado
unless
the piece was meant to be a shelf queen.

Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Morris
Odell via groups.io <vilgotch1=gmail.com@groups.io>
Sent: Friday, February 05, 2021 2:29 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

I have a 127 too and had no compunction in replacing the Se rectifiers
with Si. Nobody can tell you how long it will be before the Se
rectifiers
fail and you will probably always have it in the back of your mind
every
time you switch on the 127. It's not a diffcult or expensive
modification
to do and gives you peace of mind.
I used cheap packaged 1000 volt 1 amp Si bridge rectifiers and
replaced
R601 and R602 with 15 R 3W resistors. I aso inserted a 10R 10W resistor
in
the negative ground connection of the SR603 replacement.
The Si bridges I used had metal cases so I mounted them on insulating
posts, espaciallyu the SR607 replacement which is elevated a few
hundred
volts above the chassis.
Good luck!

Morris



























John Williams
 

Photos posted.


tgerbic
 

Should be the top rectifier in the right side of the first photo. Since the wires are color coded a look at the bottom wiring of the chassis would help confirm which pin of the rectifier is connected to the transformer and which to the regulator circuit. If you can determine which pin is which you could temporarily connect some diodes across the selenium rectifier and see if the ripple clears up. No problem having them in parallel for a test as long as you get them connected properly.

You should be able to use a DMM or scope on the connections and find the 127VAC connections and the +/- output. Watch out for ground potential, if you use a scope, between the scope ground probe and the chassis ground, ohm it out if needed.

I would wire the replacement diodes on the bottom of the chassis on a terminal strip if possible that can be screwed to the chassis. I would be careful with the terminal strip you show in the third picture as I believe these require low temp solder to keep from damaging them. It is impossible to tell from the third photo what the voltage and current rating are for the diodes but I would suggest 1A diodes from a surge perspective (selenium rectifiers behave a little different). Probably need a minimum of 400V for this circuit. I would suggest you purchase a bunch of 1N4006 (800V @ 1A) diodes to replace all the selenium rectifiers. I would also suggest you remove the wires to the selenium rectifiers on the bottom of the chassis and leave the rectifiers in place for that nostalgic look. Can't hurt to just leave them in place as long as the wires are not connected.

Regards
Tony


tgerbic
 

Can you please provide some sharp/clear photo's of the bottom of the chassis.There may be some suggestions possible to mount the diodes. I would also like to look at the wire color codes to try to determine which pin is which.

Regards
Tony


cmjones01
 

I'm looking forward to starting to use the 127, so I guess I'd better order
some rectifiers. I have experienced a selenium rectifier expiring before,
and it's not something I want to repeat...

Chris

On Sat, 6 Feb 2021, 00:24 Dave Wise, <david_wise@phoenix.com> wrote:

? Even if the Selenium stacks haven't burned up, by this time their series
resistance is out of spec so the power supplies don't have as much mains
voltage headroom as they should. I'd replace them without ado unless the
piece was meant to be a shelf queen.


Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Morris Odell
via groups.io <vilgotch1=gmail.com@groups.io>
Sent: Friday, February 05, 2021 2:29 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

I have a 127 too and had no compunction in replacing the Se rectifiers
with Si. Nobody can tell you how long it will be before the Se rectifiers
fail and you will probably always have it in the back of your mind every
time you switch on the 127. It's not a diffcult or expensive modification
to do and gives you peace of mind.

I used cheap packaged 1000 volt 1 amp Si bridge rectifiers and replaced
R601 and R602 with 15 R 3W resistors. I aso inserted a 10R 10W resistor in
the negative ground connection of the SR603 replacement.

The Si bridges I used had metal cases so I mounted them on insulating
posts, espaciallyu the SR607 replacement which is elevated a few hundred
volts above the chassis.

Good luck!

Morris