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I wish I could direct this question to the engineer who designed the
sampling bridge in the 7S14 plug-in. The original design made use of
Mercury batteries in the sampling gate back-bias (BT1 and BT2). The mercury
cells which are no longer available had a voltage of 1.35 volts. I need to
know how critical this voltage is??? Can these cells be replaced with
Zinc-Air which have a voltage of 1.25 volts? 1.25 volts should be
sufficient to back-bias the sampling diodes, but will the sampling pulse
amplitude be too much and over-drive the sampling gate? Does anyone still
have contact with the original engineers to get a definitive answer?

I wouldn't consider zinc-air batteries a good choice for this purpose. Zinc-air batteries have a relatively long shelf life unopened but once opened (activated), they discharge rather quickly, even without load, since the zinc-air reaction just proceeds.


Several good options are available:
I have replaced the batteries with a DIY photovoltaic solution, using LEDs (4 sets total). This solution is easy to build and works well.
There's been a lot of traffic on this forum re. the replacement of these bias batteries, some of it less than a month ago.


Have a look at Ed Breya's very useful article here:


7S14 repair - TekWiki


7S14 repair - TekWiki The following document was authored by Ed Breya and originally posted on the TekScopes Yahoo forum. Contents 1 Background 2 Design and construction 3 Findings and recommendations Background



View on w140.com
Preview by Yahoo






Raymond

The difference of 0.1 volts is insignificant as far as risk to the sampling gate
diodes but as Raymond points out, zinc-air batteries are a poor choice because
of their short life once activated.

I would consider either using silver oxide batteries or the photovoltaic option.

Another possibility on the photovoltaic approach is Vishay's LH1262 photovoltaic MOSFET drivers. It has two photovoltaic generators in a DIP or SOIC package. You can get a minimum of 2.6uA each (3.4uA to 6.9uA typ, depending on drive current), with an open circuit voltage of about 13.5V.

Regulation would still be needed, obviously, probably by loading the output and controlling the drive current as in the referenced artcicle. Or maybe that's enough current for an ultra-low power regulator.

The photovoltaic MOSFET approach would work of course. Current is more than enough but regulation is compulsary at >13 V open circuit. The sampler diodes in reverse bias hardly allow any current through.


Raymond


Another possibility on the photovoltaic approach is Vishay's LH1262 photovoltaic MOSFET drivers. It has two photovoltaic generators in a DIP or SOIC package. You can get a minimum of 2.6uA each (3.4uA to 6.9uA typ, depending on drive current), with an open circuit voltage of about 13.5V.

Regulation would still be needed, obviously, probably by loading the output and controlling the drive current as in the referenced artcicle. Or maybe that's enough current for an ultra-low power regulator.

You need to read the the wiki article, and message #112514 to get the background info and update. Then look at #96664 for the best implementation that I have seen, in my opinion.

Ed

Raymond:

Thanx for all the info. When it's working properly, the 7S14 is a sweet
plug-in.

I also wish there were more retired Tektronix engineers who would do repair
and calibration of old Tek equipment. Most of the repair shops in the S.F.
Bay area wanted an arm and a leg to do a few hours work.

Gary
On Dec 4, 2014 9:26 AM, "raymonddf@... [TekScopes]" <
TekScopes@...> wrote:

I wouldn't consider zinc-air batteries a good choice for this purpose.
Zinc-air batteries have a relatively long shelf life unopened but once
opened (activated), they discharge rather quickly, even without load, since
the zinc-air reaction just proceeds.


Several good options are available:
I have replaced the batteries with a DIY photovoltaic solution, using LEDs
(4 sets total). This solution is easy to build and works well.
There's been a lot of traffic on this forum re. the replacement of these
bias batteries, some of it less than a month ago.


Have a look at Ed Breya's very useful article here:


7S14 repair - TekWiki


7S14 repair - TekWiki The
following document was authored by Ed Breya and originally posted on the
TekScopes Yahoo forum. Contents 1 Background 2 Design and construction 3
Findings and recommendations Background



View on w140.com
Preview by Yahoo






Raymond


[Non-text portions of this message have been removed]

[Non-text portions of this message have been removed]

In connection with the concept of using the LH1262 (or similar) device,
I have two questions for the group members:

1. To implement the "regulation" suggested by raymonddf, applied to the
use of an LH1262 as proposed by mlitwack, would it be possible to feed
the output of the LH1262 (about 12 or 13 volts unloaded) to a resistor
in series with a string of 1n914 diodes (a shunt regulator) to obtain
the "target" 1.35V voltage (or close to it)? The Fairchild 1n914
datasheet indicates approximately 250 mV forward drop (at 25 deg C) in a
1n914 with 1.0 microamp forward bias; perhaps six 1n914 diodes in series
with a 10 megohm resistor would work?

2. Another poster (edbreya) suggested "you need to read... message
112514...then look at 96664..." If I log into the tekscopes yahoo group
site and do a "conversations" search on "112514" I can access the 112514
message, but if I attempt a similar search on "96664" I get nothing.
Can someone explain to me how I might get access to message 96664?

Mike N4MWP

On 12/04/2014 01:42 PM, raymonddf@... [TekScopes] wrote:

The photovoltaic MOSFET approach would work of course. Current is more
than enough but regulation is compulsary at >13 V open circuit. The
sampler diodes in reverse bias hardly allow any current through.


Raymond


Another possibility on the photovoltaic approach is Vishay's LH1262
photovoltaic MOSFET drivers. It has two photovoltaic generators in a
DIP or SOIC package. You can get a minimum of 2.6uA each (3.4uA to
6.9uA typ, depending on drive current), with an open circuit voltage
of about 13.5V.

Regulation would still be needed, obviously, probably by loading the
output and controlling the drive current as in the referenced
artcicle. Or maybe that's enough current for an ultra-low power
regulator.


[Non-text portions of this message have been removed]

[Non-text portions of this message have been removed]

On Thu, Dec 4, 2014 at 2:28 PM, Mike <mdinolfo@...> wrote:

2. Another poster (edbreya) suggested "you need to read... message
112514...then look at 96664..." If I log into the tekscopes yahoo group
site and do a "conversations" search on "112514" I can access the 112514
message, but if I attempt a similar search on "96664" I get nothing.
Can someone explain to me how I might get access to message 96664?

Cavid:

Thanks; that worked great!

Mike N4MWP

On 12/04/2014 05:47 PM, David DiGiacomo telists@... [TekScopes]
wrote:

On Thu, Dec 4, 2014 at 2:28 PM, Mike <mdinolfo@...> wrote:

2. Another poster (edbreya) suggested "you need to read... message
112514...then look at 96664..." If I log into the tekscopes yahoo group
site and do a "conversations" search on "112514" I can access the 112514
message, but if I attempt a similar search on "96664" I get nothing.
Can someone explain to me how I might get access to message 96664?

[Non-text portions of this message have been removed]