Topics

7A26 Ch 1 gain low

Colin Herbert
 

I have a 7A26 dual trace vertical amplifier with a Ch 1 gain problem. When trying to set the gain, I cannot set it correctly, as the GAIN adjust goes to fully clockwise but the trace on the CRT is a mm or so low. This is with the VOLTS/DIV at 10mV and a 20mV square wave from my PG506. The VOLTS/DIV VAR control won't go beyond this either. Ch 2 works fine, with about one small division of excess available using both the GAIN adjust and the VOLTS/DIV VAR. Anyone have any ideas?
TIA, Colin.

Colin Herbert
 

Doesn't anyone have any comments?

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 09 July 2019 13:20
To: TekScopes@groups.io
Subject: [TekScopes] 7A26 Ch 1 gain low

I have a 7A26 dual trace vertical amplifier with a Ch 1 gain problem. When trying to set the gain, I cannot set it correctly, as the GAIN adjust goes to fully clockwise but the trace on the CRT is a mm or so low. This is with the VOLTS/DIV at 10mV and a 20mV square wave from my PG506. The VOLTS/DIV VAR control won't go beyond this either. Ch 2 works fine, with about one small division of excess available using both the GAIN adjust and the VOLTS/DIV VAR. Anyone have any ideas?
TIA, Colin.

 

OK I'll make some guesses.

1. Have you checked that the output level from the attenuators is correct (same both channels)?

2. Does the problem occur at other volts/div settings?

3. What frequency signal are you testing with? Is the problem also present at DC?

4. You may be reduced to comparing signal levels on each channel through the various amplifier stage until you find one with low output. If you find one initially suspect any surrounding gain setting components, but equally could try swapping the ICs between channels.

HtH
David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 11 July 2019 11:03
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

Doesn't anyone have any comments?

Colin.
-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 09 July 2019 13:20
To: TekScopes@groups.io
Subject: [TekScopes] 7A26 Ch 1 gain low

I have a 7A26 dual trace vertical amplifier with a Ch 1 gain problem. When trying to set the gain, I cannot set it correctly, as the GAIN adjust goes to fully clockwise but the trace on the CRT is a mm or so low. This is with the VOLTS/DIV at 10mV and a 20mV square wave from my PG506. The VOLTS/DIV VAR control won't go beyond this either. Ch 2 works fine, with about one small division of excess available using both the GAIN adjust and the VOLTS/DIV VAR. Anyone have any ideas?
TIA, Colin.

Adrian
 

I would reinforce David's forth point about comparing channels.

I've fixed a handful of 7000 series 'ordinary' dual-channel amps (including a 26) that way, with the service manual open in front of you it is dead easy to apply the same signal to both channels and probe your way along the same points in each channel until you find the difference, in my experience that narrows it down to a few components really fast.

I may get shot for this, but in some ways it's often quicker (if not as instructive) to just do that than to sit down analyze the problem from 'first principles' It is true that an extender cable makes it physically much easier to do that than having to probe in the chassis with the side covers off though!

Adrian

On 7/11/2019 11:47 AM, David C. Partridge wrote:
OK I'll make some guesses.

1. Have you checked that the output level from the attenuators is correct (same both channels)?

2. Does the problem occur at other volts/div settings?

3. What frequency signal are you testing with? Is the problem also present at DC?

4. You may be reduced to comparing signal levels on each channel through the various amplifier stage until you find one with low output. If you find one initially suspect any surrounding gain setting components, but equally could try swapping the ICs between channels.

HtH
David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 11 July 2019 11:03
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

Doesn't anyone have any comments?

Colin.
-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 09 July 2019 13:20
To: TekScopes@groups.io
Subject: [TekScopes] 7A26 Ch 1 gain low

I have a 7A26 dual trace vertical amplifier with a Ch 1 gain problem. When trying to set the gain, I cannot set it correctly, as the GAIN adjust goes to fully clockwise but the trace on the CRT is a mm or so low. This is with the VOLTS/DIV at 10mV and a 20mV square wave from my PG506. The VOLTS/DIV VAR control won't go beyond this either. Ch 2 works fine, with about one small division of excess available using both the GAIN adjust and the VOLTS/DIV VAR. Anyone have any ideas?
TIA, Colin.








Jim Ford
 

Hi, Colin.I will be home in a couple of days and will check my 7A26's then.  Let me see if I can replicate your issue.Jim Ford Sent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: "Colin Herbert via Groups.Io" <colingherbert=blueyonder.co.uk@groups.io> Date: 7/11/19 4:02 AM (GMT-07:00) To: TekScopes@groups.io Subject: Re: [TekScopes] 7A26 Ch 1 gain low Doesn't anyone have any comments?Colin.-----Original Message-----From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.IoSent: 09 July 2019 13:20To: TekScopes@...: [TekScopes] 7A26 Ch 1 gain lowI have a 7A26 dual trace vertical amplifier with a Ch 1 gain problem. When trying to set the gain, I cannot set it correctly, as the GAIN adjust goes to fully clockwise but the trace on the CRT is a mm or so low. This is with the VOLTS/DIV at 10mV and a 20mV square wave from my PG506. The VOLTS/DIV VAR control won't go beyond this either. Ch 2 works fine, with about one small division of excess available using both the GAIN adjust and the VOLTS/DIV VAR. Anyone have any ideas?TIA, Colin.

 

On Tue, Jul 9, 2019 at 02:19 PM, Colin Herbert wrote:


Ch 2 works fine, with about one small division of excess available using both
the GAIN adjust and the VOLTS/DIV VAR.
I think that's not enough, meaning that overall gain for *both* channels isn't enough. Usually, there's at least half a major division headroom.
Did you try the other vertical slot of the mainframe?
Do you know the mainframe is ok? Do you have another plugin to check the mainframe?

If the mainframe is new to you, a previous owner may have adjusted mainframe vertical sensitivity incorrectly.

Raymond

Colin Herbert
 

Hi,
Yes, it does occur at other volts/div settings. I was quoting what I did to quantify the low gain in some way. The frequency was the "standard amplitude" 1kHz square-wave from my PG506.

As it happens, I have two 7A26 plug-ins (I like them) and swapping them around between the two vertical bays of my 7623A mainframe shows that the problem lies with the 7A26, not the mainframe or one of the bays. The "good" 7A26 shows an excess gain of about one half of a major division on both channels.

I was thinking, from a cursory inspection, that the ICs were soldered in, but I think this may not be the case on closer viewing. From what I can see of the markings on the ICs, the first three numbers differ, but then the next group of six numbers are "0078 10" followed by another group of two numbers and a letter. My service manual lists three types of ICs, the earlier ones were "SEL" for "selected" I am guessing. The later ones were designated 155-0078-10 and aren't "SEL". The 155-0078-10" are available on eBay. I might just buy a couple.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of David C. Partridge
Sent: 11 July 2019 11:47
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

OK I'll make some guesses.

1. Have you checked that the output level from the attenuators is correct (same both channels)?

2. Does the problem occur at other volts/div settings?

3. What frequency signal are you testing with? Is the problem also present at DC?

4. You may be reduced to comparing signal levels on each channel through the various amplifier stage until you find one with low output. If you find one initially suspect any surrounding gain setting components, but equally could try swapping the ICs between channels.

HtH
David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 11 July 2019 11:03
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

Doesn't anyone have any comments?

Colin.
-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 09 July 2019 13:20
To: TekScopes@groups.io
Subject: [TekScopes] 7A26 Ch 1 gain low

I have a 7A26 dual trace vertical amplifier with a Ch 1 gain problem. When trying to set the gain, I cannot set it correctly, as the GAIN adjust goes to fully clockwise but the trace on the CRT is a mm or so low. This is with the VOLTS/DIV at 10mV and a 20mV square wave from my PG506. The VOLTS/DIV VAR control won't go beyond this either. Ch 2 works fine, with about one small division of excess available using both the GAIN adjust and the VOLTS/DIV VAR. Anyone have any ideas?
TIA, Colin.

Leon Robinson
 

I havent messed with a 7A26 in some time but I think that there is a gain adjustment between the channel switch and the output that is set too low.



Sent from K5JLR

-------- Original message --------
From: "Colin Herbert via Groups.Io" <colingherbert=blueyonder.co.uk@groups.io>
Date: 07/11/2019 10:44 AM (GMT-06:00)
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

Hi,
Yes, it does occur at other volts/div settings. I was quoting what I did to quantify the low gain in some way. The frequency was the "standard amplitude" 1kHz square-wave from my PG506.

As it happens, I have two 7A26 plug-ins (I like them) and swapping them around between the two vertical bays of my 7623A mainframe shows that the problem lies with the 7A26, not the mainframe or one of the bays. The "good" 7A26 shows an excess gain of about one half of a major division on both channels.

I was thinking, from a cursory inspection, that the ICs were soldered in, but I think this may not be the case on closer viewing. From what I can see of the markings on the ICs, the first three numbers differ, but then the next group of six numbers are "0078 10" followed by another group of two numbers and a letter. My service manual lists three types of ICs, the earlier ones were "SEL" for "selected" I am guessing. The later ones were designated 155-0078-10 and aren't "SEL". The 155-0078-10" are available on eBay. I might just buy a couple.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of David C. Partridge
Sent: 11 July 2019 11:47
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

OK I'll make some guesses.

1. Have you checked that the output level from the attenuators is correct (same both channels)?

2. Does the problem occur at other volts/div settings?

3. What frequency signal are you testing with?  Is the problem also present at DC?

4. You may be reduced to comparing signal levels on each channel through the various amplifier stage until you find one with low output.  If you find one initially suspect any surrounding gain setting components, but equally could try swapping the ICs between channels.

HtH
David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 11 July 2019 11:03
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

Doesn't anyone have any comments?

Colin.
-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 09 July 2019 13:20
To: TekScopes@groups.io
Subject: [TekScopes] 7A26 Ch 1 gain low

I have a 7A26 dual trace vertical amplifier with a Ch 1 gain problem. When trying to set the gain, I cannot set it correctly, as the GAIN adjust goes to fully clockwise but the trace on the CRT is a mm or so low. This is with the VOLTS/DIV at 10mV and a 20mV square wave from my PG506. The VOLTS/DIV VAR control won't go beyond this either. Ch 2 works fine, with about one small division of excess available using both the GAIN adjust and the VOLTS/DIV VAR. Anyone have any ideas?
TIA, Colin.

Colin Herbert
 

I have looked at the Service Manual and cannot find anything about a gain adjustment between the channel switch and the output. However, on looking closer at the problematic 7A26, I think that the problem is not only with Ch 1, as the Variable gain control on Ch 2 doesn't give as much gain increase over the calibrated setting as do both channels on my "good" 7A26. It is about one half of a major division on the "good" 7A26.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Leon Robinson
Sent: 11 July 2019 19:03
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

I havent messed with a 7A26 in some time but I think that there is a gain adjustment between the channel switch and the output that is set too low.



Sent from K5JLR

-------- Original message --------
From: "Colin Herbert via Groups.Io" <colingherbert=blueyonder.co.uk@groups.io>
Date: 07/11/2019 10:44 AM (GMT-06:00)
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

Hi,
Yes, it does occur at other volts/div settings. I was quoting what I did to quantify the low gain in some way. The frequency was the "standard amplitude" 1kHz square-wave from my PG506.

As it happens, I have two 7A26 plug-ins (I like them) and swapping them around between the two vertical bays of my 7623A mainframe shows that the problem lies with the 7A26, not the mainframe or one of the bays. The "good" 7A26 shows an excess gain of about one half of a major division on both channels.

I was thinking, from a cursory inspection, that the ICs were soldered in, but I think this may not be the case on closer viewing. From what I can see of the markings on the ICs, the first three numbers differ, but then the next group of six numbers are "0078 10" followed by another group of two numbers and a letter. My service manual lists three types of ICs, the earlier ones were "SEL" for "selected" I am guessing. The later ones were designated 155-0078-10 and aren't "SEL". The 155-0078-10" are available on eBay. I might just buy a couple.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of David C. Partridge
Sent: 11 July 2019 11:47
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

OK I'll make some guesses.

1. Have you checked that the output level from the attenuators is correct (same both channels)?

2. Does the problem occur at other volts/div settings?

3. What frequency signal are you testing with? Is the problem also present at DC?

4. You may be reduced to comparing signal levels on each channel through the various amplifier stage until you find one with low output. If you find one initially suspect any surrounding gain setting components, but equally could try swapping the ICs between channels.

HtH
David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 11 July 2019 11:03
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

Doesn't anyone have any comments?

Colin.
-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 09 July 2019 13:20
To: TekScopes@groups.io
Subject: [TekScopes] 7A26 Ch 1 gain low

I have a 7A26 dual trace vertical amplifier with a Ch 1 gain problem. When trying to set the gain, I cannot set it correctly, as the GAIN adjust goes to fully clockwise but the trace on the CRT is a mm or so low. This is with the VOLTS/DIV at 10mV and a 20mV square wave from my PG506. The VOLTS/DIV VAR control won't go beyond this either. Ch 2 works fine, with about one small division of excess available using both the GAIN adjust and the VOLTS/DIV VAR. Anyone have any ideas?
TIA, Colin.

 

On Fri, Jul 12, 2019 at 02:23 PM, Colin Herbert wrote:


However, on looking closer at the problematic 7A26, I think that the problem
is not only with Ch 1, as the Variable gain control on Ch 2 doesn't give as
much gain increase over the calibrated setting as do both channels on my
"good" 7A26. It is about one half of a major division on the "good" 7A26.
... Which is what I suggested earlier: "I think that's not enough, meaning that overall gain for *both* channels isn't enough. Usually, there's at least half a major division headroom".

Raymond

Colin Herbert
 

I have purchased a number of the ICs - #155-0078-10 in order to do some swapping. I am still wondering if theses ICs are soldered in or not. Does anyone have experience of them?
TIA, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Raymond Domp Frank
Sent: 12 July 2019 18:35
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

On Fri, Jul 12, 2019 at 02:23 PM, Colin Herbert wrote:


However, on looking closer at the problematic 7A26, I think that the problem
is not only with Ch 1, as the Variable gain control on Ch 2 doesn't give as
much gain increase over the calibrated setting as do both channels on my
"good" 7A26. It is about one half of a major division on the "good" 7A26.
... Which is what I suggested earlier: "I think that's not enough, meaning that overall gain for *both* channels isn't enough. Usually, there's at least half a major division headroom".

Raymond

Chuck Harris
 

Most of the fancy IC's in the 7000 series are socketed,
using the little insert style of socket pins. Look for
a bit of white rubber stuff around each pin on the component
side for confirmation.

-Chuck Harris

Colin Herbert via Groups.Io wrote:

I have purchased a number of the ICs - #155-0078-10 in order to do some swapping. I am still wondering if theses ICs are soldered in or not. Does anyone have experience of them?
TIA, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Raymond Domp Frank
Sent: 12 July 2019 18:35
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

On Fri, Jul 12, 2019 at 02:23 PM, Colin Herbert wrote:


However, on looking closer at the problematic 7A26, I think that the problem
is not only with Ch 1, as the Variable gain control on Ch 2 doesn't give as
much gain increase over the calibrated setting as do both channels on my
"good" 7A26. It is about one half of a major division on the "good" 7A26.
... Which is what I suggested earlier: "I think that's not enough, meaning that overall gain for *both* channels isn't enough. Usually, there's at least half a major division headroom".

Raymond







 

The small ICs all over the 7A26 board are socketed. I would be inclined to suspect gain set components rather than the ICs themselves.

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 13 July 2019 12:05
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

I have purchased a number of the ICs - #155-0078-10 in order to do some swapping. I am still wondering if theses ICs are soldered in or not. Does anyone have experience of them?
TIA, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Raymond Domp Frank
Sent: 12 July 2019 18:35
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

On Fri, Jul 12, 2019 at 02:23 PM, Colin Herbert wrote:


However, on looking closer at the problematic 7A26, I think that the problem
is not only with Ch 1, as the Variable gain control on Ch 2 doesn't give as
much gain increase over the calibrated setting as do both channels on my
"good" 7A26. It is about one half of a major division on the "good" 7A26.
... Which is what I suggested earlier: "I think that's not enough, meaning that overall gain for *both* channels isn't enough. Usually, there's at least half a major division headroom".

Raymond

Colin Herbert
 

Hi,

I have done a bit more investigating on this and have found the following:

-Some of the voltages on the first CH1 IC (U1350) are different from those on a known good 7A26, so I am inclined to think the problem is associated with that circuit. Tek made some errors in Service Manual in that the polarities some DC voltages here are wrong (pins 2 & 3 are given as +0.8V but are actually -0.8V).
-Swapping the first ICs between CH1 and CH2 doesn't change anything.
-Putting the U1350 from CH1 into a known good 7A26 doesn't take the problem with the IC, so it is probably good.
-When the IC from the problematic CH1 was in place in the known good 7A26, the voltages around it were similar to when a known good IC was in place, again suggesting that there is nothing wrong with the IC.
-The +15V and -15V rails (where I have probed them) seem to be in spec.
-I have tried some of the initial calibration routines (with the correct equipment) and find odd behaviour (zero shift) when operating the BANDWIDTH and INVERT switches during some of this.
-I have also tried checking the waveforms at the various points shown in the Service Manual, but I am not too sure that Tektronix didn't make some mistakes here too, as I see large DC voltages which move the trace off the screen when using the stipulated DC input on the investigating scope.
I am at a bit of an impasse, partly because the circuits are differential and I don't understand them too well.

Any advice is appreciated.

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of David C. Partridge
Sent: 13 July 2019 15:40
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

The small ICs all over the 7A26 board are socketed. I would be inclined to suspect gain set components rather than the ICs themselves.

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 13 July 2019 12:05
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

I have purchased a number of the ICs - #155-0078-10 in order to do some swapping. I am still wondering if theses ICs are soldered in or not. Does anyone have experience of them?
TIA, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Raymond Domp Frank
Sent: 12 July 2019 18:35
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

On Fri, Jul 12, 2019 at 02:23 PM, Colin Herbert wrote:


However, on looking closer at the problematic 7A26, I think that the problem
is not only with Ch 1, as the Variable gain control on Ch 2 doesn't give as
much gain increase over the calibrated setting as do both channels on my
"good" 7A26. It is about one half of a major division on the "good" 7A26.
... Which is what I suggested earlier: "I think that's not enough, meaning that overall gain for *both* channels isn't enough. Usually, there's at least half a major division headroom".

Raymond

Albert Otten
 

Hi Colin,

Remarks only concerning the schematics:

On Wed, Jul 24, 2019 at 05:57 PM, Colin Herbert wrote:


I have done a bit more investigating on this and have found the following:

-Some of the voltages on the first CH1 IC (U1350) are different from those on
a known good 7A26, so I am inclined to think the problem is associated with
that circuit. Tek made some errors in Service Manual in that the polarities
some DC voltages here are wrong (pins 2 & 3 are given as +0.8V but are
actually -0.8V).
As far as I can see you can use there the proper values shown for CH2.


-Swapping the first ICs between CH1 and CH2 doesn't change anything.
-Putting the U1350 from CH1 into a known good 7A26 doesn't take the problem
with the IC, so it is probably good.
-When the IC from the problematic CH1 was in place in the known good 7A26, the
voltages around it were similar to when a known good IC was in place, again
suggesting that there is nothing wrong with the IC.
-The +15V and -15V rails (where I have probed them) seem to be in spec.
-I have tried some of the initial calibration routines (with the correct
equipment) and find odd behaviour (zero shift) when operating the BANDWIDTH
and INVERT switches during some of this.
-I have also tried checking the waveforms at the various points shown in the
Service Manual, but I am not too sure that Tektronix didn't make some mistakes
here too, as I see large DC voltages which move the trace off the screen when
using the stipulated DC input on the investigating scope.
I am at a bit of an impasse, partly because the circuits are differential and
I don't understand them too well.
The "Voltages and waveforms" page states that "if the 7A16A amplifier plugin is used [for measuring waveforms], the input should be AC coupled". That holds for any ordinary plugin. But if you use the 7A13 (also mentioned) then you can measure differentially, compared with a suitable internal 7A13 DC level.

Albert