Tek 151-1042-00 : Matched JFET in a 7B80


n4buq
 

I have a 7B80 which doesn't work in external triggering mode and I discovered one of a matched pair of JFETs in it (Q22A/B) decided it liked being a diode instead of a transistor. As the subject indicates, it's a Tek P/N 151-1042-00 which, apparently, are hard to find.

From what I've seen, a 2N5454 may be a possible substitute but would like to see what else could be substituted - particularly in the TO-92 form since that fits the clamp they're in. If I can find a suitable replacement, I think I'll need to do some matching and, thus, might need a handful of them (or more) to sift through for a close pair.

I think a 2N5454 is a dual JFET package and, if so, the pair may be closely matched; however, I'm not sure about that.

Anyone know what I could use and, better yet, where I could find it?

Thanks,
Barry - N4BUQ


Harvey White
 

I've seen some transistors in Tek stuff that are individual transistors, but linked by a wrap around heatsink.  No idea if they were matched, but the heatsink (it's S shaped) would keep them thermally coupled.

Harvey

On 3/6/2022 8:24 PM, n4buq wrote:
I have a 7B80 which doesn't work in external triggering mode and I discovered one of a matched pair of JFETs in it (Q22A/B) decided it liked being a diode instead of a transistor. As the subject indicates, it's a Tek P/N 151-1042-00 which, apparently, are hard to find.

From what I've seen, a 2N5454 may be a possible substitute but would like to see what else could be substituted - particularly in the TO-92 form since that fits the clamp they're in. If I can find a suitable replacement, I think I'll need to do some matching and, thus, might need a handful of them (or more) to sift through for a close pair.
I think a 2N5454 is a dual JFET package and, if so, the pair may be closely matched; however, I'm not sure about that.

Anyone know what I could use and, better yet, where I could find it?

Thanks,
Barry - N4BUQ





Tom Lee
 

Hi Barry,

The 2N5454 is a monolithic dual JFET, so the matching ought to be decent (say, better than 15mV or so). Certainly, the two FETs will be tightly coupled, thermally.

Just out of curiosity, in what way did your defective JFET fail?

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/6/2022 17:24, n4buq wrote:
I have a 7B80 which doesn't work in external triggering mode and I discovered one of a matched pair of JFETs in it (Q22A/B) decided it liked being a diode instead of a transistor. As the subject indicates, it's a Tek P/N 151-1042-00 which, apparently, are hard to find.

From what I've seen, a 2N5454 may be a possible substitute but would like to see what else could be substituted - particularly in the TO-92 form since that fits the clamp they're in. If I can find a suitable replacement, I think I'll need to do some matching and, thus, might need a handful of them (or more) to sift through for a close pair.

I think a 2N5454 is a dual JFET package and, if so, the pair may be closely matched; however, I'm not sure about that.

Anyone know what I could use and, better yet, where I could find it?

Thanks,
Barry - N4BUQ




 

Barry,

whether or not the two transistors in a 2N5454 are "matched" they will be thermally coupled, but it's a good bet that they are close to matched, merely by having been manufactured on the same line at the same time.

The 151-1042-00 were used all over the place, in the late 400-series scopes, 2213/2215/2235, T912/T921/T922/T932/T935, and in lots of 7k plug-ins, including the 7B53A, which is easy to find. I might even have the part you are looking for in a junker 7B53A (let me have a look at the schematics: I can see two pairs of transistors with bolted-on heat couplers on the trigger board).

-- Jeff Dutky


n4buq
 

The ones in my 7B80 have a wrap-around heatsink but not the S shaped kind. The JFETs are face-to-face with the heatsink wrapped around the round parts of both.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Harvey White" <madyn@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Sunday, March 6, 2022 8:18:35 PM
Subject: Re: [TekScopes] Tek 151-1042-00 : Matched JFET in a 7B80
I've seen some transistors in Tek stuff that are individual transistors,
but linked by a wrap around heatsink.  No idea if they were matched, but
the heatsink (it's S shaped) would keep them thermally coupled.

Harvey


On 3/6/2022 8:24 PM, n4buq wrote:
I have a 7B80 which doesn't work in external triggering mode and I discovered
one of a matched pair of JFETs in it (Q22A/B) decided it liked being a diode
instead of a transistor. As the subject indicates, it's a Tek P/N 151-1042-00
which, apparently, are hard to find.

From what I've seen, a 2N5454 may be a possible substitute but would like to see
what else could be substituted - particularly in the TO-92 form since that fits
the clamp they're in. If I can find a suitable replacement, I think I'll need
to do some matching and, thus, might need a handful of them (or more) to sift
through for a close pair.
I think a 2N5454 is a dual JFET package and, if so, the pair may be closely
matched; however, I'm not sure about that.

Anyone know what I could use and, better yet, where I could find it?

Thanks,
Barry - N4BUQ







n4buq
 

The signal coming in from the EXT jack makes it to the gate of Q22A but the junction of CR23/CR24 only sees a slightly negative voltage (maybe a diode drop). I removed the pair and tested them with my component tester. One tested properly, the other tested as a diode (don't recall which pins were reported as being a diode nor did I make note of which one was a diode but with an approximate diode-drop in the negative direction, I presume it is Q22B).

Oddly, I'm seeing 2N5454s as a single transistor but the datasheet appears to show a dual. That's part of what has me wondering what I should be looking for.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Tom Lee" <tomlee@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Sunday, March 6, 2022 8:33:33 PM
Subject: Re: [TekScopes] Tek 151-1042-00 : Matched JFET in a 7B80
Hi Barry,

The 2N5454 is a monolithic dual JFET, so the matching ought to be decent
(say, better than 15mV or so). Certainly, the two FETs will be tightly
coupled, thermally.

Just out of curiosity, in what way did your defective JFET fail?

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/6/2022 17:24, n4buq wrote:
I have a 7B80 which doesn't work in external triggering mode and I discovered
one of a matched pair of JFETs in it (Q22A/B) decided it liked being a diode
instead of a transistor. As the subject indicates, it's a Tek P/N 151-1042-00
which, apparently, are hard to find.

From what I've seen, a 2N5454 may be a possible substitute but would like to see
what else could be substituted - particularly in the TO-92 form since that fits
the clamp they're in. If I can find a suitable replacement, I think I'll need
to do some matching and, thus, might need a handful of them (or more) to sift
through for a close pair.

I think a 2N5454 is a dual JFET package and, if so, the pair may be closely
matched; however, I'm not sure about that.

Anyone know what I could use and, better yet, where I could find it?

Thanks,
Barry - N4BUQ







n4buq
 

Jeff,

I have a few 7B53As. I'll see if I can swap one into the 7B80 and see if that restores external triggering. Thanks for the pointer!

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Jeff Dutky" <jeff.dutky@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Sunday, March 6, 2022 8:33:56 PM
Subject: Re: [TekScopes] Tek 151-1042-00 : Matched JFET in a 7B80
Barry,

whether or not the two transistors in a 2N5454 are "matched" they will be
thermally coupled, but it's a good bet that they are close to matched, merely
by having been manufactured on the same line at the same time.

The 151-1042-00 were used all over the place, in the late 400-series scopes,
2213/2215/2235, T912/T921/T922/T932/T935, and in lots of 7k plug-ins, including
the 7B53A, which is easy to find. I might even have the part you are looking
for in a junker 7B53A (let me have a look at the schematics: I can see two
pairs of transistors with bolted-on heat couplers on the trigger board).

-- Jeff Dutky



n4buq
 

Jeff,

I see they're Q310/311 and Q410/411 in the 7B53A. I should have a few to test with.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Jeff Dutky" <jeff.dutky@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Sunday, March 6, 2022 8:33:56 PM
Subject: Re: [TekScopes] Tek 151-1042-00 : Matched JFET in a 7B80
Barry,

whether or not the two transistors in a 2N5454 are "matched" they will be
thermally coupled, but it's a good bet that they are close to matched, merely
by having been manufactured on the same line at the same time.

The 151-1042-00 were used all over the place, in the late 400-series scopes,
2213/2215/2235, T912/T921/T922/T932/T935, and in lots of 7k plug-ins, including
the 7B53A, which is easy to find. I might even have the part you are looking
for in a junker 7B53A (let me have a look at the schematics: I can see two
pairs of transistors with bolted-on heat couplers on the trigger board).

-- Jeff Dutky



Harvey White
 

eyup, same idea.

Harvey

On 3/6/2022 9:42 PM, n4buq wrote:
The ones in my 7B80 have a wrap-around heatsink but not the S shaped kind. The JFETs are face-to-face with the heatsink wrapped around the round parts of both.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Harvey White" <madyn@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Sunday, March 6, 2022 8:18:35 PM
Subject: Re: [TekScopes] Tek 151-1042-00 : Matched JFET in a 7B80
I've seen some transistors in Tek stuff that are individual transistors,
but linked by a wrap around heatsink.  No idea if they were matched, but
the heatsink (it's S shaped) would keep them thermally coupled.

Harvey


On 3/6/2022 8:24 PM, n4buq wrote:
I have a 7B80 which doesn't work in external triggering mode and I discovered
one of a matched pair of JFETs in it (Q22A/B) decided it liked being a diode
instead of a transistor. As the subject indicates, it's a Tek P/N 151-1042-00
which, apparently, are hard to find.

From what I've seen, a 2N5454 may be a possible substitute but would like to see
what else could be substituted - particularly in the TO-92 form since that fits
the clamp they're in. If I can find a suitable replacement, I think I'll need
to do some matching and, thus, might need a handful of them (or more) to sift
through for a close pair.
I think a 2N5454 is a dual JFET package and, if so, the pair may be closely
matched; however, I'm not sure about that.

Anyone know what I could use and, better yet, where I could find it?

Thanks,
Barry - N4BUQ








 

Barry,

Yes, I just found the same thing (Q310/311 and Q410/411). Those are NOT the transistors with the thermal coupling, however. The odd part is, at least in the service manuals I got off the net, the parts list claims that Q311 and Q411 are "part of Q310" or Q410, which it's quite clear, from both the board images, as well as the actual PCB in front of me, that Q310/311 and Q410/411 are a pair of discrete transistors, not two transistors in a single package.

I've only examined my parts units (which are in pieces next to my desk), and not checked any of the other (working) 7B53As that I have on the shelf, but the fact that the same service manual that claims Q311 is part of Q310 clearly shows two discrete TO-92 packages in the board images makes me think that the parts list is a misprint.

-- Jeff Dutky


n4buq
 

Jeff,

While I suppose it isn't a definite thing, usually when a single package contains two or more transistors, the package is drawn with a solid arc and a dashed arc to indicate they're not discrete parts. Those transistors in the 7B80 and in 7B53A are drawn with full, solid circles which, to me, indicates they are discrete. Apparently a 151-1042-00 is not a dual package(?).

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Jeff Dutky" <jeff.dutky@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Sunday, March 6, 2022 10:24:28 PM
Subject: Re: [TekScopes] Tek 151-1042-00 : Matched JFET in a 7B80
Barry,

Yes, I just found the same thing (Q310/311 and Q410/411). Those are NOT the
transistors with the thermal coupling, however. The odd part is, at least in
the service manuals I got off the net, the parts list claims that Q311 and Q411
are "part of Q310" or Q410, which it's quite clear, from both the board images,
as well as the actual PCB in front of me, that Q310/311 and Q410/411 are a pair
of discrete transistors, not two transistors in a single package.

I've only examined my parts units (which are in pieces next to my desk), and not
checked any of the other (working) 7B53As that I have on the shelf, but the
fact that the same service manual that claims Q311 is part of Q310 clearly
shows two discrete TO-92 packages in the board images makes me think that the
parts list is a misprint.

-- Jeff Dutky



jgunn2@...
 

Barry,

I have a parts 465 and should have 2 pairs of 151-1042-00 on hand, one of which I can send if needed. Where are you located?

Dr. G


n4buq
 

Hi Dr. G.,

I'd like to confirm that I can substitute a pair from one of my 7B53As and get the EXT triggering to work in the 7B80 before I declare the JFETs the entire problem. If so, then I'd be interested in getting a pair. Jeff also said he might have a pair as well so I don't want both of you to send them.

I'm in Alabama, USA.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: jgunn2@...
To: "tekscopes" <TekScopes@groups.io>
Sent: Monday, March 7, 2022 7:18:58 AM
Subject: Re: [TekScopes] Tek 151-1042-00 : Matched JFET in a 7B80
Barry,

I have a parts 465 and should have 2 pairs of 151-1042-00 on hand, one of which
I can send if needed. Where are you located?

Dr. G



n4buq
 

Apparently, Q310/Q311 and Q411/Q412 in the 7B53A are not JFETs - at least according to my component tester.

Assuming they were the same part numbers, I tried them in the 7B80 before testing them which was a huge mistake. I then pulled the other pair from the 7B53A, checked them, and they do not show as JFETs but as a junction transistor with a diode across C and E. They appear to have a P/N of 151-0367-00 which I think cross-references to a 2N4245 and that agrees with the parts list for the 7B53A. That still leaves me with nothing for the correct JFETs in the 7B80.

I just confused as to why the transistors are supposed to be JFETs in the 7B53A don't show up as a JFET on the component checker.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Barry" <n4buq@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Monday, March 7, 2022 7:24:21 AM
Subject: Re: [TekScopes] Tek 151-1042-00 : Matched JFET in a 7B80
Hi Dr. G.,

I'd like to confirm that I can substitute a pair from one of my 7B53As and get
the EXT triggering to work in the 7B80 before I declare the JFETs the entire
problem. If so, then I'd be interested in getting a pair. Jeff also said he
might have a pair as well so I don't want both of you to send them.

I'm in Alabama, USA.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: jgunn2@...
To: "tekscopes" <TekScopes@groups.io>
Sent: Monday, March 7, 2022 7:18:58 AM
Subject: Re: [TekScopes] Tek 151-1042-00 : Matched JFET in a 7B80
Barry,

I have a parts 465 and should have 2 pairs of 151-1042-00 on hand, one of which
I can send if needed. Where are you located?

Dr. G



Mark Vincent
 

Barry,

The 151-0367-00 usually have the diode leakage from C to E. Replace them with KSP10BU (BEC pinout). I have replaced these in things that use these 0367 transistors. Check all of the 0367 transistors for the leakage.

Mark


n4buq
 

Hi Mark,

Are you saying that the diode leakage is abnormal? I presume so.

Also, these are marked as 151367 which presume are 151-0367. Is that correct?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Monday, March 7, 2022 9:33:40 AM
Subject: Re: [TekScopes] Tek 151-1042-00 : Matched JFET in a 7B80
Barry,

The 151-0367-00 usually have the diode leakage from C to E. Replace them with
KSP10BU (BEC pinout). I have replaced these in things that use these 0367
transistors. Check all of the 0367 transistors for the leakage.

Mark



 

Barry,

I just tested all four 1042s (Q310/311/410/411) from my junker 7B53A on my Chinese component tester, and they all test as NJFETs; not BJTs with a diode from C to E (that sounds like a damaged part to me).

Was the 7B53A that you took the parts from a known working instrument, or a non-working junker?

Are you sure that you were testing the correct parts?

All of the parts that I tested are clearly marked (on the plastic case) as "1042" and are located in two clusters of three transistors, on the lower half of the trigger board at the front and back of the board. The other transistor in the cluster of three is a 151-0221 according to the markings on the part (I haven't double checked the service manual).

-- Jeff Dutky


 

Barry,

the 367s are the pairs of transistors on the top of the trigger board with a thermal link connecting them. Those are definitely the wrong parts. The 1042s are in a cluster of three, free-standing transistors below the linked pair. The 1042s are the outermost two transistors in the cluster.

-- Jeff Dutky


n4buq
 

Hi Jeff,

Sadly, I now realize that. I just looked and the 1042s are, indeed, clearly marked as such and are definitely NOT the ones in the thermal links. I pulled two of the 1042s and they do check as FETs. When I saw those links, I jumped to the wrong conclusion and I can't believe I let myself do that.

I have not checked it in a while but I believe 7B53A with the leaky 367s did work but I'll have to check it again. Of course, the delayed triggering will not work until I get those two 367s replaced in the rear part of the board.

Feeling very dumb and embarrassed this morning...

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Jeff Dutky" <jeff.dutky@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Monday, March 7, 2022 10:36:41 AM
Subject: Re: [TekScopes] Tek 151-1042-00 : Matched JFET in a 7B80
Barry,

the 367s are the pairs of transistors on the top of the trigger board with a
thermal link connecting them. Those are definitely the wrong parts. The 1042s
are in a cluster of three, free-standing transistors below the linked pair. The
1042s are the outermost two transistors in the cluster.

-- Jeff Dutky



Mark Vincent
 

Barry,

The 151-367 is the same as 151-0367-00. The leakage is not supposed to be there. Any that have the leakage need to be replaced. Jeff D. is right about the 0367s being in a thermal link. Any other items that have the 0367 in them, test them for the leakage and replace as needed.

Mark