OT: Electronic Design!

[Cursitor Doom]

In a surprise change from the usual topics discussed in this group, I'm
posting a question about electronic design!

Win kindly reposted this enhanced schematic of an amplifier board I've
been troubleshooting:-

https://www.dropbox.com/s/o4tybr81cefbk5n/8565A_amp.GIF?dl=1

My question relates to the voltage-to-current section which comprises 6
transistors plus various passive 'support' components. My previous
experience is confined to discrete amplifier stages where each transistor
is dc-isolated from its neighbours by coupling capacitors and
consequently have easily determined biasing arrangements for each stage.
This is very different as all 6 transistors are directly inter-related
and inter-dependent (for example the collector output voltage(s) of one
has to be compatible with the base input of the next and so on. Not only
this, but their emitters all share series connections with their
compliments. This kind of arrangement is a PITA to troubleshoot, as a
problem with just one active device (or its 'supporting components')
causes weird voltage readings among the other 5 as well.
Anyway, it occurred to me it must have been an even bigger PITA to
actually *design* such a beast in the first place, given all the inter-
dependence of the bias and signal voltages which all have to be
accommodated.
What thought processes would the designer of this board have had to go
through in order to come up with what is effectively a monolithic 6-
transistor "stage" with all the multiple complications that go with it?
Where does one even begin??



--
This message may be freely reproduced without limit or charge only via
the Usenet protocol. Reproduction in whole or part through other
protocols, whether for profit or not, is conditional upon a charge of
GBP10.00 per reproduction. Publication in this manner via non-Usenet
protocols constitutes acceptance of this condition.

 

On Sunday, 20 October 2019 17:57:55 UTC+1, Cursitor Doom wrote:

In a surprise change from the usual topics discussed in this group, I'm
posting a question about electronic design!

Win kindly reposted this enhanced schematic of an amplifier board I've
been troubleshooting:-

https://www.dropbox.com/s/o4tybr81cefbk5n/8565A_amp.GIF?dl=1

My question relates to the voltage-to-current section which comprises 6
transistors plus various passive 'support' components. My previous
experience is confined to discrete amplifier stages where each transistor
is dc-isolated from its neighbours by coupling capacitors and
consequently have easily determined biasing arrangements for each stage.
This is very different as all 6 transistors are directly inter-related
and inter-dependent (for example the collector output voltage(s) of one
has to be compatible with the base input of the next and so on. Not only
this, but their emitters all share series connections with their
compliments. This kind of arrangement is a PITA to troubleshoot, as a
problem with just one active device (or its 'supporting components')
causes weird voltage readings among the other 5 as well.
Anyway, it occurred to me it must have been an even bigger PITA to
actually *design* such a beast in the first place, given all the inter-
dependence of the bias and signal voltages which all have to be
accommodated.
What thought processes would the designer of this board have had to go
through in order to come up with what is effectively a monolithic 6-
transistor "stage" with all the multiple complications that go with it?
Where does one even begin??

Should you be posting on topic stuff?

 

[Cursitor Doom]

On Sun, 20 Oct 2019 10:59:46 -0700, tabbypurr wrote:

> Should you be posting on topic stuff?

*Long* term contributors here (20 years and more) will know I do
*sometimes* post on-topic stuff. I'm just a hobbyist. I'll never be able
to design stuff like this, but am fascinated by the approach real
designers take with certain challenges that I couldn't even begin to get
to grips with.



--
This message may be freely reproduced without limit or charge only via
the Usenet protocol. Reproduction in whole or part through other
protocols, whether for profit or not, is conditional upon a charge of
GBP10.00 per reproduction. Publication in this manner via non-Usenet
protocols constitutes acceptance of this condition.

 

[bitrex]

On 10/20/19 5:30 PM, bitrex wrote:

On 10/20/19 12:57 PM, Cursitor Doom wrote:
In a surprise change from the usual topics discussed in this group, I'm
posting a question about electronic design!

Win kindly reposted this enhanced schematic of an amplifier board I've
been troubleshooting:-

https://www.dropbox.com/s/o4tybr81cefbk5n/8565A_amp.GIF?dl=1

My question relates to the voltage-to-current section which comprises 6
transistors plus various passive 'support' components. My previous
experience is confined to discrete amplifier stages where each transistor
is dc-isolated from its neighbours by coupling capacitors and
consequently have easily determined biasing arrangements for each stage.
This is very different as all 6 transistors are directly inter-related
and inter-dependent (for example the collector output voltage(s) of one
has to be compatible with the base input of the next and so on. Not only
this, but their emitters all share series connections with their
compliments. This kind of arrangement is a PITA to troubleshoot, as a
problem with just one active device (or its 'supporting components')
causes weird voltage readings among the other 5 as well.
Anyway, it occurred to me it must have been an even bigger PITA to
actually *design* such a beast in the first place, given all the inter-
dependence of the bias and signal voltages which all have to be
accommodated.
What thought processes would the designer of this board have had to go
through in order to come up with what is effectively a monolithic 6-
transistor "stage" with all the multiple complications that go with it?
Where does one even begin??




So it's a V - I converter/transconductance amplifier. The design choice
to use a cascade of three is likely dictated by knowing that for
whatever purpose, you need an overall transconductance (voltage in -
current out) gain of a given amount at a certain max operating frequency.

You can increase the transistor bias current to increase the current
gain but all active devices have a gain-bandwith product / "fT" so if
you increase it too much to get the gain you need you may no longer have
the bandwidth you need.

So I suppose they ran the math on how many stages they'd need to get the
current gain but keep a wide enough bandwidth to not roll off the
bandwidth of interest too bad, using the active devices that were
available to use or at the price they wanted to pay for 'em. And came up
with three stages.

The decision to multi-stage the small-signal amp and then all the other
support components required to bias up a multi-stage differential
amplifier into the operating area it works best falls out of knowing how
much current gain you need at what max frequency, that you need an amp
that rejects common-mode, and what the performance constraints of the
active devices in your budget are (or could be attained at reasonable
cost.)

It turned out two stages is too few for performance requirements, and
four is too much money. Yeah the passives were all selected such that
the DC operating point of everything all falls into place at quiescent
very nicely. DC biasing BJT networks is pretty easy if they're all
assumed to be in the active region, the base is just assumed to be 0.6
volts above or below the emitter depending on NPN/PNP you can just about
do it by inspection.

They were also selected so the AC performance is about right too but an
adjustment trimmer was also provided to tweak that up in the AC feedback
path of the final stage, R26 and R27 don't do anything at DC.

The arrangement where Q9 and Q14 derive their base bias voltages off
their own collector voltages thru R16 and R18 is elegant and cost-saving
and can provide thermal stability, but unfortunately also does ensure
that if just one tranny or resistor is out-of-whack in the circuit all
measurements everywhere in the network will be too and the whole thing
is shitted up.

 

[bitrex]

On 10/20/19 12:57 PM, Cursitor Doom wrote:

In a surprise change from the usual topics discussed in this group, I'm
posting a question about electronic design!

Win kindly reposted this enhanced schematic of an amplifier board I've
been troubleshooting:-

https://www.dropbox.com/s/o4tybr81cefbk5n/8565A_amp.GIF?dl=1

My question relates to the voltage-to-current section which comprises 6
transistors plus various passive 'support' components. My previous
experience is confined to discrete amplifier stages where each transistor
is dc-isolated from its neighbours by coupling capacitors and
consequently have easily determined biasing arrangements for each stage.
This is very different as all 6 transistors are directly inter-related
and inter-dependent (for example the collector output voltage(s) of one
has to be compatible with the base input of the next and so on. Not only
this, but their emitters all share series connections with their
compliments. This kind of arrangement is a PITA to troubleshoot, as a
problem with just one active device (or its 'supporting components')
causes weird voltage readings among the other 5 as well.
Anyway, it occurred to me it must have been an even bigger PITA to
actually *design* such a beast in the first place, given all the inter-
dependence of the bias and signal voltages which all have to be
accommodated.
What thought processes would the designer of this board have had to go
through in order to come up with what is effectively a monolithic 6-
transistor "stage" with all the multiple complications that go with it?
Where does one even begin??

So it's a V - I converter/transconductance amplifier. The design choice
to use a cascade of three is likely dictated by knowing that for
whatever purpose, you need an overall transconductance (voltage in ->
current out) gain of a given amount at a certain max operating frequency.

You can increase the transistor bias current to increase the current
gain but all active devices have a gain-bandwith product / "fT" so if
you increase it too much to get the gain you need you may no longer have
the bandwidth you need.

So I suppose they ran the math on how many stages they'd need to get the
current gain but keep a wide enough bandwidth to not roll off the
bandwidth of interest too bad, using the active devices that were
available to use or at the price they wanted to pay for 'em. And came up
with three stages.

The decision to multi-stage the small-signal amp and then all the other
support components required to bias up a multi-stage differential
amplifier into the operating area it works best falls out of knowing how
much current gain you need at what max frequency, that you need an amp
that rejects common-mode, and what the performance constraints of the
active devices in your budget are (or could be attained at reasonable
cost.)

It turned out two stages is too few for performance requirements, and
four is too much money. Yeah the passives were all selected such that
the DC operating point of everything all falls into place at quiescent
very nicely. DC biasing BJT networks is pretty easy if they're all
assumed to be in the active region, the base is just assumed to be 0.6
volts above or below the emitter depending on NPN/PNP you can just about
do it by inspection.

They were also selected so the AC performance is about right too but an
adjustment trimmer was also provided to tweak that up in the AC feedback
path of the final stage, R26 and R27 don't do anything at DC.

 

[bitrex]

On 10/20/19 6:56 PM, Robert Baer wrote:

bitrex wrote:
On 10/20/19 12:57 PM, Cursitor Doom wrote:
In a surprise change from the usual topics discussed in this group, I'm
posting a question about electronic design!

Win kindly reposted this enhanced schematic of an amplifier board I've
been troubleshooting:-

https://www.dropbox.com/s/o4tybr81cefbk5n/8565A_amp.GIF?dl=1

My question relates to the voltage-to-current section which comprises 6
transistors plus various passive 'support' components. My previous
experience is confined to discrete amplifier stages where each
transistor
is dc-isolated from its neighbours by coupling capacitors and
consequently have easily determined biasing arrangements for each stage.
This is very different as all 6 transistors are directly inter-related
and inter-dependent (for example the collector output voltage(s) of one
has to be compatible with the base input of the next and so on. Not only
this, but their emitters all share series connections with their
compliments. This kind of arrangement is a PITA to troubleshoot, as a
problem with just one active device (or its 'supporting components')
causes weird voltage readings among the other 5 as well.
Anyway, it occurred to me it must have been an even bigger PITA to
actually *design* such a beast in the first place, given all the inter-
dependence of the bias and signal voltages which all have to be
accommodated.
What thought processes would the designer of this board have had to go
through in order to come up with what is effectively a monolithic 6-
transistor "stage" with all the multiple complications that go with it?
Where does one even begin??




So it's a V - I converter/transconductance amplifier. The design
choice to use a cascade of three is likely dictated by knowing that
for whatever purpose, you need an overall transconductance (voltage in
-> current out) gain of a given amount at a certain max operating
frequency.

You can increase the transistor bias current to increase the current
gain but all active devices have a gain-bandwith product / "fT" so if
you increase it too much to get the gain you need you may no longer
have the bandwidth you need.

So I suppose they ran the math on how many stages they'd need to get
the current gain but keep a wide enough bandwidth to not roll off the
bandwidth of interest too bad, using the active devices that were
available to use or at the price they wanted to pay for 'em. And came
up with three stages.

The decision to multi-stage the small-signal amp and then all the
other support components required to bias up a multi-stage
differential amplifier into the operating area it works best falls out
of knowing how much current gain you need at what max frequency, that
you need an amp that rejects common-mode, and what the performance
constraints of the active devices in your budget are (or could be
attained at reasonable cost.)

It turned out two stages is too few for performance requirements, and
four is too much money. Yeah the passives were all selected such that
the DC operating point of everything all falls into place at quiescent
very nicely. DC biasing BJT networks is pretty easy if they're all
assumed to be in the active region, the base is just assumed to be 0.6
volts above or below the emitter depending on NPN/PNP you can just
about do it by inspection.

They were also selected so the AC performance is about right too but
an adjustment trimmer was also provided to tweak that up in the AC
feedback path of the final stage, R26 and R27 don't do anything at DC.
  It has been over 40 years since i was "into" such things, but as long
as your transistor emitter current did not materially increase Rbb',
increasing the operating current would increase Ft.

For a given gain, sure. But for any active device of a given type your
max potential gain/bandwidth ratio is a constant determined by the
physical characteristics of the device. It applies for op amps and it
applies for discrete transistors, too.

 

[Robert Baer]

bitrex wrote:

On 10/20/19 12:57 PM, Cursitor Doom wrote:
In a surprise change from the usual topics discussed in this group, I'm
posting a question about electronic design!

Win kindly reposted this enhanced schematic of an amplifier board I've
been troubleshooting:-

https://www.dropbox.com/s/o4tybr81cefbk5n/8565A_amp.GIF?dl=1

My question relates to the voltage-to-current section which comprises 6
transistors plus various passive 'support' components. My previous
experience is confined to discrete amplifier stages where each transistor
is dc-isolated from its neighbours by coupling capacitors and
consequently have easily determined biasing arrangements for each stage.
This is very different as all 6 transistors are directly inter-related
and inter-dependent (for example the collector output voltage(s) of one
has to be compatible with the base input of the next and so on. Not only
this, but their emitters all share series connections with their
compliments. This kind of arrangement is a PITA to troubleshoot, as a
problem with just one active device (or its 'supporting components')
causes weird voltage readings among the other 5 as well.
Anyway, it occurred to me it must have been an even bigger PITA to
actually *design* such a beast in the first place, given all the inter-
dependence of the bias and signal voltages which all have to be
accommodated.
What thought processes would the designer of this board have had to go
through in order to come up with what is effectively a monolithic 6-
transistor "stage" with all the multiple complications that go with it?
Where does one even begin??




So it's a V - I converter/transconductance amplifier. The design choice
to use a cascade of three is likely dictated by knowing that for
whatever purpose, you need an overall transconductance (voltage in -
current out) gain of a given amount at a certain max operating frequency.

You can increase the transistor bias current to increase the current
gain but all active devices have a gain-bandwith product / "fT" so if
you increase it too much to get the gain you need you may no longer have
the bandwidth you need.

So I suppose they ran the math on how many stages they'd need to get the
current gain but keep a wide enough bandwidth to not roll off the
bandwidth of interest too bad, using the active devices that were
available to use or at the price they wanted to pay for 'em. And came up
with three stages.

The decision to multi-stage the small-signal amp and then all the other
support components required to bias up a multi-stage differential
amplifier into the operating area it works best falls out of knowing how
much current gain you need at what max frequency, that you need an amp
that rejects common-mode, and what the performance constraints of the
active devices in your budget are (or could be attained at reasonable
cost.)

It turned out two stages is too few for performance requirements, and
four is too much money. Yeah the passives were all selected such that
the DC operating point of everything all falls into place at quiescent
very nicely. DC biasing BJT networks is pretty easy if they're all
assumed to be in the active region, the base is just assumed to be 0.6
volts above or below the emitter depending on NPN/PNP you can just about
do it by inspection.

They were also selected so the AC performance is about right too but an
adjustment trimmer was also provided to tweak that up in the AC feedback
path of the final stage, R26 and R27 don't do anything at DC.

It has been over 40 years since i was "into" such things, but as long
as your transistor emitter current did not materially increase Rbb',
increasing the operating current would increase Ft.

 

[bitrex]

On 10/20/19 5:49 PM, bitrex wrote:

On 10/20/19 1:59 PM, tabbypurr@gmail.com wrote:
On Sunday, 20 October 2019 17:57:55 UTC+1, Cursitor Doom  wrote:

In a surprise change from the usual topics discussed in this group, I'm
posting a question about electronic design!

Win kindly reposted this enhanced schematic of an amplifier board I've
been troubleshooting:-

https://www.dropbox.com/s/o4tybr81cefbk5n/8565A_amp.GIF?dl=1

My question relates to the voltage-to-current section which comprises 6
transistors plus various passive 'support' components. My previous
experience is confined to discrete amplifier stages where each
transistor
is dc-isolated from its neighbours by coupling capacitors and
consequently have easily determined biasing arrangements for each stage.
This is very different as all 6 transistors are directly inter-related
and inter-dependent (for example the collector output voltage(s) of one
has to be compatible with the base input of the next and so on. Not only
this, but their emitters all share series connections with their
compliments. This kind of arrangement is a PITA to troubleshoot, as a
problem with just one active device (or its 'supporting components')
causes weird voltage readings among the other 5 as well.
Anyway, it occurred to me it must have been an even bigger PITA to
actually *design* such a beast in the first place, given all the inter-
dependence of the bias and signal voltages which all have to be
accommodated.
What thought processes would the designer of this board have had to go
through in order to come up with what is effectively a monolithic 6-
transistor "stage" with all the multiple complications that go with it?
Where does one even begin??

Should you be posting on topic stuff?


If something were for-sure shitted up in that circuit (and we'd know
this by testing the stages previous and after for performance) I'd
probably replace all the resistors with modern 1% types and then replace
the transistors with 6 modern equivalents hand-matched for similar
Vbe/current performance, it would prolly be faster than trying to figure
out exactly what the fault is.

if you replace one tranny there you should probably replace 'em all.

 

[bitrex]

On 10/20/19 1:59 PM, tabbypurr@gmail.com wrote:

On Sunday, 20 October 2019 17:57:55 UTC+1, Cursitor Doom wrote:

In a surprise change from the usual topics discussed in this group, I'm
posting a question about electronic design!

Win kindly reposted this enhanced schematic of an amplifier board I've
been troubleshooting:-

https://www.dropbox.com/s/o4tybr81cefbk5n/8565A_amp.GIF?dl=1

My question relates to the voltage-to-current section which comprises 6
transistors plus various passive 'support' components. My previous
experience is confined to discrete amplifier stages where each transistor
is dc-isolated from its neighbours by coupling capacitors and
consequently have easily determined biasing arrangements for each stage.
This is very different as all 6 transistors are directly inter-related
and inter-dependent (for example the collector output voltage(s) of one
has to be compatible with the base input of the next and so on. Not only
this, but their emitters all share series connections with their
compliments. This kind of arrangement is a PITA to troubleshoot, as a
problem with just one active device (or its 'supporting components')
causes weird voltage readings among the other 5 as well.
Anyway, it occurred to me it must have been an even bigger PITA to
actually *design* such a beast in the first place, given all the inter-
dependence of the bias and signal voltages which all have to be
accommodated.
What thought processes would the designer of this board have had to go
through in order to come up with what is effectively a monolithic 6-
transistor "stage" with all the multiple complications that go with it?
Where does one even begin??

Should you be posting on topic stuff?

If something were for-sure shitted up in that circuit (and we'd know
this by testing the stages previous and after for performance) I'd
probably replace all the resistors with modern 1% types and then replace
the transistors with 6 modern equivalents hand-matched for similar
Vbe/current performance, it would prolly be faster than trying to figure
out exactly what the fault is.

 

[Cursitor Doom]

On Sun, 20 Oct 2019 14:56:37 -0800, Robert Baer wrote:

It has been over 40 years since i was "into" such things, but as long
as your transistor emitter current did not materially increase Rbb',
increasing the operating current would increase Ft.

I was quite surprised when I scoped the signal path of this section as
after each transistor, going from source to load, the signal level
halves. I'd have expected this if they were using transformers to bump up
the current, but not with active devices. If I can't find the cause of
the fault I might just bypass these 6 transistors altogether and replace
them with a pair of op amps. Did they not have op amps in 1977? ;-)



--
This message may be freely reproduced without limit or charge only via
the Usenet protocol. Reproduction in whole or part through other
protocols, whether for profit or not, is conditional upon a charge of
GBP10.00 per reproduction. Publication in this manner via non-Usenet
protocols constitutes acceptance of this condition.

 

[bitrex]

On 10/20/19 6:52 PM, Cursitor Doom wrote:

On Sun, 20 Oct 2019 14:56:37 -0800, Robert Baer wrote:

It has been over 40 years since i was "into" such things, but as long
as your transistor emitter current did not materially increase Rbb',
increasing the operating current would increase Ft.

I was quite surprised when I scoped the signal path of this section as
after each transistor, going from source to load, the signal level
halves. I'd have expected this if they were using transformers to bump up
the current, but not with active devices. If I can't find the cause of
the fault I might just bypass these 6 transistors altogether and replace
them with a pair of op amps. Did they not have op amps in 1977? ;-)

Yes but they were bad and your idea is bad.

 

[Winfield Hill]

bitrex wrote...

if you replace one tranny there you should probably replace 'em all.

But can probably keep the four HV parts.


--
Thanks,
- Win

 

[Phil Allison]

Cursitor Doom wrote:

--------------------

-
What thought processes would the designer of this board have had to go
through in order to come up with what is effectively a monolithic 6-
transistor "stage" with all the multiple complications that go with it?
Where does one even begin??

** You have posted a pic of a weird looking dog and ask us to unravel its pedigree. 100% absurd.

Like with the dog, one has to know it's ancestry first in order to explain why it looks the way it does.

Do you even know where that schem comes from at all?

What commercial product ?

So many scopes have been designed over the decades and they all have some similarities - designers are terrible plagiarisers.

Products from the same maker and period likely have whole sections copied from earlier ones or copied from similar products from other makers.

Sometimes with improvements, but more likely with fewer and cheaper parts to save pennies.

Really, only the actual designer can explain how he arrived at a particular circuit - and none of them are talking.



..... Phil

 

[bitrex]

On 10/20/19 11:05 PM, Phil Allison wrote:

Cursitor Doom wrote:

--------------------
-
What thought processes would the designer of this board have had to go
through in order to come up with what is effectively a monolithic 6-
transistor "stage" with all the multiple complications that go with it?
Where does one even begin??



** You have posted a pic of a weird looking dog and ask us to unravel its pedigree. 100% absurd.

Like with the dog, one has to know it's ancestry first in order to explain why it looks the way it does.

Do you even know where that schem comes from at all?

What commercial product ?

So many scopes have been designed over the decades and they all have some similarities - designers are terrible plagiarisers.

Products from the same maker and period likely have whole sections copied from earlier ones or copied from similar products from other makers.

Sometimes with improvements, but more likely with fewer and cheaper parts to save pennies.

Really, only the actual designer can explain how he arrived at a particular circuit - and none of them are talking.



.... Phil

HP 8565A, is in the dropbox URL he posted my dude

<https://www.youtube.com/watch?v=NFW8Qwmbpo0>

 

[bitrex]

On 10/20/19 11:53 PM, bitrex wrote:

On 10/20/19 10:45 PM, Winfield Hill wrote:
bitrex wrote...

if you replace one tranny there you should probably replace 'em all.

  But can probably keep the four HV parts.



I think (when it works) that V -> I -> V topology helps simplify making
the wide-band differential HV signal to drive the CRT quite a bit. You
get a 150 volt P2P swing out of a four-transistor TIA. gain is
approximately Iin * C10||R29. Bypassing bias resistor R35(?) makes sure
both output transistors can get enough AC drive current. Idk if there's
a name for that topology it's a little like a White cathode follower,
but not.

Particularly since all the loop gain is in the output stage and output
is taken from the collectors rather than emitters.

 

[bitrex]

On 10/20/19 10:45 PM, Winfield Hill wrote:

bitrex wrote...

if you replace one tranny there you should probably replace 'em all.

But can probably keep the four HV parts.

I think (when it works) that V -> I -> V topology helps simplify making
the wide-band differential HV signal to drive the CRT quite a bit. You
get a 150 volt P2P swing out of a four-transistor TIA. gain is
approximately Iin * C10||R29. Bypassing bias resistor R35(?) makes sure
both output transistors can get enough AC drive current. Idk if there's
a name for that topology it's a little like a White cathode follower,
but not.

 

On Sunday, 20 October 2019 19:46:37 UTC+1, Cursitor Doom wrote:

On Sun, 20 Oct 2019 10:59:46 -0700, tabbypurr wrote:

Should you be posting on topic stuff?

*Long* term contributors here (20 years and more) will know I do
*sometimes* post on-topic stuff. I'm just a hobbyist. I'll never be able
to design stuff like this, but am fascinated by the approach real
designers take with certain challenges that I couldn't even begin to get
to grips with.

But it's on topic, that's the problem.

 

[Winfield Hill]

Bill Sloman wrote...

But it's on topic, that's the problem.

Not exactly. The problem is the usual one - he hasn't said
where it came from. or what it was actually intended to do,
as Phil Allison has pointed out

Aren't we talking about the horizontal deflection amplifier
for the CRT in HP 8565A spectrum analyzers, c. 1977? See:

https://www.dropbox.com/s/o4tybr81cefbk5n/8565A_amp.GIF?dl=1


--
Thanks,
- Win

 

[Bill Sloman]

On Monday, October 21, 2019 at 8:10:30 PM UTC+11, tabb...@gmail.com wrote:

On Sunday, 20 October 2019 19:46:37 UTC+1, Cursitor Doom wrote:
On Sun, 20 Oct 2019 10:59:46 -0700, tabbypurr wrote:

Should you be posting on topic stuff?

*Long* term contributors here (20 years and more) will know I do
*sometimes* post on-topic stuff. I'm just a hobbyist. I'll never be able
to design stuff like this, but am fascinated by the approach real
designers take with certain challenges that I couldn't even begin to get
to grips with.

But it's on topic, that's the problem.

Not exactly. The problem is the usual one - he hasn't said where it came from. or what it was actually intended to do, as Phil Allison has pointed out

This does get mentioned from time to time when people post ostensibly on-topic items, but with way too little context.

It's not unusual for a crucial detail to only emerge after a dozen or so posts have beaten around quite a different bush from the one the OP wanted beaten.

--
Bill Sloman, Sydney

 

[Bill Sloman]

On Tuesday, October 22, 2019 at 12:10:15 AM UTC+11, Winfield Hill wrote:

Bill Sloman wrote...

But it's on topic, that's the problem.

Not exactly. The problem is the usual one - he hasn't said
where it came from. or what it was actually intended to do,
as Phil Allison has pointed out

Aren't we talking about the horizontal deflection amplifier
for the CRT in HP 8565A spectrum analyzers, c. 1977? See:

https://www.dropbox.com/s/o4tybr81cefbk5n/8565A_amp.GIF?dl=1

The string "8565" didn't trigger anything in my memory. Cursitor Doom just talked about an "amplifier board".

The circuit diagram does mention that it's driving a horizontal CRT deflection plate. Somebody who knew a bit about HP 8565 might be able to work out how fast it might be going, but that's more digging that I'd be bothered doing for Cursitor Doom.

The fact that it's an HP part suggests that it probably worked when it was new, but it's no guarantee that it was well-designed. Some of their stuff that I did have to dig into wasn't impressive.

My favourite American botch job was a Princeton Applied Research boxcar integrator that we got stuck with when we had to copy a Siemens development prototype for Thompson CSF. The spec sheet said that it could sample at up to 5MHz, but when we checked it died at 2.3MHz. I could get it to go faster by sticking in a 74H part, but that only got it up to 2.7MHz.

When we contacted PAR, they agreed that it didn't meet it's spec.

Not its only defect - but a particularly blatant one.

--
Bill Sloman, Sydney