I as well can attest that the feet turned out great. I bought and put them in service on a 7834 and TM-503 both of which benefited greatly as I like this cord wrap design far and above the originals they came with. ? ?
toggle quoted message
Show quoted text
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of George Lydecker
Sent: Thursday, January 03, 2013 5:48 PM
To: TekScopes@...
Subject: [TekScopes] Re: Rear feet for 475? ? Gang,
Here are some pictures of the 3D printed feet Tim created;
I am very happy with the results.
Goerge
--- In TekScopes@..., Jim wrote:
>
> I would add -- Tim was kind enough to send me one of his prototypes so I could use it to make a mold, and compare my castings from his prototype vs. an original foot that Tom Jobe sent me.
>
> I owe both of them some preliminary samples as soon as I find the time to make molds and cast something.
>
> Fortunately, I'll have a whole lot more time in my shop during the next two weeks. ?A bunch of unplanned projects finally concluded in the last couple of days.
>
> 73
> Jim N6OTQ
>
>
>
> >________________________________
> > From: Glydeck
> >To: "TekScopes@..."
> >Sent: Thursday, January 3, 2013 5:13 PM
> >Subject: Re: [TekScopes] Rear feet for 475
> >
> >
> >
> >I bought some and they work great. I plan on posting some pictures, but have not got around to it. ?Tim's Email address is;
> >
> >
> >lemonscentedmoisttowelette@...
> >
> >George
> >
> >On Jan 3, 2013, at 2:14 PM, Paul Amaranth wrote:
> >
> >
> >?
> >>I saw him offering those on ebay along with the handle endcaps.
> >>
> >>they look very nice. Search for tektronix feet and it should
> >>come up.
> >>
> >>On Thu, Jan 03, 2013 at 07:56:09PM -0000, Gerald Krizek wrote:
> >>> Does anyone know if the fellow that was trying to 3D print rear feet for the Tek 475 has had any success? I seem to have lost his e-mail address.
> >>> Thanks,
> >>> Jerry
> >>>
> >>
> >>--
> >>Paul Amaranth, GCIH | Rochester MI, USA
> >>Aurora Group, Inc. | Security, Systems & Software
> >>paul@... | Unix & Windows
> >>
> >>
> >
> >
> >
> >
> >
>
|
The real question is why DON'T you see the 7A26 noise?
IF (and that's a big if!) the input impedance of an amplifier were purely
resistive then the thermal noise contributed by the resistor (Vn)is
Vn(RMS)= sqrt(4*KB*T*R*(F2-F1)) in Volts, where
T is the temperature of the resistor (in degrees Kelvin). (70F = 294K)
K is Boltsmann's constant (1.38 x 10^-23)
R is the resistance in ohms
F2-F1 is the bandwidth you are measuring the noise across. In the case of a
Tektronix 7A19 amplifier it would be DC to the full bandwidth of the plugin
(500,000,000Hz - 0Hz) = 500,000,000
So, if a 7A19 had a purely resistive 50 ohm input its thermal noise at room
temperature would be 20uV which is far too small to see on the scope even at
the most sensitive setting of the 7A19.
For a 7A26, with 20,000 times input resistance (if it was purely resistive),
and 2/5 the bandwidth, the thermal noise would be 1.8mV which would be so
great you couldn't help but notice it on the most sensitive scale because it
would occupy more than 1/3 division of the CRT. So why don't you see it?
The answer to my question is simple: the input isn't purely resistive. The
amplifier specs say there is resistance and capacitance at the input. With
1Mohm input impedance the effect of even a small capacitance becomes
significant as the frequency of the signal goes up. Another way of looking
at this is that with a lower 50ohm input impedance, a capacitor will have
little effect until you reach very high frequencies. For high frequency work
1MOhm amplifiers are not a good choice, 50 ohm inputs are better. The
capacitance at the input of the 7A26 kills a lot of the noise and keeps the
trace sharp. The 7A19 will have no visible noise.
Dennis
toggle quoted message
Show quoted text
-----Original Message-----
From: micpreamp, Sent: Thursday, January 03, 2013 12:58 PM
Hi all,
I have a 7904A with a 7A19 plugin (plus lots of slower ones, e.g. 7A24 and
7A26). I'd love to have two channels of > 500 MHz, and I'm contemplating if
I should chase another 7A19 or get two 7A29 (and sell the 7A19). The 7A29
will make better use of the mainframe bandwidth and has a variable vertical
deflection feature.
What I wonder: how do things look noise wise? I hate blurry traces, and I'm
rather surprised how sharp the 7A19 is (much better than the 7A26!). Anyone
who could compare them side-by-side?
Any other difference I should know about?
Thanks a lot,
Samuel
|
Re: 2246 Mod A - Is it OK to replace just one SMPS switching transistor?
--- In TekScopes@..., "Tom Jobe" wrote: Thank you to all that have contributed to this interesting discussion!
tom jobe... Hiya Tom ! Wish you a wonderful New Year ! I have now 7 Teks working flawlessly, plus a dozen of plug-ins for my 7904's. (I have some Philips too...). The only which will be out of rescue is a 2213 which has dropped from a desk and has a defective CRT. Sad...
|
Re: Is it safe to power on a 2246 power supply over the bench to check the voltages?
I have done some work on the 2213/2215 (with prereg board),
224X A and non-A, 2232 and now a 2235A. All these scopes use
the saturable transformer, if I am right, and, for example,
the 2213 use the same transformer than the 2245A, although
the design is very different.
The main difference is that on the 224X models, the regulation
loops controls the prereg output for a coarse ajustment of the
43V output via the IC switching command control AND the FET
conduction for an accurate regulation of the 7.5V reference.
On the 2235A, the -8.6 reference is regulated by the control
of the conduction of the switching transistors, which are not
working in their saturation aerea, as stated in the service
manual (theory of operation-inverter). This explains the much
higher value of the base resistor (47 Ohms), which has only
IC/?? flowing through.
My 2235A has some issues: the mains filter (schaffner) is defective,
it smells awfully. The SMPS ticks. I found a 10??F 100 filtering
cap with a huge ESR, which caused problems in the regulation loop,
and overcurrent though the 0.75 Ohm sense resistor, which furthermore has drifted out of spec to 0.85 Ohm. All that fired the thyristor.
Problem solved! Remains somme ripple needing partial recapping.
|
Gang,
Here are some pictures of the 3D printed feet Tim created;
I am very happy with the results.
Goerge
toggle quoted message
Show quoted text
--- In TekScopes@..., Jim wrote:
I would add -- Tim was kind enough to send me one of his prototypes so I could use it to make a mold, and compare my castings from his prototype vs. an original foot that Tom Jobe sent me.
I owe both of them some preliminary samples as soon as I find the time to make molds and cast something.
Fortunately, I'll have a whole lot more time in my shop during the next two weeks. ?A bunch of unplanned projects finally concluded in the last couple of days.
73
Jim N6OTQ
________________________________
From: Glydeck
To: "TekScopes@..."
Sent: Thursday, January 3, 2013 5:13 PM
Subject: Re: [TekScopes] Rear feet for 475
I bought some and they work great. I plan on posting some pictures, but have not got around to it. ?Tim's Email address is;
lemonscentedmoisttowelette@...
George
On Jan 3, 2013, at 2:14 PM, Paul Amaranth wrote:
?
I saw him offering those on ebay along with the handle endcaps.
they look very nice. Search for tektronix feet and it should
come up.
On Thu, Jan 03, 2013 at 07:56:09PM -0000, Gerald Krizek wrote:
Does anyone know if the fellow that was trying to 3D print rear feet for the Tek 475 has had any success? I seem to have lost his e-mail address.
Thanks,
Jerry
--
Paul Amaranth, GCIH | Rochester MI, USA
Aurora Group, Inc. | Security, Systems & Software
paul@... | Unix & Windows
|
Re: 2246 Mod A - Is it OK to replace just one SMPS switching transistor?
Hi Priya
I have followed every discussion of 22xx and 224x scopes on
Tekscopes for years, and I do not remember?much talk about base resistor
problems.
But I do think you?might be?on to something because
of the common 224x heat related failures around, and including, Q2209 and
Q2210.
With the help of the Tekscope membership I once fixed a 224x
power supply that had the PCB badly charred around Q2209 and Q2210. There was no
cause ever discovered for that damage, and I do believe that I used some
TIP41C's for the successful repair.
?
It was interesting that Francis reported changing the base
resistor values on his 224x. Francis is very knowledgeable and has?quite a
bit?of experience with these 224x power supplies including adding Baker
clamps which David has mentioned in?this discussion thread.
A?couple of?years ago Francis taught me how to make
a test setup to look at the?storage time Ts?of transistors?one
might use for Q2209 and Q2210 as part of an off group?Baker Clamp
discussion we were having. For example, the tests showed that?TIP41C's from
different manufacturers had widely different storage times and characteristics,
as did many other similar transistors that could potentially be used for Q2209
and Q2210. I think I tested 14 different transistor brand and part number
combinations for their Ts time, and that made me wonder how a circuit like this
in the 224x could possibly function with all of those different transistor
characteristics. All of that, was what Francis was trying to open my eyes
to.
?
Thank you to all that have contributed to this interesting
discussion!
tom jobe...
?
?
?
?
toggle quoted message
Show quoted text
----- Original Message -----
Sent: Thursday, January 03, 2013 1:08
PM
Subject: [TekScopes] Re: 2246 Mod A - Is
it OK to replace just one SMPS switching transistor?
?
The 2246 has an active protection circuit with a feedback loop that
monitors the Drain voltage of the FET and keeps the heat dissipation of the
FET under check. The FET also has a huge heat sink. There's no such active
protection for the switching transistors, so it is probably safer to allow the
FET to take more heat than the transistors. Since driving the transistors
harder with a smaller resistor will reduce transistor heat dissipation, your
logic of retaining smaller resistors fits right in other than of course if it
blows the resistors!
Have others seen overheated and blown
resistors?
Priya.
--- In TekScopes@...,
"Albert" wrote:
>
> Could it be that after your modification the
switching transistors get hotter? At first sight it is a trade-off between
saturation voltage of the switching transistors and voltage across the FET.
The sum of these voltages must remain more or less the same, in order to have
the same voltage swing at the primaries of the inverter power
transformer.
> I only have experience with 7000 SPMSs. Good to know that
these "modern" supplies are so different, in case my 2232 breaks
down...
> BTW the manual says those small resistors are 0.2 W types. In
for instance a 7704A supply Ic/Ib = 4 but the collector currents are smaller.
The resistors are 0.5 W.
>
> Albert
>
> --- In TekScopes@...,
"Francis" wrote:
> >
> > Hi,
> >
> >
don't forget that the transistors are operating in switching mode,
>
> and therefore should have a Ic/Ib ratio not greater than 10,
> >
which gives 200mA of base current for a 2A collector current.
> >
> > This can be easily verified by a simple voltage probing on the
> > leads of the resistors. The base current is even over the above
> > value. (be careful, this part of the SMPS is connected to the
> > mains)
> >
> > I have found 0.3A and over. A
simple calculation gives 5.1x0.3x0.3=459mW, which is way above the possible
dissipation
> > of these components. BTW, I not sure that their size
can allow
> > 1/4W dissipation. 1/8 looks more realistic....
>
>
> > I must say that the power is not maintained all the time,
only
> > when the transistor is ON, roughly 1/3 of the time. Still
too
> > much in my opinion. I decided to increase the value as
follow:
> > 2.7 -> 5.1 and 5.1 -> 10. I don't use TIP41 either,
only TIP31
> > The FET is much cooler, so are the resistors. The PCB
says "thanks",
> > and will be less cooked than with the original
values. The
> > waveforms are perfect.
>
|
Re: 2246 Mod A - Is it OK to replace just one SMPS switching transistor?
That's right. On the 2246, the pre-regulator 44 VDC needs the inverter circuit to be functioning correctly to get to a stable 44 VDC because of the Q2008 feedback from the inverter FET Q2214 Drain back into the pre-regulator.
Without the inverter switching working correctly, the internal voltage sits at about 40VDC with ripple. This wrongly suggests that there is a pre-regulator problem when there is really an inverter issue.
Cheers .... Priya.
toggle quoted message
Show quoted text
--- In TekScopes@..., David wrote:
On the 2230 which uses the same basic design, the preregulator output
is fixed. On the 2246, it looks like Q2208 adjusts the preregulator
output to keep the same voltage across the switching transistors and
power pass element Q2214.
On Thu, 03 Jan 2013 22:04:38 -0000, "Albert"
wrote:
You are right. At first sight I thought that the 44 V would be regulated independently always to that value, but it's not. This also destroys my argument that the sum of saturation voltage and FET voltage must be more or less constant.
Albert
--- In TekScopes@..., "Mover" wrote:
The 2246 has an active protection circuit with a feedback loop that monitors the Drain voltage of the FET and keeps the heat dissipation of the FET under check. The FET also has a huge heat sink. There's no such active protection for the switching transistors, so it is probably safer to allow the FET to take more heat than the transistors. Since driving the transistors harder with a smaller resistor will reduce transistor heat dissipation, your logic of retaining smaller resistors fits right in other than of course if it blows the resistors!
Have others seen overheated and blown resistors?
Priya.
--- In TekScopes@..., "Albert" wrote:
Could it be that after your modification the switching transistors get hotter? At first sight it is a trade-off between saturation voltage of the switching transistors and voltage across the FET. The sum of these voltages must remain more or less the same, in order to have the same voltage swing at the primaries of the inverter power transformer.
I only have experience with 7000 SPMSs. Good to know that these "modern" supplies are so different, in case my 2232 breaks down...
BTW the manual says those small resistors are 0.2 W types. In for instance a 7704A supply Ic/Ib = 4 but the collector currents are smaller. The resistors are 0.5 W.
Albert
|
I would add -- Tim was kind enough to send me one of his prototypes so I could use it to make a mold, and compare my castings from his prototype vs. an original foot that Tom Jobe sent me.
I owe both of them some preliminary samples as soon as I find the time to make molds and cast something.
Fortunately, I'll have a whole lot more time in my shop during the next two weeks. ?A bunch of unplanned projects finally concluded in the last couple of days.
73 Jim N6OTQ
From: Glydeck
To: "TekScopes@..."
Sent: Thursday, January 3, 2013 5:13 PM
Subject: Re: [TekScopes] Rear feet for 475
I bought some and they work great. I plan on posting some pictures, but have not got around to it. ?Tim's Email address is;
On Jan 3, 2013, at 2:14 PM, Paul Amaranth < paul@...> wrote:
?
I saw him offering those on ebay along with the handle endcaps.
they look very nice. Search for tektronix feet and it should
come up.
On Thu, Jan 03, 2013 at 07:56:09PM -0000, Gerald Krizek wrote:
> Does anyone know if the fellow that was trying to 3D print rear feet for the Tek 475 has had any success? I seem to have lost his e-mail address.
> Thanks,
> Jerry
>
--
Paul Amaranth, GCIH | Rochester MI, USA
Aurora Group, Inc. | Security, Systems & Software
paul@... | Unix & Windows
|
I bought some and they work great. I plan on posting some pictures, but have not got around to it. ?Tim's Email address is;
On Jan 3, 2013, at 2:14 PM, Paul Amaranth < paul@...> wrote:
?
I saw him offering those on ebay along with the handle endcaps.
they look very nice. Search for tektronix feet and it should
come up.
On Thu, Jan 03, 2013 at 07:56:09PM -0000, Gerald Krizek wrote:
> Does anyone know if the fellow that was trying to 3D print rear feet for the Tek 475 has had any success? I seem to have lost his e-mail address.
> Thanks,
> Jerry
>
--
Paul Amaranth, GCIH | Rochester MI, USA
Aurora Group, Inc. | Security, Systems & Software
paul@... | Unix & Windows
|
I'm interested also¡. 6 connectors???¡. ?to make 3 extenders¡. ?solid pcb inside the scope, then wire outside i think¡. ? shielded microstrip inside to smb panel mouts, then smb coax ?to socket¡ outside¡
jim
???
toggle quoted message
Show quoted text
On Jan 3, 2013, at 10:41 AM, John Griessen < john@...> wrote:
?
On 01/03/2013 12:17 PM, Dave Daniel wrote:
> I personally would be willing to buy several.
> What is the G&A for purchasing one lot, aside from transportation charges? Is it small, medium, large?
Since the testing is already done and it has a part number, just one day's effort spread over
a few days to start the transaction.
Is a lot of 25 simply too
> small to futz with?
no. If 25 sell, I'll buy another 25 for other like the rigid extender and Jerry's idea
of an easier to assemble 7k_flex development.
>
> Anybody else think similarly?
>
> Regarding the time investment conflict that you currently have, is there anything that anyone else (perhaps myself) can do do to
> help? I am not in a particular hurry, but the longest journey, etc.
It's mostly getting 25 units pledged, ( and a few more for no show possibilities when money is needed).
The same old kit with its soldering required can have the same price, $35 for 2 foot wires basic version.
It's possible a rigid one could go fast track before Chinese new year, but not a redesigned flex one.
John Griessen
|
Re: 2246 Mod A - Is it OK to replace just one SMPS switching transistor?
--- In TekScopes@..., David wrote: Isn't the 2246 inverter design just another variation of the saturable
core oscillator with T2205 designed to saturate before T2204? Yes that's right. Inductor L2206 limits dI/dt and helps to collapse the voltage across T2205. ---
Tektronix used that saturable core oscillator technique a lot in their
high voltage inverters and I thought I remembered seeing it in some of
the 7000 series. I have just a few different 7000 'scope types. You may well be right there. The surprise for me was that control of the dead time in the switching transistor pair with the famous IC was abandoned. Albert
|
Re: 2246 Mod A - Is it OK to replace just one SMPS switching transistor?
On the 2230 which uses the same basic design, the preregulator output is fixed. On the 2246, it looks like Q2208 adjusts the preregulator output to keep the same voltage across the switching transistors and power pass element Q2214. On Thu, 03 Jan 2013 22:04:38 -0000, "Albert" <aodiversen@...> wrote: You are right. At first sight I thought that the 44 V would be regulated independently always to that value, but it's not. This also destroys my argument that the sum of saturation voltage and FET voltage must be more or less constant.
Albert
--- In TekScopes@..., "Mover" wrote:
The 2246 has an active protection circuit with a feedback loop that monitors the Drain voltage of the FET and keeps the heat dissipation of the FET under check. The FET also has a huge heat sink. There's no such active protection for the switching transistors, so it is probably safer to allow the FET to take more heat than the transistors. Since driving the transistors harder with a smaller resistor will reduce transistor heat dissipation, your logic of retaining smaller resistors fits right in other than of course if it blows the resistors!
Have others seen overheated and blown resistors?
Priya.
--- In TekScopes@..., "Albert" wrote:
Could it be that after your modification the switching transistors get hotter? At first sight it is a trade-off between saturation voltage of the switching transistors and voltage across the FET. The sum of these voltages must remain more or less the same, in order to have the same voltage swing at the primaries of the inverter power transformer.
I only have experience with 7000 SPMSs. Good to know that these "modern" supplies are so different, in case my 2232 breaks down...
BTW the manual says those small resistors are 0.2 W types. In for instance a 7704A supply Ic/Ib = 4 but the collector currents are smaller. The resistors are 0.5 W.
Albert
|
I saw him offering those on ebay along with the handle endcaps.
they look very nice. Search for tektronix feet and it should
come up.
toggle quoted message
Show quoted text
On Thu, Jan 03, 2013 at 07:56:09PM -0000, Gerald Krizek wrote: Does anyone know if the fellow that was trying to 3D print rear feet for the Tek 475 has had any success? I seem to have lost his e-mail address.
Thanks,
Jerry
--
Paul Amaranth, GCIH | Rochester MI, USA
Aurora Group, Inc. | Security, Systems & Software
paul@... | Unix & Windows
|
Re: 2246 Mod A - Is it OK to replace just one SMPS switching transistor?
On Thu, 03 Jan 2013 20:18:17 -0000, "Albert" <aodiversen@...> wrote: Could it be that after your modification the switching transistors get hotter? At first sight it is a trade-off between saturation voltage of the switching transistors and voltage across the FET. The sum of these voltages must remain more or less the same, in order to have the same voltage swing at the primaries of the inverter power transformer.
I only have experience with 7000 SPMSs. Good to know that these "modern" supplies are so different, in case my 2232 breaks down... Isn't the 2246 inverter design just another variation of the saturable core oscillator with T2205 designed to saturate before T2204? I see the same configuration in the "good" 22xx series (2230 and 2232) but the "evil" 22xx units (2225) use a completely different fixed frequency push-pull stage based on the TL594. I suspect one of its advantages is that the soft switching makes for less noise but I suspect saturation causes more interference from magnetic flux leakage. Maybe the later is why they went to a two transformer design. Tektronix used that saturable core oscillator technique a lot in their high voltage inverters and I thought I remembered seeing it in some of the 7000 series.
|
Re: 2246 Mod A - Is it OK to replace just one SMPS switching transistor?
You are right. At first sight I thought that the 44 V would be regulated independently always to that value, but it's not. This also destroys my argument that the sum of saturation voltage and FET voltage must be more or less constant.
Albert
toggle quoted message
Show quoted text
--- In TekScopes@..., "Mover" wrote:
The 2246 has an active protection circuit with a feedback loop that monitors the Drain voltage of the FET and keeps the heat dissipation of the FET under check. The FET also has a huge heat sink. There's no such active protection for the switching transistors, so it is probably safer to allow the FET to take more heat than the transistors. Since driving the transistors harder with a smaller resistor will reduce transistor heat dissipation, your logic of retaining smaller resistors fits right in other than of course if it blows the resistors!
Have others seen overheated and blown resistors?
Priya.
--- In TekScopes@..., "Albert" wrote:
Could it be that after your modification the switching transistors get hotter? At first sight it is a trade-off between saturation voltage of the switching transistors and voltage across the FET. The sum of these voltages must remain more or less the same, in order to have the same voltage swing at the primaries of the inverter power transformer.
I only have experience with 7000 SPMSs. Good to know that these "modern" supplies are so different, in case my 2232 breaks down...
BTW the manual says those small resistors are 0.2 W types. In for instance a 7704A supply Ic/Ib = 4 but the collector currents are smaller. The resistors are 0.5 W.
Albert
|
Re: 2246 Mod A - Is it OK to replace just one SMPS switching transistor?
On Thu, 03 Jan 2013 21:11:08 -0000, "Francis" <jekker@...> wrote: The switching transistors are rather cold than anything else.
I put an heatsink on each of them for the peace of my mind....
You can put your fingers (be careful!) anywhere with no feeling
of excessive heat.
Please take into consideration that the excess of base current
increases the dissipation as well. The FET has roughly 3V VDS, 0.3/0.4 Amps come from the command, then ~1W is dissipated in it.
A 0.3 IC/IB ratio is too much. The only benefit is a lower "on"
switching time, which is useless here due to the softness of the
waweform edges in the transformers. The cons are a higher "off"
time. Since the inverter uses a seperate transformer, drived by
the main transformer for the base command, this can cause troubles. I think if the designer was concerned about the switching time, then they would have used some sort of active saturation control like a Baker clamp. I would be tempted to add Baker clamps anyway if the switching stage had a history of mysterious failures. I have a bag of TIP31. All tested at 250v DC. None failed or
even avalanched. They are Motorola, rated at 40v VCE. Stunning, no?
Maybe Tek sorted their "specials" this way? Who knows... This would not be the first time I suspected that Tektronix graded a common part but did not document it in their Common Design Parts Catalog.
|
Re: HP question (yeah I know, this is the TEK forum :-)
No, there's no 608C manual there, only a Service Note. I have a photocopy of a 608C manual;
if you still need it, contact me off-list and I'll scan it as time permits.
Dave Wise
toggle quoted message
Show quoted text
-----Original Message----- From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Albert Sent: Sunday, December 16, 2012 3:28 AM To: TekScopes@... Subject: [TekScopes] Re: HP question (yeah I know, this is the TEK forum :-) Hallo Leonhard, This one? Albert Guys I am about to buy an HP608C VHF Generator (real boat anchor).
First of all I am looking for the manual in pdf, it's not at the BAMA site. Anyone able to help? (send to tubes.leonard@...)
---
Thanks Leonard
|
Re: Electrical Differences between 015-0611-00 (Tektronix) and 015-0611-01 (Tegam)
Hi Dave,
The 150ps versus 200ps difference is in my view only the result of the better RF-environment of the new relais(design) in the 015-0611-00 Pulse Head. I have both BNC Pulse Heads (311-01 and 611-00) and also their Manuals. Victor Silva has a 015-0611-01 Pulse Head. And this Tegam made one has the SMA-Connector as Output-Connector. I don't have a 015-0611-01 Manual (I can order one for you). According to Tegam's CG5011 Documentation the Trr is slightly worse (<160ps).
Greetings,
Egge Siert
|
Re: 2246 Mod A - Is it OK to replace just one SMPS switching transistor?
There is something strange about those switching transistors. I just looked in more detail in the 2232 manual. There the base resistors are 33 Ohm (!) but still 0.2 W (!). Same transistors, 151-0852-00 or SJE6447. At internet I only read NPN 100 V / 6A bipolar transistors, but could these be Darlington transistors? BTW these are currently listed at ebay from a parted-out 2232.
I realize my impression was just "at first sight".
Albert
toggle quoted message
Show quoted text
--- In TekScopes@..., "Francis" wrote:
The switching transistors are rather cold than anything else.
I put an heatsink on each of them for the peace of my mind....
You can put your fingers (be careful!) anywhere with no feeling
of excessive heat.
Please take into consideration that the excess of base current
increases the dissipation as well. The FET has roughly 3V VDS, 0.3/0.4 Amps come from the command, then ~1W is dissipated in it.
A 0.3 IC/IB ratio is too much. The only benefit is a lower "on"
switching time, which is useless here due to the softness of the
waweform edges in the transformers. The cons are a higher "off"
time. Since the inverter uses a seperate transformer, drived by
the main transformer for the base command, this can cause troubles.
I have a bag of TIP31. All tested at 250v DC. None failed or
even avalanched. They are Motorola, rated at 40v VCE. Stunning, no?
Maybe Tek sorted their "specials" this way? Who knows...
--- In TekScopes@..., "Albert" wrote:
Could it be that after your modification the switching transistors get hotter? At first sight it is a trade-off between saturation voltage of the switching transistors and voltage across the FET. The sum of these voltages must remain more or less the same, in order to have the same voltage swing at the primaries of the inverter power transformer.
I only have experience with 7000 SPMSs. Good to know that these "modern" supplies are so different, in case my 2232 breaks down...
BTW the manual says those small resistors are 0.2 W types. In for instance a 7704A supply Ic/Ib = 4 but the collector currents are smaller. The resistors are 0.5 W.
Albert
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On Thu, 3 Jan 2013 14:26:04 -0700, David DiGiacomo <daviddigiacomo@...> wrote: On Thu, Jan 3, 2013 at 1:57 PM, micpreamp <micpreamp@...> wrote:
I have a 7904A with a 7A19 plugin (plus lots of slower ones, e.g. 7A24 and 7A26). I'd love to have two channels of > 500 MHz, and I'm contemplating if I should chase another 7A19 or get two 7A29 (and sell the 7A19). The 7A29 will make better use of the mainframe bandwidth and has a variable vertical deflection feature.
What I wonder: how do things look noise wise? I hate blurry traces, and I'm rather surprised how sharp the 7A19 is (much better than the 7A26!). Anyone who could compare them side-by-side? Isn't that just due to the termination on the 7A19? Have you tried
putting a good inline termination on the 7A26?
Anyway, I haven't noticed a difference in trace noise between the 7A19 and 7A29. The difference in noise between the 7A19 and 7A29 should be there (maybe about 30%) but it will not amount to much and I doubt it would be visible. The 7A26 is so much noisier because it has a 1 MOhm input impedance. The higher noise levels involved with high impedance inputs is why you do not find high bandwidth and high sensitivity in the same amplifier for a given input impedance. I actually prefer the 7A26 (200 MHz switchable to 20 MHz) or 7A13 (100 MHz switchable to 5 MHz) to the 7A18 (100 MHz only) because they have switchable bandwidth limiting which makes a large difference in displayed noise when you do not need the higher bandwidth.
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Glydeck
David Hess