Same problem that I have. I suspect a short to ground (or to the HV coil)on the LV side of the transformer as the measured resistance between the two coils is only about 5.5 KOhms.

Will have to keep this one for parts unless I can find another cheap 7603 for parts (with a good HV board).

--- In TekScopes@..., "Robert" <go_boating_fast@> wrote:

My purpose in adding a resistor was to limit the inrush current. I believe my problem was a bad HV transformer. Based on symptoms, I think there was a partial short in the output. This could have the effect of changing the input-output ratio. So the regulator circuit would have to drive the transformer harder to get the output voltage up to spec. However this level of regulation would be outside the regulator design.

Without the current limit ( snubber?) resistor, the regular circuit would first overdrive than over correct and under-drive resulting in a ringing waveform instead of a sine wave. Much like a new car driver learning to steer. By limiting the input a bit, the regulator circuit was just able to work within its design. I finally got a working HV output and trace. But after about an hour it started to go unstable and then slowly got worse. I finally turned it off figuring to replace the transformer before it failed completely and took out more components. This of course is just a guess on my part. It's still on the shelf as I have gone on to other things.

Also it seemed to me that the whole HV box heated up a lot faster than other 7603s I have worked on. This is subjective, but would fit as the HV circuit would be driving more energy that normal.

Bob

--- In TekScopes@..., "gshashte@" <gshashte@> wrote:

Bob,

My Q1216/Q1218 are overheating.
Looking at the Diagram from your website, the 15.7 ohm resistor was added to the Q1216/Q1218 loop. I am in "first grade" electronics, lacking some elementary knowledge but comparing my circuit with yours(I have a much older 7603), mine does not have resistor R1217 (47 Ohms). Seems to me that in later versions they added R1217 to limit the current opening Q1216/Q1218 and consequently their current. Did you try limiting the Q1216/Q1218 by using a higher value of R1217.

I am not totally sure how the circuit works but it seems to me that there are 2 ways that the current on Q1216/Q1218 can increase:
1) Decreased resistance on the gate (Q1214 is failing, the driver is putting higher current or a resistor has lower value).

2) Q1216/Q1218 are deteriorating and allowing higher current or a short on the transformer.

In the end, were you able to completely solve the problem?

--- In TekScopes@..., "Robert" <go_boating_fast@> wrote:

This might help. It is some extensive tests on a 7603 HV I did a few
years ago.

;
=a096bf125eb899358c4d218d16fd078d
<;
D=a096bf125eb899358c4d218d16fd078d>

Bob


--- In TekScopes@..., "gshashte@" wrote:

I bought a broken 7603, the fan was working but nothing else.

I opened the power supply and started looking but did not really find

anything.

I did not find on the power supply board the TO transistors that

Stefan mentioned (referenced below). After moving to the LV Regulator
Board, they were there - 6 of them. Re soldered all those connections
and now I have graticule lights and power led on.

After I reconnected the power supply to go to the LV Regulator I

noticed something was smelling. Turned the power off and decided to
change the position of the cable that goes to the Z-Axis Amplifier board
and supplies 15V unregulated. P870 on the PS has 3 pins but the cable
only two wires. The smell got stronger and I noticed that the big
transistors connected to the HV board were getting hot. So that was a
bad move I made.

After accessing the High voltage board I noticed that it was just

floating, the one metallic post and the 3 plastic ones were not secured.
It seemed to me that the contacts of the transformer could have been
making contact with the metallic enclosure.

I also noticed that the big HV disk cap had some melting on both

sides.

I secured the board with the metallic post (don't have plastic screws

for the other 3)and made sure that there was a good separation between
the big HV disk cap and adjacent caps.

At this moment nothing smelling burned so I suspect the transformer

was making contact with the metallic enclosure.

However, Q1152 on the Z-Axis board which seems to supply 130v to the

HV board on line B overheats. When I remove the cable that supplies 15V
unregulated the problem goes away.

At this point I am not sure how to continue. I only have a Multimeter
to check continuity. I may get a cheap old Radio Shack Transistor

tester and a cheap Chinese ESR tester like this one:

;
PN-PNP-MOSFET-RESISTOR-METER-/160943358553?pt=LH_DefaultDomain_0&hash=it&#92;
em2578f8c259

or this one:

;
984.m1438.l2649

How do I go about testing the transformer on the HV board?

Stefan Trethan wrote:
The 7603, unlike the 7623A, has a linear power supply.

There are big TO-3 series pass transistors on a heatsink, the

problem

is that they are in sockets that combine an unfortunate choice of
plating with an unfortunate mounting arrangement. Thermal expansion
and contraction stresses the solder joint between socket pin and

PCB,

which eventually causes even the leaded solder to fail. The failure
line is very thin and difficult to see even if you are used to

looking

for this. Usually the driver transistor has failed from carrying the
load current, but the TO-3s are still good once you reestablish
contact.

Another common failure is big Sprague capacitors with internally
corroded terminations (reading no capacitance / excessive ripple on
the supply).

As you can see I know more about the 7603 than the 7623A, my 7633

has

been working fine so I was never in there.

ST

Hello all (and Ha Happy New Year)

I am looking for the images of the version -02 of the 7854 scope. All I could find are either the -00 or the old ones from Pentti. Anybody have them?
I was able to read the FPLAs (82S107 ver -00 and -02), but the ROMs themselfs are bad...

Best regards,

Bogdan

On 01/02/2013 11:34 AM, bogroca wrote:

Hello all (and Ha Happy New Year)

I am looking for the images of the version -02 of the 7854 scope. All I could find are either the -00 or the old ones from Pentti.
Anybody have them?

My 7854 was working last I checked. Probably still is even though I may not be getting
back to using it for a couple more months...

Is there a quick indication of which version they have?
I could warm it up and see! (Then back to work on less fun tasks, dang...)

My purpose in adding a resistor was to limit the inrush current. I believe my problem was a bad HV transformer. Based on symptoms, I think there was a partial short in the output. This could have the effect of changing the input-output ratio. So the regulator circuit would have to drive the transformer harder to get the output voltage up to spec. However this level of regulation would be outside the regulator design.

Without the current limit ( snubber?) resistor, the regular circuit would first overdrive than over correct and under-drive resulting in a ringing waveform instead of a sine wave. Much like a new car driver learning to steer. By limiting the input a bit, the regulator circuit was just able to work within its design. I finally got a working HV output and trace. But after about an hour it started to go unstable and then slowly got worse. I finally turned it off figuring to replace the transformer before it failed completely and took out more components. This of course is just a guess on my part. It's still on the shelf as I have gone on to other things.

Also it seemed to me that the whole HV box heated up a lot faster than other 7603s I have worked on. This is subjective, but would fit as the HV circuit would be driving more energy that normal.

Bob

--- In TekScopes@..., "gshashte@" <gshashte@> wrote:

Bob,

My Q1216/Q1218 are overheating.
Looking at the Diagram from your website, the 15.7 ohm resistor was added to the Q1216/Q1218 loop. I am in "first grade" electronics, lacking some elementary knowledge but comparing my circuit with yours(I have a much older 7603), mine does not have resistor R1217 (47 Ohms). Seems to me that in later versions they added R1217 to limit the current opening Q1216/Q1218 and consequently their current. Did you try limiting the Q1216/Q1218 by using a higher value of R1217.

I am not totally sure how the circuit works but it seems to me that there are 2 ways that the current on Q1216/Q1218 can increase:
1) Decreased resistance on the gate (Q1214 is failing, the driver is putting higher current or a resistor has lower value).

2) Q1216/Q1218 are deteriorating and allowing higher current or a short on the transformer.

In the end, were you able to completely solve the problem?

--- In TekScopes@..., "Robert" <go_boating_fast@> wrote:

This might help. It is some extensive tests on a 7603 HV I did a few
years ago.

;
=a096bf125eb899358c4d218d16fd078d
<;
D=a096bf125eb899358c4d218d16fd078d>

Bob


--- In TekScopes@..., "gshashte@" wrote:

I bought a broken 7603, the fan was working but nothing else.

I opened the power supply and started looking but did not really find

anything.

I did not find on the power supply board the TO transistors that

Stefan mentioned (referenced below). After moving to the LV Regulator
Board, they were there - 6 of them. Re soldered all those connections
and now I have graticule lights and power led on.

After I reconnected the power supply to go to the LV Regulator I

noticed something was smelling. Turned the power off and decided to
change the position of the cable that goes to the Z-Axis Amplifier board
and supplies 15V unregulated. P870 on the PS has 3 pins but the cable
only two wires. The smell got stronger and I noticed that the big
transistors connected to the HV board were getting hot. So that was a
bad move I made.

After accessing the High voltage board I noticed that it was just

floating, the one metallic post and the 3 plastic ones were not secured.
It seemed to me that the contacts of the transformer could have been
making contact with the metallic enclosure.

I also noticed that the big HV disk cap had some melting on both

sides.

I secured the board with the metallic post (don't have plastic screws

for the other 3)and made sure that there was a good separation between
the big HV disk cap and adjacent caps.

At this moment nothing smelling burned so I suspect the transformer

was making contact with the metallic enclosure.

However, Q1152 on the Z-Axis board which seems to supply 130v to the

HV board on line B overheats. When I remove the cable that supplies 15V
unregulated the problem goes away.

At this point I am not sure how to continue. I only have a Multimeter
to check continuity. I may get a cheap old Radio Shack Transistor

tester and a cheap Chinese ESR tester like this one:

;
PN-PNP-MOSFET-RESISTOR-METER-/160943358553?pt=LH_DefaultDomain_0&hash=it&#92;
em2578f8c259

or this one:

;
984.m1438.l2649

How do I go about testing the transformer on the HV board?

Stefan Trethan wrote:
The 7603, unlike the 7623A, has a linear power supply.

There are big TO-3 series pass transistors on a heatsink, the

problem

is that they are in sockets that combine an unfortunate choice of
plating with an unfortunate mounting arrangement. Thermal expansion
and contraction stresses the solder joint between socket pin and

PCB,

which eventually causes even the leaded solder to fail. The failure
line is very thin and difficult to see even if you are used to

looking

for this. Usually the driver transistor has failed from carrying the
load current, but the TO-3s are still good once you reestablish
contact.

Another common failure is big Sprague capacitors with internally
corroded terminations (reading no capacitance / excessive ripple on
the supply).

As you can see I know more about the 7603 than the 7623A, my 7633

has

been working fine so I was never in there.

ST

----- Original Message -----

From: David

To: TekScopes@...

Sent: Wednesday, January 02, 2013 2:12 PM

Subject: Re: [TekScopes] Re: 7587 Nuvistor Tetrode Substitutions in the Tektronix 184 Time-Mark Generator

?

Did you have a particular FET in mind? All of the enhancement mode
FETs I am familiar with have way too high a gate to source threshold
voltage. Small signal and power depletion mode MOSFETs used to be
available which would have been perfect or at least worth trying but
they are no longer commonly available.

On Wed, 02 Jan 2013 02:21:17 -0500, "Tom Miller"
<tmiller11147@...> wrote:

>Change the 125 volt plate supply on the switch to 12 volts. Leave the screen open, it's not needed. Just put a pair of RF N-Ch e-mode fets to the plate, grid, and cathode and tune away. Pick a fet that runs about 10 ma at Vgs=0. It's worth a try if you can't find the tubes.
>
>Good luck,
>Tom
>
> ----- Original Message -----
> From: David
> To: TekScopes@...
> Sent: Wednesday, January 02, 2013 1:02 AM
> Subject: Re: [TekScopes] Re: 7587 Nuvistor Tetrode Substitutions in the Tektronix 184 Time-Mark Generator
>
> I should have linked this in my original post. Here is a cleaned up
> copy of the 184 schematics that I made when I printed them out. The
> tetrodes are all on schematic 1:
>
>
>
> The full 184 manual is available here:
>
>
>
> The control grid DC biasing is all high impedance so unless I want to
> change that, I will have to stick with FETs. The screen grid DC
> biasing is not much lower.
>
> After further thought, I believe that a dual JFET cascode would work
> as a drop in replacement for each one if the anode supply is lowered.
> A source degeneration resistor can be added to lower the gain if
> necessary. The screen grid circuit voltage would need to be about
> half of the drain supply.
>
> On Wed, 02 Jan 2013 02:01:48 -0000, "Ed Breya" <edbreya@...>
> wrote:
>
> >I'm not familiar with the 184, but it seems that for a 500 MHz type circuit, it's low-Z RF anyway, so you could probably design in some bipolar transistor circuits to replace the tubes, with new bias and much lower operating voltage, and some changes to RF matching components - easier yet if they're class-C amplifiers.
> >
> >UHF JFETs should work OK as amplifiers, but may not have enough power output available - it depends on the circuit RF power levels needed. Even for low power, you'll probably still have to significantly change the bias and operating voltage range. Big MESFETs would be way overkill, and probably unsuitable for the operating voltages and currents available in the 184.
> >
> >With the vast number of choices in bipolar NPN RF transistors, it should be fairly easy to match the power of a 7587, as long as you can change enough other stuff to go with it.
> >
> >Ed
> >
> >--- In TekScopes@..., David wrote:
> >>
> >> I have a mostly functional Tektronix 184 Time-Mark Generator that I am
> >> restoring. It has six 7587 nuvistor tetrodes and they all appear to
> >> work except for maybe the pair in the 500 MHz multiplier.
> >>
> >> You can still pick up used but probably good and NOS 7587 nuvistors
> >> for reasonable prices but how feasible would it be to replace them
> >> with say JFETs with cascodes?
> >>
> >> I will probably try it no matter what is said here just for the fun of
> >> it but any advice would be appreciated.

Bob,

My Q1216/Q1218 are overheating.
Looking at the Diagram from your website, the 15.7 ohm resistor was added to the Q1216/Q1218 loop. I am in "first grade" electronics, lacking some elementary knowledge but comparing my circuit with yours(I have a much older 7603), mine does not have resistor R1217 (47 Ohms). Seems to me that in later versions they added R1217 to limit the current opening Q1216/Q1218 and consequently their current. Did you try limiting the Q1216/Q1218 by using a higher value of R1217.

I am not totally sure how the circuit works but it seems to me that there are 2 ways that the current on Q1216/Q1218 can increase:
1) Decreased resistance on the gate (Q1214 is failing, the driver is putting higher current or a resistor has lower value).

2) Q1216/Q1218 are deteriorating and allowing higher current or a short on the transformer.

In the end, were you able to completely solve the problem?

--- In TekScopes@..., "Robert" <go_boating_fast@> wrote:

This might help. It is some extensive tests on a 7603 HV I did a few
years ago.

;
=a096bf125eb899358c4d218d16fd078d
<;
D=a096bf125eb899358c4d218d16fd078d>

Bob


--- In TekScopes@..., "gshashte@" wrote:

I bought a broken 7603, the fan was working but nothing else.

I opened the power supply and started looking but did not really find

anything.

I did not find on the power supply board the TO transistors that

Stefan mentioned (referenced below). After moving to the LV Regulator
Board, they were there - 6 of them. Re soldered all those connections
and now I have graticule lights and power led on.

After I reconnected the power supply to go to the LV Regulator I

noticed something was smelling. Turned the power off and decided to
change the position of the cable that goes to the Z-Axis Amplifier board
and supplies 15V unregulated. P870 on the PS has 3 pins but the cable
only two wires. The smell got stronger and I noticed that the big
transistors connected to the HV board were getting hot. So that was a
bad move I made.

After accessing the High voltage board I noticed that it was just

floating, the one metallic post and the 3 plastic ones were not secured.
It seemed to me that the contacts of the transformer could have been
making contact with the metallic enclosure.

I also noticed that the big HV disk cap had some melting on both

sides.

I secured the board with the metallic post (don't have plastic screws

for the other 3)and made sure that there was a good separation between
the big HV disk cap and adjacent caps.

At this moment nothing smelling burned so I suspect the transformer

was making contact with the metallic enclosure.

However, Q1152 on the Z-Axis board which seems to supply 130v to the

HV board on line B overheats. When I remove the cable that supplies 15V
unregulated the problem goes away.

At this point I am not sure how to continue. I only have a Multimeter
to check continuity. I may get a cheap old Radio Shack Transistor

tester and a cheap Chinese ESR tester like this one:

;
PN-PNP-MOSFET-RESISTOR-METER-/160943358553?pt=LH_DefaultDomain_0&hash=it&#92;
em2578f8c259

or this one:

;
984.m1438.l2649

How do I go about testing the transformer on the HV board?

Stefan Trethan wrote:
The 7603, unlike the 7623A, has a linear power supply.

There are big TO-3 series pass transistors on a heatsink, the

problem

is that they are in sockets that combine an unfortunate choice of
plating with an unfortunate mounting arrangement. Thermal expansion
and contraction stresses the solder joint between socket pin and

PCB,

which eventually causes even the leaded solder to fail. The failure
line is very thin and difficult to see even if you are used to

looking

for this. Usually the driver transistor has failed from carrying the
load current, but the TO-3s are still good once you reestablish
contact.

Another common failure is big Sprague capacitors with internally
corroded terminations (reading no capacitance / excessive ripple on
the supply).

As you can see I know more about the 7603 than the 7623A, my 7633

has

been working fine so I was never in there.

ST

I should have linked this in my original post. Here is a cleaned up
copy of the 184 schematics that I made when I printed them out. The
tetrodes are all on schematic 1:



The full 184 manual is available here:



The control grid DC biasing is all high impedance so unless I want to
change that, I will have to stick with FETs. The screen grid DC
biasing is not much lower.

After further thought, I believe that a dual JFET cascode would work
as a drop in replacement for each one if the anode supply is lowered.
A source degeneration resistor can be added to lower the gain if
necessary. The screen grid circuit voltage would need to be about
half of the drain supply.

On Wed, 02 Jan 2013 02:01:48 -0000, "Ed Breya" <edbreya@...>
wrote:

I'm not familiar with the 184, but it seems that for a 500 MHz type circuit, it's low-Z RF anyway, so you could probably design in some bipolar transistor circuits to replace the tubes, with new bias and much lower operating voltage, and some changes to RF matching components - easier yet if they're class-C amplifiers.

UHF JFETs should work OK as amplifiers, but may not have enough power output available - it depends on the circuit RF power levels needed. Even for low power, you'll probably still have to significantly change the bias and operating voltage range. Big MESFETs would be way overkill, and probably unsuitable for the operating voltages and currents available in the 184.

With the vast number of choices in bipolar NPN RF transistors, it should be fairly easy to match the power of a 7587, as long as you can change enough other stuff to go with it.

Ed

--- In TekScopes@..., David <davidwhess@> wrote:

I have a mostly functional Tektronix 184 Time-Mark Generator that I am
restoring. It has six 7587 nuvistor tetrodes and they all appear to
work except for maybe the pair in the 500 MHz multiplier.

You can still pick up used but probably good and NOS 7587 nuvistors
for reasonable prices but how feasible would it be to replace them
with say JFETs with cascodes?

I will probably try it no matter what is said here just for the fun of
it but any advice would be appreciated.

I have a mostly functional Tektronix 184 Time-Mark Generator that I am
restoring. It has six 7587 nuvistor tetrodes and they all appear to
work except for maybe the pair in the 500 MHz multiplier.

You can still pick up used but probably good and NOS 7587 nuvistors
for reasonable prices but how feasible would it be to replace them
with say JFETs with cascodes?

I will probably try it no matter what is said here just for the fun of
it but any advice would be appreciated.

It's probably best to use at least the same type of transistor in both spots - they don't need to be matched closely, but may cause problems if way different. Also, when one is shot, it's possible the other is damaged, so if you have two new ones available, it's a safe bet to just do both and reduce the risk of having to repeat the whole process. Just test the apparent good one and save it as a spare for just in case.

Ed

--- In TekScopes@..., "Mover" <priya_jakatdar@> wrote:

On my 2246 ModA, I first thought that both inverter switching transistors Q 2009 and Q 2010 are fried, but it looks like one is fine after I removed the obviously shorted device. Since I am using an after-market replacement TIP 41C, should I still replace both or is it fine to operate with a mismatched switching transistor pair? is, leave the good one alone and replace just the bad one with a mismatched part?

Thanks in advance.

Priya Jakatdar.

--- In TekScopes@..., "mockba_the_borg" <marcelo.f.dantas@> wrote:

So I felt brave enough and swapped the CRT (OMG, it is 1:40 AM already).

I did it right after turning off the scope (really brave huh?), so when I pulled the anode connector I could hear the hissing of it discharging into the air before touching the chassis.
The sound of the hissing kill-o-volts made me feel like pulling a rattlesnake out of her nest by the tail ... LOL.

Once replaced, the CRT works perfectly, in fact so good that I didn't even bother put the other one back. So now I have a spare CRT.
So now the only two things left for testing are the Power Supply and the mainboard.

This weekend I may feel brave again and work on the SMPS. I will keep everyone posted.

Thanks everyone. Time to go sleep.

Cheers,
Marcelo.


--- In TekScopes@..., "mockba_the_borg" <marcelo.f.dantas@> wrote:

Thanks everyone who is contributing. I am taking note of each suggestion so I can do the troubleshooting safely this weekend.
This way if the world spirals down into oblivion it won't be my fault, but the Mayan's. :-)

Cheers,
Marcelo.


--- In TekScopes@..., "Tom Jobe" <tomjobe@> wrote:

Hi Marcelo,
In my limited experience there is no danger of swapping 2246 CRT's around between scopes.
Tektronix gear is usually made so that a dead short will not ruin much anyway.
tom jobe...




----- Original Message -----
From: mockba_the_borg
To: TekScopes@...
Sent: Thursday, December 06, 2012 1:43 PM
Subject: [TekScopes] Re: Is it safe to power on a 2246 power supply over the bench to check the voltages?



Sorry ... I forgot to mention ... this test I made when the oscilloscope was still in one piece.
From that moment on I started taking it apart and testing its parts onto another 2246 I have, everything I tried worked fine.
The only things still present on this scope now (and therefore not tested) are the PS, the mainboard and the CRT.

Do you think putting this CRT on another scope could in any way fry that scope? I dont think so, unless there is some crazy short on it, but again, I am no specialist on these things, so I am always scared of breaking something else ... :(

Thanks,
Marcelo.

--- In TekScopes@..., "Mover" <priya_jakatdar@> wrote:
>
> Hi Marcelo,
>
> So all you have in the chassis is the SMPS and it makes a brief "ween" sound? In this case, as suggested by others on this site, you can unplug the scope and make resistance measurements in the SMPS, starting with all the transistors so see if there are obvious shorts. Something will show up based upon my experience and this test has no voltages or floating ground risks.
>
> I'm more fearful than you and therefore no help with CRTs and high voltages unfortunately as you can see from my other postings.
>
> Priya.
>
> --- In TekScopes@..., "mockba_the_borg" <marcelo.f.dantas@> wrote:
> >
> > Thanks U both.
> >
> > Hi Pryia,
> >
> > I was indeed following your thread on the SMPS.
> > The only symptom I have so far is that when I turn on the scope I can hear a very brief "weeen" like if the SMPS is going to start oscillating, but then it stops.
> > My next step would be to pull the PS and check if it just runs when over the bench. Of course trying not to kill myself in the process.
> >
> > I have moved the CPU board and knob boards from this scope to another one and all works fine.
> > As this one was used as parts donor I was thinking about getting it back to work, but I was scared of switching boards around and end up killing something that is working on the other scope.
> >
> > I am either going to rebuild this scope or take it apart and try selling the parts (If I can't manage to find another 2246 CPU board).
> > Right now I have the CRT out, which I don't know if it is safe to test by plugging it to my other scope. Any advice on testing the CRT is also welcome.
> >
> > The mainboard is also out, which I belive would be safe to plug into the other scope after checking that there are no shorts on it (I will follow you sugestion of injecting the low voltage).
> >
> > And the power supply, which is what scares me the most for some reason ... :) ... I,ve had a SMPS pull a lightshow on me once, and since then I have kept myself away from them.
> >
> > And of course ... Even if I decide to sell the parts, I wouldn't try to sell anything without being 100% sure it is working. I don't want other people to buy broken stuff from me.
> >
> > Any insight you may give me at this point would be awesome.
> >
> > Thank you so much.
> >
> > Marcelo.
> >
> >
> > --- In TekScopes@..., "Mover" <priya_jakatdar@> wrote:
> > >
> > > I just went through this very same experience of trying to determine if the fault is inside or outside the 2246 SMPS. My SMPS was running in chirp mode verified by an isolation transformer, a series light bulb for safety and a 2nd test oscilloscope displaying internal SMPS waveforms 11A through 11F. It showed showing high current pull from the internal 44VDC rail, not allowing the voltage to even reach 10V. Following the guidance of some of the folks on this group, I took the following actions:
> > >
> > > (1) Measured resistance of each of the low voltage supply lines under three conditions (a) SMPS connected to the main board, (b) main board alone by lifting up the SMPS about a half inch to disconnect the pins from the main board without touching the high voltage lines and (c) SMPS alone while still lifted up. Found no obvious shorts.
> > >
> > > (2) Injected a small amount of voltage into the main board for each of the supplies. Found no shorts there and correctly concluded that the issue was internal to the SMPS.
> > >
> > > (3) Ran resistance checks on all devices active and passive on the SMPS and found a pair of blown switching transistors Q2009/2010 shorted at the C-E junction.
> > >
> > > (4) I then disconnected the jumper W2201 connecting the switching transistors to the switching FET Q2214, powered up the SMPS still lifted up a half-inch and bingo: The 44 VDC rail looks great and all waveforms upto the inverter section look exactly like the service manual.
> > >
> > > Just keep in mind that the isolation transformer and series light bulb are critical to prevent shorts or ground currents from blowing up the scope or test gear. This process worked for me and may be useful for you depending upon what your exact symptoms are. If you describe your symptoms I may be able to help you.
> > >
> > > One additional point: Unlike most SMPS designs, this SMPS does NOT contain a feedback loop from the secondary voltages to manage regulation, so it is fine to run it disconnected just like I did. All voltage regulation is on the primary side of the final transformer T2204. Still, I believe that a lot of investigation is possible to identify the bad component before attempting to power it up isolated.
> > >
> > > Good luck,
> > >
> > > Priya.
> > >
> > > --- In TekScopes@..., "Albert" <aodiversen@> wrote:
> > > >
> > > > I didn't read those messages about 2246 but I suppose the power supply is a switching mode power supply. A SMPS needs some (well, considerable) load to operate normally. You will find many messages about this with respect to the 7000 series 'scopes. Without load the Overvoltage protection will force the supply in tick mode.
> > > > The power supply (the Low Voltage regulators) will also use voltage Sense lines which will get disconnected when you have the power supply at the bench. In most 7000 supplies those sense lines are connected (via resistors) to the output lines so that the sense lines can be left open without problem. Check that such is also the case in the 2246.
> > > >
> > > > Albert
> > > >
> > > > --- In TekScopes@..., "mockba_the_borg" <marcelo.f.dantas@> wrote:
> > > > >
> > > > > I am thinking about having this power supply removed from the scope and turned on to see if it starts when disconnected (maybe indicating some problem with the mainboard).
> > > > > Of course I will stay VERY away from the high voltage side, I just want to make sure the low DC voltages are present on the connector (yes, I know there's 130v there too).
> > > > > Just wondering if the power supply would require any signaling from the mainboard to start working.
> > > > >
> > > > > Sorry if this is so dumb/newbie question ... I just don't want get into that situation where some voltage runs wild without a load, creates a singularity and sends the galaxy spiraling down into oblivion. It is not Dec/21st yet. (I'll leave that to the guys at CERN).
> > > > >
> > > > > Thanks,
> > > > > Marcelo.
> > > > >
> > > >
> > >
> >
>

----- Original Message -----

From: jerry massengale

To: tekscopes@...

Sent: Wednesday, January 02, 2013 9:47 AM

Subject: [TekScopes] bad -15v on 7704a

?

Hi,

I had a 7704A power supply that would lose -15V after it had been running awhile. It might be after minutes or a few hours but it would fail and the screen go blank. I could never find anything wrong with the power supply out and on the min load.

Since the power supply did not go into tick mode I guess that the problem must be in the pass circuit of the -15V. I had previously replaced the pass transistor. I decided to replace it again just in case. I happened to see R93, the 0.12 ohm resistor in series with the pass transistor. It checked okay but I decided to jumper the resistor and test again. It worked fine. The -15V went to -15.1V but it worked for 24 hours. I have replaced R93 with a resistor from a donor and it is running fine.

Looking at the resistor with a magnifying glass I can see a very small crack.

Jerry Massengale