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On Wed, Sep 2, 2020 at 2:07 PM Vince Vielhaber <vev@...> wrote:

I had actually done that last nite, but didn't have the parts layout
handy to see which probe was on which pin (I had them on the diodes>.

Here they are with voltage measurements:

Uh, this makes no sense to me.
The base of Q870 should be pulled toward +15V unreg through R864(?
hard to read on the schematic), and the two op amps shunt it towards
ground when they detect too high voltage or too high current,
respectively. Whichever of pins 1/7 is lower is the op-amp in control of the output.

... time passes ...

Yeah, still doesn't make sense to me. I'm going to guess that either
the op-amps are (pins 1/7 are) reversed on the schematic, or that your
channel designations are.
Even so, it still doesn't make sense, as the voltage-control op-amp
starts dialing more voltage (Channel 2 normal at 5.54V rises to
16.94V), while the current limit op-amp goes on the limit (Channel 1
normal at 16.76V dips to 6.16V). What doesn't make sense here is that
the current control op-amp is dialing a voltage that exceeds the voltage control's usual/normal voltage.
I haven't done the maths, but as there's a bias on the current limit
op-amp's feedback input, it might be that it's simply kicking in
because the output voltage has sagged.

I think you need to look further afield, how's the +8V rail doing, the
+-15V rails, raw and regulated? How is the -5V rail doing? Any other
rails you can find...

If you can't find a bad rail, start looking at the biasing for Q870 &
Q879, have the resistors drifted? If R864(?) or R478 have drifted
significantly upward, that'd explain why the transistors can't meet the current demand.
If the biasing is OK, you need to look at the transistors themselves.
If their gain is down, that'd also explain things. You can measure the
base current to Q879 indirectly by the voltage drop over its base
resistor. Even looking at what's happening on Q870's emitter under
collapse - is that transistor going to saturation, indicating that
Q879 is dozing off on the job?

Good luck!

On ebay, 362868002250; not sure what it really is- what caught my eye was
the two 125ohm GR connectors on the rear. Maybe someone's accessory for
part of a 519 system?
-Dave

On Sep 9, 2020, at 20:50, Jim Ford <james.ford@...> wrote:

?Maybe it does for shorted tantalum capacitors. Not guaranteed to work on shorts inside the PCB. Oh, it will show you where the short is alright, but that doesn't mean the short will clear or the PCB will be usable afterward. Don't ask....

Jim Ford

------ Original Message ------
From: "Stephen Hanselman" <kc4sw.io@...>
To: [email protected]
Sent: 9/9/2020 3:18:34 PM
Subject: Re: [TekScopes] Help Troubleshooting 466, No +15VDC

Our favorite way was to use a HP 2100 power supply 5VDC at 50Amps. Finds’um every time

Regards,

Stephen Hanselman
Datagate Systems, LLC

On Sep 9, 2020, at 07:00, Martin <musaeum@...> wrote:

?Hello Sparky,

my preferred method for instant finding of shorts in these caps is with my IR-Cam.
You may supply the +15V rail with a current limited supply, or shortly apply mains power - the component will instantly reveal itself.

IR-cams are getting more and more accessible, perhaps you can borrow one?

cheers
Martin

Maybe it does for shorted tantalum capacitors.? Not guaranteed to work on shorts inside the PCB.? Oh, it will show you where the short is alright, but that doesn't mean the short will clear or the PCB will be usable afterward.? Don't ask....

Jim Ford

------ Original Message ------
From: "Stephen Hanselman" <kc4sw.io@...>
To: [email protected]
Sent: 9/9/2020 3:18:34 PM
Subject: Re: [TekScopes] Help Troubleshooting 466, No +15VDC

Our favorite way was to use a HP 2100 power supply 5VDC at 50Amps. Finds’um every time

Regards,

Stephen Hanselman
Datagate Systems, LLC

?On Sep 9, 2020, at 07:00, Martin <musaeum@...> wrote:

??Hello Sparky,

?my preferred method for instant finding of shorts in these caps is with my IR-Cam.
?You may supply the +15V rail with a current limited supply, or shortly apply mains power - the component will instantly reveal itself.

?IR-cams are getting more and more accessible, perhaps you can borrow one?

?cheers
?Martin

Our favorite way was to use a HP 2100 power supply 5VDC at 50Amps. Finds’um every time

Regards,

Stephen Hanselman
Datagate Systems, LLC

On Sep 9, 2020, at 07:00, Martin <musaeum@...> wrote:

?Hello Sparky,

my preferred method for instant finding of shorts in these caps is with my IR-Cam.
You may supply the +15V rail with a current limited supply, or shortly apply mains power - the component will instantly reveal itself.

IR-cams are getting more and more accessible, perhaps you can borrow one?

cheers
Martin

Our favorite way was to use a HP 2100 power supply 5VDC at 50Amps. Finds’um every time

Regards,
Stephen Hanselman
Datagate Systems, LLC

On Sep 9, 2020, at 07:00, Martin <musaeum@...> wrote:

?Hello Sparky,

my preferred method for instant finding of shorts in these caps is with my IR-Cam.
You may supply the +15V rail with a current limited supply, or shortly apply mains power - the component will instantly reveal itself.

IR-cams are getting more and more accessible, perhaps you can borrow one?

cheers
Martin

Ok, here's where I'm at.

ALL of the electrolytics, except the two big ones in the back, have been
replaced. I tested them and they were all good with very low ESR so
I'll toss them in a box for now.

Q870 and Q879 have been replaced. Had to order these.

All resistors in the 5V Regulator have been checked and are all well
within spec.

The few small caps (.001uf) were also checked and are right on.

Problem still exists.

Before I was toying around with the idea of hanging an external 5V
supply on it. I did try that last week and it did NOT do it and
according to the meter on the supply, it wasn't really drawing much
current from it - less than 100ma.

Tomorrow I'll go thru the supply again and take more measurements, there
were a few points I was looking at and couldn't remember if I checked
them and if so what they were. So tomorrow I'll check them and take
pictures and better notes.

That's it for tonite.

Vince.



On 09/02/2020 02:50 PM, Siggi wrote:

On Wed, Sep 2, 2020 at 2:07 PM Vince Vielhaber <vev@...> wrote:

I had actually done that last nite, but didn't have the parts layout
handy to see which probe was on which pin (I had them on the diodes>.

Here they are with voltage measurements:

Uh, this makes no sense to me.
The base of Q870 should be pulled toward +15V unreg through R864(?
hard to
read on the schematic), and the two op amps shunt it towards ground when
they detect too high voltage or too high current, respectively.
Whichever
of pins 1/7 is lower is the op-amp in control of the output.

... time passes ...

Yeah, still doesn't make sense to me. I'm going to guess that either the
op-amps are (pins 1/7 are) reversed on the schematic, or that your
channel
designations are.
Even so, it still doesn't make sense, as the voltage-control op-amp
starts
dialing more voltage (Channel 2 normal at 5.54V rises to 16.94V),
while the
current limit op-amp goes on the limit (Channel 1 normal at 16.76V
dips to
6.16V). What doesn't make sense here is that the current control
op-amp is
dialing a voltage that exceeds the voltage control's usual/normal
voltage.
I haven't done the maths, but as there's a bias on the current limit
op-amp's feedback input, it might be that it's simply kicking in because
the output voltage has sagged.

I think you need to look further afield, how's the +8V rail doing,
the +-15V rails, raw and regulated? How is the -5V rail doing? Any other
rails you can find...

If you can't find a bad rail, start looking at the biasing for Q870 &
Q879,
have the resistors drifted? If R864(?) or R478 have drifted significantly
upward, that'd explain why the transistors can't meet the current demand.
If the biasing is OK, you need to look at the transistors themselves. If
their gain is down, that'd also explain things. You can measure the base
current to Q879 indirectly by the voltage drop over its base resistor.
Even
looking at what's happening on Q870's emitter under collapse - is that
transistor going to saturation, indicating that Q879 is dozing off on the
job?

Good luck!

On Sep 9, 2020, at 07:00, Martin <musaeum@...> wrote:

?Hello Sparky,

my preferred method for instant finding of shorts in these caps is with my IR-Cam.
You may supply the +15V rail with a current limited supply, or shortly apply mains power - the component will instantly reveal itself.

IR-cams are getting more and more accessible, perhaps you can borrow one?

cheers
Martin

From: "Harrison" <buma7@...>
To: [email protected]
Sent: Wednesday, September 9, 2020 3:47:17 PM
Subject: Re: [TekScopes] Tek 4654M; No Trace, No +5V, No +95V; Help

I am afraid that I have been jumping around too much and now, even with good
shop notes, am not sure what elements/modules I think are good and which
might be bad. Looking at the troubleshooting diagrams every thing still
points to the HV power supply. Currently there doesn't seem to be any out
there so it is a case of building a complete unit or repairing what is
there. I am coming close to declaring this unit a parts machine and closing
the book on it but I hate to do so. A repair might be right around the
corner or I am missing the obvious and the fix is staring me in the face.

Chuck,

The 7D20T did not require anything extra from the 7D20. Any production 7D20 will function in the T package without modification. You could add the rear access cable for the GPIB that had a companion mod for the R7603. The mainframe for the 7D20T has the necessary 7k interface, rear GPIB wiring and power in a TM5003 skin (the notorious modular package system (MPS)).

Regards,
Clark Foley