Priya,
Wow, I didn't know it was all channels! Check your trigger
setting, try a single shot too. maybe your focus is so far out you can't see
it. Look at it in the dark too. Check the rejection section in the trigger
area, set it for DC. Use the menu to set things to default just in case you
got into unfamiliar territory. I don't see how putting in that socket would
have done anything else barring you accidently damaged something in the
process or zapped something that is static sensitive.
Pull back for a
second. Hopefully you just overlooked something!
Nick
Nielsen
--- In TekScopes@...,
"Mover" wrote:
>
> Hi Nick,
>
> There is currently no
trace on any of the channels after installing the IC socket at U174. However,
the beam finder works as do all the menus and also the vertical time base
cursors and readouts. I suspect something with the motherboard now at or near
U174. Using your plan, I will apply an identical calibration signal to Ch2 and
Ch3 and compare what happens to the signal using my R7704 as a test scope.
More to come on this ...
>
> Yes, I will be careful as you say to
not blow up something else along the way!
>
> Priya.
>
> --- In TekScopes@...,
"nielsentelecom@" wrote:
> >
> > Priya,
> >
> > You could probably use ch3 to probe for the comparisons as a
last resort on the following.
> >
> > I would set up
identical conditions on both ch2 and ch1. then on ch 2 compare signal traces
starting at the input. Place the same signal on both leads(channels). Check
the U175 relay driver on ch2 and find out if any are high. You can also check
the U171/U172 if your input to the U174/U175 does not match. But they appear
to do different jobs on given pins. Probably best to test at the inputs of
U174 & U175. Then go forward till you lose identical signal. Since you
don't have U174 installed, just the socket instead, U171 may be calling for
one of the relays in the Ch1 circuit to be pulled in or activated if you
prefer. Then jumper where U174 should be on the affected driver. If as you say
the relays are all operating as they should, you should expect the ch1 and ch2
relays to all align identically. It is possible one of the relays just isn't
pulled in correctly. You can also check test point 1A (AT117 pin 7) in the ch1
circuit, and At127 pin 7 to ensure the they match once you get both
attenuators aligned the same.
> >
> > Don't forget, BE
CAREFUL! Don't blow up ch2!!
> >
> > Nick Nielsen
>
>
> >
> > --- In TekScopes@...,
"Mover" wrote:
> > >
> > > Update: I soldered an IC
socket in U174 thinking that I would connect leads to it externally to
diagnose the Ch1 issue and now the trace has completely disappeared! The beam
finder works, but when I release the beam finder button, there is nothing on
the screen other than the vert and hor selection displays. The menus are all
fine and the time cursors work fine too - just no signal trace. Checked all
the voltages and they are fine.
> > >
> > > I assume
that I have to use another scope to trace the vertical amplifier area, but
would appreciate any tips here - if the vertical amplifier was bad, would it
not also affect the beam finder, displays and menus?
> > >
> > > Priya Jakatdar.
> > >
> > > --- In
TekScopes@...,
"Mover" wrote:
> > > >
> > > > I put the scope
back together and it runs fine on Ch2, Ch3 and Ch4 including time &
frequency measurements. All secondary voltages are well within the spec range
(7.48V, -7.47V, 5.2V, -5.2V, 15.1V, -15.9V etc), so I have not made any
adjustment to the 44VDC rail to avoid calibrating the scope from scratch. Will
use it this way for now and let it run for some time to be sure nothing else
is wrong or will fail before attempting further repair.
> > > >
> > > > The issue with Ch1 and voltage measurements is a
charred U174 transistor relay driver IC (Intersil 3082) that appears to have
caused the SMPS to fail in the first place. I have not replaced the chip
thinking there may be a larger issue with or around U174. The Ch1 individual
relays are all working fine (tested with a Kikusui constant-voltage
constant-current supply) and drawing 25 mA or 12 mA according to their spec.
When I do get some time later, I am thinking about validating the Shift
Register output pins of U171 that drives U174 first and then maybe installing
an IC socket rather than soldering a new U174.
> > > >
> > > > I have searched the archives and not found anything
specific with respect to U174. Any thoughts or previous experience with this
will be greatly appreciated.
> > > >
> > > > I
want to thank everyone's help in getting my scope back into working condition.
I have learned a lot about my 2246 and it is mainly because of you
guys.
> > > >
> > > > Priya.
> > >
>
> > > > --- In TekScopes@...,
"Mover" wrote:
> > > > >
> > > > >
Tom,
> > > > >
> > > > > I was only able
to get enough time to assemble the SMPS into the chassis with just the main
board and it runs fine - without the 40W series bulb:). The voltages are a bit
off (5.28V, 7.46V etc) but I won't adjust anything till the proc board and the
pot boards are in and providing normal operating load.
> > > >
>
> > > > > That's for the weekend. More to
come.
> > > > >
> > > > > Thanks for
taking the time to make resistance measurements - very similar to
mine.
> > > > >
> > > > > Priya.
>
> > > >
> > > > > --- In TekScopes@..., "Tom
Jobe" wrote:
> > > > > >
> > > > > >
Hi Priya,
> > > > > > Resistances were taken on a nice
working 2246 with a Fluke 110 which does not have enough voltage to forward
bias the diodes.
> > > > > > I tried this Fluke meter on
some standard diodes and some Schottky diodes and there was not much of a
connection either way, just a little leakage depending on the polarity
>
> > > > > There is not much difference from your readings, and
no obvious indication of a problem on any of the voltages.
> > >
> > > I will add my readings on to the end of your list.
> >
> > > >
> > > > > > > +5:100 ?? - mine is
99 ohm - 106 ohm
> > > > > > > -5:50 ?? - mine is 41
ohm - 53 ohm
> > > > > > > -15:500 ?? - mine is 750
ohm - 590 ohm
> > > > > > > +15:400 ?? - mine is 583
ohm - 483 ohm
> > > > > > > +7.5:150 ?? - mine is 134
ohm - 152 ohm
> > > > > > > -7.5:180 ?? - mine is 143
ohm - 150 ohm
> > > > > > > +58: 7500 ?? - mine is
almost open ( 8M ohm) - 2.5 M ohm? impossible to get a good reading
>
> > > > > > +130: 9000 ?? - mine is 10,000 - 9000
ohm
> > > > > >
> > > > > > I guess
the next step is to see what happens when you put the scope back together
tonight.
> > > > > > tom jobe...
> > > >
> >
> > > > > >
> > > > > >
> > > > > > ----- Original Message -----
> >
> > > > From: Mover
> > > > > > To: TekScopes@...
> > > > > > Sent: Friday, January 04, 2013 12:20
PM
> > > > > > Subject: [TekScopes] Re: 2246 Mod A main
board overloading the SMPS
> > > > > >
> > >
> > >
> > > > > >
> > > > >
> Tom & Francis,
> > > > > >
> > >
> > > I discovered what it is - the ultra cautious in me had left a
40W light bulb in series with the isolation transformer secondary providing AC
line power to the scope. It was there originally to prevent further collateral
damage to the scope through my repair and testing. With any load exceeding the
load provided by the fan, the SMPS pulls enough current through the light bulb
that the AC line power to the scope drops below the 80V required to keep the
SMPS running. This shuts down the SMPS and the 40W bulb comes on as the SMPS
restarts - absolutely normal behavior. I had completely forgotten about the
light bulb till this afternoon when I chanced to glance at it cycling with the
SMPS and then I had an AHA moment!
> > > > > >
>
> > > > > Regardless, I did measure resistances (with a Fluke
116 - don't own a meter recommended by TEK) and here they are:
> >
> > > > > +5:100 ?? - mine is 99 ohm
> > > >
> > > -5:50 ?? - mine is 41 ohm
> > > > > > >
-15:500 ?? - mine is 750 ohm
> > > > > > > +15:400 ??
- mine is 583 ohm
> > > > > > > +7.5:150 ?? - mine is
134 ohm
> > > > > > > -7.5:180 ?? - mine is 143
ohm
> > > > > > > +58: 7500 ?? - mine is almost open (
8M ohm)
> > > > > > > +130: 9000 ?? - mine is
10,000
> > > > > >
> > > > > >
Other than the 58V resistance probably becuase my meter can't drive the
diodes, my measurements looks fine.
> > > > > >
>
> > > > > I ran it without the series light bulb and it happily
drives a 10 ohm resistor from the 5V and regulation is excellent at 5.05 VDC.
With the light bulb in series, it won't run anything more than the fan.
Indeed, the fan is not sufficient load to test the SMPS.
> > >
> > >
> > > > > > Believe I am ready to put the
SMPS back into the scope tonight. I apologize to everyone who I misled with my
earlier post on this and thanks as always for quick and thoughtful responses.
You guys are great!
> > > > > >
> > > >
> > Priya.
> > > > > >
> > > > >
> --- In TekScopes@...,
"Francis" wrote:
> > > > > > >
> > > >
> > > Hi,
> > > > > > >
> > >
> > > > several things:
> > > > > > >
> > > > > > > as Tom Jobe suggested, you can do the
following:
> > > > > > > with the scope disconnected
from the mains, check the resistances
> > > > > > >
between ground and each supply. As Tek says, use an ohmeter that is able to
forward bias diodes, usually a low range.
> > > > > >
>
> > > > > > > Values are
> > > >
> > > +5:100 ??
> > > > > > > -5:50
??
> > > > > > > -15:500 ??
> > > >
> > > +15:400 ??
> > > > > > > +7.5:150
??
> > > > > > > -7.5:180 ??
> > > >
> > > +58: 7500 ??
> > > > > > > +130: 9000
??
> > > > > > >
> > > > > >
> these values are taken from a 2245A manual, but should not be
>
> > > > > > very different on your scope. If one is
significantly lower,
> > > > > > > check the related
circuitry.
> > > > > > >
> > > > >
> > The fan is not a sufficient load for the SMPS. When I make
>
> > > > > > my trials, I use 2 4.7 ?? paralleled on the +5v.
> > > > > > > If the supply still ticks, try to
disconnect the crowbar protection,
> > > > > > > or
tie together the gate and the cathode of the SCR. Maybe some
> >
> > > > > overvoltage due to a dying cap can fire it. Same
recommendation
> > > > > > > about Q2208 (base tied
to ground).
> > > > > > >
> > > > >
> > --- In TekScopes@...,
"Mover" wrote:
> > > > > > > >
> > >
> > > > > Alas - if only it were that simple ...
> >
> > > > > >
> > > > > > > > I
finally fixed my SMPS and put it back into the chassis with only the main
board. It runs in chirp mode - about every 1 sec there is a clicking sound -
the secondary voltages rise and then something gets overloaded and the SMPS
shuts down and then cycles again.
> > > > > > > >
> > > > > > > > I quickly disconnected power,
isolated the SMPS and again verified that the SMPS is fine on its own with
just the cooling fan load with a steady internal 44 VDC. No new or collateral
damage thankfully.
> > > > > > > >
> >
> > > > > > There are no obvious shorts on any of the supply
lines on the main board. The issue may still very well be with the SMPS being
unable to handle load even though the fan runs fine and the other voltages are
fine.
> > > > > > > >
> > > > >
> > > I assume that chirp mode is a common issue with these SMPS
supplies. What have others done to (1) find whether the issue is with the SMPS
or with the main board and if the latter, (2) which supply line or lines is
drawing too much current and how to isolate the problem further?
> >
> > > > > >
> > > > > > > >
Thanks in advance.
> > > > > > > >
> >
> > > > > > Priya.
> > > > > > >
>
> > > > > > >
> > > > >
>
> > > > >
> > > >
> >
>
> >
>
