Chris

The problem could be in the 87V regulator

Or

Their could be a short/low Resistance some where in the load

I have a 2465B open and I measured a load resistance of 25.3K to ground on
Pin 7 of J121 (87V, the blue wire) with the power supply unplugged from

unit

John

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On

Of Chris
Sent: Sunday, February 17, 2013 5:40 PM
To: TekScopes@...
Subject: [TekScopes] Re: Tektronix 2465 capacitors etc.

All voltages measured in the scope with everything hooked up. I just got
done checking the voltages pre-regulator.

At J233A I get:
pin 5 (87v unregulated) 97.6v
Pin 3 (42v unregulated) 49.0v

And at J234A I get:
pin 5 (15v unregulated) 18.94v
Pin 4 (5v unregulated) 6.64
Pin 2 (-5v unregulated) -6.92v
Pin 1 (-15v unregulated) -18.54v

Based on the unregulated values, I believe the issue must lie in the
regulator portions, and since I'm getting the 97.6v pre-regulated it would
seem to me the voltage doubler is working. I also double checked and
verified the 10v reference voltage at the test point on the main board as
well as the J121 connector that feeds the main board. (I figured this was
important to double check as the regulators all are based off the 10v
reference as I read it). Of course this is where I wish to learn more - I
understand what the individual components do but I still somewhat struggle
to see what the assembly as a whole is doing, which is why I'm trying to

into all this, so maybe my understanding is skewed. :)

At any rate, it would seem to me that with the unregulated voltages where
they're at, that the issue would have to lie within the follow area:


Yes? No? I mean I have the correct voltages on the left side of those

groups, and the wrong voltages on the right side, so it would seem that's
where I should look next?

Thanks for helping this newb out!

Chris


--- In TekScopes@..., "Tom Miller" <tmiller11147@> wrote:

The +87 is from a doubler and added to the +42. I would take a close

at the three 10 uF caps and all the diodes in that area. Be sure to only

fast diodes for any replacements.

Since you recapped this unit, don't overlook the fact that bad

can come from the factory new. Did you measure all these voltages in the
scope or on an external load?

Regards,
Tom

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

Yahoo! Groups Links

Anyone familiar with the TM500 counter, model DC504 ?
I just picked one up & I noticed a couple of the digits are missing the middle horizontal segment.
So, it displays an "8" as a "0", or a "0" as a "U".
In your experience, is this a display problem or one of the chips driving the display ?

HankC

?

Going by the schematic, the display digits are multiplexed which is
the common configuration so if the middle horizontal segment is
working on at least some digits, then the 7 segment display driver is
fine. If some segments of every 7 segment display work, then the
column display driver is fine as well.

That pretty much narrows any problem to bad display segments or
possibly an open connection to the missing display segments. I would
normally consider the later less likely than the former except that
two of the same segment are missing.

On Mon, 18 Feb 2013 20:52:57 -0800 (PST), HankC hankc918@...>
wrote:

>Anyone familiar with the TM500 counter, model DC504 ?
>I just picked one up & I noticed a couple of the digits are missing the middle horizontal segment.
>So, it displays an "8" as a "0", or a "0" as a "U".
>In your experience, is this a display problem or one of the chips driving the display ?
>
>HankC

Be careful with that!

Danger, 640 Ohms!


On 2/18/2013 5:50 PM, Alex wrote:

640 ohms ought to be enough for anyone.

?

Anyone familiar with the TM500 counter, model DC504 ?
I just picked one up & I noticed a couple of the digits are missing the middle horizontal segment.
So, it displays an "8" as a "0", or a "0" as a "U".
In your experience, is this a display problem or one of the chips driving the display ?

HankC

Chris

The problem could be in the 87V regulator

Or

Their could be a short/low Resistance some where in the load

I have a 2465B open and I measured a load resistance of 25.3K to ground on
Pin 7 of J121 (87V, the blue wire) with the power supply unplugged from the J121 connector
unit

John

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf
Of Chris
Sent: Sunday, February 17, 2013 5:40 PM
To: TekScopes@...
Subject: [TekScopes] Re: Tektronix 2465 capacitors etc.

All voltages measured in the scope with everything hooked up. I just got
done checking the voltages pre-regulator.

At J233A I get:
pin 5 (87v unregulated) 97.6v
Pin 3 (42v unregulated) 49.0v

And at J234A I get:
pin 5 (15v unregulated) 18.94v
Pin 4 (5v unregulated) 6.64
Pin 2 (-5v unregulated) -6.92v
Pin 1 (-15v unregulated) -18.54v

Based on the unregulated values, I believe the issue must lie in the
regulator portions, and since I'm getting the 97.6v pre-regulated it would
seem to me the voltage doubler is working. I also double checked and
verified the 10v reference voltage at the test point on the main board as
well as the J121 connector that feeds the main board. (I figured this was
important to double check as the regulators all are based off the 10v
reference as I read it). Of course this is where I wish to learn more - I
understand what the individual components do but I still somewhat struggle
to see what the assembly as a whole is doing, which is why I'm trying to get
into all this, so maybe my understanding is skewed. :)

At any rate, it would seem to me that with the unregulated voltages where
they're at, that the issue would have to lie within the follow area:


Yes? No? I mean I have the correct voltages on the left side of those three
groups, and the wrong voltages on the right side, so it would seem that's
where I should look next?

Thanks for helping this newb out!

Chris


--- In TekScopes@..., "Tom Miller" <tmiller11147@> wrote:

The +87 is from a doubler and added to the +42. I would take a close look

at the three 10 uF caps and all the diodes in that area. Be sure to only use
fast diodes for any replacements.

Since you recapped this unit, don't overlook the fact that bad capacitors

can come from the factory new. Did you measure all these voltages in the
scope or on an external load?

Regards,
Tom

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

Yahoo! Groups Links

> >
> > Another I have studied is an old Fairchild that is in good condition that
> has a resistance of 640 ohms end to end on the center conductor. To what use
> could one put to such a probe?
> >
>

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" <priya_jakatdar@> 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.
> >
>

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

From: Don Black

To: TekScopes@...

Sent: Monday, February 18, 2013 10:35 PM

Subject: Re: [TekScopes] T935A seized pots help

?

I have a T912 which had tight controls on the brightness and something else (focus?). I managed to free them up a bit but they need more TLC. These pots are located back inside the scope with plastic shafts running from the knobs to the pots. I'm not sure if it's the pots are tight or the shafts in the front panel bushings. These bushes release with a snap action by pulling the shaft forward; it's detailed in the manual which warns considerable force may be required. I'm worried about aging plastic breaking if I try to remove them. I intend to try some lubricant as a first step. I hadn't considered deoxit, have you found that effective on plastic? I was going to try isopropyl alcohol first, then perhaps some oil. I've had success with silicon lubricant on plastic if you can get it into the bearing. Are the pots you're dealing with mounted on the front panel or remotely like mine. Please keep posting on your progress. I have also considered the plastic might have swollen slightly, any thoughts?
The T912 is a storage scope and a in a brief trial I couldn't seem to get it to store, but I may not have been holding my mouth right. Have to read the instructions again. Nice little scope.

Don Black.

On 19-Feb-13 8:11 AM, Tom Miller wrote:

?

I would try to put a little thin oil down the shaft. Take the knobs off and just apply a drop or two to the shaft and allow it to soak in for a while. It may take a few applications to break it free.

?

?

Tom

?

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

From: Andrew

To: TekScopes@...

Sent: Monday, February 18, 2013 3:44 PM

Subject: [TekScopes] T935A seized pots help

?

Hi,

First posting on this group, hope someone can help....

I have a T935A scope which has developed a problem and I'd like to try and fix it as its a very useful portable scope.

Basically two of the front panel variable pots, the Y2 axis variable and the X axis variable have seized solid. Y1 seems okay.

So before I strip the scope down, which looks to be quite a job to get the relevant pcbs out, has anybody come across this before?

Is there much hope that I can fix the existing pots or do they need to be replaced (any suggestions for good suppliers that will ship to the UK?).

Many thanks,

Andrew

?

I would try to put a little thin oil down the shaft. Take the knobs off and just apply a drop or two to the shaft and allow it to soak in for a while. It may take a few applications to break it free.

?

?

Tom

?

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

From: Andrew

To: TekScopes@...

Sent: Monday, February 18, 2013 3:44 PM

Subject: [TekScopes] T935A seized pots help

?

Hi,

First posting on this group, hope someone can help....

I have a T935A scope which has developed a problem and I'd like to try and fix it as its a very useful portable scope.

Basically two of the front panel variable pots, the Y2 axis variable and the X axis variable have seized solid. Y1 seems okay.

So before I strip the scope down, which looks to be quite a job to get the relevant pcbs out, has anybody come across this before?

Is there much hope that I can fix the existing pots or do they need to be replaced (any suggestions for good suppliers that will ship to the UK?).

Many thanks,

Andrew