Date   
Re: 7904 problem

 

Hi David,
There are several reasons the 7B92 is superior to the 7B71.
1) The 7B71 was designed to be used in conjunction with the 7B70 to provide
delayed sweep capability. The 7B92 (and 7B92A) does this in one plugin (with
a few restrictions). This frees up a slot for other uses.
2) The 7B92 has superior triggering capability compared to the 7B70 and
7B71. It will trigger up to and beyond 500MHz which is desirable in a 500MHz
scope.
3) The 7B92 (and 7B92A) have faster sweep speeds which come in handy when
measuring the kind of fast signals the 7904 can display.
Dennis Tillman W7PF

-----Original Message-----
Sent: Saturday, September 17, 2016 11:03 AM
Subject: Re: [TekScopes] 7904 problem

Is there really that big of a difference? I have never used a 7B70 or 7B71
(or 7B50 or 7B51) (I guess none of these are that common which makes sense
since they were produced for a shorter time then their replacements) but
except where the delay line length is an issue, I rarely see any difference
between a 7B53A and 7B92A except of course where the fastest sweep speeds
are needed.

I like the 7B92A more for its alternate delayed sweep and I prefer the
7B80/7B85 for their automatic peak-to-peak triggering and delta delayed
sweep. In both cases, their higher frequency triggering and fastest sweep
speeds are of secondary importance.

On Fri, 16 Sep 2016 22:10:43 -0700, you wrote:

Assuming that you will eventually get this scope working I would like
to suggest you buy a timebase that is a better match to the 500+MHz
capability of your 7904. The 7B92A would be ideal. The 7B71 is a very
old plugin and not great at triggering or capable of triggering at the
higher frequencies the 7904 can display.

Dennis Tillman W7PF
------------------------------------
Posted by: David <@DWH>
------------------------------------

Re: Tektronix 1240 / 1241 innards to give away

 

Hi Marian, I sent you a message via PM.
Since I live in NL, shipment seems affordable.
Kind regards,


Raymond

Tektronix 1240 / 1241 innards to give away

Marian B
 

Hi

got most innards of said system to give away for someone who needs 'em.
Including power supply, CRT drive board (suction cup torn), card cage
main board, and cards.
Not for sale are the front panel boards (incl. IR screen boards).

Working condition: Unknown. When turned on, no display + burning smell.
No visible burns. Likely a CRT drive board or PSU failure. It would seem
that no repair was attempted.

Price: Shipping costs from Germany. If you want all boards this can be a
bit costly, depending on target country, since they're heavy, maybe 5
kilos total.

Cheers, Marian

Re: 7904 problem

 

Is there really that big of a difference? I have never used a 7B70 or
7B71 (or 7B50 or 7B51) (I guess none of these are that common which
makes sense since they were produced for a shorter time then their
replacements) but except where the delay line length is an issue, I
rarely see any difference between a 7B53A and 7B92A except of course
where the fastest sweep speeds are needed.

I like the 7B92A more for its alternate delayed sweep and I prefer the
7B80/7B85 for their automatic peak-to-peak triggering and delta
delayed sweep. In both cases, their higher frequency triggering and
fastest sweep speeds are of secondary importance.

On Fri, 16 Sep 2016 22:10:43 -0700, you wrote:

...

Assuming that you will eventually get this scope working I would like to
suggest you buy a timebase that is a better match to the 500+MHz capability
of your 7904. The 7B92A would be ideal. The 7B71 is a very old plugin and
not great at triggering or capable of triggering at the higher frequencies
the 7904 can display.

Dennis Tillman W7PF

Re: Tek 2465 attenuator rebuild

Daniel Kolbach
 

Good tip, thank very much.

Re: End of the road, scrapping 7000 scopes

Craig Sawyers <c.sawyers@...>
 

Craig, when did you give the scopes away?
About a year ago

Re: End of the road, scrapping 7000 scopes

bobworsley92
 

Hi, a few items seem to be going.

Am sorting out some rack mount scopes for my use, they seem to be in flakier condition than the cabinet version, possibly many more hours on?

The only MCP CRT I have is a 2467 in a parts mule so no way to test.

No one want any tunnel diodes? Just found that the ones in the HP 1821 timebase are actually plugged in. Need to fix the curve tracer and check them, but the timebase triggering says they are working.

Craig, when did you give the scopes away? The world has changed in the last 3 or 4 years, I can't give away working scopes, only 20MHz but perfectly good to start with. I had a complete 7704A with plug ins at auction and didn't get a start bid of £10.

What I would say is, if you think you want to keep some spares then now is the time to do it. None of these scopes will exist in 10 years. I haven't seen one for sale commercially for at least 2 or 3 years. Gone!

[Non-text portions of this message have been removed]

Re: Tek 2465 attenuator rebuild

Roger Evans
 

I did a small repair to an attenuator on a 2440, it had received an impact to the BNC connector and had a broken wire from the connector to the ceramic substrate so a very easy fix. There are detailed instructions, either here or in the manual on how to remove the attenuator module. On the 2440 ch1 and ch2 attenuators have different part numbers but can be swapped easily, the difference is on which side the probe detection wires emerge and you can very easily reroute them once the lid is off.

Roger

Re: 7904 problem

simoniep
 

I am reading a document found on Tekwiki about 7904 power supply servicing. Hope it will aid.

http://w140.com/Tek_7904_Power_Supply.pdf http://w140.com/Tek_7904_Power_Supply.pdf

Re: 7904 problem

 

On Sat, 17 Sep 2016 07:45:07 +0100, you wrote:

At power up, there is a very bright unfocused spot on the screen with absolutely no reaction at
any
front panel command.
I only saw this once on my 7904. I had set up a sampling set up (7T11, 7S11 and either an S4 or
S6). Fortunately I was watching the screen at the time, when suddenly bam! A maximum brightness
spot bang in the middle of the screen. I hit the power switch FAST.

After trying other plugins, and confirming that it wasn't the 7904, I eventually found a shorted
tubular tantalum (it was a small black one with a taper on one end) in the sampling head. I forget
which line it was taking down, but whichever it was failed to trip the protect circuits in the power
supply.

Craig
The linear post regulators have foldback current limiting and that was
the protection circuit which got tripped. The switching regulator
only sees the preregulated outputs.

Re: 7904 problem

Craig Sawyers <c.sawyers@...>
 

At power up, there is a very bright unfocused spot on the screen with absolutely no reaction at
any
front panel command.
I only saw this once on my 7904. I had set up a sampling set up (7T11, 7S11 and either an S4 or
S6). Fortunately I was watching the screen at the time, when suddenly bam! A maximum brightness
spot bang in the middle of the screen. I hit the power switch FAST.

After trying other plugins, and confirming that it wasn't the 7904, I eventually found a shorted
tubular tantalum (it was a small black one with a taper on one end) in the sampling head. I forget
which line it was taking down, but whichever it was failed to trip the protect circuits in the power
supply.

Craig

Re: 7904 problem

 

Hello Pascal,

It is always wise to read the Theory of Operation in the Service Manual
before trying to troubleshoot a Tek product. It sounds like you have done
that.

Next I would check all the power supply voltages against what the manual
says they should be.

I would pay particular attention to the beam (Z-Axis) blanking and to the
HV.

Two things you can do without any test equipment are
1) Try turning off the lights in the room so it is completely dark. Then
press the beam finder to see if you see any sign of brightness on the CRT.
2) If there is none then try to see inside the neck of the CRT back by the
socket to determine if the filament is glowing. If there is no filament glow
then the CRT might be bad.

Assuming that you will eventually get this scope working I would like to
suggest you buy a timebase that is a better match to the 500+MHz capability
of your 7904. The 7B92A would be ideal. The 7B71 is a very old plugin and
not great at triggering or capable of triggering at the higher frequencies
the 7904 can display.

Dennis Tillman W7PF

-----Original Message-----
Sent: Friday, September 16, 2016 10:52 AM
Subject: Re: [TekScopes] 7904 problem

The intensity knob have no effect on brightness.

------------------------------------
Posted by: fcbtt3.nina@...
------------------------------------

Re: 7854 ROMS

bobh@joba.com
 

I am not sure about this and I am interested myself. I know some 7854 bin files are available on TekWiki.
Bob.


From: mailto:TekScopes@...
Sent: Thursday, September 15, 2016 3:49 PM
To: TekScopes@...
Subject: [TekScopes] 7854 ROMS


Does anyone know the highest revision level of the ROMS in a 7854?

[Non-text portions of this message have been removed]

Re: FBSOA or RBSOA?

Dave Wise
 

An update. I discovered another criterion. Quasi-saturation. Since the part is employed as a non-saturating switch, if the on voltage and current are in the quasi-saturation region, the part will be too slow. The only place to check this beforehand in the datasheet is the common emitter output characteristics, the Ic vs Vce family for a variety of base currents.

A good example is the 2SC4614. At 300mA, you have to push Vce over 5V before the transistor moves out of quasi-saturation region into the active region. At the normal on voltage of 1.5V, storage time is over half a microsecond. This will be true for most parts rated over 100V. So I have worry not only about my basement, but my ceiling!

Dave Wise
________________________________________

Re: Hello from newcomer Fabio Trevisan - My first Tek Scope 464 + DM44

Fabio Trevisan
 

Hi David, Just to give a feedback:
It's a long message and OF COURSE don't feel obliged to either to read it or answer... telling it to someone is my way to organize the ideas and - maybe - the outcome can be disclosed later and help someone.


On the DM44 issues, I installed the missing R1142, R1143 and R1147 on the A&B Timing Switch board, and now it can measure time on all horizontal scales.


Oh... And there's a TEK trick there...


On the 464 service manual "A&B Timing Switch Diagram 7", it shows the 3 resistors above as 2K, 1K and 1K respectively... and I naively installed them exactly as such... just to find out that the measurements were wrong in scale. (but at least, this time it was measuring in all horizontal time-bases, and not just on the 2s multiples).
Went to the Parts list of the 464, and it stated: Check DM44 manual for Resistor Values...!!!
Then, went to DM44 manual and, next to the DM44's parts list, there's an additional section describing all the values of all components that need to be added, removed, or changed values, for each scope model that the DM44 fits in...
Guess what, the correct values are 500R, 250R and 250R..!!! And not 2k, 1k and 1k as it was drawn on 464's diagram.


DM 44 is still sometimes crazy (less now than it was at the beginning), but I assume it's some bad contact somewhere which I will eventually find out.


Now, regarding the HV supply... I did some progress in making various checks but still didn't find the root cause for the drop of HV after the scope heats-up...


1. When the HV drops, cathode voltage drops to around 1000V, and then, in 20~30 seconds, it drops to 920V. when I shut it off.


2. Checked the Unregulated +15 V and it reads about 24V (just a tad higher than what shows in the schematics). The ripple is about 1Vpp while in normal operation and, after the HV drops, the ripple increased to almost 2Vpp until when I shut it off...
2.b. Still at the Unreg +15V, as the manual doesn't state how much is the acceptable ripple, I did the following... I added a brand new 6800uF cap in parallel (to the original which is nominal 5500uF +100% -10%)... and the ripple under normal condition dropped from 1Vpp to 0.5Vpp... I didn't care to look after the HV drops because it doesn't add anything to the conclusion.
So, IMHO, the original cap is performing roughly the same as the brand new 6800uF cap... In other words... C1487 is GOOD.


3. I don't have a temperature probe, but using my finger (with the scope off, and after discharging!), and while the HV is still normal... both the driving transistor and the HV transformer are hot, but not too hot... I can keep my finger on them, so they're about or below 70 Deg Celsius.
But when HV drops and I leave it on for about 20~30 secs which is as far as I went, both the transistor and the HV transformer becomes VERY Hot, to the point that I cannot keep my finger on one of the pins from the HV transformer (I can keep the finger on its plastic body though)


4. I don't have a reference stated on the manual, but the Post Acceleration anode is - while in normal operation - at 7.22kV. I didn't measure it yet after HV is down. I suppose that it will be down by the same amount (33% below the 7.22kV)...


4b. I measured the 5 resistors of 3M Ohms in the HV multiplier and they're all up by 10%, like 3.3M each.
It's not ideal, but I don't think it's the cause for the cathode HV drop, which is the main issue.


4c. I managed to find some ESJA53-12 diodes, which are 12KV, 5mA fast recovery diodes (actually better than the originals, at 100ns, while the originals are 250ns, only the forward voltage is worse, at 37V while the originals are about 25V and this doesn't seem to be a problem here).
I already tested them reverse biased at the first stage of the HV multiplier (3.61kV) and they're quite promising at 1.1nA (an indirect measure... I actually measured how much voltage it allowed to leak to a 10Mohm voltmeter, and it read 11mV!!!)
I will try to replace - first only CR1503 at the cathode - and see if it helps.
Specially this one, which is mounted right under the transformer and soldered very closely to its pins, if it's leaking too much, as it gets heated by the transformer, it may be causing the whole mess.


If replacing the diode doesn't help, however, I'm afraid I will need to start looking for an HV transformer, as I`m not seeing much more that can be ruled out...
The HV caps seem to be in rather good condition, and they're HUGE (I imagine they're very robust)...


Rgrds,


Fabio

Re: Tek 2465 attenuator rebuild

Siggi
 

On Fri, 16 Sep 2016 at 14:13 dkolbach@... [TekScopes] <
TekScopes@...> wrote:



I will give it a try, I have not even open it. My scope will not calibrate
the second channel, I was advised that the attenuator needed to be replaced
as there is no way to fix them.
What seems to be the problem? When you "will not calibrate" what does that
mean?
Note that you can diagnose some attenuator problems with a DMM, by simply
measuring the resistance from the center of the BNC connector to the
shield, as you run the vertical gain through the range. The resistance
should stay smack-on 1MOhm throughout.
When you then click in the 50Ohm termination, you should get near as 50Ohm
as makes no difference.

Re: 7904 problem

 

The high voltage side of the anode connector is fully enclosed so
assuming that the CRT is discharged, it is safe to leave disconnected.
I might put some tape over the end to keep contamination from getting
inside.

The CRT side can be discharged by touching the exposed metal pin to
the chassis.

On Sat, 17 Sep 2016 00:09:09 +0200, you wrote:

What I did was to disconnect the HV cable to the CRT. With extreme care and
thick rubber gloves.

Göran

Re: 7904 problem

Göran Krusell
 

What I did was to disconnect the HV cable to the CRT. With extreme care and
thick rubber gloves.
Göran


[Non-text portions of this message have been removed]

Re: 7904 problem

 

The +130 volt supply comes directly from the off-line power supply
which uses it for regulation and I see no fuse in the schematics
although there is a 10 ohm series resistor before the CRT circuits. If
the +130 volt output was completely missing, then the switching power
supply would shut down.

On 16 Sep 2016 13:22:58 -0700, you wrote:

From the discussion so far, it seems to me that the +130V supply may be gone. As I recall, there's a small fuse protecting it on the power supply regulator board. If plug-ins are installed and timebase set up for a proper free-running sweep, that should spread the beam out to a workable non-damaging level. If the sweep doesn't happen on-screen even though it should, then the 130V is probably out. In that case, I'd take David's suggestion and unplug the CRT socket or the anode connection to protect the CRT while servicing.

Ed

Re: 7904 problem

simoniep
 

With CRT disconnected, a tic tic is now audible and 2 neon lamps flash on the Z axis board.