Topics

Another scope 7854

Joe Laffey
 

Hello all,

Well, I am now the proud owner of another scope I don't need... a Tek 7854.

It came with a 7A18 (70MHz dual input) and a 7B53A (100MHz dual time base).

The 7A18 has some issues. Channel 1 seems fine, but Channel 2 has a lot of noise on it (need to check the exact value, but the line is about 4 times thicker than Channel 1). It also seem to have all of its attenuator values off by around one click of the switch (not exactly). All waveforms have much greater vertical amplitude then they should. The readout display is correct based on the switch position.

I tried cleaning all the basic switches and pots, but I have not disassembled the cam based switches on the attenuator controls. I was wondering what special tricks there were to dealing with those. I am guessing one of the contacts is dirty/bad.

I will try to use the Open/Closed table for the attenuator and the amount of increase vertical scale to determine which contacts are bad. I just need to sit down with the unit and the service manual and see what I can discern. Any info is appreciated.

The scope also seems to have trouble triggering cleanly off Channel 2. I will occasionally see a glitch waveform. I do not see this on Channel 1. Also the glitches go away if I toggle the Channel 2 polarity switch to invert mode. They also go away if I use DC Offset (Option 06) mode on the 7A18.

I am guessing I should start with the cam switches, but do those trigger glitch criteria ring any bells with anyone?

Also, what is the fastest setup I can get with this 7854? The 7A24 with its 50 Ohm 400MHz inputs? And the 7B85 ? Just trying to figure out the ideal plugins to look out for. I'd like this to be my fast scope, possibly supplanting the 2465 (300MHz).

What happens if you try to run the 1GHz plugins in the 7854? Are they just limited to 400MHz or what?

This thing is pretty amazing considering the date of manufacture. Of course it was over $30k in today's dollars...

Thanks,

--
73
Joe Laffey
The Stable
Visual Effects
http://TheStable.tv/?e40763M/

John Griessen
 

On 05/03/2017 07:50 PM, Joe Laffey joe@... [TekScopes] wrote:
What happens if you try to run the 1GHz plugins in the 7854? Are they just
limited to 400MHz or what?
I think they are limited to 100MHz+.

For fast try a 7904A, and then of course, a 7104.

David DiGiacomo
 

On Wed, May 3, 2017 at 7:20 PM, John Griessen <@jgriessen> wrote:
On 05/03/2017 07:50 PM, Joe Laffey joe@... [TekScopes] wrote:
What happens if you try to run the 1GHz plugins in the 7854? Are they just
limited to 400MHz or what?
I think they are limited to 100MHz+.

Why would you say that? The 7854 is a 400MHz mainframe.

John Griessen
 

On 05/03/2017 08:24 PM, David DiGiacomo telists@... [TekScopes] wrote:
Why would you say that?

Forgot. I use mine in store mode where there IS a 100MHz sampling speed that limits one shot events.

Thanks for correcting. It's one of 3 that are above my bench all the time: R7844, 2230, 7854

ArtekManuals
 

JOE

The 1GHz plug-ins will be just fine in the 7854 up to 400MHz. Yes you are limited by the 7854 but you might find it is good to 450MHz or even 600MHz with reduced sensitivity , dimmer waveform etc.

Dave

manuals@...

On 5/3/2017 8:50 PM, Joe Laffey joe@... [TekScopes] wrote:

What happens if you try to run the 1GHz plugins in the 7854? Are they just
limited to 400MHz or what?
--
Dave
Manuals@...
www.ArtekManuals.com

 

what is the fastest setup I can get with this 7854? The 7A24 with its 50 Ohm 400MHz inputs?
The 7854 is specified to have 400 MHz vertical bandwidth (BW) when using either a 7A19 or 7A29 plug-in amplifier.
The 7A24 amplifier is specified to have 400 MHz BW when used in a 7104 mainframe. The 7104 is specified as quite a bit faster (1 GHz with 7A29) than the 7854. It is therefore to be expected that the 7854 / 7A24 combination may have trouble reaching the 400 MHz mark if you ignore the fact that actual BW of mainframes and plug-ins usually is significantly higher than the specification.

And the 7B85 ?
The 7854 has a specification of 500 ps/div. maximum horizontally. The 7B85 is specified at only 1 ns/div. max, when using the 10x magnifier and so would be the limiting factor.

Just trying to figure out the ideal plugins to look out for.
To fully utilise the horizontal and vertical speeds that the 7854 is capable of, a 7A19 (or 7A29) amplifier and a 7B92(A) would be a very nice setup. Be aware that the 7A19 and 7A29 are single-channel and that the 7B92(A) is a dual time base with delay.
With a 7B92(A) you'd have one horizontal slot unused. A 7D15 counter/timer would be nice to have real-time frequency and timing info.

What happens if you try to run the 1GHz plugins in the 7854? Are they just limited to 400MHz or what?
See above. It's not a matter of "trying to run": It just runs at at least 400 MHz BW without a problem, as described above.

If you like cheating:

The fastest setup by far would use a sampling set (7S12 or 7S11 with 7T11A). Top BW would be around 14 GHz, depending on sampler. That would only work for repetitive signals though.
Although a sampling setup only needs a very slow 'scope (a few MHz would do nicely), the storage and processing capabilities make for a very nice set and beautiful screen traces.

Raymond

Joe Laffey
 

Here's a question for somebody with a 7854:

When I use one of the Average functions to acquire an average waveform the readout display characters are kind of pixelated with random noise (like you overlaid random "off" pixels on top of the characters) during the acquisition. Once all the waveforms have been acquired and averaged the readout appears normal.

From the manual it seems like this should not happen and I should see a count of waveforms acquired. I can almost see this, but it is really rough looking. Again, once the waveform is acquired it looks fine.

Thanks.

--
Joe Laffey


On May 3, 2017, at 8:29 PM, John Griessen @jgriessen [TekScopes] <TekScopes@...> wrote:

On 05/03/2017 08:24 PM, David DiGiacomo telists@... [TekScopes] wrote:
Why would you say that?
Forgot. I use mine in store mode where there IS a 100MHz sampling speed that limits one shot events.

Thanks for correcting. It's one of 3 that are above my bench all the time: R7844, 2230, 7854

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

 

Joe,
The behaviour you describe is normal for the 7854, including the garbled number indicating the acquisition count while averaging.

Raymond

Joe Laffey
 

On May 3, 2017, at 8:44 PM, @Raymond [TekScopes]
To fully utilise the horizontal and vertical speeds that the 7854 is capable of, a 7A19 (or 7A29) amplifier and a 7B92(A) would be a very nice setup. Be aware that the 7A19 and 7A29 are single-channel and that the 7B92(A) is a dual time base with delay.
With a 7B92(A) you'd have one horizontal slot unused. A 7D15 counter/timer would be nice to have real-time frequency and timing info.
Thanks for the info. Just because I am new to these dual trace mainframes... would you be able to use two 7A19 amplifiers with one 7A29 time base?

Could I use 7A19 and the 7A18 that came with it, along with a time base and get three channels, albeit limited by the bandwidth of the 7A18?

Thanks.
--
Joe Laffey

 

On Wed, 3 May 2017 19:50:55 -0500 (CDT), you wrote:

The 7A18 has some issues. Channel 1 seems fine, but Channel 2 has a lot of
noise on it (need to check the exact value, but the line is about 4 times
thicker than Channel 1). It also seem to have all of its attenuator values
off by around one click of the switch (not exactly). All waveforms
have much greater vertical amplitude then they should. The readout display
is correct based on the switch position.
Sounds like some of the cam switches are not making complete contact
but also check the attenuator modules. They are socketted and
sometimes need to be reseated. In some cases, the connections between
the exposed pins and the hybrid inside can go open; if this is the
case, they can be resoldered with silver solder after removing the
plastic top from the module which just pops off.

I tried cleaning all the basic switches and pots, but I have not
disassembled the cam based switches on the attenuator controls. I was
wondering what special tricks there were to dealing with those. I am
guessing one of the contacts is dirty/bad.
Clean the cam switches by drawing isopropyl alcohol soaked clay free
paper or card stock through the closed switch; I like to use business
cards or 3x5 card stock. Vellum paper would also work well. It is
important to avoid common paper which has clay because the clay will
abrade the gold plating on the switches.

It is possible that some of the switches are not making good contact.
Tweezers can be used to bend the switch element where it meets the
leaf spring so that harder contact is made.

Do not lubricate these switch contacts.

Check the attenuator modules (under the aluminum covers) before
messing with the cam switches.

The scope also seems to have trouble triggering cleanly off Channel 2. I
will occasionally see a glitch waveform. I do not see this on Channel 1.
Also the glitches go away if I toggle the Channel 2 polarity switch to
invert mode. They also go away if I use DC Offset (Option 06) mode on the
7A18.

I am guessing I should start with the cam switches, but do those trigger
glitch criteria ring any bells with anyone?
The socketted transistors and ICs inside the 7A18 should be reseated.
The trigger source switching is done electronically but maybe the
mechanical wafer switch is dirty and needs to be cleaned.

Note that all of the above applies to the 7A26 as well.

Also, what is the fastest setup I can get with this 7854? The 7A24 with
its 50 Ohm 400MHz inputs? And the 7B85 ? Just trying to figure out the
ideal plugins to look out for. I'd like this to be my fast scope, possibly
supplanting the 2465 (300MHz).
It is not quite that simple. The 200MHz 7A26 is only 180MHz in the
7854 and almost all of the faster high input impedance vertical
amplifiers are 50 ohms only. So in a practical sense, your 2465 is
usually 300MHz while the 7854 is limited to 180MHz when used with
common x10 high impedance passive probes.

On the other hand, high impedance passive probes are anything but high
impedance at high frequencies where low-z probes or FET probes should
be used. But the dual trace 7A24 is only 300MHz in the 7854 which is
no faster than your 2465.

The recommended timebases are the 7B85 and 7B87 and operate up to
1ns/div. The 7B92A dual timebase can operate at up to 500ps/div. In
theory the 7B15 could replace the 7B85 to get 500ps/div capability
while keeping the 7B87.

The 7B87 timebase allows low sample rate single shot pretrigger
capability. It replaces the 7B80 which would normally be used but the
7B80 works fine also.

What happens if you try to run the 1GHz plugins in the 7854? Are they just
limited to 400MHz or what?
The 1984 Tektronix catalog has a table showing the vertical bandwidth
for different combinations of vertical amplifier and mainframe. For
the 7854:

7A11 200MHz Low Capacitance FET Single Trace
7A13 95MHz 1M Differential Comparator
7A16A 200MHz 1M Single Trace 20MHz Bandwidth Limit
7A18A 75MHz 1M Dual Trace
7A22 1MHz 1M Differential Amplifier 10uV/div
7A26 180MHz 1M Dual Trace 20MHz Bandwidth Limit
7A42 275MHz 1M Quad Trace Double Wide Logic Triggering

7A24 300MHz 50 ohms Dual Trace
7A19 400MHz 50 ohms Single Trace
7A29 400MHz 50 ohms Single Trace

The best general purpose vertical amplifier is the 7A26 because of its
dual channels, high bandwidth, high impedance inputs, and 20 MHz
bandwidth limit. The 7A29 is preferred to the 7A19 because it has
better input protection and is a better design.

This thing is pretty amazing considering the date of manufacture. Of
course it was over $30k in today's dollars...
Add in the cost of the vertical amplifiers and timebases and it is
more like $40k in today's dollars.

 

On Wed, 3 May 2017 22:04:32 -0500, you wrote:

Thanks for the info. Just because I am new to these dual trace mainframes... would you be able to use two 7A19 amplifiers with one 7A29 time base?
I think you mean 7B92A timebase.

Could I use 7A19 and the 7A18 that came with it, along with a time base and get three channels, albeit limited by the bandwidth of the 7A18?
This would absolutely work. I commonly use a 7A13 single trace and
7A26 dual trace like this.

 

On Wed, 3 May 2017 20:29:48 -0500, you wrote:

On 05/03/2017 08:24 PM, David DiGiacomo telists@... [TekScopes] wrote:
Why would you say that?
Forgot. I use mine in store mode where there IS a 100MHz sampling speed that limits one shot events.

Thanks for correcting. It's one of 3 that are above my bench all the time: R7844, 2230, 7854
The sample rate is more like 400kHz with the 7B87.

 

That is normal. The processor has better things to do during
aquisition than maintain the storage display and readout.

On Wed, 3 May 2017 20:46:19 -0500, you wrote:

Here's a question for somebody with a 7854:

When I use one of the Average functions to acquire an average waveform the readout display characters are kind of pixelated with random noise (like you overlaid random "off" pixels on top of the characters) during the acquisition. Once all the waveforms have been acquired and averaged the readout appears normal.

From the manual it seems like this should not happen and I should see a count of waveforms acquired. I can almost see this, but it is really rough looking. Again, once the waveform is acquired it looks fine.

Thanks.

Joe Laffey
 

On May 3, 2017, at 10:43 PM, David @DWH [TekScopes] <TekScopes@...> wrote:

On Wed, 3 May 2017 19:50:55 -0500 (CDT), you wrote:

The 7A18 has some issues. Channel 1 seems fine, but Channel 2 has a lot of
noise on it (need to check the exact value, but the line is about 4 times
thicker than Channel 1). It also seem to have all of its attenuator values
off by around one click of the switch (not exactly). All waveforms
have much greater vertical amplitude then they should. The readout display
is correct based on the switch position.
Sounds like some of the cam switches are not making complete contact
but also check the attenuator modules. They are socketted and
sometimes need to be reseated. In some cases, the connections between
the exposed pins and the hybrid inside can go open; if this is the
case, they can be resoldered with silver solder after removing the
plastic top from the module which just pops off.
So by attenuators modules you mean the little plastic boxes under the aluminum shielding? Each box has two trim caps?? I just pull those up and re-seat them? Any trick or special tool required other than patience and care?

I already cleaned and aligned the cam switches. They seem to be working better, but I still have an overall gain issue on ch2 (about 5X too much gain on all volts/div settings). I turned down the gain setting using the push in with a screwdriver method on the variable gain on the front of the unit. That did not go low enough. I checked the resistors around the gain pots and they check out in spec. That push in gain pot was going down to about 1 Ohm.

Perhaps this is caused one of those modules.

I will reseat the ICs. I don't think I see any socketed transistors.

It is not quite that simple. The 200MHz 7A26 is only 180MHz in the
7854 and almost all of the faster high input impedance vertical
amplifiers are 50 ohms only. So in a practical sense, your 2465 is
usually 300MHz while the 7854 is limited to 180MHz when used with
common x10 high impedance passive probes.

On the other hand, high impedance passive probes are anything but high
impedance at high frequencies where low-z probes or FET probes should
be used. But the dual trace 7A24 is only 300MHz in the 7854 which is
no faster than your 2465.

Interesting indeed. That 2465 family is a great bang for the buck. I was mainly trying to justify buying (albeit cheaply, locally, and with a real nice Tek cart included) the 7854. The GPIB might be interesting to mess with, and there are other features to justify it.

Thanks for the compatibility info as well. This is all great data.

--
73
Joe Laffey

 

On Thu, 4 May 2017 02:10:45 -0500, you wrote:

So by attenuators modules you mean the little plastic boxes under the aluminum shielding? Each box has two trim caps?? I just pull those up and re-seat them? Any trick or special tool required other than patience and care?
Those are them. They are mounted just like big 4 pin ICs.

I already cleaned and aligned the cam switches. They seem to be working better, but I still have an overall gain issue on ch2 (about 5X too much gain on all volts/div settings). I turned down the gain setting using the push in with a screwdriver method on the variable gain on the front of the unit. That did not go low enough. I checked the resistors around the gain pots and they check out in spec. That push in gain pot was going down to about 1 Ohm.

Perhaps this is caused one of those modules.
5 times too much gain at all volt/div settings? No, that is some
other problem and an odd one. The frequency response should be
screwed up also.

A shorted C375 shown on schematic 4 would cause about that much high
gain. A shorted C445 shown on schematic 3 could cause it if R445 is
adjusted to a low value.

Comparing the DC levels at different points between channel 1 and
channel 2 should help find the problem.

Interesting indeed. That 2465 family is a great bang for the buck. I was mainly trying to justify buying (albeit cheaply, locally, and with a real nice Tek cart included) the 7854. The GPIB might be interesting to mess with, and there are other features to justify it.
The averaging capability is very nice in some applications. Except
for that, the 7854 is a slightly slower 7904 to most users.

Cliff Carrie
 

I had a 7A26 with intermittent vertical instability in channel 2 only. I found that alongside the attenuator plug-in modules, there are discrete 1/8 watt resistors. They are also plugged in and one had developed high contact resistance. I replugged it about 4 times after using a q-tip to swab its leads with contact cleaner, being careful not to get it anywhere except on the ends of the leads. The problem never came back.


Cliff Carrie

________________________________
From: TekScopes@... <TekScopes@...> on behalf of David @DWH [TekScopes] <TekScopes@...>
Sent: Thursday, May 4, 2017 7:22:26 AM
To: TekScopes@...
Subject: Re: [TekScopes] Another scope 7854



On Thu, 4 May 2017 02:10:45 -0500, you wrote:

So by attenuators modules you mean the little plastic boxes under the aluminum shielding? Each box has two trim caps?? I just pull those up and re-seat them? Any trick or special tool required other than patience and care?
Those are them. They are mounted just like big 4 pin ICs.

I already cleaned and aligned the cam switches. They seem to be working better, but I still have an overall gain issue on ch2 (about 5X too much gain on all volts/div settings). I turned down the gain setting using the push in with a screwdriver method on the variable gain on the front of the unit. That did not go low enough. I checked the resistors around the gain pots and they check out in spec. That push in gain pot was going down to about 1 Ohm.

Perhaps this is caused one of those modules.
5 times too much gain at all volt/div settings? No, that is some
other problem and an odd one. The frequency response should be
screwed up also.

A shorted C375 shown on schematic 4 would cause about that much high
gain. A shorted C445 shown on schematic 3 could cause it if R445 is
adjusted to a low value.

Comparing the DC levels at different points between channel 1 and
channel 2 should help find the problem.

Interesting indeed. That 2465 family is a great bang for the buck. I was mainly trying to justify buying (albeit cheaply, locally, and with a real nice Tek cart included) the 7854. The GPIB might be interesting to mess with, and there are other features to justify it.
The averaging capability is very nice in some applications. Except
for that, the 7854 is a slightly slower 7904 to most users.




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

Joe Laffey
 

yOn Thu, 4 May 2017, David @DWH [TekScopes] wrote:


A shorted C375 shown on schematic 4 would cause about that much high
gain. A shorted C445 shown on schematic 3 could cause it if R445 is
adjusted to a low value.
Wow! I want to be you when I grow up! C375 was indeed shorted. I understand why this affects the gain, and would be able to follow it better if it were a modern OpAmp rather than discreet transistors (I can't visualize those as well yet).

I got a suitable replacement from my local store and will swap it out later today.

The occasional triggering glitch I think must by the cam finger switches on the AC/DC/GND/DC Offset switch. I got them to go away on channel 1 through exercising the switch, but channel 2 still has this glitch. I need to figure out the best way to physically access these to clean them with as little disassembling as possible. Will have a look at that too.

Thanks so much for the help.

Also, in addition to the averaging the 7854 does have the cursors and measurements built-in over the 7904. They are actually relatively easy to use, about the same as my 2232, though having a knob for the cursors is nicer than the buttons.

--
73
Joe Laffey
The Stable
Visual Effects
http://TheStable.tv/?e40768M/

Joe Laffey
 

The new C375 fixed the gain issues. Thanks again.

I am still having these triggering glitches.

Test waveform is a sine wave. Frequency doesn't seem to matter much.

Triggering off Ch1 is working fine.

Triggering off Ch2 has the occasional glitch.

Trigger off "Mode" (in between Ch1 and Ch2) also has the glitches

Trigger off external trigger is fine.

When the glitch happens I see for a split second an additional waveform out of phase, but the same amplitude.

When I have Ch2 inverted the glitches are much less frequent.

When I set the coupling to DC offset the glitches go away. They are present on AC and DC Coupling.

If I use variable gain on Ch2 and reduce the gain to around 50% or less the issue goes away. (something clipping?)

Trigger slope has no impact.

High frequency reject triggering fixes the issue as well.

I cleaned the trigger source wafer switch, and it seems to be OK with a meter.

I cleaned the cam switches for both volts/div and coupling mode. This did not seem to make much difference. Also, it looks like if this were the problem then D.C. Offset would have the issue as well.

I twiddled R521-- DC balance for Ch2-- back and forth to no avail


I will keep digging around... any insights appreciated.

Thanks.

--
Joe Laffey

Joe Laffey
 

When this glitch occurs I noticed that the glitch waveform that appears joy for an instant looks like the triggering has the trigger slope inverted from the main waveform.

Cleaning and exercising the slope switch on the time base has no effect (which makes sense since it happens only with channel 2. Actually I will very very occasionally see something with channel 1 as well.

Still searching... I tried some percussive troubleshooting with a chopstick, but couldn't find anyplace where tapping made it better or worse.


--
Joe Laffey

On May 4, 2017, at 9:14 PM, Joe Laffey joe@... [TekScopes] <TekScopes@...> wrote:

The new C375 fixed the gain issues. Thanks again.

I am still having these triggering glitches.

Test waveform is a sine wave. Frequency doesn't seem to matter much.

Triggering off Ch1 is working fine.

Triggering off Ch2 has the occasional glitch.

Trigger off "Mode" (in between Ch1 and Ch2) also has the glitches

Trigger off external trigger is fine.

When the glitch happens I see for a split second an additional waveform out of phase, but the same amplitude.

When I have Ch2 inverted the glitches are much less frequent.

When I set the coupling to DC offset the glitches go away. They are present on AC and DC Coupling.

If I use variable gain on Ch2 and reduce the gain to around 50% or less the issue goes away. (something clipping?)

Trigger slope has no impact.

High frequency reject triggering fixes the issue as well.

I cleaned the trigger source wafer switch, and it seems to be OK with a meter.

I cleaned the cam switches for both volts/div and coupling mode. This did not seem to make much difference. Also, it looks like if this were the problem then D.C. Offset would have the issue as well.

I twiddled R521-- DC balance for Ch2-- back and forth to no avail

I will keep digging around... any insights appreciated.

Thanks.

--
Joe Laffey


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

 

You reminded me that I know of another failure in these vertical
plug-ins. Some of the early ones have a heavy gold plating on the
printed circuit board traces and the tin-lead solder can eventually
suffer from gold embrittlement breaking the solder connection.

If this happens, resoldering the connection will reveal what I suspect
is Purple of Cassius under the solder.

<https://en.wikipedia.org/wiki/Purple_of_Cassius>

The solution I found was to use a soldering iron on low heat to
repeatedly add and remove solder until the Purple of Cassius and gold
is removed. The tin in the solder dissolves the gold until the base
metal is reached. This is actually the recommended soldering
technique for heavy gold plating if tin solder is used.

Modern flash gold plating is deliberately thin enough to completely
dissolve into the solder preventing this problem.

On Thu, 4 May 2017 19:23:10 +0000, you wrote:

I had a 7A26 with intermittent vertical instability in channel 2 only. I found that alongside the attenuator plug-in modules, there are discrete 1/8 watt resistors. They are also plugged in and one had developed high contact resistance. I replugged it about 4 times after using a q-tip to swab its leads with contact cleaner, being careful not to get it anywhere except on the ends of the leads. The problem never came back.

Cliff Carrie