HP Agilent 141T

[Luze]

Hello,
I have owned this oldie for some time now. Unfortunately, the device
has a defect. Now I'm hoping to find someone here, who might still
own such a device (the last post here about this Specki was
a while ago) or is familiar with it and can give me a few pointers.
Since double postings are undesirable in some forums, I just want to add,
that I have already posted about this topic in the RBF forum.
The situation is as follows:
The 141T has the IF8552B and the LF8556A as plug-in units.
There is no zero line written. Since I am not a specialist for
spectrum analyzers, I hope that this does not mean that there is an operating error on my part.

The following signals can be measured at the Vertical/out and Scan/in outputs:

Vertical out:
demanded: 100 mV
measured: 380 mV

Scan In/Out
required: -5V to +5V

Since I can't get the illuminated dot to be in the center of the screen and therefore don't know
where 0V is, I set the scan control (Man) to the left stop and took that as 0V.

Scan mode
man Left stop 4V (goes back to 3.8V when turning further,
then rises constantly)
Right stop 12V
single 11V stable (independent of the scan regulator)
exit 2V stable (shouldn't this be 0V, isn't there an input?)

Int 11V stable

(measured with the oscilloscope)

Since Scan Trigger was set to "auto", shouldn’t it have been deflected in the Scan
mode Int position? Since there the voltage is stable at 11V, I guess
the illuminated dot is probably outside the screen. With manual deflection, the deflection voltage (4V) is sufficient to get it on the screen.
With the oscilloscope, I would say that the sawtooth generator does not start.

Now, I have already been told that the best (and safest?) method of finding the failure,
would be to plug the IF slot into a working device to see if it works.
Unfortunately, I don't know anyone close by  who owns such a device. Are there any other
ways of localizing the fault (display unit or IF plug-in unit)? I would be grateful for any information.

I hope this text makes sense – my mother tongue is German and I translated as best as I can.

Best regards

Luze

[mag_therm]

(This subject would be better moved to "REPAIR" section)
Hi,Luze,    I have 141T/8553B/8552B made in about 1972
Present problem here maybe similar to yours,  started this winter when it has not been used for weeks, there is a trace position problem until warm up time of > 1 hour.
When cold, Trace is off screen to lower left; Beam Finder does not work; Manual Scan @ max clockwise  will move end of trace only to second division from left.
Trying it today after your post, it appears that the trace is not going to become correct after warm up.
However, the external oscilloscope method (below) indicates the the IF section is good.

EXTERNAL OSCILLOSCOPE connection for diagnosis:
With another oscilloscope with 10W by 8H screen, it is possible to accurately replicate the 141 display.
with (2) BNC cables, plug SCAN OUT to scope ext trigger; plug VERTICAL OUT to scope Y; set scope Y gain to 100mV/div  ; Timebase 50ms/div; TRIG: EXT

Set 141T : Video OFF; Mode INT; Trigger AUTO ; Bandwidth 30 kHz; Scan width 0.5 MHz/div.
These settings will allow easy initial setup of the oscilloscope, to view a -30dBm 14 MHz signal from generator.
 Keep Oscilloscope timebase always same as 141 SCAN TIME
Be aware that the 141T normally always shows the IF peak at trace start,far left of screen
 and the generator signal can be centralized with the FREQUENCY knob . Having a strong generator signal may help diagnose problem with trace below bottom of screen

I have had the 141 apart about 3 times to fix faults. ( I think some of my repairs are doc on EEV)
Need a large, strong workbench and up to 3 days of infinite patience, in my experience.
Get a full set of service manual however the problems I had were all in display section , and maybe this present one also.
Also note thst there are various High Voltage levels on display section with a max of 6 kV.

I hope you can initially get an oscilloscope connected as above to help diagnose your problem.
Good Luck

Edit: about 1 hour after this post, I moved the 141 and the trace became normal intermittently . Grabbed the side handle and  pressed the IF section firmly back into its cage ( the connectors are on rear.)
This has fixed my problem. Oh Lucky Day!
So a suggestion for Luze is to pull the IF section out and examine /clean the connectors

[factory]

The PSU section in the 140/141x series is a well known problem area, have you verified the voltages are present and correct?
Also what is the serial number prefix, this is the first set of numbers on the serial number plate, the 141T was in production for several decades and they updated the PSU board for later ones.

P.S. no idea why you put Agilent in the title, this was long discontinued before the stupid renaming.

David

[trackersoft]

I have two of these both working (after LOTS of work!). There is an excellent five part youtube series on repair and calibrate that includes how to use an oscilloscope if your display is bad.



Also, pull your plugins and make sure the fuses in the back of the plugins (I think there are 3 of them) have not blown.

[Luze]

Hello,

@ David:
The voltages on the 141T are correct.
The serial no. is: 1337A 0 7 4 8 3

@ trackersoft:
I checked the fuses right at the beginning because I knew about this error. They are ok.

@mag-therm.
It's true, the topic is better placed in "Repair". Maybe a moderator can move it there, unfortunately I don't have the ability.
I removed the plug-in units and cleaned and checked the contacts again. But nothing changed.
There was no change.

Now I wanted to do the check with the oscilloscope, as you have thankfully described so precisely. But there is a problem. I have the 8556A LF plug-in unit. I cannot set the bandwidth to 30 kHz (max. only 10 kHz) and the sampling width cannot be set to 0.5 MHz (max. 20 kHz).  What values should I set?

A question just so I understand correctly: At the Scan Out output, a DC voltage should change continuously between -5V and +5V in the INT mode setting, which triggers the oscilloscope. On my oscilloscope screen, regardless of whether I apply the Y signal or trigger signal or not, only the point passing through the zero line (because of the 50 ms/div) can be seen. There is no deflection in the vertical direction at all. But perhaps the sampling width and bandwidth should be set correctly first. What values should I choose?

Many thanks for your help so far.

With best regards

Claus

[factory]

OK 1337A is an older one, 1973 design revision, I have 1320A here which will be similar, later ones have a relay to disable the PSU with the modules removed & have the oscilloscope calibrator outputs removed.

Do you have anything on the CRT, from memory in "store" mode you should be able to adjust the persistence & get a background illumination.
Beware of using it in "conv" mode, as the storage mesh in the CRT will be damaged by excessive intensity, there are many warnings in the manual about this.

David

[mag_therm]

Hi Luze,
Based on your comments, I have amended the scope diagnostic check.
On the external ocsilloscope:
 Channel 1 ( upper trace) set to 5V per division  is plugged to 141T SCAN OUT
 Channel 2 ( lower trace) set to 200mV/div  is plugged to 141T VERTICAL OUT
Time /div is 100 msec /div, (exactly 2 * the 141T setting of 50ms/div) , so that both the 141T and external oscilloscope traces stay in sync
Trigger Source is set to Ch1, negative trigger on the SCAN OUT
Ch2 (being 141T Vertical out) is inverted, because the 141T IF section puts out a NEGATIVE signal 0 to -10V

These settings are better for diagnostic because we can now see more than one full scan, and the Output is aligned with the Scan.

I reduced the generator frequency to 2 MHz and set 141T BANDWIDTH 10 kHz and WIDTH 20 kHz /div to be similar to your RF section
That setting is not critical to just get a fat trace of the signal that is easy to find.

Here is a photo : https://app.box.com/s/slydji1vuzj7d23sxw18xixbn7mn685g

The SCAN OUT starts at -5V and slopes to +5V at the end of that slope there is a sharp negative edge back to -5V which is used to trigger the external scope.
The Vertical output ( being the 2MHz signal generator) is within the SCAN OUT slope, and the position can be adjusted using the 141T FREQUENCY control on the RF Section.
Note on the SCAN OUT there is a period of flat -5V before the slope starts. I think this is for the Horizontal position adjustment.
I will leave this set-up for a few days in case you need any more.
Regards

[Luze]

Hello,
 I apologize, but there is one thing I do not yet quite understand . You are giving a frequency of 2 MHz of a generator to the input of the upper plug-in unit (in my case the 8556A).
You seem to have a different plug-in unit, because on the photo I can see a set range of 0-11 MHz. My plug-in unit works max. up to 300 KHz! How can this work?
Or did I not understand correctly?

Best regards

[mag_therm]

Hi Luze,
 The 8553B RF section here ( upper module) has a range of 0 ~11 MHz and 0 ~ 110 MHz.
Sorry I did not look up the range of your RF section. Now I know it is 300 kHz max.
The RF Sections were interchangeable, so I expect the SCAN and VERTICAL signal given by IF Section to HP141T display will be similar for all RF Sections.

So do you have a signal generator capable of giving a signal in range of your RF section? eg 100 kHz or say, 250 kHz?
If so, connect it.
If not, maybe you can turn the gain up and just view noise.

Anyway, the IF section should give the SCAN sawtooth similar to as shown in my photo.
And the VERTICAL signal should be in range 0 to -10V depending on whatever input signal you can use.
Regards,

[Luze]

Hello,
I just tried to do the the measurement set up according to the last description and the picture. I noticed the following.
Channel 1 : Connection to Scan Out
Channel 2: Connection to Vertical Out
External trigger input of the oscilloscope: Connection to Scan Out. But this one is already connected to channel 1 ?
My oscilloscope has no trigger source switching channel 1/channel 2. I assume that with external triggering, both channels are triggered by the external signal.

As generator I could use a test generator for radio equipment. It delivers 200 kHz (6 Vpp). Not particularly clean (a bit of distortion), but it might work.

How high should the output voltage of the generator (200 kHz) in Vpp be?


With best regards

[mag_therm]

Hi Luze,
1) the test signal level:
The max RF input signal on the 8553B here is 1 volt
The termination (internal to the RF section) is 50 Ohm.
This info is printed near the RF input BNC conector.
Check that data for your RF section.
Don't exceed the maximum input level.

I suggest to initially set the test signal generator to a level between -30 dBm and -10dBm
( that is a voltage range of 7 mV rms to 70 mV rms)

2) Triggering the test oscilloscope:
You can only select EXT Trigger on your test oscilloscope, if you have the SCAN Out connected to that input.
But we want SCAN Out to be visible as a diagnostic, so we want to connect it to Y1 on test oscilloscope.
So you need to trigger scope on Y1.
I am fairly sure your test oscilloscope can do that.
I you need assistance with trigger setting, post up the model number of the test oscilloscope.
Regards

[Luze]

Hello,

The 8556A can handle 10 V rms, but for some reason this should not be fully utilized.
So I start with 7 mVrms (20 mVpp).
Re 2. connection of the scope

 Channel 1 ( upper trace) set to 5V per division  is plugged to 141T SCAN OUT
OK

 Channel 2 ( lower trace) set to 200mV/div  is plugged to 141T VERTICAL OUT
OK

Trigger Source is set to Ch1,
I can't find an option to set this?

negative trigger on the SCAN OUT
Not sure if I’m understanding this correctly (maybe my translation isn’t good enough). Are you saying that I should connect Scan Out to the External Triggering input of the oscilloscope?
But the cable to Channel 1 (Y) is already connected to Scan Out? So I would have to connect 2 cables to Scan Out. Is that what you mean?
I have a Hameg 203 - 7 and will try to attach a picture of the control panel.  (I dont found to do it, sorry).
Best regards

[mag_therm]

OK,...  ..No Channel selection of the trigger.... on the external oscilloscope
 I have a picture now of the Hameg controls and the Hameg 203 - 7 english instruction manual.
You can read the manual and maybe a Hameg user can assist with trigger setting.
Read the "Trigger" pages (no page numbers shown)
In my experience these old oscilloscopes were sometimes difficult to trigger when not familiar with the particular unit.
I suggest to initially disconnect CH2 BNC connector (disconnect VERTICAL OUT) and try to get the Hameg to
trigger correctly display ( follow the instructions)  only with the SCAN OUT signal on CH1 as shown on the photo I posted.

[LarryAndrews]

Luze, It seems that I had a similar problem as you're having, but I have an HP141B. My issue was a broken fuse holder at the rear of the 8552B IF housing. I repaired the fuse holder and replaced the 1 Amp fuse with a fast blowing fuse. When I powered the HP141B, I got a trace and was able to you use an RF generator across the range of my RF section. The next time I tried to use the SA, I got no trace. I then rechecked the fuse on the rear of the 8552B and found the two fuses open. I then tried a slow-blow version of the 1 Amp fuse. This worked, but I found that I had to turn my SA on and off every once in a while or I didn't get a trace. I noted that there was no trace when the "Display Adjust" dial + and - lamps were not illuminated. When I turned the unit on and one of the lamps illuminated, I got a trace. It didn't happen a lot, but I knew if i didn't see one of those lamps illuminated, There would be no trace. I didn't trace out the fault on a schematic because it was working and I had other, more pressing projects to do. It did pan out to be a fault in the IF section and I will track down the fault in some future adventure. Just throwing it out there for grins

[Luze]

Hello,
@mag-therm
I have now only connected Scan Out to channel 1 of the oscilloscope and fed a signal into the input of the LF plug-in unit (10 mV). Only the zero line can be seen on the oscilloscope. If it
does not trigger or triggers incorrectly, at least the zero line should be broadened if an AC voltage is present. But it remains as narrow as it is. I therefore assume that Scan Out does not supply an AC voltage signal. Switching from channel 1 to DC then showed a constant DC voltage level of 11V.  This can only be changed using Scan Mode Man and the Man Scan control.

(translated with deepl!).

Best regards
Luze

[mag_therm]

OK. You have found a fault!
Next step is to look for a signal on VERTICAL, per my photo
Fault could be:
1) Failure in IF Section 8552B
2) Problem in interconnection to 141T Display section  causing SCAN_OUT to be pulled to +12V
3) Problem in 141T Display section causing SCAN_OUT to be pulled to +12V.

I think 1) is highest probability. You will need to diagnose using the circuit diagrams in Service Manual.
It will be a tough job, based on my experience with repairing the 141T here.
Good Luck

Edit : ........I see your your last comment "This can only be changed using Scan Mode Man and the Man Scan control."
This could be  confirmation the the fault is inside IF Section 8552B

[Luze]

Hello,
I have found someone who will put the IF insert into his 141T to test whether it works there. I think that will be the quickest way to find out if the IF insert is faulty. That's what I would assume. What do you think?

Best regards

(translated with deepl)

[mag_therm]

Good idea