HP 141T restoration

[ealex]

Hello

I've managed to "save" an old HP141T unit form being scrapped with a sledge hammer.
It's got the IF and AF plugins.

I'll like to keep this tread for questions, photos, etc of trying to clean up and restore the unit.
I might try to get a RF front end plugin for it, or sell it once i get it working ( it's damn heavy so i'll try to sell it locally  )

From what i've seen it's almost black from dirt and most of the knobs seem stuck - spent it's last 30 years on a shelve in a dirty storage space, but  free from corrosion. It did power up - at least nothing blew up, but no trace or anything on the screen - it blinked when pressing the "erase" button.

I'll get the unit home tonight - i'll add some photos then.

[ealex]

Sorry for the delay.
This is the unit, after removing the first cm of dust and grime:

At a quick glance: still dirty.
On the LF section: 8556A
    the frequency nob feels rough - like grinding sand
    the adj pots are filled with dust
    the 3 switches are stiff but seem fine otherwise

On the IF section - 8552B
    the 2 pots on the left ( black knobs ) are stuck
    all linear switches look fine
    the "log ref" selector seems to turn forever but it still switches something


On the display all pots feel rough and the push buttons seem ok.

I'll try to power up the display unit without the plugins once it gets warm, if i can unstuck the plugin release lever - i can pull it for 15 degrees then it sticks somewhere

Sorry for the bad lighting - i don't have a good diffuse light source availabe

[KJDS]

I've not used one in 20 years and it was way past its best then.

[ealex]

first power up went bad: no display / nothing on the CRT with or without plugins but no smoke or odd smells ( the R&S sig. gen stank of capacitor electrolyte )
none of the display unit buttons work.
i can see the filament lighting up

i'll start reading the service manual for the display unit and checking the power supplies.

[ealex]

it seems it wasn't that dead: i've moved it around a little and it power up

The knobs do something, the CRT memory seems to work - even if the stored image is blurry, the beam finder still does nothing

I've attached a photo that's displaying the "20kHz markers"

(edit - fixed photo )

[ealex]

I've managed to get it open - take a quick look inside

The plastic paint that's covering the case is sticky / partially melting - how can i fix this ?
Also, the interior seems to be covered with some sticky layer even if there's not much dust.

From the side you can see a ceramic trimmer package ( first time seeing this ) and the HV crt connector.
There is also a flex cable - no idea what it's used for

From the top you can see the CRT, a huge transformer, fan, some power transistors and a row of electrolytic caps. that i'll have to replace.
Is it a good idea to keep the outer casing of the caps ? New caps will be smaller / thinner and I will not be able to mount them on the same plate. It should be easy enough to remove their interior and keep the outer shell and the mounting tabs intact.

All the interesting stuff is on the bottom side.
On the left side it's what looks like the CRT driving amplifiers.
On the right side it's the HV supply under a thick metal plate with a lot of warnings all over

I'll have to be extra careful with this unit - there are a lot of old / hard to find parts - destroying some strange transistors will pretty much junk it if there is no data available.

The next step is to look over the schematics and find replacements for all caps - there are also some axial caps that look bad - some corrosion marks.

Also i might have a problem with the work bench - this thing is huge ... i'll have to use the floor or find a place - i don't think there is any hacker space here - i could 'donate' it in exchange of some work space ( the unit, as it is, it's almost useless for me - the RF plugins are expensive, but the experience of restoring it should be enough )

[gilbenl]

This looks like a neat project. If you don't already have them, here are all of the docs you could need for the 141T. They'll have schematics and parts lists:
http://www.ko4bb.com/manuals/index.php?dir=HP_Agilent/HP_141T_Spectrum_Analyzer

I'm fairly new to restoration myself and have found some great tips here. Currently working on a meter calibrator and encountered a similar issue with the old sprague caps. If you don't intend to replace all of them, and you feel it is safe to do so, I recommend leaving the instrument powered up for a few days. After sitting on a shelf for a while, the caps may look good and work for a while, then boomski-True story, happened to me. I chose to re-stuff with nichicon. There's a lot of info on the process on antique/vintage radio sites. You can see how I did it here:

https://www.eevblog.com/forum/repair/hp-6920b-meter-calibrator-schematic/

And the final product:

[Smith]

Caps can measure and look good, but fail under load. Happened to me multiple times. It's hard to find and sometimes replacing whatever cap you suspect is the only way to fix your isue.

[termen11243]

I have a 141t mainframe. The manual was easy to find on Boat anchors website. My trouble begins with noise on the vertical display. Checking the supplies in the 141t I found the +248 to be low, so I fixed that. It was stuck at 210 volts. I checked all the transistors and the gain was OK pulling them out into a DVM with transistor testor. I did find an NPN that was open and replaced that. Still stuck at 210volts.
Staring at the schematic and measuring voltages, the adjustment never went low enough to turn on the regulator which pulls the low end of the transformer up to 70 volts, that sat at 5 volts. I spied R9 a 100K and bugged it, OPEN! Looking through my collection of resistors I found a metal film rated at 200volts 100K 0.1 watt and replaced the open resistor. Success! now the 248 will adjust.

Problem 2 there is an impulse noise on the -12.6volts. On a DVM it adjusts but the noise propogates to the vertical deflection.
Checking all the capacitors in the -12.6 they were all correct. Measuring the timing it was about 30KHz, Hmm maybe thats from the high voltage supply under the cover. There is a 5000uF capacitor across the transformer, and this regulator works by modulating one side of the transformer to acheive a steady -12.6, but its getting this impulse noise on it. There is a darlington formed by q9 and q4 but they are separated by the cable bundle and thus can pick up and amplify this stray impulse, it does not look like oscillation its induced.
I tried a 4.7uF ceramic 50v across the main rectifier capacitr C6 of the main frame. I may also put more capacitance on the -12.6 i f that doesnt work.
More work needed.....unless someone else has solved this. I think its design flaw as the parts on the board (c48, r46) were larger values then shown on the schematic.

[coppercone2]

you do have a capacitor ESR meter right?

Just making sure because I heard transistor tester and those can be really dodgy. Unless its a curve tracer but thats a really tall order

for HV things, keep in mind some resistors like to short out at high voltages but measure OK at low voltages.

I have been meaning to make a fixture to HV test resistors.

[termen11243]

I tried a ceramic 4.7uf across the 5000uF and the impulse is still there.
Looks like more troubleshooting. But wait, the tek465 needs some work off to that for awhile see other thread.
As far as transistor tester its a B&K portable with transistor socket on it to measure beata and leakage current.
Also bugged in circuit. I think it took a lot of probing to find the open 100K. 100 volts on it for the life of the instrument, Pfft.
So the power NPN and the driver PNP are separated by a wire bundle about 8 inches long. This would likely be a source of noise pickup.
It could be one of the other supplies is high ESR and contributing to the 30khz impulse amplitude. So I'll take the scope and look at the rest of the lineup and see. They all were approximately correct on a c meter. ESR who knows. So further tracking down in order.