HP 1741A oscilloscope - restoration project

[tekfan]

This is to all of you who are afraid to spend too much money on a scope that they don't know if it works or not. I'll try to show (hopefully) how to restore a scope in the most awful condition you can imagine.

OK so recently I got a HP 1741A 100MHz dual channel analog storage scope from ebay for an insanely low price. This picture was in the auction. Although a bit dirty, it looked quite promissing.


The package arrives. The scope was very well packed indeed. The box was almost a meter high.


This is how the scope looked when taken out of the box. As for the picture in the auction, appearances can be deceiving.


Shot of the scope from above. Notice I immediately unplugged the anode lead and touched it to ground just incase.
As for the carpet......dot't ask.


So first thing to check in an analog scope is the CRT. Prying off the rusty base connector wasn't easy.


Tube lifted to slide out from the CRT shield.


CRT on the table. It doesn't look good. The getter evaporated.


The neck of the CRT is broken so this one is useless. (unless you've got an induction heater, diffusion pump, glass blowing lathe.... you get the picture).
Sadly I don't so what now? The electron gun makes a nice ornament though.


It's a too nice scope to throw away or just use for parts so after a bit of searching I found a used CRT on ebay and the restoration will continue.

I will post more information as the restoration continues. Comments appreciated.

[Bored@Work]

What did you pay for it on eBay?

[saturation]

Very nice tekfan,  if you photograph the before and after as you move forward, it would make a great documentary!

[Kiriakos-GR]

I love restorations, but among the others damages, looks that there is too much salt in there.
This item looks to was near to sea water for long time.

I wish you all the luck of this world, you will need it. 

[Alex]

Lovely scope, shame to the seller! Nice HV connection too. How much was it? Have you restored any valve scopes like...

[tekfan]

What did you pay for it on eBay?

It was slightly more than 30 EUR. Which for an oscilloscope is cheap, especially in europe. They normally go from 100 to 150 EUR.
America is a different story. I've seen better working scopes go for 0.99USD (shame really)

I love restorations, but among the others damages, looks that there is too much salt in there.
This item looks to was near to sea water for long time.

It looked like salt but it really isn't. It doesn't have that salty sea smell. I don'tknow what it is. It came from Germany so it can't be sea water damage.

Lovely scope, shame to the seller! Nice HV connection too. How much was it? Have you restored any valve scopes like...

I don't think it was the seller's fault. It must have stood in water somewhere (flood maybe?).

Sadly I haven't got a 547. Even the tube Teks are getting more expensive in europe. The Tek 555 can sometimes reach 300 EUR!!!
I've only got a Tek 561 with a missing HV transformer. Might restore that one in the near future.

[Neganur]

It came from Germany so it can't be sea water damage.

Oh, there's the Baltic Sea and the North Sea. It's salty too
(not trying to be smart, I was born there )

Good luck finding a replacement, the scope looks really nice otherwise.

[tekfan]

Sorry for the wait. I was at a concert the other day.
Anyway now the scope is being dismantled so each individual part can be cleaned.


I've found out that you can unscrew the back half of the scope off. The power supplies can now be easily accessed.
The 4 long extender shafts  can also be seen. (top to bottom: focus, intensity, beam finder, power)


Power supply board with many filtering capacitors. I figure it's easier to start troubleshooting some components at this stage since everything is easily accessible
Transformer is disconnected and checked for shorted or open windings. Capacitors checked for ESR and capacitance. All test good.


Crystalised deposits from water damage (probably). It is not salt and doesn't smell of anything. Maybe mineral content from hard water?
The scope must have been standing face up with water dripping on it.


Luckily the scope is all made from aluminium so there's no rust on the chassis but the only thing that's iron is of course the transformer which looks like this...


Back plate, power supply board and transformer cleaned and ready for assembly.

It came from Germany so it can't be sea water damage.

Oh, there's the Baltic Sea and the North Sea. It's salty too
(not trying to be smart, I was born there )

Good luck finding a replacement, the scope looks really nice otherwise.

The scope actually came from Stuttgart so I think it's safe to rule that one out.

More later.

[Frant]

Crystalised deposits from water damage (probably). It is not salt and doesn't smell of anything. Maybe mineral content from hard water?

Before you continue the restoration, you should make sure it isn't asbestos or something like that.

[tekfan]

Crystalised deposits from water damage (probably). It is not salt and doesn't smell of anything. Maybe mineral content from hard water?

Before you continue the restoration, you should make sure it isn't asbestos or something like that.

I think it's very unlikely because the scope is very well sealed. Unless it was mixed with water dripping onto the scope. Who knows. Anyway it's too late now because the thing is cleaned and doesn't pose any real threat now.

[bilko]

It looks like the scope may have been stored in a damp basement. Good look with the restoration and its great to see the progress shots.

[tekfan]

More progress.

The vertical preamplifier board removed. One custom HP hybrid can be seen in the middle of the board.


Gold plating has to be cleaned on the vertical attenuator contacts.


Attenuator cam switch gold plated fingers. These move up and down in a certain sequence when turning the volts/div knob and
route the signal trough attenuators and compensation networks.


Bare frame without any circuit boards ready for assembly.


Front panel being cleaned. Looks nice without too many scratches .


The things that go around the buttons being cleaned. The LEDs are probably going to be replaced because they are now 30 years old and quite dim.

[tekfan]

Now the scope is assembled. Knobs still have to be put on.


All the tools and crap leftover after assembly


Ready for a power up without the CRT. A few knobs are still missing because I had to drill out the screws because they rusted and couldn't be removed normally. The knobs will be tapped and a slightly larger screw will be used.


First power up. The transformer makes a terrible buzzing noise. The problem was a shorted rectifier.


No rectifier available. What now? Four diodes of course! It looks ugly but it works. Four 1N4007 diodes were used. The reverse breakdown voltage and current is higher than the original rectifier so they should work nicely. After that the scope turns on and all the power supply voltages are present and nicely regulated.


I connected the output amplifiers that originally connect to the deflection plates to another oscilloscope in XY mode with differential inputs.


Here I'm feeding a sine wave into channel 1 input and YAY!!! The amplifiers work and I get a display on the external scope.


Scope in a scope. This is what I would see if the CRT were in the scope. Now I just have to be patient and wait for the CRT to come from America Hope it doesn't break on the way here


Should I do the calibration procedure as well?

[alm]

The service manual may give give more details, but I would at least expect that you have to go through geometry and storage circuit adjustments after a CRT replacement. If you have the equipment, you might as well do a full calibration for peace of mind.

[slburris]

Very nice!  Of course while I'm staring at the photos, I can't help but notice
the Datron 1072 multimeter in the background.  What's that, a 7 1/2 digit
meter?

I'm about to try for a scope repair myself, on a Tektronix 2246.  It works,
but the trace is noisy, even when the input is grounded, so I'm suspecting
high ripple in the power supplies.  Probably at least a nice recapping job
ahead of me.

Scott

[tekfan]

Very nice!  Of course while I'm staring at the photos, I can't help but notice
the Datron 1072 multimeter in the background.  What's that, a 7 1/2 digit
meter?

I'm about to try for a scope repair myself, on a Tektronix 2246.  It works,
but the trace is noisy, even when the input is grounded, so I'm suspecting
high ripple in the power supplies.  Probably at least a nice recapping job
ahead of me.

Scott

If it has a focus problem you might want to check this link out:
http://www.cromwell-intl.com/radio/tek2445a.html

It looks like this is a common problem in the bigger analog Tek scopes. The resistor opens up as it is right on the limit of its power rating. And yes the capacitors should definitely be replaced with low ESR types.

Here's what can happen to the capacitors in the SMPS (German but you get the picture):
http://www.spurtikus.de/basteln/tek2432_reparatur/index.html

The Datron 1072 is actually a 6 1/2 digit meter but 7 1/2 with averaging and filter on.
Here's a review/teardown I did a while ago: https://www.eevblog.com/forum/index.php?topic=3013.msg41015#msg41015

[Alex]

Hey tekfan, is that mica capacitor next to the rectifier for some sort of 'line trigger'?

[ivan747]

I wish I had the experience to restore things 
This looks awesome

[tekfan]

Hey tekfan, is that mica capacitor next to the rectifier for some sort of 'line trigger'?

Yes! It is exactly that capacitor. It's tapped off the 120VAC secondary of the transformer.

I wish I had the experience to restore things 
This looks awesome

Don't be afraid to buy old non working stuff. This is how I got started in electronics. Well that and taking things apart to see how they work never to put them back again! Looking at a schematic and trying to identify the faulty part can be a very rewarding learning experience itself.

[Alex]

taking things apart to see how they work never to put them back again!

I used to put them back together without the PCBs and everyone was wondering why they were so lightweight.

[ivan747]

Don't be afraid to buy old non working stuff. This is how I got started in electronics. Well that and taking things apart to see how they work never to put them back again! Looking at a schematic and trying to identify the faulty part can be a very rewarding learning experience itself.

I have just tossed away a radio that looks from the 80's after taking it apart. I took the pots and interesting, expansive and hard to find components. I hate cheap bakelite PCB's. If it only depended on me, I would be buying this kind of stuff on eBay on a weekly basis, but I don't pay them and the shipping of this huge test equipment is as huge in cost as the equipment itself.

[tekfan]

The project continues:

At last the CRT came from the US.

Here are the broken one and the new one


Face of the old CRT


Face of the new CTR. The graticule lines are slightly thicker than the old one.

[MikeB]

Hi Tekfan, this is my first post to this site. I did a similar resto to you - only mine was not quite so bad - still have one problem though.
The 'X' scan sometimes just scans half the screen - tapping some buttons can sometimes restore it - any ideas?

I do not have the manual yet!

Great resto BTW!

[Spiro]

How much was shipping from Germany?

Does it work in the end?

[amspire]

Hi Tekfan, this is my first post to this site. I did a similar resto to you - only mine was not quite so bad - still have one problem though.
The 'X' scan sometimes just scans half the screen - tapping some buttons can sometimes restore it - any ideas?

I do not have the manual yet!

Great resto BTW!

I use an HP 1740A as my analog scope. Bought it cheaply with a dead power supply and fixed it. A really nice oscilloscope.

The operator and service manuals (including schematics) can be downloaded from Agilent:

Operator:  http://cp.literature.agilent.com/litweb/pdf/01740-90911.pdf
Service:  http://cp.literature.agilent.com/litweb/pdf/01740-90909.pdf

Application Note: http://cp.literature.agilent.com/litweb/pdf/5952-2040.pdf

Also the HP Journal article on the HP1740A is great:

http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1975-12.pdf

Richard