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My Wonderful HP1740 Died

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wings515

New Member
I hope I do not offend this group by asking what seems to be a question previously posted. As I stated this is my 1740 scope problem. while using it yesterday to design a PWM to control the DC motor on my wood lathe I heard a crack and then the trace disappeared. If I push the Beam Find button the trace will be about 2 cm from the top. I printed out the schematics of the previous posting and measured the input voltages to what I think is the A5 board. Correct me if I am wrong since I do not have a layout page, this assembly attaches to the big assembly that the front panel controls are connected to. I measured the voltage at the delay line input and it is 11.63V. I can control the differential by varying the Vertical Position control.
So I think everything into the A5 is OK.
My question is the correct location of the A5 assembly, just so I know I should continue looking at this PCB.
The previous poster was given such wonderful information I can only hope I will be able to solve my problem.
Best regards,
Dan Kahn
Retired HP Field Systems Engineer
 

wings515

New Member
Thank you so much for the service manual. I did some T/S and it looks like the 4.8 volt zener going to pin 13 of the A5
A1 assembly is at 3.63 V. The 243 ohm resistor reads 240 and the resistance to ground with the power OFF at pin 13 is 650K. Looks like the zener is bad. The 12 volt supply is at 11.84
Do you concur?

Thanks,
Dan
 

JLNY

Active Member
Thank you so much for the service manual. I did some T/S and it looks like the 4.8 volt zener going to pin 13 of the A5
A1 assembly is at 3.63 V. The 243 ohm resistor reads 240 and the resistance to ground with the power OFF at pin 13 is 650K. Looks like the zener is bad. The 12 volt supply is at 11.84
Do you concur?
Er, well, I had a look at the schematic where you mentioned (I've never used or worked on one of these, so I'm not super knowledgeable about them, sorry!), and a bad zener sounds reasonable. Probably worth unsoldering one leg of the zener to check it outside the circuit if that is easily doable.

I'm assuming that the HP 1740 he is referring to is a 1740A? I don't know if there is a 1740B or just a 1740 with no letter.
 

wings515

New Member
This one is an A. I think I'll remove the assembly and apply an external +12 to the lifted zener thru a 240 ohm resistor to see if it is 4.8 volts
 

wings515

New Member
Removing the PCB from the metal plate allowed the ceramic IC to be removed from the PCB. Supplying +12 to the circuit the zener measured 4.06V when it should be 4.64V. This is greater than 10% under voltage. I think I'll order a new zener and see if it does the trick. Now I have to wait for shipping.
Dan
 

schmitt trigger

Well-Known Member
If I push the Beam Find button .............
I imagine that younger readers, who may have never used an analog scope, do not appreciate the usefulness of the Beam Find button.

Anyways, good luck fixing your scope. I wish I had the money to purchase a vintage analog scope.
 

Tony Stewart

Well-Known Member
Most Helpful Member
"heard a crack" should cause visible damage to some part. It is unlikely to be the Zener but rather excess load on the Zener. , 12V and 43V supply.

Replace large electrolytics, inspect magnetics and check and markup schematic with DC/AC voltages for the A5A1 interface pins.
 

BobW

Active Member
Replace large electrolytics
Yes. And the small electrolytics too. The zener may be bad, but likely as a result of a major failure of another component. Zeners don't just fail due to old age. On the other hand, electrolytic capacitors only last so long. After about 10-15 years, they are a ticking time bomb. Your scope is 40 years old. Best to replace all electrolytics near ground zero of the bad zener, and then plan to re-cap the whole thing in the near future.

Coincidentally, I'm in the process of recapping my own 30 year old scope, because of erratic behaviour.
 

JLNY

Active Member
Yes. And the small electrolytics too. The zener may be bad, but likely as a result of a major failure of another component. Zeners don't just fail due to old age. On the other hand, electrolytic capacitors only last so long. After about 10-15 years, they are a ticking time bomb. Your scope is 40 years old. Best to replace all electrolytics near ground zero of the bad zener, and then plan to re-cap the whole thing in the near future.

Coincidentally, I'm in the process of recapping my own 30 year old scope, because of erratic behaviour.
Replacing the caps certainly can't do any harm, but I'm not certain that completely recapping the whole unit is strictly necessary for the moment. I can't speak for the HP 1740A specifically, but I have worked on a number of HP 140/141-series spectrum analyzers from this era, and the failures I tend to see are blown transistors (the power transistors in the linear supply regulators tend to run hot) and bad resistors. In my limited experience, the big Sprague capacitors used in a lot of HP equipment of this era are very robust, so in the 6 or so HP 141T mainframes that I've fixed up (and the even older 140A in my avatar), and my 436A power meter, I have never really had to replace the caps.

YMMV. Units from as little as a decade later are a different story; I'm probably planning to re-cap my 1980's Hitachi V-1100 scope soon, as sometimes the horizontal sweep randomly starts jittering, and I'm pretty sure that the caps are to blame.

I have heard of folks carefully powering up 40-50 year-old equipment that hasn't been used in decades using a variac, and then having it work just fine once the polarization of the electrolyte in the caps has reformed a bit. It sounds like the OP has been using this scope recently, though, so I don't think that would be of use here. Often it seems that the capacitors in 60Hz linear supplies tend to last longer than the caps in more modern switch-mode power supplies. I also had an HP 5342A counter (also from the 1970s, but with a switch-mode supply) which would crash and lock up during the startup routine when I first got it. I don't know if I was the first person to use it in a long time or not, but I left the unit running for a few hours, then reset it and it booted up just fine from then on.
 

wings515

New Member
I ordered the correct zener from Canada, should be here in about a week. I'm going to go back and make detailed measurements of all the DC voltages. I redrew the A5A1 assembly and it looks like it could be replaced with a modern day Op Amp. Just have to figure out how to bias the output so it is nominally 18 volts with .5V input. May be concerned with the input filter circuits on the A5 board. Also Op amp needs to have at least 100 MHz BW to be the same as the A5A1 chip.
Still working to resurrect this great instrument.
I will also inspect all the power supply caps and measure the main PS Dc voltages.
If the A5A1 is at fault and I can't replace it, I am considering a lower BW scope seen on ebay. Then I'll put mine up for sale. If there are folks here that might be interested I'll keep you in mind.
Regards,
Dan Kahn
 

BobW

Active Member
Replacing the caps certainly can't do any harm, but I'm not certain that completely recapping the whole unit is strictly necessary for the moment.
No, sorry. I didn't mean to suggest a complete recap, but I do recommend replacing electrolytics that are in the vicinity of the failed component.
 

wings515

New Member
Well I got back to it this morning and did some in depth voltage measuring. Starting at the input of the A5 assembly, I can adjust the vertical position control to balance the inputs. Checking on the input to the A5A1 again I can adjust the inputs to be balanced. Continuing, at the output of the A5 using the position control I can set each output to be 18.87 and 18.97. I can also almost get them to be equal without seeing a trace on the screen. This leads me to believe there must be a power supply voltage way off changing the grid voltages on the CRT. I just printed out the P/S schematic to review all the places to measure. This goes along with the Crack I heard when the trace was lost.
Any ideas where I should start to look?

Regards,
Dan Kahn
 

wings515

New Member
Poking around the Power Supply all voltages were within spec. Tracing back there is a Gate voltage at TP1 of the A12 assembly. It is supposed to be 15-20 volts. By adjusting the Intensity control I got it up to 18 volts and much to my surprise the trace appeared. It was only at full CW on the intensity control. The voltages look close to those on the schematic but going back there is a DIP with some different voltages on the pins. Cant figure out the pin numbering since there is no mark for pin 1. Going to look for detail on the part. Moving along.
Dan Kahn
 

wings515

New Member
Continuing with this saga, the Q3 transistor of U1 on the A12 board has a .103 volt drop in either direction(B to E) when using the Diode mode on my HP DVM. I think this is wrong but I wanted to confirm the drive into this transistor. Tracing the interconnect, it shows it goes back to the A3 Vertical Amp board. There is just one problem, on my schematic I can not find the source of this signal. It is supposed to be on pin 10 of P1 but I can not find it. Anyone have a revised schematic that shows where this Chop Blanking signal originates? I have ordered a new CA3046 IC for replacement but I want to make sure there is not a driver problem.
Did some tracing into the A3 board. It appears my scope with date code 1941 has a later revision to the schematic I have.
Anyone have a later schematic?

Dan
 

wings515

New Member
With Dave's help at Artek I was able to find the elusive circuit on a different page. Confirming the drive was OK, I lifted pin 7 of A12 U1 and the 'normal' intensity returned. It has been a long time since I did T/S of this depth but I guess you do not loose it even with intervening years. Now I have to wait a week for the new CA3046 to arrive. Other than killing a lot of trees in printing massive amount of schematics, my only other investment was the $9.00 for the zener diode I did not need. With a total investment of about $15, I think I did pretty well in rescuing my trusty 1740. I still can not associate the snap I heard with the failure mode of the CA3046. I do see the Power Button sticks some times and it may have been just coincidence that the failure occurred as the button was resetting itself. Sometimes it is Pure F Magic how all these electrons really work.
Thanks to all for the insight and help.
Regards,

Dan
 
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