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HP 1741A Oscilloscope

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Maybe it's time for me to throw in the towel. I just went out to test LVPS after the transistor swaps. With absolutely NO load, all of the rails run to zero within seconds.

Did you keep the transistors you removed, and do you know where they were positioned?
 

tryppyr

Oct 22, 2013
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Did you keep the transistors you removed, and do you know where they were positioned?

Sure, I still have them and remember their positions. They are, however, compromised, so I'll not be putting them back in. However, I will use some other new components I still have, taking out the Texas transistors and using the NTE replacements that Kris recommended.

One of the reasons I'm starting to think I may be near the end of this project is because I'm seeing increasing signs of the stress I'm applying to these boards by constant removal and reinstallation. As I was setting up for the last test I noticed that one of the wires to the off-board transistor connectors had broken due to metal fatigue from being bent too often. I cut off a small bit of the insulation and reinstalled it, checking the others at the same time and finding another on the verge of failure. During a previous test I noticed one of the pins from the rails had broken completely loose. I soldered it back into place then. But the cumulative evidence is starting to suggest I may be creating more problems by this too frequent stressing of the boards for tests.

Anyway, I'll plug on for a bit more.
 

atsu

Dec 13, 2013
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Dear tryppyr,
Please read my message in the other thread. Thank you, atsu
 

tryppyr

Oct 22, 2013
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Being thoroughly confused by the results I was getting on the LVPS, I decided to test the AC outputs coming from the transformer. After the few seconds it took me to set up the test, all were at or near zero.

So at least now I think I know where to look for the problem.

Edit: Confirmed. The line switch is broken. It no longer locks into ON position. It's acting like a momentary switch. This presents an interesting problem... or opportunity.

Follow-up: The switch problem proved to be purely mechanical and easily solved. However, I then noticed one of the chassis mounted transistors (the one on the +15V circuit) has been damaged and needs to be replaced. The pins have been bent so often they no longer stay put as the cap is being fitted over them.

It's always something. But at least I can be pretty sure I haven't trashed the LVPS yet.

However, the unloaded LVPS now has almost nothing on the +120V rail. Which means the +156V rail is also not working.

Ugh.
 
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tryppyr

Oct 22, 2013
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I just finished doing the wholesale zener swap on HVPS (plus one of the rectifiers that also had a toasty look to it). I also finished the rebuild of the 120V rail on LVPS. Still need to get to VETCO and pick up a couple of parts before I go into the next test, so there's no immediate way to know if my efforts will bear fruit. I feel pretty good about it, though.
 

tryppyr

Oct 22, 2013
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Today I received the CA3094AT I ordered. It did indeed come from China, which at first had me concerned. But when I saw what was inside, I was delighted. It's in a hermetically sealed anti-static bag, mounted to a piece of spongy foam. When I opened the package, all of the legs are full length, none shows the least sign of usage.

Knowing how rare these things are (and how prized by musicians), I almost feel guilty using it. Almost. I will at leas withhold its use until such time as I am sure the rest of the scope is operational.
 

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tryppyr

Oct 22, 2013
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Still trying to get LVPS working again, still having no luck. When I power the unit up with almost no load on LVPS I hear the transformer buzzing. One of the bridge rectifiers gets warm almost instantly. No voltage shows up on any rails. I don't leave it powered up long enough to do spot checks. The feeling I get is that the problem is before I get to the 120V rail circuitry that I have replaced twice now.
 

Pioneer621

Aug 3, 2010
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Still trying to get LVPS working again, still having no luck. When I power the unit up with almost no load on LVPS I hear the transformer buzzing. One of the bridge rectifiers gets warm almost instantly. No voltage shows up on any rails. I don't leave it powered up long enough to do spot checks. The feeling I get is that the problem is before I get to the 120V rail circuitry that I have replaced twice now.

Could be a shorted bridge rectifier, I would test them (likely the one over heating). There are a couple threads on other forums & that was a problem in at least two of those scopes that I seen posted about. That is often the symptom of a buzzing transformer & blowing fuses. Make sure you did not create any solder bridges when changing parts; I’ve seen that happen a lot.

Just seen your thread & thought I would give my thoughts on that, I am not a professional either & cannot help otherwise, I simply do not have the knowledge to do so.

Hope you get it up & running though, with all your effort put in. I find with stuff I’m have trouble repairing, setting it on the shelf or in the closet for a few months & coming back with a fresh start often seems to help get me through a repair (or make some progress), gives me a little time to try to understand the circuit in the mean time if possible & helps clear the frustration of not being able to find the fault. Again, hope you get it running!

All the best.
 

tryppyr

Oct 22, 2013
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Thanks for the feedback, Pioneer.

Like you I find that time does lend one a bit of perspective. I may be getting to the point where breaking away for a bit makes sense. Not quite there yet, though. Still enjoying the challenge. When it starts to make me angry I'll put it away. ;)
 

tryppyr

Oct 22, 2013
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I removed the bridge rectifier that is heating up and did a continuity test on the + and - terminals. Aligned correctly (black lead to the -, red to the +) I get nothing. Reversed, I get about 300K resistance. By contrast, a brand new NTE168 reads closer to 30M in reverse mode.

I also got weird readings from the other two of this type of bridge rectifiers still attached to the circuit, but I suppose I expected that since they are still in the circuit.

As an aside, these rectifiers are hard to remove from the board because their leads are so beefy.
 

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tryppyr

Oct 22, 2013
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For those interested, Talonix just delivered a genuine HP replacement part. A bit pricey, but definitely the right part. They only had the one that I needed (not five others I checked on), but hey... one is better than none.
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Tryppyr, I recommend you try to get the unit back into almost working condition again before you consider using these genuine parts.

I might go as far as to suggest you do some tests on this part and see if you can find something that might be similar to use as a crash test dummy. Better to toast something cheap than something expensive.
 

Saul Britto

Mar 4, 2016
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I know this 1741A fix is a bit old but, if your scope never lights the TRC your problem was maybe
that optocoupler that has on the HVPS,it controlls the cathote voltage.If you still has this scope try it tryppyr and, if you no more have this scope it is a tip to the other members.


I am new to the forum,my name is Saul, I am from Brazil.
 
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tryppyr

Oct 22, 2013
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Thanks for the tip. I do still have the scope, but haven't worked on it in quite a while. I previously spent a lot of time on the HVPS module, and replaced many of the components there, including some not mentioned on this forum. If I get a chance I'll check into it and see if there's anything I overlooks.
 

Saul Britto

Mar 4, 2016
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Thanks for the tip. I do still have the scope, but haven't worked on it in quite a while. I previously spent a lot of time on the HVPS module, and replaced many of the components there, including some not mentioned on this forum. If I get a chance I'll check into it and see if there's anything I overlooks.
Ok. at the last week I did fix with the help of a good friend an unit of 1741A.It justs did lights the flood gun but,no trace then we figured out the cathode was in cut,the optocoupler was the culprit it's internal diode was dead.when it was replaced the scope returned to life.
Look: http://www.eevblog.com/forum/repair/hp1741a-in-a-big-shappe-but-i-bet-in-a-focus-problem-and-you/
 

Saul Britto

Mar 4, 2016
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If you has a VTVM and HV probe measure "with care" 5kv at the tripler out and 2295 V at cathode TP if this is right than propably HVPS is good.If it is good and TRC do not lights then the cathode protection maybe the culprit (optocoupler circuit).Dont use 10MOhms input impedance DMM with HV probes.
 

X-Ray

May 17, 2018
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Hello there,
I know it’s a death topic but still relevant to the ones trying to fix this baby with the same symptoms..
had exactly the problems and it turn out to be the hv transformer on the A15 hvps .
Desolder the hv transformer and measured the 2 yellow wires connected PIN number 9 and 10 if you measure this to GND pin 6 wen it shown a value ohms then its leaky and need to be replaced ..

My scope is fixed now..
 
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