Author Topic: HP 1740A on the bench, help please?  (Read 32593 times)

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Online bd139

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Re: HP 1740A on the bench, help please?
« Reply #150 on: October 19, 2017, 05:37:20 pm »
You shouldn't need to touch the HT itself to see if it's running or not. I don't know where TP3 is in the following diagram but popping a scope on that should indicate if it's running or not. That is the primary side so is peaking at 15v.

You should be able to work back from that to the oscillator and/or feedback or the HT side of things:

 

Offline SpecmasterTopic starter

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Re: HP 1740A on the bench, help please?
« Reply #151 on: October 19, 2017, 06:28:30 pm »
Ok, I'll give it go if it happens again, switched it off at the moment while I have my dinner. TP3 is in the HV enclosure, its within the dotted lined area which is the HV enclosure. If its working I should get a sawtooth wave, if not, check C1?
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Online bd139

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Re: HP 1740A on the bench, help please?
« Reply #152 on: October 19, 2017, 06:42:45 pm »
Mot sure what sort of wave you will get, probably a bastardised square wave of some description on that. Main thing thing is to check it is actually doing something. 

C1 is possible.
 

Offline SpecmasterTopic starter

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Re: HP 1740A on the bench, help please?
« Reply #153 on: October 19, 2017, 06:47:54 pm »
The manual shows a triangle wave at TP3, scope to be set at 10v/div and 10uS/div.
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Offline tautech

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Re: HP 1740A on the bench, help please?
« Reply #154 on: October 19, 2017, 06:58:36 pm »
You shouldn't need to touch the HT itself to see if it's running or not. I don't know where TP3 is in the following diagram but popping a scope on that should indicate if it's running or not. That is the primary side so is peaking at 15v.
The simplest way to check if the EHT oscillator is working is to wave a probe close to it, ahla near field style.
They run at 20-40 KHz. The feedback circuitry can give problems as can the secondary side caps and DC restorer circuit. In a CRO it's one of the areas under some stress, all the high value resistors should be checked, they drift and can go OC.
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Offline SpecmasterTopic starter

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Re: HP 1740A on the bench, help please?
« Reply #155 on: October 19, 2017, 09:09:57 pm »
Oscillator is not running when the display vanishes, TP3 is not marked on the board at all but there is a testing post that is unlabelled and visually tracing the circuit through, that is indeed TP3. TP1 is a 3Kv one accessible with a probe though a rubber grommit but no point in probing that with my AVO as just holding a scope probe near the transformer confirms when it is running without any need  for direct contact to be made.

I have just noticed that there is occasionally a light buzzing sound emanating that quarter of the scope, it could be I suppose transformer laminations buzzing at times. I turned off the lights in nicely darkened room and looked for any sign of tracking / arcing when the sound is audible but nothing is visible.
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Offline SpecmasterTopic starter

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Re: HP 1740A on the bench, help please?
« Reply #156 on: October 20, 2017, 12:03:34 am »
Update.
Been checking the HV board & it is slightly different to the schematic as it is one of the areas that they made changes to and the thing that I have discovered is that Q1 and Q2 checked while in circuit with a transistor checker are reported as double diodes? Q3 seems OK but on a quick check earlier tonight to see if it was still firing up, shows that there is no HT at all at the moment, all the LEDs light up ok. I'll check this tomorrow evening and make sure I have not accidently disconnected anything.
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Online bd139

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Re: HP 1740A on the bench, help please?
« Reply #157 on: October 20, 2017, 07:35:37 am »
Ok next thing to do is check the HV disable isn't jammed on. If you check the collector of Q1 that shouldn't be close to ground. HV disable shuts the whole show down and kills the oscillator. If there are any out of spec or dodgy components (Q1/Q2/R30-R32) it'll just shut itself down instantly. If it's ok, next thing is check the feedback loop. Debugging this can be a dick but it's great fun at the same time :)

I'm starting to wonder why I do this because this week I have spent the entire week debugging hideously broken asynchronous threading stuff built by the lowest bidder so fixing scopes is taking repair work home  :o
 

Offline SpecmasterTopic starter

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Re: HP 1740A on the bench, help please?
« Reply #158 on: October 20, 2017, 08:01:36 am »
Why does a transistor tester indicate that Q1 and Q2  are double diodes, I just don't understand why that should be?
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Re: HP 1740A on the bench, help please?
« Reply #159 on: October 20, 2017, 08:05:10 am »
Did you check them in or out of circuit?

If it’s one of those Chinese transistor testers they can be right dumbasses sometimes. That’s what a transistor looks like to a naive algorithm.
 

Offline SpecmasterTopic starter

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Re: HP 1740A on the bench, help please?
« Reply #160 on: October 20, 2017, 08:07:28 am »
In circuit tested ,same for Q3 which showed up as a transistor.
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Offline tautech

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Re: HP 1740A on the bench, help please?
« Reply #161 on: October 20, 2017, 08:08:32 am »
Yep, testing active components in circuit can be a crap shoot even with the best of gear.
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Re: HP 1740A on the bench, help please?
« Reply #162 on: October 20, 2017, 08:24:16 am »
Yep. You should be able to spot if it’s working by measuring voltages in circuit.
 

Offline SpecmasterTopic starter

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Re: HP 1740A on the bench, help please?
« Reply #163 on: October 20, 2017, 09:04:28 am »
Ok, just ran a quick test (have to take Mother in Law out shortly to East London), switched it on there was this light buzzing as mentioned earlier coming from the rear quarter, took all the covers off, the oscillator is not running and the buzzing is definitely coming from mains transformer, sounds different to most transformer hums, maybe because this one is fully screened?

Checked main power rails, all present and correct, will probe the primary side of the HV section later today on my return.
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Re: HP 1740A on the bench, help please?
« Reply #164 on: October 20, 2017, 09:07:09 am »
If it's buzzing it's under load which suggests a short somewhere.

I'm wondering if this is something else. Check the rails on the HT board coming in as there's an opamp on there if I remember correctly.
 

Offline SpecmasterTopic starter

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Re: HP 1740A on the bench, help please?
« Reply #165 on: October 20, 2017, 09:18:20 am »
If its got a short then its going to drag one or more of the rails down, but they are all spot on within 0.1V of their specs?
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Re: HP 1740A on the bench, help please?
« Reply #166 on: October 20, 2017, 10:24:19 am »
Depends if there is any decoupling on the rails. If there is a tant after a resistor then the resistor will just get hot :)
 

Offline SpecmasterTopic starter

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Re: HP 1740A on the bench, help please?
« Reply #167 on: October 20, 2017, 03:18:05 pm »
Just got back and checked the voltages in the HV section and in doing so discovered that the schematic for that board has been changed but fully documented, attached is the schematic for that board and is the only one that I can find on line. I purchased a manual from Artekmedia and they have made impossible to print a page as a pdf or to copy a page and that manual gives some of the changes but the schematic and board overlay is still the old version so I have taken a photo of the listed mods.

All the incoming supplies to the HV section are on spec and the voltages measured on Q1 and Q2 are as follows

Q2
Base         +16.8v
Emitter     +15v
Collector    0v

Q1
Base         0v
Emitter     0v
Collector    +7.27v
 
« Last Edit: October 20, 2017, 06:09:52 pm by Specmaster »
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Offline SpecmasterTopic starter

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Re: HP 1740A on the bench, help please?
« Reply #168 on: October 20, 2017, 03:29:26 pm »
Just got back and checked the voltages in the HV section and in doing so discovered that the schematic for that board has been changed but fully documented, attached is the schematic for that board and is the only one that I can find on line. I purchased a manual from Artekmedia and they have made impossible to print a page as a pdf or to copy a page and that manual gives some of the changes but the schematic and board overlay is still the old version so I have taken a photo of the listed mods.

All the incoming supplies to the HV section are on spec and the voltages measured on Q1 and Q2 are as follows

Q2
Base         +16.8v
Emitter     +15v
Collector    0v

Q1
Base         0v
Emitter     0v
Collector    +7.27v
Oops forgot to post the drawings I will send them when I get home again.

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Online bd139

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Re: HP 1740A on the bench, help please?
« Reply #169 on: October 20, 2017, 03:56:38 pm »
I hate it when people send protected PDFs. Quick tip:

1. Open it in Google Chrome.
2. Print it to a PDF.
3. All copy/paste/print protection gone! :D

If that doesn't work, qpdf (available on sourceforge) does it.
 

Offline SpecmasterTopic starter

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Re: HP 1740A on the bench, help please?
« Reply #170 on: October 20, 2017, 05:33:12 pm »
I hate it when people send protected PDFs. Quick tip:

1. Open it in Google Chrome.
2. Print it to a PDF.
3. All copy/paste/print protection gone! :D

If that doesn't work, qpdf (available on sourceforge) does it.
Me to so annoying, I'll try that out later, good tip, thanks [emoji3]

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Offline SpecmasterTopic starter

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Re: HP 1740A on the bench, help please?
« Reply #171 on: October 20, 2017, 06:11:14 pm »
The pdf's are in the post
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Online bd139

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Re: HP 1740A on the bench, help please?
« Reply #172 on: October 20, 2017, 06:44:00 pm »
Cool. I reckon Q2 is wonky. Should be max 0.6-0.7v between base/emitter.

Looks like they added some hysteresis to the old design based on the Zener and it’s getting stuck in one state.
 

Offline SpecmasterTopic starter

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Re: HP 1740A on the bench, help please?
« Reply #173 on: October 20, 2017, 06:53:29 pm »
Thanks, I'll have a look and see if I have an equivalent that I could try.
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Re: HP 1740A on the bench, help please?
« Reply #174 on: October 20, 2017, 06:56:01 pm »
Make sure you have two because if you replace it and it blows up there’s another problem :)
 


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