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HP 1740A HV Oscillator has stopped running
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Hi everyone, the last time I asked a question I got some very good advice, so lets see if we have same luck this time. I have the 1740A that I'm trying to restore, its in really good condition inside and out generally and certainly does not belong as landfill etc. Last time I had distorted waveforms and your advice pointed me in the right direction and we resolved that issue and so while attempting to calibrate the scope a bit, the trace disappeared completely, did this a couple of times and never recovered. Pressing beam find confirmed that there was no HT. Removed the HV cover and checked the supply rails coming in and all is correct but the oscillator is not running and I've confirmed this by using another scope and holding the probe in close proximity to the transformer and there is high frequency bleed at all. There is a 50Hz one if I turn the sensitivity all the way up, normally there is a high strength signal of around 30 to 40Khz but not on this one at present. The voltages I'm getting on Q1, Q2 and Q3 are: Q2? Base? ? ? ? ?+16.8v Emitter? ? ?+15v Collector? ? 0v Q1 Base? ? ? ? ?0v Emitter? ? ?0v Collector? ? +7.27v Q3 Base? ? ? ? ?+7.27v Emitter? ? ?0v Collector? ? +22v I should say as well that this particular board has the modifications to the Q1 and Q2 configuration which I have a photo of but not too sure how I can attach it in a fashion that you can see it, any ideas on how I submit the photo and on how I can resolve the oscillator issue? All assistance is greatly appreciated. Graham?? |
开云体育By "Q3", do you mean "HV OSC Q1", the off board power transistor connected to T1-3?? The 7.27v base-emitter voltage would indicate either a bad connection to? "HV OSC Q1" (open emitter?) or a blown "HV OSC Q1".? If that part is socketed, pull it out and re-install.Dave On 10/21/2017 7:59 AM,
graham.l.n.butcher@... [hp_agilent_equipment] wrote:
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Yes, you are correct, it is the HV Osc Q1, I was mixed with another section that was giving problems but resolved.
I'll pull it out and test it, I have already done that HV Disable Q1 and Q2. Someone else suggested on another group that Q2 was dicky, despite it checking ok on my tester, it was replaced as I have loads of equivalents but that wasn't the problem.? So I'll try this test and let you now if is OK or not. Graham |
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Graham,
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By chance did you check the secondary is not being loaded down by a bad component? That would likely stall the oscillator. Dennis -------------------------------------------- On Sat, 10/21/17, graham.l.n.butcher@... [hp_agilent_equipment] <hp_agilent_equipment@...> wrote:
Subject: [hp_agilent_equipment] Re: HP 1740A HV Oscillator has stopped running To: hp_agilent_equipment@... Date: Saturday, October 21, 2017, 6:59 PM Yes, you are correct, it is the HV Osc Q1, I was mixed with another section that was giving problems but resolved. I'll pull it out and test it, I have already done that HV Disable Q1 and Q2. Someone else suggested on another group that Q2 was dicky, despite it checking ok on my tester, it was replaced as I have loads of equivalents but that wasn't the problem.? So I'll try this test and let you now if is OK or not. Graham |
开云体育No, the bias on Q2 is correct if 120v is good.? The 16.8v base voltage reverse biases the b-e junction of Q2 turning it OFF.? The 7.27v b-e bias on Q1 HV OSC is problematic.? This shoud be no more than 1vdc or so with the oscillator running.? Power transistor sockets can get flaky with age, and sometimes simply removing and re-installing the transistor will cure the problem.Dave On 10/21/2017 5:59 PM,
graham.l.n.butcher@... [hp_agilent_equipment] wrote:
Yes, you are correct, it is the HV Osc Q1, I was mixed with another section that was giving problems but resolved. |