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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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8th Nov 2018, 11:51 pm | #1 |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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Tektronix 475A intermittent sweep
Got a weird one on my hands. Have a 475A here which only sweeps intermittently. When cold it'll quite happily sweep constantly but after it has warmed up it'll sit there for 20-30 seconds and then suddenly sweep 2-3 times and then go back to sleep for 20-30 seconds.
I've checked power supply out. All ok. Have checked every rail voltage on sweep circuit and trigger generator. Works entirely fine in X-Y mode. For lack of a better option I pulled the trigger tunnel diodes out and stuffed them in a working 465 and that triggers fine so TD's are ok. All waveforms are about right when it's working. I've checked both ends of pot travel. I've checked all reference voltages and they're spot on. I've wiggled every transistor and reseated every IC in the signal chain. It just won't keep triggering. Anyone got any ideas? If not I'm going to see if I can inject a signal into the loop and follow it round that way. Not ideal though! |
9th Nov 2018, 12:04 am | #2 |
Octode
Join Date: Dec 2010
Location: St Ives, Cambridgeshire, UK.
Posts: 1,180
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Re: Tektronix 475A intermittent sweep
Not the actual sweep switchery is it?
Andrew
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Invisible airwaves crackle with life. Or they should do. BVWS Member |
9th Nov 2018, 12:40 am | #3 |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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Re: Tektronix 475A intermittent sweep
No have gone through that and tested it. Seems to be trigger related. If I inject something into ext trigger it doesn’t reliably trigger, deciding to do so randomly and intermittently.
I’m watching the input of the monolithic input preamp and the gate input. |
9th Nov 2018, 1:27 am | #4 |
Octode
Join Date: Dec 2010
Location: St Ives, Cambridgeshire, UK.
Posts: 1,180
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Re: Tektronix 475A intermittent sweep
Ah OK, one of my 465s needed some repeating switching of the Auto and Norm buttons before the sweep worked properly.
Andrew
__________________
Invisible airwaves crackle with life. Or they should do. BVWS Member |
21st Nov 2018, 1:39 pm | #5 |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
|
Re: Tektronix 475A intermittent sweep
Found the cause of this in the end. U600 sweep IC was dicky. After some assistance on tekscopes list, I sprayed some freezer spray on U600 and it started working until it warmed up again. I had a spare 465 in the cupboard (with numerous far more serious problems) so I put the sweep IC out of that in the 475A and it works perfectly now. Phew.
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