Author Topic: 475A Sweep  (Read 9497 times)

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Offline SoundTech-LG

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Re: 475A Sweep
« Reply #25 on: December 18, 2014, 03:45:45 pm »
Absolutely.  :clap: So when the amplitude of the sweep signal gets smaller by turning the pot, that is faithfully reproduced on screen as a SHORTER sweep.
Normally the H. amp faithfully reproduces the same amplitude sweep as the pot is turned, and changes only the sweep frequency.
That's why the H. amp is working so faithfully.  :-+ It reproduces exactly the signal it is fed, and that has been verified, and checked several times. :-BROKE

Maybe I am failing to accurately describe the failure. Maybe the fault will become clearer with screen shots of the failed, and good scope, and those waveform signals also shown on the test scope???   Maybe. ;D

One interesting thing is that at the highest sweep speed, I do not see the problem. I have to check this further, checking amplitude, and frequency.
« Last Edit: December 18, 2014, 04:02:19 pm by SoundTech-LG »
 

Offline oldway

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Re: 475A Sweep
« Reply #26 on: December 18, 2014, 04:20:39 pm »
Quote
TIME/DIV Variable knob shortens sweep length down to about an inch of the left side of CRT when all the way from the CAL position.
Probable fault: defective Q980
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #27 on: December 18, 2014, 07:14:56 pm »
Yes. Q980 (and around 8-10 others) were replaced initially after the firestorm from the 60 V unregulated AC ripped through the whole scope. So that is taken care of, and I have checked it again since, but will check it again as well. Thanks!


Actually I swapped in Q980 from the working buddy. It worked just fine, as well as the problem scope Q980, in the buddy.

Can't remember now if Q1010 got the same treatment. Need to check that one and the whole circuit in, and around there. Likely in that ckt.  :-+
« Last Edit: December 18, 2014, 11:00:56 pm by SoundTech-LG »
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #28 on: December 19, 2014, 04:40:52 pm »
Discovered that the sweep malfunction in "A lock knobs" (shortening with the VAR ctl.) appears identical to the good buddy scope in "B Dly'd" mode when also turning the "VAR" time/div knob. Need to check the signal and ascertain if only sawtooth signal amplitude is changing, not frequency, and that would mimic the operation I am seeing in the victim  :wtf:. Also "B Dly'd" sweep is not visible/working in the victim. I knew that, but had been concentrating on getting A working to begin with, might be time to shift gears...
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #29 on: December 21, 2014, 01:20:01 am »
Repair complete. Normal functionality restored. Q904 leaky between Coll. to Emit.
 

Online tautech

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Re: 475A Sweep
« Reply #30 on: December 21, 2014, 01:34:49 am »
Congrats, I can see your grin from here.  :phew:
Avid Rabid Hobbyist
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #31 on: December 21, 2014, 01:44:15 am »
... and that's a long way! my buddy at work just came back from 2 1/2 weeks vacation down in NZ, and loved it. I've always wanted to go. ;D
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #32 on: December 24, 2014, 12:10:25 am »
so here's the Grand Total of destruction +/- a few... U600 the sweep control IC did end up with a bad "ready" lamp output. Thought it survived but no. Probably could have lived with that, but had a donor 465 that gave that IC up. Oh, almost forgot it torched the "ready" lamp itself too! 19 transistors, 3 diodes the IC, etc. etc. :palm:
« Last Edit: December 24, 2014, 12:16:50 am by SoundTech-LG »
 


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