[sdiy] [OT] My Scope Saga

Adam Inglis 21pointy at tpg.com.au
Mon Mar 25 08:08:01 CET 2019


> On 22 Mar 2019, at 8:51 AM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> 
> Adam, Oscilloscope HV problems are a nightmare because if the HV is wrong it messes up the calibration for the horizontal and vertical deflection too :-(
> 
> For example, if the HV is too low the electrons pass through the deflection plates slower than intended and the resulting trace on the CRT screen is larger than it should be.  Also usually dimmer and probably out of focus too.

That explains why all I could see was glowing green fog!  
I don’t believe it was one single thing that I replaced that got it working again. Both high and low voltage supplies had their faults. The +/- 50 and +100 v supplies were needed to drive the coil to generate - 1000 and +3000 volts for the CRT.

Now, it’s quite functional and useful, but perhaps a little dim. I measure only 2500 volts at the front of the CRT screen, when the SM tells me there should be 3000v. 
Perhaps some more tweaking to be done ...

A














> 
> -Richie,
> 
> 
> -----Original Message----- From: Tim Ressel
> Sent: Thursday, March 21, 2019 6:42 PM
> To: synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>
> Subject: Re: [sdiy] [OT] My Scope Saga
> 
> 
> 
> My replacement power supply came in this morning. I am no longer scope-negative!
> 
> 
> On 3/17/2019 5:25 PM, Adam Inglis wrote:
> 
> 
> 
> On 18 Mar 2019, at 9:56 AM, Brian Willoughby <brianw at audiobanshee.com> wrote:
> 
> 
> I think it’s kinda funny, because sometimes I repair vintage synths for local musicians, but I have to look to someone else to get my ‘scope repaired.
> 
> 
> 
> 
> I’ve recently finished repairing a 50 year old CRO. There were a variety of (mostly power related problems) but the area that really stumped me was the high voltage circuit for the CRT itself. I had no experience with this type of circuit and no way of testing the high voltages. I made a divide-by-10 test probe, took a voltage measurement at virtually every point on the schematic and muddled through, ended up virtually rebuilding the entire HT circuits either side of the coils, and eventually got those electrons hitting the screen in a coherent way. Took a couple of years between all the other jobs….
> 
> Adam
> 
> _______________________________________________ Synth-diy mailing list Synth-diy at synth-diy.org http://synth-diy.org/mailman/listinfo/synth-diy -- --Tim Ressel Circuit Abbey timr at circuitabbey.com
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