Re: Trace Issues on 7A18 in a 7603


cmjones01
 

On Sat, Feb 17, 2018 at 04:53 am, David Berlind wrote:
I've got a problem with one channel on one of the 7A18's in a 7603.
Basically, the trace on channel 2 is very jumpy at some settings and then
flatlines altogether when going from 200mv to 500mv and beyond. Also, after I
fine tune the peak to peak voltage with the calibration knob (based on what I
know the input peak to peak to be), I can't lock it in.
I've had a look at your video and your 7A18 is showing the classic symptoms of dirty switches and controls. The gold-plated leaf switches in the attenuator assembly get dirty, and the little plastic attenuator modules suffer from poor contacts and dry solder joints internally. It's fiddly to clean everything properly (takes about an hour per plugin if you've done it before and have all the right tools, allow 3-4 hours if you don't) but a squirt of good switch cleaner (I use DeoxIT D5) can help. The variable volts/div potentiometer also needs cleaning and lubricating. I can't remember what type the 7A18 has in it, but sometimes Tek used 'Modpots' which can be unscrewed and lubricated. Other types of potentiometer have enough holes in the housing that you can get cleaner in there, and then work them backwards and forwards a few dozen times.

When you say you're trying to 'lock in' your adjustment to the gain setting, I don't think you're doing it right. You're just using the variable volts/div mode (note the '>' symbol in the readout) and then switching it off by pushing the red knob in. To change the gain setting properly, you need to push in the centre knob while turning it with a small screwdriver - see the slot in the knob? That's what it's for. As you turn it, at some point there will be a small click as the drive to the gain setting potentiometer engages, then you can turn it to get the gain right, and then release the knob. It will pop out in to the 'VAR' position, but just push it back in to latch it. The gain setting will be saved. I find the that the gain preset often needs readjusting after one of these plugins has been transported. The various metal and plastic parts on the shaft of the gain pot are fairly heavy and tend to move around when the plugin gets bumped.

However, before adjusting anything, get the attenuator working properly first so that you have a trace on all V/div settings.

I hope this helps
Chris

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