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Re: Tektronix 475 with no display
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So I replaced the two capacitors (C1414 and C1442), reviewed the schematic and made sure that any connected points were still making contact (need to add a jumper from C1442 to CR1415, due to my removal accident where I damaged a trace).?
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I backed off the +50V adjustment, fired it up, adjusted it back to 50.0V and now I get the following: +50=50.0V +110=91.0V +15=15.0V +5=5V -15=-14.8V -8=-7.9V
+105=132.0V UNREG 50=~66V Now, those all look to be within spec except for +110 (at 91V) and possibly the 105 (at 132, schematic shows 136). I did some searching/testing and didn't see anything major wrong, so I tested the resistance values.
TP - expected - measured 110 - 11K - >20K (too high) 50 - 2.7K - 2.7K (perfect) 15 - 63 - 66.3 (within range) 5 - 46 - 49.9 (within range) -15 - 480 - 12.9K (too high)
-8 - 32 - 31.2 (within range) UNREG 50 - 14K - >20K (too high) 105 - 12K - >20K (too high) So my 110 and 105 lines are both showing low voltage, so the higher resistance made sense to me. But the high resistance on the -15 line seemed odd. My assumption is that it means the voltage compensation is working overtime and properly correcting for an issue somewhere. Is that correct?
In reading the service manual, it states to attempt to test the rectifier, the current-limiting transistor, and the op amp. I see that the 105 and 110 both get fed from the +50V rectifier as well as the 110 op-amp pulling from the regulated 50V line.
When I get home from work, I intend to test the 50V supply area more. What I'm wondering is if anything I've stated throws an "obvious" flag for anyone with more experience. When I tested the current-limiting transistor for the 50V section, it had some voltage (the manual states it should be "off", which I take to mean 0V). I'm just not sure if that's the cause of the issue, or a result.
Based on some time staring at the manual, I'm assuming the rectifier is more of an issue, but that's a guess. Also, as a side note, I can get a line when I press the beam finder button now (I did not previously). So some progress is being made (aside from the obvious progress of mostly-proper voltages).
One final thing to mention: This morning I powered it up before heading to work, just to make sure I had noted the proper values. When I did that, the voltage readings were better on 110 and 105 (94, 136.9 respectively). But as a minute or two went by, and things warmed up, the voltages started to drop. Again, this tends to make sense to me as something is most likely expanding, providing more resistance, and cutting down the potential. Any confirmation/correction would be appreciated.
Also, again, those caps are tricky. I've been soldering (hobbiest, not full-time) for at least 15 years, and I had a rough time replacing the caps. Everything works and tests out nicely, but it doesn't look so great (good joints, but heat marks, etc). I'm definitely not used to this older style of circuit board. Way too easy to lift traces, but that was a good lesson learned.
As always, thanks for all of the information. It's been greatly useful for my learning process.
-Josh On Mon, Jul 16, 2012 at 11:08 AM, David <davidwhess@...> wrote:
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