1. Home
  2. Electronics Repair
  3. more about (oh, no) the NAD MR-20

more about (oh, no) the NAD MR-20

If you remember, I was having slow warm-up, focus, and black-level problems with this set.

Over the past two months, the picture gradually stabilized, with these problems fading away (for no obvious reason). Nevertheless, I thought it would be a good idea to replace the caps on the video driver board, just to be safe.

I hadn't ordered them from Digi-Key (or any other company), because those that had the values I wanted required a $25 minimum order, and there was nothing else that I needed. But I needed an RS-232 cable, and Fry's -- not far from my home -- had the cable and all the caps (NTEs, if it's of any interest).

Well, I replaced two of the three caps on the board -- the ones I'd replaced

10+ years ago -- and guess what -- the set is _worse_ than it was before I replaced them.

The black level is unstable and "flickers" occasionally. The focus sometimes jumps out of line. Warm-up (that is,, the first appearance of a picture) is more or less normal, though it's sometimes darker than it should be.

I saved the existing caps, and am thinking about putting them back. (Here's a case where I could really use an ESR meter!)

They are the correct values, except one is 350V when it should be 250V. (I checked before buying the parts, and found that the 250V caps in the set had

200V across them. So I couldn't safely use the 160V cap.)

Assuming the replacement caps aren't bad (and they might very well be, as they're probably five years old), does anyone have an idea of why "new" parts should (apparently) degrade the set's performance?

Thanks.

Reply to
William Sommerwerck
Loading thread data ...

Probably highlighting other problems.

--
Dec. 6 (Bloomberg) -- Government officials and activists flying to Bali,
Indonesia, for the United Nations meeting on climate change will cause
as much pollution as 20,000 cars in a year.
Reply to
clifto

s

Just physically moving the chassis may have aggravated some cold solder joints into becoming more active again.

H. R. (Bob) Hofmann

Reply to
hrhofmann

I appreciate your comments, but the chassis was not moved. And the symptoms are qualitatively no different than they were when the problems first surfaced two or three months ago.

I wouldn't mind ripping into the set and replacing most of the electrolytic caps in the video, horizontal, and HV sections. But this is not a set designed to be easily serviced -- the main board is large and sits above a wooden base.

I welcome further thoughts. I think later today I'll put the "old" caps back and see what happens.

Reply to
William Sommerwerck

s
c

ck

I am a little confused. How did you replace the capacitors without moving the chassis?

Anyway, I would monitor the power supply voltages with a voltmeter or scope, if they are changing enough to produce the effects you mentioned, you should be able to see some shifting of voltages, then it's just a matter of tracing backwards. Do you have the schematic? That would be a tremendous help?

H. R(Bob) Hofmann

Reply to
hrhofmann

s
o

ed

es

s
s

ad

Is it possible that the tolerance of the new caps is way larger than the originals?

I've measured many larger-value electrolytics and have had so many of them read way beyond their stated capacitance that I've begun to consider the face value as "minimum guaranteed capacitance."

Just a thought...

Reply to
Mr. Land

Easy. They're on the video driver board, which pops right off the CRT. This is a common (though not universal) arrangement.

Yes, I have the schematic. I think. (It's disappeared at the moment.) The problem is that the schematic doesn't give voltages. So you don't always know for sure whether a fluctuation is a cause or effect, and are, to some extent guessing.

Reply to
William Sommerwerck

Is it possible that the tolerance of the new caps is way larger than the originals?

It's possible. As far as I can tell, tolerances have gotten tighter over the years, with the errors leaning more in the direction of "too much" rather t han too-little (as your observations suggest).

As electrolytics are most-often used for coupling, decoupling, bypassing, and filtering -- all applications where "more" capacitance is not usually a problem -- this seems to be the right way to spec them.

The set appears to be stabilizing; the problems aren't as severe as they were two days ago.

Reply to
William Sommerwerck

"William Sommerwerck" wrote in news:p9adnRup8pTsaO_anZ2dnUVZ snipped-for-privacy@comcast.com:

De-aging may be the cause.

Most ceramic capacitors [and perhaps some electrolytics] decrease in value at a consistent aging rate.

A plot of cap vs natural log of time gives a straight line. Let us say a particular cap ages 1% per decade. Its value will drop 1% in the first hour, 1% in the next 10 hours, 1% in the next 100 hours, 1% in the next 1000 hours, etc.

The STARTING point for aging is when the cap cools down after being heated above the ceramic's curie point.

In other words, heating up the capacitor above a particular temperature [that varies with the ceramic's formulation] "de-ages" the capacitor.

It is quite possible that soldering a part into a circuit can 'deage' it.

If HIGH cap at some point in the circuit is causing your problems, de-aging may exacerbate the problem and aging may 'cure' it.

--
bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap
Reply to
bz

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.