| Electronics > Beginners |
| NAD 218 THX amplifier Repair. |
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| james_s:
What DMM? I have an older Fluke that has an audible diode test function where a short beep indicates a good junction and a steady tone is shorted. Every meter works differently though, it's important to know how to properly use your test gear in order to interpret what it's telling you. What happens when you measure a known good diode? |
| SantaClaw:
I used my Anegh 8008 https://www.aliexpress.com/item/ANENG-AN8008-True-RMS-Digital-Multimeter-9999-counts-Square-Wave-Backlight-AC-DC-Voltage-Ammeter-Current/32810125781.html From what I understand it's a relatively good DMM for it's price and type. It's 2 in the morn, so I'm going to sleep now. I do appreciate the help |
| james_s:
It may be a very good DMM, I don't know, either way the first step is to learn how the diode test feature works and then you'll know how to interpret the tests you do. Find a good diode and test it with the meter to see how it responds, usually it will display the forward drop which is somewhere around 0.6V for a normal silicon diode. |
| Richard Crowley:
--- Quote from: SantaClaw on February 23, 2019, 12:30:37 am ---I put my DMM meter in diode check mode, and it briefly beeped when I measured between the legs. --- End quote --- That is an extremely crude way of "testing" a transistor. It is not good enough for the kind of analysis and repairs you are attempting. In reply # 21 you said "my cheapo component tester." Perhaps it is time to look properly at your test gear and how you are using it. As I mentioned before, this is a high-power, direct-coupled circuit with an expensive set of transistors. It would be easy to make a minor mistake and blow up $100s worth of transistors and other components instantly. You could end up spending much more on replacement transistors than the amplifier is worth. --- Quote ---So I need to de-solder q317 and q319 ? --- End quote --- It is the only way of properly comparing the transistors in the good channel with the equivalent devices in the failed channel. Repairing a high-power amplifier puts you well above the level of sophistication where you can use a cheap DMM in diode test mode to evaluate transistors. At best, you might be able to use a diode-test DMM to make a very crude go/no-go test. But unless you can get something that really tests a transistor using all three leads, you can't really repair this amplifier properly. At least IMHO. Even one of those $10 component testers (which some people here refer to as "POS") is infinitely better than trying to test a transistor two leads at a time with a "diode test". |
| SantaClaw:
Well, I have been using a cheap transistor tester with all the transistors I have de-soldered. I figured using the DMM was ok for testing for shorts |
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