NAD 218 THX amplifier Repair.

[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]

I put my DMM meter in diode check mode, and it briefly beeped when I measured between the legs.

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.

So I need to de-solder q317 and q319 ?

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

[Richard Crowley]

Well, I have been using a cheap transistor tester with all the transistors I have de-soldered.

WHAT "cheap transistor tester". Specifically.

I figured using the DMM was ok for testing for shorts

We are past that stage. We need better detail at this stage.
Maybe those transistors are not dead.  But how close are they to their twins over on the good side?

[SantaClaw]

I have two identical kits, one in a nice plastic case, the other just as is.

https://www.banggood.com/Supporting-Shell-For-DIY-M12864-Graphics-Version-Transistor-Tester-Kit-p-989949.html

https://www.banggood.com/DIY-M12864-Graphics-Version-Transistor-Tester-Kit-LCR-ESR-PWM-p-986954.html

Basically, a kit. It seems to be accurate from what I can tell, with repeatable values that measure the same on both kits.

[Richard Crowley]

OK. going forward, we are going to use the component testers to get more detailed intelligence of what is going on inside the transistor package.
1) To confirm that they are behaving properly as a bipolar NPN or PNP transistor (as appropriate for each device)
If they have developed some weird behavior, even those cheap transistor testers will produce results that appear bizarre. That is an indication that the device is no longer behaving the way we want it to do
2) To compare the details (particularly Beta = hFE = gain) for parallel array matching. Specifically those places where there are four power transistors working in parallel. But also to compare how alike the transistors are to their twins over on the good channel for troubleshooting purposes.  That is what we are trying to do now. Compare the failed channel vs. the good channel. Component by component, and starting with the transistors as they are the prime suspects in this mystery.

[SantaClaw]

OK. going forward, we are going to use the component testers to get more detailed intelligence of what is going on inside the transistor package.
1) To confirm that they are behaving properly as a bipolar NPN or PNP transistor (as appropriate for each device)
If they have developed some weird behavior, even those cheap transistor testers will produce results that appear bizarre. That is an indication that the device is no longer behaving the way we want it to do
2) To compare the details (particularly Beta = hFE = gain) for parallel array matching. Specifically those places where there are four power transistors working in parallel. But also to compare how alike the transistors are to their twins over on the good channel for troubleshooting purposes.  That is what we are trying to do now. Compare the failed channel vs. the good channel. Component by component, and starting with the transistors as they are the prime suspects in this mystery.

Right, I'll start de-sodlering, labling, and posting the comparison numbers here.
Thanx for the help, I really appreciate it. As I said I'm a newbie at this.

[Doctorandus_P]

https://octopart.com/search?q=2SC5200