Part substitution for diodes?

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Old 03-08-17, 09:43 PM
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Part substitution for diodes?

Working on a Denon DRA-397 Receiver with a blown left channel going into protect mode. Output transistors dead, Emitter resistors too, at least one connected cap. All parts on order but on the underside of the board were two SMD mounted diodes in series - a 1N4148W and a Zener UDZS3.6B, I think I found a overseas source for the 1N4148w, but no luck on the zener. Any ideas on a source or appropriate substitution?

Also, working on SMD size parts is not my thing to be honest though will get it done if that's what the parts must be. Are there any "normal" sized alternatives to both that I don't need to rig up triple magnification and pinpoint tip soldering iron for? Thanks!
Jim
 
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Old 03-08-17, 10:17 PM
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1N4148 is the signal diode part number.
1N4729 is the zener replacement you can use. ([email protected])

These parts probably used for bias. Doubtful they are bad unless burned.
You can check them with a meter in the diode test position.

As far as sources..... Ebay, Mouser electronics, MCM electronics.

http://www.mcmelectronics.com/
 
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Old 03-08-17, 10:32 PM
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Thanks pete! They were burned up. One destroyed, the other tested bad.
 
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Old 03-08-17, 10:50 PM
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Gotcha.

I have accounts with a lot of parts suppliers but Mouser is my go to company.
For you.... MCM would be perfect. They carry a lot of stuff.

Fairchild Semiconductor Small Signal Diode 100v 200ma Do-35 | 1N4148
Fairchild Semiconductor Zener Diode 1W 3.6v Do-41 | 1N4729A
 
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Old 03-08-17, 11:31 PM
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I actually have both those parts in stock. Must have bought a set a long time ago. Now I just need to be careful to put them in correct, given the upside down view of the board etc. Also wondering how much other damage was done, beyond the output transistor and their resistors. Plus C508 which measured very high ESR, though that could be coincidence or could be a source/effect of the problem. Name:  denon_DRA-397_left channel.jpg
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Old 03-09-17, 12:25 AM
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The two transistors in red get changed as a pair. The four emitter resistors in pink need to be checked. The transistors in green are probably ok but need to be checked.

Click on pic for larger view.
 
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Old 03-09-17, 10:09 AM
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That 4 and the 4 resistor were the ones I had found failed. I will definitely check the other two you mentioned.
Question - Int he service manual, it lists most of the resistor as Metal film - except those emitters resister. They are list just as "fixed. Does that mean that are metal film ocide instead of just metal film? Or carbon? or something else?
 
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Old 03-09-17, 10:24 AM
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Usually the emitter resistor are ceramic and wirewound due to their low resistance. (.22ohm)
The type is not critical there. They are basically being used as fuses.

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Old 03-09-17, 12:01 PM
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i actually have some of those from another repair. That is the type I normally see of these more "modern" receivers. The interesting thing is on this particular unit, they do not use that type. They look like regular color banded resistors.
 
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Old 03-09-17, 12:04 PM
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You can use those too. They can be wirewound or metal oxide but in a different form.

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Old 03-09-17, 08:18 PM
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Thank you very much for your help! I've tried a variety of other online forums in the past and usually either get no answer, or more often than not the type of answer that is a perfect world scenario with no constraints to time or budget or technical reason (e.g. you need to replace every single part on the board, even if they all test good.) It's rare to get help that actually points you in a specific direction for resolution, things to look out for, and also helpful explaining things along the way. I'm a retired software guy - working out the next chapter of my life (to keep busy) as a hardware guy. Learning as I go of course and loving it!
 
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Old 03-09-17, 08:40 PM
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Here's a hint that servicing guys know..... we use a variable AC supply to power up just repaired electronic equipment. Some use a variac. I use a Sencore AC supply. The piece gets plugged into it and the voltage is turned up slowly. If there is a defective component the AC current will rise.... usually quickly..... signifying a problem.

If you repair a piece of equipment like that Denon and there is a shorted part you miss..... since the stages are direct coupled..... you can wipe out everything that was just repaired.

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Old 03-09-17, 09:28 PM
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I do have a variac. Have only really used it for powering up used radios. Your Sencore is much nicer but pricey!.
Not exactly sure how I would use a variac with this denon, given is has this standby mode and fault detection etc. With a regular receiver I could just turn it on and start at zero volts (just like a radio) and bring up slowly, but this unit might not react as expected when coming up from standby. Not sure what it would do exactly. Might even turn off from not seeing enough voltage? Assuming it works, where/what would you recommend to measure?
Int he meantime, it will be some time before my parts arrived, so I am going to check the surrounded components for sure
 
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