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denon pma-500v amplifier dead channel repair?

T

TrevorBarten

Well-Known Member
I got this amplifier a while ago and I noticed that the fuse on one of the channels was blown (I think it's the right one). I measured the output transistors and the ones for the defective channel where shorted. I ordered some new ones and installed them (for both channels since I read online that you need to be careful with them matching) and the working channel is still working but after installing and testing the fuse instantly blew again. The transistors didnt die though. I measured the diodes close to the transistors but those measure fine. I also measured the other 2 transistors close to the output transistors and those also measured fine. Any tips on how to proceed?
 
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What did you replace the outputs with?
Can you build a dim bulb tester for any further adventures?
 
eBay???? They're fakes. Buy from Digikey or Mouser.
Check all transistors and resistors on that channel.
 
TrevorBarten said:
I replaced them with output transistors that are supposedly the same as the originals. I found them on ebay. And yes I can.
Click to expand...

If you do a little research here you can learn a lot.
Put the part number of the original transistor in the search bar and you'll find the modern suitable sub. Buy a complete shipset from Mouser or Digikey and do not power up again until you are hooked up to a dbt.
You probably have some more damage than just the outputs, search how to test a transistor and check the others on the bad channel.
 
Milo and Ivan provide excellent advice. In my case (in the link provided by Milo above), I used 2SA1294 / 2SC3263 pairs, but the 2SA1695 / 2SC4468 would be a good choice too. Get them from a trusted supplier: Digikey/Mouser/Newark are such but sometimes don't have stock in these Sankens; I got mine at BDent.

I also replaced the driver transistors, which you indicate you checked and they were ok; mine tested ok too, but...I follow the advice of those more knowledgeable than me, which is to replace one level upstream from the last transistor that failed. I would recommend you do the same.

And to second Ivan's advice above, carefully check around the failed output transistors for damage. For example, looking at the schematic, it is easy to see that if an output transistor failed with a C-E short, then the 0.22Ω emitter resistor would have had full rail voltage (51VDC) across it, and current through it would have ramped up sharply until the rail fuse let go. Mine were ok, so the fuse acted quick enough. 0.22Ω resistors are a little difficult to measure and differentiate from a short, but look for heat stress, etc., although I suspect they are more likely to fail open.
 
ivandezande said:
eBay???? They're fakes. Buy from Digikey or Mouser.
Check all transistors and resistors on that channel.
Click to expand...
Why exactly are they fakes if I purchased them from ebay? How does that even work considering the other channel for which I also replaced the transistors with the fakes works just fine?
 
TrevorBarten said:
Why exactly are they fakes if I purchased them from ebay? How does that even work considering the other channel for which I also replaced the transistors with the fakes works just fine?
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Because no legit seller is using eBay. This has been covered time after time, why do you think three users have all said only buy from Newark/Digikey/Mouser?
Because fakes can work, but won't when you turn it up.
Also why did you replace working output devices?
 
ivandezande said:
Because no legit seller is using eBay. This has been covered time after time, why do you think three users have all said only buy from Newark/Digikey/Mouser?
Because fakes can work, but won't when you turn it up.
Also why did you replace working output devices?
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Just because a ot of people tell me something doesn't mean I instantly get why that is the case. I replaced the working ones because I remember hearing that they need to be matched. If this is not the case and these fake ebay transistors are worse than the originals I'll put the original ones back in and order some replacements from a trusted source. What does it mean to sell a fake transistor. What is even the point in that?
 
Crooks will do anything for a buck, there are countless videos on youtube that show they will even wrap crappy Wonhunglo capacitors with fake Nichicon and Panasonic labels. Its nothing new, some used to remove the labeling on old tubes and relabel them as a desirable tube when production ended. The old transistors that are desirable and hard to come by are the ones that are most likely relabeled, there are pictures and videos that prove it is being done. There are markets in CHina, Mexico and even here in the states where pretty much everything in there is fake rebrands.
 
TrevorBarten said:
Just because a ot of people tell me something doesn't mean I instantly get why that is the case. I replaced the working ones because I remember hearing that they need to be matched. If this is not the case and these fake ebay transistors are worse than the originals I'll put the original ones back in and order some replacements from a trusted source. What does it mean to sell a fake transistor. What is even the point in that?
Click to expand...
So you believed hearing somewhere they need to be matched (not true at all) but three people saying the fact a 3rd party marketplace sells fakes is something that needs to be vetted? Hmm.....
Even if they did need to be matched, you didn't think the working ones would be correct from the factory?

It means to manufacture a cheap part that doesn't meet specs or doesn't work at all, print a legit brand name and part number on it, and sell it for probably a 400% profit. It's actually a great business if you can contract almost free labor in China.
 
ivandezande said:
So you believed hearing somewhere they need to be matched (not true at all) but three people saying the fact a 3rd party marketplace sells fakes is something that needs to be vetted? Hmm.....
Even if they did need to be matched, you didn't think the working ones would be correct from the factory?

It means to manufacture a cheap part that doesn't meet specs or doesn't work at all, print a legit brand name and part number on it, and sell it for probably a 400% profit. It's actually a great business if you can contract almost free labor in China.
Click to expand...
I thought that you'd create problems if you had different ones for either channel. So even if the originals where matched it would cause problems when switching out only 1 channel. I have heard this dozens of times in repair videos and the like but I probably just misinterpreted what they meant. Obviously I would be less inclined to believe the option that creates trouble for me IE the transistors I bought are fake and I need to buy new different (probably more expnsive) ones. But now I know that that is indeed the case. So I purchase new ones, and put the original ones I had back in the working channel. I also check the other transistors in the dead channel again. And I need to make a dbt. Anything else I should do? I checked the diodes on the dead channel but I didn't check the resistors out of circuit. Would that be worth doing?
 
petes'picks said:
Crooks will do anything for a buck, there are countless videos on youtube that show they will even wrap crappy Wonhunglo capacitors with fake Nichicon and Panasonic labels. Its nothing new, some used to remove the labeling on old tubes and relabel them as a desirable tube when production ended. The old transistors that are desirable and hard to come by are the ones that are most likely relabeled, there are pictures and videos that prove it is being done. There are markets in CHina, Mexico and even here in the states where pretty much everything in there is fake rebrands.
Click to expand...
Ok, I was aware of this but I guess I didn't really connect the dots. Sometimes there are NOS listings from aliexpress and the like for transistors or ics etc though. Is there anyway to know if those are or aren't real or is it always just gonna be a gamble? I have ordered V-fet transistors from china before around 10 years ago and those where from what I can tell 100% legit (the amp is still in use and working just fine) But they did also cost a price which you would expect to pay for real ones (I think it was around 100 for a pair at the time I don't really remember).
 
TrevorBarten said:
I didn't check the resistors out of circuit. Would that be worth doing?
Click to expand...

If you have pulled the output transistors, you can leave the resistors in circuit, just measure directly across them at their leads. Note that Denon used two 0.22Ω 1W resistors in parallel for each emitter resistor location, so the measurement will be 0.11Ω. As stated before, this will be hard to tell from a short for most meters (and if you do get a reading, remember to subtract any resistance you can measure when crossing your probes), but will reveal if both are open or have run high.

As for checking the other transistors, again the recommendation is that you replace the transistor one place upstream from the last failed transistor, even if it measures ok. This means replacing the two transistors located just to the left (on the schematic) of the two output transistors on your failed channel.
 
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