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Problems with my Denon PMA-520

Lelle

Lelle

Active Member
Hello everyone at Audiokarma, hope you all doing well.

I have huge problem with my Denon PMA-520 amp.
So I will try my best to describe my problem as well as I can.

I've purchased a faulty Denon PMA-520 from eBay a week ago.
The power supply was dead, the transformer and the caps were bulged.

Replaced with a similar transformer, also replaced the rectifier and replaced the four 5600uF caps (two per channel). The 5600uF caps are in parallell in pair, so two 5600 is 11200uF for each channel. Replaced them with two 12000uF caps instead, one for each channel. The amp now powered up and relay clicked as it should, but I heard a low buzz sound from the right channel. So, I increased the volume at almost on top to see if the buzz gets loader, not max. And suddenly... BANG! transistor TR454 (2SA1358) was busted, as well as three zenerdiodes D410, D416 and D418. Replaced TR454 with KSA1220AYS, and the three diodes with BAV20-TR. R452 (220 ohms) was replaced too. The power amp transistors TR460 and TR462 were completely shorted, so these I replaced with A1695 and C4468. Couldn't find anymore suspicious, so I powered the amp once again. This time BANG! from TR452 (2SC3421), zenerdiodes D408, D412 and D414 is shorted, and one of the power amp transistors (TR462) is shorted as well, but not TR460. R452 is a bit burned. I'm going to replace all the busted components, but before I'm powering it up again I wanted to hear your answers and I need your advices. I'm so devastated right now because I've fixed exactly the same amp model many years ago with the same fault, and I ruined it this time. :( I forgot to check the power transistors before powering up at the first time, I regretted that I didn't checked them first.
 
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You need to always power the amplifier up with a DBT - find threads on this site on how to make one. Only power it up thru this - it will save your bacon, believe me - and you will have a failsafe bright lightbulb going instead of blasted components! What was the "same fault" in the same amp that u fixed last time, shorted outputs?
 
good point redk' dbt, too. Ouch! Must have got hit pretty good or dropped? abused?

(btw.. if you come across a sony? most likely the VR adjustment is counter clockwise to zero and cheap plastic vrs. just a tip.)
 
slimecity said:
You need to always power the amplifier up with a DBT - find threads on this site on how to make one. Only power it up thru this - it will save your bacon, believe me - and you will have a failsafe bright lightbulb going instead of blasted components! What was the "same fault" in the same amp that u fixed last time, shorted outputs?
Click to expand...
By DBT you mean a lightbulb in series with the primary voltage to the amp?
The first amp I had, I replaced the transformer, the caps were ok that amp.
It powered up, but it ran very hot after a while. It turned out to be two power transistor with a low value, replace those and that amp worked like a charm again. I don't have that amp anymore, but when I saw this one on eBay with the same fault, I couldn't resist to buy it. But this amp was a bit worse, it had bulging caps as I mentioned earlier.
 
Binkman said:
good point redk' dbt, too. Ouch! Must have got hit pretty good or dropped? abused?

(btw.. if you come across a sony? most likely the VR adjustment is counter clockwise to zero and cheap plastic vrs. just a tip.)
Click to expand...

Probably, power problems or shorted somewhere. The original transformer cannot be repaired. Cosmetically the amp is almost in mint cond. Haven't touched the VR at all.
 
Not sure if going up that high in uf's is a good idea. you more than doubled the originals. The circuit after it might not handle that increased power rush/surge well.

Athanasios
 
Hi - yes to confirm - powerup always with a lightbulb in series with the live. Not sure if you are doing this already - but I doubt it if things are blowing up.....?
 
DBT....the poor mans safety net. Definitely agree...has saved me from very expensive oversights more times than I can count. Personally, I would take the time to systemically go through this unit and ensure you have all these key component checked and validated before attempting any further power on. if you have the sm, dial down the bias all the way and then recalibrate it up from there once you have verified you are not blowing any more components. I would also have recommended against the use of a two 11200uf caps compared to the 4 5200's. there is a reason why they did that and you could be possibly changing the design of the power supply by impacting the ESR/Impendence rating. perhaps not to the extent where it would cause things to blow up, but it adds a level of complexity and unknown.
 
nabeelsayegh said:
DBT....the poor mans safety net. Definitely agree...has saved me from very expensive oversights more times than I can count. Personally, I would take the time to systemically go through this unit and ensure you have all these key component checked and validated before attempting any further power on. if you have the sm, dial down the bias all the way and then recalibrate it up from there once you have verified you are not blowing any more components. I would also have recommended against the use of a two 11200uf caps compared to the 4 5200's. there is a reason why they did that and you could be possibly changing the design of the power supply by impacting the ESR/Impendence rating. perhaps not to the extent where it would cause things to blow up, but it adds a level of complexity and unknown.
Click to expand...

Maybe I should find some 5600's instead, 2 for each channel. I just found it a bit strange that just the right channel was damaged. About the DBT, will a 100w lamp do the job? We have 230 volts here. Maybe there was something wrong with the right channel from the beginning. It could have been so if the power transistor already had low value. But I was to eager to power it on, before measuring the transistors first. I will continue with the repair as soon as I have the time.
 
100W bulb will be fine for a poweramp. I would personally prefer to go lower - say 75W, but opinions differ. The bulb needs to allow the amp enough grunt to fire up, so 75W-100W will be fine.
 
I agree...you wont need a very big bulb, in fact, if it is too big, it may not do what you want it to. I also would agree that more than likely, there was a pre-existing condition (or failure) already there. that being said, it would not have taken long to before the fireworks started.
 
Sorry for not being logged in for a while, much to do at work. I've been catching a cold a couple days ago, but i'm feeling better now. I will continue with the amp and build an DBT when I have the time. Just wanted to give you all an update.
 
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