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Denon PMA425R no sound left channel blown
Denon PMA425R no sound left channel blown
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Old 24th February 2019, 04:37 PM   #1
ssi is offline ssi  United Kingdom
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Default Denon PMA425R no sound left channel blown

Hi All


I fancied attempting a repair for fun and picked this up for 9 on ebay. Dim bulb tester stays off. Relay clicks but no sound through headphones other than the faint "leakage" sound you get on input selectors.


DC at both speaker terminals is around zero. Right channel DC at bias adjust points is 17mv which is normal but around 2v on the left channel. Can I assusme that the power transistors and possibly drivers are blown on the left. There is also a hum when the AUX input is selected.



If so need replacements for 2SA1633 / 2SC4278 and possibly a 2SD1762 driver. Can you suggest good equivalents which are easily available.


Any advice would be appreciated.



Best Wishes
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Old 25th February 2019, 05:27 PM   #2
ssi is offline ssi  United Kingdom
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Default Help!

I have done some further investigation. If you go through "Source Direct" the amp appears to work normally with audio through both channels via headphones. It reads around 0.5 v across the biasing resistors for the left channel (too high ), right channel normal. With the tone circuit engaged (Source Direct off) the left channel becomes just a hum and it reads around 22v (very high!) across the left channel biasing resistors, right channel normal. The relay enagages as normal.


What faulty componenet am I looking for here or where should I look?? Can I assume the transistors are OK as it works with source direct OR could it be the biasing resistors? I don't want be desoldering everything without a theory. All I have is a DVM and a transistor checking device.


Any help would be appreciated.
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Old 25th February 2019, 07:01 PM   #3
Mooly is offline Mooly  United Kingdom
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Denon PMA425R no sound left channel blown
Assuming the 17mv bias is the voltage across the 0.22 ohms then 17mv sounds OK and would equate to about 77ma (or half that if across both in series).

The 2 volts bias (and 22 volts and 0.5 volt you later mention) all point to (again assuming you are reading across the 0.22 ohms) the resistors being open circuit which in turn suggests the output stage semiconductors will have failed short circuit.
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Old 26th February 2019, 04:47 PM   #4
ssi is offline ssi  United Kingdom
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Thanks for replying Mooly. You have helped me many times before.


Just out of interest why does the relay not prevent the amp from operating and why does it appear to work OK when used via source direct?


Any suggestions for suitable transistors? I did find some potentials for the power transistors but the details are on another PC. I am not sure about the drivers though.
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Old 26th February 2019, 07:07 PM   #5
Mooly is offline Mooly  United Kingdom
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Denon PMA425R no sound left channel blown
Quote:
Originally Posted by ssi View Post
Just out of interest why does the relay not prevent the amp from operating and why does it appear to work OK when used via source direct?
The honest answer to that one is that is that there could be several reasons why it works with headphones and so it is easier to work with something definite like the voltage you mention.

Just think for a moment... you say you have 22 volts across the channel biasing resistors, and as I mentioned earlier I'm assuming this is across the 0.22 ohm emitter resistors as shown here.

So 22 volts across 0.44 ohm (the two 0.22's in series) means a current of 50 amps is flowing... which isn't really possible. So the most likely scenario is that the resistor/s are open.

Although a relay is shown on the circuit diagram it isn't immediately obvious what it does. The speaker feed as far as I can see is direct (apart from switches).

So that is the starting point, to confirm what is or has happened. Are those resistors open and are the transistors shorted.

It would also be worth measuring the DC voltage at the input to the power stages as the source direct switch is operated. It should be zero volts across R201 and R202.
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Old 26th February 2019, 07:15 PM   #6
ssi is offline ssi  United Kingdom
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Thanks for this. Will have to put it on hold for now as suddenly very busy at work. Will post again once I have done some more investigating.
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Old 27th February 2019, 05:37 PM   #7
ssi is offline ssi  United Kingdom
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Hi all


Got home early today due to a poer cut at work! Did some dismantling of the unit to get a better look. It appears that the 4 power transistors have been replaced along with one of the voltage regulator transistors. Will remove these and test when I have more time along with resistors. The transistors had no pads, just heat sink compound so they could have shorted that way?
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Old 27th February 2019, 05:49 PM   #8
Mooly is offline Mooly  United Kingdom
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Denon PMA425R no sound left channel blown
So the amp has unknown history Its a case of being methodical and doing all the usual checks and tests. No insulating pads could be a big issue unless the transistors are all plastic types.
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Old 27th February 2019, 06:00 PM   #9
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by ssi View Post
Thanks for this. Will have to put it on hold for now as suddenly very busy at work. Will post again once I have done some more investigating.
The problem needs some basic component checking.
Open circuit resistor is easy with a multi-meter.
Look up transistor checking for the transistors.
They often go short circuit across CE or DS depending on type.
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Old 9th April 2019, 02:53 PM   #10
ssi is offline ssi  United Kingdom
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OK one faulty resistor found and replaced but now will not power on anymore. I can hear transformer humming so power on. Dim bulb tester comes on for a second then instantly goes out. May have broken something else!


Any ideas?
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