• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Burnt resistor on M-25 power amp

Hauxon

Hauxon

New Member
I'm considering buying this amp from a local fellow. He told me he burnt a resistor last time he used it (some months ago) and he hasn't gotten around fixing it. I was wandering if someone here could share his/her thoughts regarding the issue and possible causes. I'm not an electrician and no good at reading the schematics to track down the correct resistor and judge possible causes. I will let a professional amp guy fix it if I will buy it but I'm a bit nervous about it.

Here's a picture of the burnt resistor:
27224369835_53e9550e1c_k_d.jpg


Here's a picture I found on the net showing the whole board:
26949102840_a23496a4a2_b_d.jpg


Thanks, Hrannar
 
Register to hide this ad
I can't see the resistor due to the camera flash (at least on my monitor). Resistors burn for a reason. 99% chance if you just replace the resistor, the new one will burn up also.
Bob
 
Shorted outputs and or drivers on the one channel. It might be marginally working if the fuses are not blown but not for long.
Everything needs checked on that board.
I would love to mess with one of those!
 
Last edited:
WE6C said:
I can't see the resistor due to the camera flash (at least on my monitor). Resistors burn for a reason. 99% chance if you just replace the resistor, the new one will burn up also.
Bob
Click to expand...
Using a different monitor now. I can see that fried resistor.
Bob
 
R32, 910 ohms 1/2 watt... q11,q13 one or both are hurt. it is q11's emitter resistor.

I don't LIKE that component that seems to be touching the to-220's mounting screw.
is that a metal end?
 
Thanks for the help, I knew I was posting in the right place! :)

I met my amp guru last night, he had just finished fixing my (no so exotic) Pioneer A-656 amp and I showed him the above images and the schematics. His feeling was it was too much of a risk for me unless I'd get the M-25in for analysis.

He also told me that the M-25 is a quality amp but it would not be wiping the floor with my A-656. The A-656 was a quality amp an much of it's design was inherited from the earlier top of line amps. The worth of having the M-25 would be
10% for getting better sound and 90% possession of a rarity. I asked him why the A-656/757/858 were not that sought after, he said just wait 10 more years. :) Have in mind that he is a tube kind of guy.

So my next step is probably to speak to the owner and see if he agrees on sending it in for a check. ...of just let someone else have it.

Best, Hrannar
 
markthefixer said:
R32, 910 ohms 1/2 watt... q11,q13 one or both are hurt. it is q11's emitter resistor.

I don't LIKE that component that seems to be touching the to-220's mounting screw.
is that a metal end?
Click to expand...

Thanks for the insightful reply. According to the M-25 service manual these are the part numbers:
Resistor
* R32: RD1/2PSF 910J
Semiconductors
* Q11. 25A899A-V
* Q13: 2SB536A-L or K

How hard are these to come by? Available from every electronics shop? Pioneer only? The Unobtanium shop?
 
You can cross reference equivalent transistors.
It can be repaired.
 
zebulon1 said:
You can cross reference equivalent transistors.
It can be repaired.
Click to expand...

My electronics man strongly advises to use original parts if possible and also suggests changing the equivalent parts on the other channel to ensure exact same tonality in both channels. He also told me that there is no assurance that the problem was actually with the transistors before the burnt resistor, it could be further back leaving you with the same problem again and having to search again for parts and more trial and error. This is not that much of a problem if you're repairing it your self and have time and patience to wait for parts. ..so he has warned me! ...but a demon inside of me wants that amp... :crazy:

So, can anyone point me to a good place to buy these parts and help me with what might be considered equivalent parts??

Thanks, Hrannar
 
You can't find original electronic components for that set. Your will have to use better equivalents from Mouser, Digikey, or Parts Express. On Semi, Fairchild and such. Do not use any NTE products on that amp.
We were not saying that the obvious damage you presented is the only damage. Yes the whole amp will need to be checked.

zebulon1 said:
Shorted outputs and or drivers on the one channel. It might be marginally working if the fuses are not blown but not for long.
Everything needs checked on that board.
I would love to mess with one of those!
Click to expand...
 
Does not look like the original resistor. Hard to tell from blackened body the value. Maybe the replacement was not 910 ohm?
Anyway, Q11, Q13 and surrounding components have to be pulled and checked, including C9, VR4.
Forget finding genuine original transistors.
Possible replacements for Fujitsu A899 are Fairchild KSA1381, Sanyo 2SA1381, Toshiba 2SA1360.
I have all kinds of Pioneer amplifiers which I have rebuilt. M-25 and A-27 are better than any Pioneer black face stuff I have.
 
You must log in or register to reply here.
Back
Top Bottom