• 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

Help with marantz 2245

jFF_Bulbo

jFF_Bulbo

Active Member
hello AK'ers.
I´m working on 2245.
on P750 power amps, i recapped both boards. DBT pass ok

on both unit i´ve changed all electrolitic caps.

BIAS is imposible to set properly on both channels. h756/h757 are extremely hot on both. When i blow on to the board, bias downs for a while, after few seconds bias start to climb again.

on the left i´ve changed h754/755 (2sc735) and h766/766 (2sa562)with 2n3904/2n3906. h753 replaced with new 2sc945. h760 now is a MJE243g. no bias good response anyway.


can anyone helpme to stabilish the temperature on the transistors?


thank you in advance.

right amp photo, not new h760 or h754-765 yet.......
WwF31uO.jpg
 
Register to hide this ad
If you don't find a cap problem, you are going to have to systematically test every transistor, diode and resistor on the board. One or more of those components is bad, allowing too much current in the resistors that are getting hot (could even be those resistors). Since you have changed many of the transistors, maybe start checking diodes in circuit with the diode test function on your DMM. You should measure a voltage drop in silicon diodes of about 0.6V one direction and OL (no conduction) in the other. If you don't get that with them in circuit, lift one leg and test again. Same with resistors. Check resistance across each resistor. If it isn't close to spec, lift one leg and test again. Unless you find a problem there, you will have to pull each transistor and test out of circuit. Have you changed the output transistors? If not, they are the first thing to pull and test. I hope the new transistors you put in didn't come from eBay. If so, that might be part of the problem. In general, shotgunning semiconductors is not a good idea. Better to test and find the problem first.
 
Last edited:
Steven Tate said:
If you don't find a cap problem, you are going to have to systematically test every transistor, diode and resistor on the board. One or more of those components is bad, allowing too much current in the resistors that are getting hot (could even be those resistors). Since you have changed many of the transistors, maybe start checking diodes in circuit with the diode test function on your DMM. You should measure a voltage drop in silicon diodes of about 0.6V one direction and OL (no conduction) in the other. If you don't get that with them in circuit, lift one leg and test again. Same with resistors. Check resistance across each resistor. If it isn't close to spec, lift one leg and test again. Unless you find a problem there, you will have to pull each transistor and test out of circuit. Have you changed the output transistors? If not, they are the first thing to pull and test. I hope the new transistors you put in didn't come from eBay. If so, that might be part of the problem. In general, shotgunning semiconductors is not a good idea. Better to test and find the problem first.
Click to expand...
Thank You.
I'm working on one board. I'll follow your sugestions.
In the other hand.. in R784/785 there is a single wire installed. I'll try changing them for 5.6 ohms resistors. Hope it solve the problem.

I'll report what results.

XDLpW77.jpg
 
Hello. Updating
New resistors installed. 5.6 ohm. Nos bias si more accurate., But THE h756/757 transistors keep being hot.
I measured them with IR termometer, 55 celcius on the transistor in right Chanel. Left a bit less.

Does anyone know about the normal operaron temp on this transitoria?
Keep un touch.
 
You must log in or register to reply here.
Back
Top Bottom