• 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

Marantz receiver 2245 amplifier board oscillations.

T

Traverse

Member
Hello
I am working on Marantz 2245 board and trying to figure cause of oscillations when I increase supply voltage. This happened when voltage reach 21VDC or so and see HF oscillations of the sinusoidal signal. When DC is back down to 20.5VDC there is no oscillation . I have checked several decoupling capacitors on the board with no progress so far. Output transistors are SJ2513 and SJ2514 .


20191002_143824.jpg 20191002_143707.jpg 20191002_124411.jpg
 
Register to hide this ad
Following. And welcome to the group!
I'm by no means an expert and have not seen this before so interested in cause. I'm sure some of the more seasoned tech will have some input for you.

Have you performed the P/S (35V), DC Offset and Bias adjustments prior to inputting the signal?
Do both Left and Right amps do same thing?
 
Thank you JD. No I have not put full P/S 35V yet as I believe amp should be stable with lower rails as well with no load connected to the output . I will check all the components in feedback loop to see if any caps or resistors are off the values. Then I will add Hi freq stabilization caps on drivers transistors base to collector 10pF to see what happened. I do have also feedback loop analyzer to see how the gain and phase margin is. Wondering if there is some common problem with resistors on the board ageing over the time but I will know more by testing them. I have tested only one channel so far. MB
 
Is that a ground lug I see hanging from a wire in the picture of the PCB? If so, was it grounded while you were testing? That is, was it connected to the same point as any other ground terminals that the board has?

(Sorry if I suggest the obvious.)

Cheers and welcome to AK,

chazix
 
Those power supply leads are quite long. Try installing a 0.1µF film capacitors from the Plus and Minus rails to ground right at the amp.

Tom
 
Thank you chazix Yes I have input GND wire grounded as J752 is J758 GND on the PCB . J758 is GND which I have connected to DC supply. So I turned up it again and tweaked R763 a bit and is seems that oscillation disappear. I will try connect the board to the Marantz original transformer to see if I can mitigate oscillation in real PS. Thank you Tom good advice and I will add these 100nF caps and will twist the wires as well. BTW do you guys know why there are actually two versions of power amplifier ?
The version I am working on similar to 1001 to 3300 and another two boards I do have seems to be 3301 to 3600 versions..
Yes I do have 3 boards all together :)
catrafter said:
Those power supply leads are quite long. Try installing a 0.1µF film capacitors from the Plus and Minus rails to ground right at the amp.

Tom
Click to expand...
chazix said:
Is that a ground lug I see hanging from a wire in the picture of the PCB? If so, was it grounded while you were testing? That is, was it connected to the same point as any other ground terminals that the board has?

(Sorry if I suggest the obvious.)

Cheers and welcome to AK,

chazix
Click to expand...
 
The 2270 has at least two power amp versions, not sure about the 2245. Your info on this could be helpful!

Tom
 
catrafter said:
The 2270 has at least two power amp versions, not sure about the 2245. Your info on this could be helpful!

Tom
Click to expand...
It seems that schematic (where power transistor emitters are connected directly to rails ) is applied to the units for the EU market. I spent 30 min debugging this then I realized that I was looking at the incorrect version of the schematic and the board I was working on.
 
20191004_173541.jpg 20191004_173550.jpg Amplifier gain Av= 20V / 0.5V = 40 (32 dB) Is this correct ? some pictures R channel connected to the 4 Ohm resistor. (25 Watts power out) transistor were bit hot :)
 
Check the pre-drivers: Marantz used glue to attach the plastic spacer on the pre-drivers and as it ages this glue is known to become somewhat corrosive and conductive (encounter this problem in a 2270).
 
Now I realized that Stereo light is on when TAPE and PHONO is selected but will not go on with FM mode . Any suggestions where to look? I have not touched anything in that section . 20191004_235103.jpg 20191004_235109.jpg 20191004_235116.jpg
 
This and the service manual explains stereo FM operation very well. The stereo lamp is controled by the P300 board when a stereo station is detected.
An except:
If the 19kHz pilot is present from the broadcast station, then it is doubled to 38kHz by H304 and if of sufficient amplitude to decode stereo, H305 turns on causing the stereo lamp to illuminate.

Tom
 

Attachments

catrafter said:
This and the service manual explains stereo FM operation very well. The stereo lamp is controled by the P300 board when a stereo station is detected.
An except:
If the 19kHz pilot is present from the broadcast station, then it is doubled to 38kHz by H304 and if of sufficient amplitude to decode stereo, H305 turns on causing the stereo lamp to illuminate.

Tom
Click to expand...
Thank you for an explanation on this. Martin :)
 
Traverse said:
Hello Tom, Would the STEREO indicator be ON , even selector switch is in PHONO, TAPE or AUX position?
Click to expand...

Yes, the "STEREO" light is on for all sources and when receiving a FM stereo broadcast, it is off when AM or the MONO switch is selected.
 
Leestereo said:
Yes, the "STEREO" light is on for all sources and when receiving a FM stereo broadcast, it is off when AM or the MONO switch is selected.
Click to expand...
Issue here is that STEREO indicator used to be on FM select only , but now is on all the time. I have not done anything to this baby :) Even there is no FM wire antenna connected.
 
You must log in or register to reply here.
Back
Top Bottom