my mcintosh mac 1900 keeps blowing 5 amp fuses!

VFET747

Member
hi everyone, this is continuation of thread that I started in year 2009! when I first posted about this receiver and then let this receiver aside in my basement and after all these years have only now! find time to work on it again, does any mcintosh expert know why it keeps blowing 5 a fuses?! checking the power supply with a digital meter shows that dc voltage after the 600ohms 5 watt resistence shows 150vdc instead of the schematic reading of 110v dc, also one one of the driver board all the group of three transistor show defective replacing these , a diode and all new electrylitc capacitor on the board did not solve the problem, also like to mention have replaced the 9000 uf /50v electrolytic and one of the multi sec capacitor 80/80/50 uf , the pre-amp section is working great, the am is working too, there is no sound from fm wonder why the signal strength meter show tuned station but no audio, and lastly there is problem with balance control when you move it in the center the audio disappear but when you move it to extreme left or right the audio for that channel appear , I have the service manual, any kind help by resident experts here will be much appreciated, thanks! Ali

the other driver board where the parts are all original and not replaced the transistors get so hot you can see smoke rising all this happens powering with a variac at only 40-60 volts, the other voltages like -40 and + 40 dc are correct the smaller voltages from power supply seem o be correct as the light light up well in the panel, the pre amp section is working, as is the line input and the am section,but no sound from the fm section, this was verified using the pre-amp out jacks connected to a Nad integrated amp aux input and listing to the sound via nad stereo headphone socket
 
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I would suspect multiple outputs have shorted.......and then who knows what else went as you kept swapping in fuses.

Never ever add fuses until you have monitored the current draw through the fuse with your meter as you slowly creep up the variac. After all these decades though if i see a blown 5 amp in one of these vintage units I just add some more MJ15003Gs to my next part order to make sure i have some for the next one that comes in........
 
I would suspect multiple outputs have shorted.......and then who knows what else went as you kept swapping in fuses.

Never ever add fuses until you have monitored the current draw through the fuse with your meter as you slowly creep up the variac. After all these decades though if i see a blown 5 amp in one of these vintage units I just add some more MJ15003Gs to my next part order to make sure i have some for the next one that comes in........
 
hi cdk, thanks for your hint about measuring the current draw through the fuses . I am replacing the blown transistors one the power amp driver board of my mac 1900 receiver, the service manual parts numbers make no sense, would you be knowing the value of the Q 405, and D412, thanks for your help!
 
hi cdk, thanks for your hint about measuring the current draw through the fuses . I am replacing the blown transistors one the power amp driver board of my mac 1900 receiver, the service manual parts numbers make no sense, would you be knowing the value of the Q 405, and D412, thanks for your help!
sorry diode D413
 
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