Burning Power Resistor in Organ PS

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meatwave

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Joined 2016
#1
Hi everyone, long time lurker, first time poster.

I'm having trouble bringing this classic Lowrey DSO-1 organ back to life.

After a catastrophic capacitor explosion in the power supply section the organ went dead, no sound at all. I've replaced all of the filter capacitors, and now I have just a fairly loud hum coming from the speakers, and a 1.5k 20w resistor that is smoking hot.

Here is the schematic, I've circled the part in question, its on the right side of this schematic in the power supply section, you may have to zoom in with your browser.

Could anyone tell me what would be causing this? I'm thinking it could be a faulty capacitor in that section, but I'd love a second opinion before I go replacing things blindly!

Thanks everyone
 
#2
That node supplies the two phase inverters. Discharge all caps before doing any work on this and unplug the amp. Disconnect the wire to those phase inverters to isolate the cap from other parts of the circuit. Then do a resistance reading from that cap to ground. It should read as an open. If this is the case, then do a resistance reading from the wire you disconnected to ground to see if it has a short. This will tell you where a short or partial short is at. When a cap explodes, a good visual check helps too. You never know if other parts were destroyed along with the cap.
 
#3
Thank you, I did as you said and realized I had it wired wrong. Now that I've rewired it correctly, the 1.5k ohm resistor isn't heating up anymore, but I'm back to my original problem.

Something is making the 7.5 ohm 5w resistor get way too hot. I've updated the schematic and re-posted it HERE. Am I right to assume that the first 150uf capacitor in that series is likely shorted? I assume that would cause this problem.

Thanks everyone, and thanks boobtube for your help.
 
#5
Thanks, so far I've replaced all the polarized capacitors in the power supply. Other parts of the power supply seem to be working fine. Is there a difference between a smoothing and a filter capacitor?

I might just have to buy a new set of caps and start over. This is driving me crazy. Now the 8 ohm power resistor is not getting hot, but sound is still dead and I seem to have no B+ power. I'm wondering if I fried all my newly replaced caps, and maybe cooked that power resistor while I had things wired wrong and was making the magic smoke.

I really appreciate your help!
 
#7
By the way, welcome to the forum.

Smoothing and filter caps are the same. Just different names. It sounds like it might be best to check the whole power supply. With everything changing all the time, it's hard to tell just what is wrong.

Another thing you should do is to take an AC voltage reading for the output of the secondary of the power tranny to make sure it hasn't been damaged with all of this. Also check the diodes, the resistors, all of it. With no B+ you need to start at the AC output and figure out what is still working.

Thanks for the updated schematic. Much easier.
 
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