Line AC on B+??

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Hey all I need some tips! I am currently repairing a Randall RH100 and RH200sc. I have finished repairing the power section of the RH100 but I have 110ac on the B+?? It is a push-pull head using complimetary pairs of 2sc4793/2sa1837, 2sc1815/2sa1015 and 2sc5198/2sa1941 at the final stage. The transformer is 110primary to dual secondary 80VAC center tapped and 30VAC center tapped. I have already replaced the 1n5402 rectifier bridge completely with no effect. What is puzzling me is that the AC gets through the B+ side of the rectifier bridge at full cycle??....the diodes should clip half of that off... If there is short from the primary of the transformer to the secondary shouldn't the transformer output voltage drop signifigantly?? The real kicker is that I still have my + and - 40VDC after the rectifer???? ..................i have a headache!

-Brian
 
Are you using a scope to see this 110V AC ??? or DMM???
Is this 110V AC you are seeing at 60Hz or 120Hz????
Sometimes if the scope and the circuit your measuring share the same ground at different outlets..you get a ground loop when measuring and this shows up on the scope screen as 60Hz AC...Sometimes I plug the scope into an isolation transformer to avoid this problem... or you use a ground breaker on the scope and then use an aligator clip to ground the scope to the amp chassis, in which the amp is grounded properly at the outlet...
If the filter cap is working and or wired properly after the rectifier, then you should have a low impedance path for the AC component to ground return at that node....therfore should eliminate this AC.... check the AC ripple current through this cap...
Is this a grounded 3-prong chord for the main????
Check your outlet... Measure the AC voltage between Neutral and Ground... Make sure Neutral is no greater than .5V above ground..
It really should be no greater than 250mV in most cases...
Make sure you get 120V AC or whatever you have between HOT to Ground as well as HOT to Neutral....

Chris
 
metallifornia said:
Unfortunately I am using a DMM to see the 110 AC so I don't think the other ideas apply in my case. I did not check the reference of the Line ac at the wall outlet. I'll try that when I get home.

-B


It's possible that this 110V is a paper monster... It may be that unless it's loaded you won't see it...
The pri-sec AC leakage current on the transformer is normally small, uAmps...It can show a 110V into a high impedance but dissapears when loaded...
Did you observe this with the power supply properly connected and loaded, with the circuit running????

Chris
 
yes, with a speaker enclosure attached and the power supply properly connected all that I get is a loud HUMM. This was the reason to start digging around how I found the 110AC. Yes I suspected that the super high input impedance of my DMM might exaggerate the measurement after the rectifier. Unfortunately I don't have the scratch to purchase an oscilloscope.........

Thanks!!

-Brian
 
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