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Post by roly on Mar 10, 2020 21:23:17 GMT -5
Amp Is a Blues Deluxe.
With all tubes removed, the fuse holds while on standby.
Brief hum before 2 amp breaker clipped to fuse socket kicks out when amp is taken off standby.
How is AC voltage getting to the output transformer with all tubes removed?
Thanks for your help.
cheers Roly
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Post by roly on Mar 10, 2020 22:40:03 GMT -5
With the B+ disconnected, the clipped in breaker holds, but there there is still a subtle hum.
So....no B+....any guesses as to why there is hum?
Thanks folks.
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Post by roly on Mar 10, 2020 23:26:21 GMT -5
I think the hum is coming from the output transformer itself and not the speaker.
cheers
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Post by LTB on Mar 10, 2020 23:58:57 GMT -5
I think the hum is coming from the output transformer itself and not the speaker. cheers As long as the Power Switch is on the transformer has power. Tube's heaters have power at all times when Power switch is on. When Standby switch is switched off standby the Power Supply produces power whether tubes are in or out. I would suspect bad power supply cap/s or Rectifier diode/s Beware of very lethal high voltages. Do not stick your fingers in there unless you know for sure there is no power on the caps. If you are not experienced in amp repair or electronics I suggest you take it to someone who does. Specifically C31,C33 (Page 2 lower drawing) Fender Blues Deluxe Schematic |
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Post by roly on Mar 11, 2020 0:51:27 GMT -5
Hi LTB
Thanks for your advice, I am well versed with the risks inherent with tube amps.
I have probably built at least 30 amps.
Here's what has me confused....with all tubes removed the amp briefly hums very loud before the clipped in 2 amp breaker kicks out. That's also the case with all leads to and from the 47@500 and three 22@500 caps disconnected.
With the B+ disconnected, the breaker holds but a subtle hum is present.
So.... all large value caps are out of circuit. My limited skills suggest that there is no way that the output transformer can be seeing any AC voltage to amplify, but that's not the case.
I have not yet checked diodes.
cheers
Roly
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pdf64
Wholenote
Posts: 556
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Post by pdf64 on Mar 11, 2020 6:21:36 GMT -5
Where’s your light bulb limiter in all this 🤔
In cases like this I use my LBL and then work my way down the HT line etc, first breaking the circuit between the winding and rectifier. If LBL shows no fault then reinstate that break and move the break to between the rectifier and reservoir. And so on
Yes, check those diodes.
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Post by Peegoo 🏁 on Mar 14, 2020 0:07:12 GMT -5
I wonder if the PT's magnetic field is inducing current in the OT, causing the hum.
Just a wild-arse guess...I'm not an expert in this stuff.
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Post by modbus on Mar 15, 2020 15:41:12 GMT -5
I would guess that, given the fuse blowing when you go off standby, something on the high voltage rail is shorting to ground (maybe not a dead short, but enough to draw a lot of current). The hum you hear is probably the PT oscillating from driving an excessive amount of current into the short prior to the fuse blowing.
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Post by LTB on Mar 17, 2020 22:43:41 GMT -5
Where’s your light bulb limiter in all this 🤔 In cases like this I use my LBL and then work my way down the HT line etc, first breaking the circuit between the winding and rectifier. If LBL shows no fault then reinstate that break and move the break to between the rectifier and reservoir. And so on Yes, check those diodes. Like pdf64 stated a lightbulb limiter in series with the amp's power cord is an effective tool in troubleshooting over current problems. I not only limits current to the maximum amount the bulb draws but the amount of illumination can show when a component is the culprit by lifting and and reinserting each component one at a time. |
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Post by modbus on Mar 18, 2020 9:41:39 GMT -5
Also, if you've got one of these you can see what's warming up when you turn it on:
You said it takes a second or two for the breaker to trip, so it might be enough time to warm up whatever is shorting.
Since all the tubes are out, you won't have to worry about the heaters giving a false reading.
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Post by LTB on Apr 4, 2020 16:07:12 GMT -5
Amp Is a Blues Deluxe. With all tubes removed, the fuse holds while on standby. Brief hum before 2 amp breaker clipped to fuse socket kicks out when amp is taken off standby. How is AC voltage getting to the output transformer with all tubes removed? Thanks for your help. cheers Roly Any news on troubleshooting and/or repair of your amp yet??? |
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Post by roly on Apr 26, 2020 11:34:53 GMT -5
I handed the amp over to a friend whose skills are far better than mine.
Here's his conclusion.
D5 on the tube pcb has failed.
The part number is VCB or VC8 over R3000.
With the output transformer primaries disconnected, the breaker holds when the amp is taken off standby.
Add centre tap....good.
Add one side of the primary (wasn't told which one)...good.
Add other side....breaker trips.
He figures a shorted power tube caused the problem.
cheers
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Post by Peegoo 🏁 on Apr 26, 2020 13:37:06 GMT -5
Wow...good info!
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Post by LTB on May 1, 2020 23:43:09 GMT -5
Very cool you found the problem.
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