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innercityman

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Check your wiring from the power transformer, I got smoked diodes, when I misswired the power transformer in one build.

Thanks Tim but the wiring of the transformer I have is very simple, you can't go wrong... 1rst secondary is for the 220V input, one 15V and one 12V. It's something else. I triple checked my solders and can't find any short.
 

innercityman

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Your transformer is fine. Without load the AC voltage is higher.

I know that but almost 310VAC is a huge voltage IMO. Even if I already triple checked my solders, I'll do it again, but what is crazy is that I have the same diodes rectifiers smoke with the HT psu out of the board. I did everything advised on this thread and read almost the 115 pages of it... Star ground, D.I. traces cut mod, 470uf/400V caps instead of the 100uf on the schematic... I'm considering buying two 30VA 15V secondary transformers and go with the classic build.
 

innercityman

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Your transformer is fine. Without load the AC voltage is higher.

The problem is somewhere in your build. Smoking rectifier diodes is sign for a very low resistance betwen HT and Ground, probably a dead short.

It is pretty hard to help you from the distance. Check your build again and show some photos of it.

I've checked every solder pads and I'm 200% sure I don't have no short anywhere. My tension measurements are good. Both channels work, condenser mic works with 48V engaged. But 100HZ noise issue on both channels. Have the noise even when the unit is off, but less of course. I've tried to move the power transformer but no change. Maybe I should move my star ground place ?

Still have the little smoke cloud from the bridge rectifier at first start. If I turn the unit out, and power it on again few minutes later, no smoke anymore.

Here's a picture of my build...
 

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innercityman

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Picture of my star ground wiring...

I'm thinking about replacing the Don Audio PSU transformer with 2 transformers, following the original design.
 

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rock soderstrom

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That's just my humble opinion:

Your setup has a lot of problem areas. Your AC cabling is much too close to the sensitive parts of a microphone amplifier like the signal transformers. For example, the red HT cables are 2cm away from the output transformers. Furthermore, you run the cabling to the 470uf capacitors across your case. Why? You are begging for trouble. The regulator for the heater voltage is also wired across the case under the board, I wouldn't do it that way either. Keep the distances short. What about the shielded cables? These must not build up ground loops. Each part of the circuit must have only one path to ground.

Your central ground point does not comply with the legal requirements in Europe, please google. I would completely rebuild the HT power supply. Get a decent board that is worth the project. Turn the transformer so that the point where the cables exit does not point to the transformer or PCB. Overall, I would make all AC wiring as short as possible and as far away as possible from the PCB/ transformers.

I don't understand how your amp can hum in 100Hz when it's turned off?

The smoking diodes are not normal either.

From my point of view, your build would not be better with two transformers instead of one.

What is this blue thing under the HT PCB? A choke or a temporary spacer?
 

innercityman

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Thanks rock soderstrom.

What is this blue thing under the HT PCB? A choke or a temporary spacer?

A temporary spacer.

You're right, I should clean up my wiring.

I think I found out about the hum issue... I thought my audio input/output cables were shielded, in fact they're not, I've just checked. So I'm pretty sure that replacing them with a proper shielded cable will solve the noise problem.

I'm still lost about the smoking diode at first start... I'll try to move the star ground point to the bottom of the chassis, drilling a hole.
 

innercityman

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I would check the wiring.
Especially from the small psu board to the main board.
From the first look it seams wrong.

Can you explain what seems wrong to you ?

I don't think my wiring from psu board is wrong. My tension measurements are good... 246V at R31/131, 48V at R42 and 11,9V. I'll replace audio input/output cables today with shielded ones I guess it will solve the hum problem. For the smoking bridge rectifier, I had the same behaviour with D17 to D20 diodes when HT psu was on the main board.

make sure there is good continuity between all enclosure parts and ground

I already checked that of course, I'll try to move my star ground point and see if it helps.
 

rock soderstrom

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If your enclosure is higher than the diameter of your transformer, it might be a good idea to mount the transformer upright on the right side wall. This will give you space and clearance.

Can you sketch your HT power supply? Which diodes do you use? Type?
 

innercityman

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Can you sketch your HT power supply?

Sketch ? What do you mean ? I used the spare psu board that I finally find out.

Which diodes do you use? Type?

I used a 2W10G-E4/51 Vishay Bridge Rectifiers instead of 4 diodes, but as I wrote before, Same issue when psu was on main board with 1N4007 rectifier diodes.
 

rock soderstrom

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innercityman said:
Sketch ? What do you mean ? I used the spare psu board that I finally find out.

I mean if you can make a drawing of your used circuit for the HT power supply so we can see what exactly is going on.

I think your smoking diodes come from the current flowing into the gigantic capacitors at the moment of switch-on. That's why the problem disappears when you switch on again after the device has already been switched on once - the capacitors are then still charged, the inrush current flow is lower,
 

innercityman

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I think your smoking diodes come from the current flowing into the gigantic capacitors at the moment of switch-on. That's why the problem disappears when you switch on again after the device has already been switched on once - the capacitors are then still charged, the inrush current flow is lower,

Very interesting... This is probably the explanation. So these caps could be faulty or maybe two big ? What would be the solution... Replace them with smaller ones ?

I'll draw my psu set up and upload it.
 

innercityman

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I made an error on my drawing... I bring back the 245 VDC to main board  from Cap 2 of course, not from Cap 1 like I've sketched.
 

rock soderstrom

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We need more detail, a schematic! I can not see whats going on on the HT PCB.

Is there a resistor betwen the two 470uf capacitors? Like R1 in the attached schematic? If so, what value?
 

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rock soderstrom

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innercityman said:
Very interesting... This is probably the explanation. So these caps could be faulty or maybe two big ? What would be the solution... Replace them with smaller ones ?

The caps are ok, just to big for this setup. You can test it, disconnect one of them.

You can fix it with smaller caps or more resistance, so that the inrush current is limited.
 

innercityman

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The caps are ok, just to big for this setup. You can test it, disconnect one of them.

You mean disconnect one cap from the circuit and power up the unit ?

You can fix it with smaller caps or more resistance, so that the inrush current is limited.

Which cap value would you advice me ? I have a pair of 330uF 450V so lower in Farad but Bigger in voltage and even size, should they work better ?

Also you tell me that I could solder resistors before the caps ? Which value and I guess at least 2W resistors right ?
 

rock soderstrom

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rock soderstrom said:
We need more detail, a schematic! I can not see whats going on on the HT PCB.

Is there a resistor betwen the two 470uf capacitors? Like R1 in the attached schematic? If so, what value?

Please answer the questions first so I can help you! I need to know exactly how your circuit is built, without that it is a blind flight. This is too dangerous for me, HT and oversized capacitors are an effective combination to blow you out of your socks!

 
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