• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Kofi Annan in: "Kofi's Baby Huey"

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#121
I measure megaohms. My VOM keeps climbing until it runs out of digits...

Does that mean it's likely to be a short in the PSU? I noticed that initially the DC voltage climbed to about 390V before the fuse popped. Subsequent attempts with, I assume, some charge remaining in the caps popped the fuse right away.

Maybe a bad cap? How can I check?

Thanks again for all the help. This is frustrating as the answer is right in front of me, no doubt, and I'm too stupid to see it.

Kofi
 
#123
Must be the PSU then. 2 caps, 2 diodes and a choke. So the diodes are not short circuit, the caps are not short circuit, and the polarity is correct on the caps? If you disconnect both leads to the choke and power up, does it blow the fuse? I hope your filter caps are rated at least 400V and 450V surge. Best to wear some protective glasses in case.

To measure the caps, disconnect either terminal from the circuit, and measure the ohms across it with the red lead on the positive terminal. The ohms should climb into the megohm range like when you measured the OPT centre tap to ground.

What is the part number of the diodes you are using?
 
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#124
The diodes have NF9426 and U860 printed on them so I assumed they were MUR860s, which I normally order. I found them in a parts bin, so I could have made a mistake, but I don't think so.

I'll re-wire tomorrow with some N4005s that I have and see if that does the trick.

Both caps are in the 5 - 8Mohm range on the VOM, so hopefully they're OK. Still, there should be something wrong here...

Kofi
 
#125
For center tapped full wave rectification the inverse across each of two diodes will be 2 times the output. Its not like in bridge. I would use 1000 PIV for your 310Vo PSU. How much is it for DC before the choke?
 
#127
If you need to test the choke, maybe you could connect a 100ohm or 150ohm 5W resistor in place of the choke. If the fuse doesn't blow, put the choke back in, if the fuse then blows, then the choke is no good. Yves suggested a faulty choke back in post 114, I was hoping it would not be that, for your sake.
 
#128
Success! I replaced the diodes with Rat Shack 1N4007s and the supply is up and working! Here's the bad news... the supply is at 380V and the 5V supply is not lighting the LEDs on the CCS. It plays music but it sounds awful, likely because the CCS ain't working.

I'll need to investigate the issue with the CCS, but I don't know what's wrong yet. I suspect that once the CCS is working the B+ will come down to normal...

Any ideas?

Kofi
 
#130
So, when I check for continuity on the CCS, the LED lights, which should mean that it's seeing voltage from the 5V rectifier. It just doesn't light when the power is applied, which makes me think that I have the PSU wired backwards or that it's not pushing enough voltage.

I will review the circuit tomorrow and reply, but any other recommendations are welcome.

Thanks!
Kofi
 
#132
NEW POST

Here's the PSU. I had some trouble understanding the 5V negative supply, so I may have screwed up the wiring in concept. Any explanation of the supply would be helpful.

Thanks in advance.... we're close here...

Kofi
 

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#137
Kofi Annan said:
and shorted the base of the BC547B to ground. This is probably an error, right?
Click to expand...

You bet it is.😉 You shorted the CCS's reference. No chance it could work like that. Ropt-Copt are described as extra R-C filtering of LED's noise towards the transistor's base. LED is very low noise if not blue anyway. Short Ropt and just delete Copt. If your 547s are intact it will pull and light up as it should.
 
#138
So, I was actually NOT that big of a doomasse and I did NOT short the base of the BJT. I checked for continuity on the LED and during that testing I observed it lighting. However, I'm tryong now to re-create that effect and it won't light-- possibly due to the caps being charged or something, I'm guessing...

What's the best way to check and see if I've wired the LEDs backwards?

Kofi
 
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