Problem build [resolved]

[Kevin]

I've had nothing but problems with this build so far... I have been in email contact with PB because I was having some issues posting on the forum. He has been very responsive and helpful. I know he's super busy, so I decided to move it here to the forum to see if anyone has any suggestions too.

I have completed the Mainline and S.E.X. amps previously without any issues, so I am fairly confident in my ability in building amps. This is the first time I'm using 2% silver solder, but I don't see how the problems I ran into below have to do with poor connections since I didn't need to re-solder anything.

• Fuses blew during initial wall voltage check. I blew 2 2a fuses (fast blow). Didn't see anything wrong. For some odd reason, when I tried the next day after looking everything over again and not finding anything wrong. 2a fuse (fast blow) worked. I proceeded with the build and went on to complete both glow tests.
• Fuse blew during high voltage check. Checked everything again, found nothing wrong. PB suggested using a 3 light tester on the wall outlet I've been using and trying a 5a fuse. Wall outlet was wired correctly per 3-light tester. Tried again with 5a fuse, and it worked. No more blown fuses. High voltage test passed with flying colors.
• PB had suggested I flow some extra solder onto the joints since some of my joints looked like lead-free solder. I added some solder on after the high voltage test. The C4s boards, however, were made with 2% silver solder, since I made them over the weekend while I was trying to figure out the fuse issue.
• Now, for both boards, only the LEDs on A side were lit.  The voltages were the following:

• IA on both boards were 524v
• IO on the A board was 365v. IO on the B board was 363v
• 32u = 0.147v (does not respond to potentiometer)
• 46u = 0.865v (does not respond to potentiometer)
• 10u = 215v
• 15u = 240v

I attached some pictures below with what I have done so far. Any help would be greatly appreciated.

[Deke609]

Hi Kevin - I am just a hobbyist with a shaky at best understanding of a tiny fraction of this stuff, so this is just a poorly-educated shot in the dark. 524V at IA/IB seems pretty high given that the target is approx. 450V.  This suggests to me that the problem lies with the power supply circuit with the six electrolytic caps and the two 120 ohm 10W dropping resistors. I would double check that everything is wired to the correct terminals. If that checks out, I would then focus on the the 120 ohm resistors. Maybe desolder them completely and resolder them after crimping the leads to the solder terminals. 


Going by your pics, I might also touch up the power supply cap soldering by bending and crimping the cap leads around the terminals and adding a touch more solder.


Not sure if this will help at all - just my best guess.


cheers and good luck, Derek

[Deke609]

I'd also check that none of the leads of the 120R 10W resistors isn't making contact with the chassis -- and in particular, that a lead of the 120R closest to the front of the amp isn't in contact with a screw or nut securing the solder terminal strip.


[edited to correct typo: wrote "is" when I meant "isn't" - oops]

[Kevin]

Hey, Derek. Thank you very much for the suggestions. I'll try those as soon as I get back.

[Doc B.]

What's your AC mains voltage reading?

[Kevin]

Hey, Doc. ~123.6v from the wall

[Doc B.]

RE the fuses, we run them pretty close to the limit so they will definitely blow before any other damage occurs. My experience over the years tends to indicate that fresh electrolytic caps may draw more current at start up than they do once they have cycled a couple of times. That may be the reason they blew the first couple of cycles but the fuse in there now has held OK.

With respect to the LEDs not lighting and the high voltages at OA and OB, those both tend to indicate that only the Zener string is regulating voltage and the shunt regulator is not drawing the current it should be. I would be inclined to double check that the wires leading off the boards to various terminals are connected to the correct terminals, and that there are no hidden breaks in those wires under the insulation right where the strippers would have cut it.

[Paul Birkeland]

What voltages do you get at 53 and 65?

I know this is unrelated to your C4S questions, but if the 300B tubes are conducting properly, the IA voltage will be drawn down by their operation.

[Kevin]

I'm a dumbass.

I went through all your suggestions and found nothing wrong. I decided to go back and look for issues with the board. I didn't think it was the boards, because the issue was symmetrical. So I figured that the issue might be somewhere in the middle of the amp. However, because I built the C4s boards along sides one another, when I missed the step where I connect breg on B and bB, I missed it for both boards.

I added the wire, and now everything is working.

IA on both boards now 515v.
IO on the A board is 297v. IO on the D board is 298v.
32u and 46u both now adjustable to 175v.

Thanks again for your help and infinite patience... Can't wait to knock this out and start on BeePre next when that comes in!

[Paul Birkeland]

Excellent!