[brach]

Even if the solder joints on those parts i mentioned in my previous post look fine, resolder them anyway. Don’t add anymore solder just heat them up until they flow onto the pad. We call it “reflowing” the solder joints. Sometimes it’s easier to reflow all of them than try to trace out the single bad joint. Also make sure that all of those components are in the correct place and nothing is switched. Since you said the volume control has no effect on the sound then it must be in the signal path after pot VR9. Look very closely at all the solder joints around the audio amp ic U3, make sure nothing is bridged…use a magnifying glass if you need to. Good luck. Let me know what you find.
-Brach

[brach]

That is very peculiar. I’ve never heard a sound that could be described as a rattle coming through the headphone jack. Does it sound more like a mechanical rattling noise or an electrical oscillation? An oscillation could sound like anything from a
“thump thump” to a high pitched whine.
Since the sound is coming from the headphone jack, I’m suspecting the circuit around the LM386 audio amplifier chip could be the issue. Please double check (and even re-solder) the pads in the audio path…R14-16, R19-20, R22, C8-11, C13, C3-4, U3, and of course make sure the speaker and the headphone jack are properly soldered.
Good luck.
-Brach

[brach]

Matthew,
I don’t remember experiencing this particular wooshing sound before.
I know you have experience building circuits, but i have to say this…please make sure all the wires (xlr and piezo) are hooked up to the correct place (don’t forget to check in the xlr jack). Also check with your multimeter to make sure you are getting a very high resistance (almost infinite) between the piezo disc wires…I’ve built hundreds of these things and I still make wiring mistakes sometimes.
It sounds like you know what you are doing on the pcb side of things, which makes me think this issue might be related to the piezo disc. If you unsolder the piezo disc wires from the pcb and plug the XLR in to the preamp, do you still get that sound? If you don’t, i would suggest trying to hook up a different “test” piezo disc (without even shielding it) just to see if you get more expected results.
If you still get the noise when the disc is unsoldered then there’s a good chance that it’s a component failure. It’s pretty unlikely to have a component failure, but still possible.
Let me know what you find.
-Brach

[brach]

I’m sorry to hear you are having trouble. I have never heard of that particular issue before. Do you hear anything through your headphones when you plug them in to the output jack (keep the volume low when testing this)? Let me know what you find out.
-Brach

[brach]

Giovanni,
Sorry for the late response.
In reference the “quick start guide”…First of all make sure the depth, attack, and release knobs is all the way to the left. Plug in headphones or computer speakers and then test it again. If you still can’t hear any sound, then double check your solder joints in the audio section of the circuit board…which is all the components connected to the LM396 amplifier chip. Make sure everything is connected properly and nothing is connected that shouldn’t be.
If you still can’t figure it out, let me know and we’ll start looking at the voltages around the audio section to see if everything is functioning properly.
Good luck.
-Brach

[brach]

Great! I’m glad to hear that it’s up an running. Yes, the socket should be tight enough to make good contact with the tube pins…I’ve also learned that lesson the hard way!
Enjoy the amp!
-brach

[brach]

It looks like you fixed that issue. What does TP9 read now?…i’m assuming that it’s around 70V?
Are all the other test points reading correctly?
Just for good measure you may want to reflow the other joints around the signal path and tighten the other tube pins. To keep from having to pull the board out of the chassis just solder as many components as possible on the component side of the board and see if that fixes the signal path problem.
Do you, by chance, happen to have access to an oscilloscope or other signal tracing tool? If not, no problem…just thought i’d ask.

[brach]

…and yes, R7 and R14 would measure about 750R in the circuit because they are nearly in parallel. One side of both resistors are on the same node. R7 goes to ground and R14 goes to ground through the secondary of the output transformer which has a negligible dc resistance.
-brach

[brach]

Let’s get this fixed…
According to your voltage chart (by-the-way, thanks for that…it’s very helpful) R8 only has 1 volt dropped across it, which tells me that there is very little current flowing into pin 4, which is why TP7 only has 32mV across it. The problem seems to be with either pin 4 or 7 on the tube. The fact that you are getting no sound at all leads me to think there is an open somewhere in your circuit (probably at pin 4 on the tube). Resolder pins 4 and 7 on the tube to make sure they are solid, as well as R8 and R7. Also check to make sure the tube socket itself is making good contact with the tube pins. If you pried the tube socket contacts open too far then try to pinch them closed a bit more. When you get TP7 and TP9 fixed then the voltage on TP5 will probably normalize. Make sure you drain the caps before soldering pin 4! Keep me posted on what you find.
Good luck!
-brach

[brach]

ChristopherM,
Thanks so much for letting us know. I just fixed it. Yesterday I uploaded a newer version of the manual an I forgot to update the links.
Anyway, it’s all good now.

[brach]

Thanks for the questions. Yes, the schematic for the Percolator is provided with the kit as an assembly aid and for help in troubleshooting.

[brach]

Exceltent! That was very helpful.
It looks like your switch went bad in the explosion. Unfortunately, you’ll have to take the pcb out of the chassis to replace the switch. The best way to remove the bad switch is to clip the off the 3 leads and heat up the 2 front posts at the same time until you can pull the switch out of the pcb…be careful, it gets hot!
I’m glad to see that none of the switch contacts are connected to ground. But also please check the round pad on P4 to see if that is connected to ground….hopefully it’s not.
Next, you’ll have to scrape off all the carbon from the burned spot. It’s easy to do with a wire brush, but if you’re careful you can do it with a razor blade or other scraping device. You should notice that there was a trace on the pcb going from the round pad of P4 to the middle pin of the switch. Surrounding the trace on either side is the pcb ground plane (a layer of copper covering most of the board that is connected to ground). Both the trace and the ground plane are usually covered by a layer of blue paint. In the photo i have scraped off the paint on the trace and some of the ground plane so you can see the copper underneath.

The explosion was most likely caused by a short between the trace and the ground plane. Possibly a drop of solder or small piece of wire was bridging the trace and the ground plane. In scraping off the carbon it is important to make sure that there are no tiny little drops of melted copper hanging around that could possibly short it out again. You’ll have to replace this trace with a jumper wire…you can use one of the wires you cut from the transformers. cut it to length and strip off only the amount of insulation necessary to solder the ends to the middle pin of the switch and the round pad of P4 (tin both ends first). make sure there is not strands of wire or anything that could bridge to the ground plane or to other pads near by.
When you re-wire the pcb back in the chassis make sure the transformer wires are not touching anything on the solder side of the board.

• This reply was modified 9 years ago by brach.

[brach]

Thanks for answering so quickly.
Just to be clear…i’m assuming you are testing the switch with the chassis upside down (the transformers sitting on the table) while the bottom of the chassis is open facing up…is this correct? in this position was the switch up or down (down would be pointing to the word “power” on the faceplate) when you got continuity between the middle pin and the pin furthest away from the front of the chassis?
Please re-check the continuity between the middle pin on the switch and the round pin on P4. If you measured 122V on the middle pin after the explosion, you should have continuity between these two points. Make sure your meter probes are making good contact. Also please re-check to see if you have continuity between any of the pins on the switch and one of the square ground pads near the IEC power socket.
Does your meter actually make a beep noise or are you using the resistance setting on your meter to determine continuity? I’m asking because i can give you better instruction if i know what method you are using.

[brach]

Thanks for taking those tests.
It looks like you need a new fuse. You can get one from us, but you’d probably be better off finding one locally…it’s a pretty generic part. You need to get a 1A, 250V fast-acting (not slo-blo) fuse. I think 15mm is the length, but you can measure that yourself.
Next, your pcb needs some help. But before we get to that let me ask…when you measured the 122V (am i correct to assume AC voltage?) on the switch, when was that? Was it before or after the fireworks? Was it plugged in to the wall when you measured that? It would be helpful to know this before we proceed with the pcb repair
In the meantime you can check out the switch to make sure it is still working correctly. With your meter still on the continuity setting measure between the switch’s middle pin and both the outside pins (one at a time). It should beep while touching one outside pin, but not beep while touching the other. Flip the switch and test again. It should now be just the opposite…it should beep on the other pin now and not beep on the pin it beeped on originally. Let me know what you find (or if this is too confusing).
Good luck.
-brach

[brach]

The wedding isn’t happening until later this afternoon so I have a bit of time right now to help figure this out…
First of all, sorry for the trouble. Secondly, don’t worry…we’ll get this fixed real soon and you’ll have a nice amp to play through. I’m out of state right now so i don’t have an amp in front of me, but we can get started with what i remember.
Thanks so much for the photos. That really helps. The voltages you read seem correct when the amp is plugged in to the wall and the switch is on, but from now on keep it unplugged until we get some more measurements done.
With the meter on the continuity setting (it’s the one that makes a beep when the two probes touch…not the voltage setting) test weather you have continuity between the round pad of P4 and the middle pin on the switch (it should beep if there is continuity). Also check weather you have continuity between either of these points and ground.
Also, check for continuity across the fuse to see if the fuse blew when you saw the fireworks.
Do these tests and let me know what you find.
Good luck.
-brach

[Author]

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