I was trying to add some pin jacks to my SE board and scewed something up. I took it apart and one of 1 watt, 10 ohm resistors was shot. Ok, problem solved. Nope, still can't get any bias. What should I check next? Thanks.
Ishold add that all other tubes light up,
like normal. I must have shorted something to grnd.
Ishold add that all other tubes light up,
like normal. I must have shorted something to grnd.
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I pulled D1,IC1, IC2, IC3, Q1 and Q2 and checked them, they measured the same as new ones, but I replaced them with new ones, just to play it safe. When I turned it back on, I was able to get the rectifier lit and the 5842 set to 175 VDC. Still nothing out of the 300Bs though. There is 6.3 V to T1 Grn. Resistors and diodes check out. I get 465VDC to 560 VDC across the small pins of the 300B socket, depending on the position of the pots. Across the big pins I get .166 VDC and .163 VDC. Across the big pins of the 300B tubes, I get .9 ohms, across the small pins, I get 0. Any suggestions on where to go next?
Just some more info,Edcor power transformer has 5.6 VAC on the Yellow to Yellow Orange to Red is 679 VAC, Brown to brown is 6.7 VAC, Orange to White center tap is 337 VAC, Red to White Center tap is 338 VAC. This all seems about right to me. Output transformers show identical ohmage across the input side. The choke measures 89 ohms. I'm wondering if it could be the 300B sockets not making pin contact? Was working tho, before I started screwing around with it.
Your filament voltage (across the big pins) should be 5 volts. You may have shorted something in the filament supply. Check for sloppy solder bridges where you were trying to add pin jacks. Can you post close up pictures of your pcb?
I'll have to see how to load pictures. When I was trying to hook up the bias probes, I shorted R29 or R18 to the chassis, sparks and all DOH! So I took the probe jacks out ( they came up through the top plate). Won't do that again. Matter of fact, on future boards, I'll just put the board flush with the top. I've done that on George's other boards. If the Grandson comes over, I'll just leave it unpluged.Tell me the 5v supply comes from the regulater, does it not? Seems that way from the schematic. You can see I know just enough to be dangerous. There's a guy on the web, I think it's called Mr Carlson's Lab. He has an online electronics course and I should probably take it.
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Yes, the 5 volts comes from the regulator chip, IC-3.
You say that the 5842's light up like normal, so there must be voltage going into the regulator chip. This means that the chip is dead, the resistors, R1 or R2 are bad, or something is shorting out the output.
Does the regulator chip get hot?
Measure the resistance between the two fat pins on the 300B socket whet the amp off and no tubes installed. If it's less than 1 ohm there is a short somewhere. The likely suspect is C3, or some misplaced solder around the 300B tube sockets.
You say that the 5842's light up like normal, so there must be voltage going into the regulator chip. This means that the chip is dead, the resistors, R1 or R2 are bad, or something is shorting out the output.
Does the regulator chip get hot?
Measure the resistance between the two fat pins on the 300B socket whet the amp off and no tubes installed. If it's less than 1 ohm there is a short somewhere. The likely suspect is C3, or some misplaced solder around the 300B tube sockets.
I get .3 ohms so onto pulling and replaceing C3. I had already pulled it to test and it tested fine. I'll replace it
I replaced C3 and now I get 5V. I had already replaced it once and both times the cap checked out OK. But here is where the story gets interesting....To test for the 5V, I just hooked up the 6.3V wires from the PT. Bingo 5V on the big pins. I then hooked up all the rest of the PT wires, one set at a time. Still getting 5V...Yahoo! but when I bolted the board down in the chassis, fractional V. Undid all the bolts and putthem back in one at a time. The center bolt, by the regulator was the culprit. Was it shorting to the chassis? I thought the top plate should be grounded? I took the washer from under the mounting nut and I'm back to 5V. I'm wondering if I should use nylon mounting bolts/nuts. Maybe tightening pulls a trace apart?
Not familiar with the design or the filament regulator IC used, but depending on the device used it may or may not be OK to GND the tab. Some fixed regulators have grounded tabs, and some adjustable regs reference the tab to the output. (or input)
Quick way to check - is there 5V on the regulator tab?
Quick way to check - is there 5V on the regulator tab?
Grounding the tab on the 5 volt regulator is OK in this case. Grounding the tab on the dual Schottky rectifier is NOT. Fortunately, it only needs a small heat sink.
Your washer was probably biting through the solder mask on the board near the mounting hole.
The mounting hole between the regulator and the 300B socket has a gold ring around it which is isolated. There is a space around that ring, and a large solid copper trace under solder mask nearby. The copper trace is the 5 volt supply feed to the output tube heaters. It runs on both sides of the board, and is huge since it would carry over 5 amps in a 2A3 build.
A mounting bolt, washer and nut must stay in the gold area, be insulated, or made of non conductive material to avoid shorting out the 5 volt supply. If the bolt and nut are small enough, leave the washer out or use a non conductive washer.
Your washer was probably biting through the solder mask on the board near the mounting hole.
The mounting hole between the regulator and the 300B socket has a gold ring around it which is isolated. There is a space around that ring, and a large solid copper trace under solder mask nearby. The copper trace is the 5 volt supply feed to the output tube heaters. It runs on both sides of the board, and is huge since it would carry over 5 amps in a 2A3 build.
A mounting bolt, washer and nut must stay in the gold area, be insulated, or made of non conductive material to avoid shorting out the 5 volt supply. If the bolt and nut are small enough, leave the washer out or use a non conductive washer.
Yes, I left all the washers out on that side of the board and all is working. I'll replace with nylon washers on both sides, later. Thanks to all for the help.
Something else I'd like to add. After I hooked up the cheap Habor Freight meters, for volts and bias. I couldn't get it to bias on one channel....would only come up to 8MA. So I switched the meter positions and all was well. If you use these cheap meters ( which I think are great for the price) watch out for meter failure or wire failure. It could put you on a goose chase.