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 Post subject: Fried PI trim pot
PostPosted: Thu Oct 24, 2019 9:19 pm 
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Hello all,

I've just built the OSD kit, did my final double and triple checking and started the start up procedure.

I first started up with a bulb limiter and everything looked ok. The amp lamp lit up brightly and I got 6.5VAC at the heaters. Then I powered down, removed the bulb limiter and powered up with my meter set to measure B+. When I turned it on the voltage jumped quickly to around 450VDC as expected but I quickly heard a sizzling and then the small PI trim pot on the board popped and smoked. Obviously, I powered down immediately.

I again checked all the connections to and from the pot and didn't see any problems. Does anyone have any idea why the trim pot would get fried? I need to replace the trim pot but need to figure out what went wrong first.

Thanks in advance for your help.

Sincerely,

Josh


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 Post subject: Re: Fried PI trim pot
PostPosted: Fri Oct 25, 2019 1:27 am 
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Hi Josh, welcome to the forum.

Sounds like your power supply is working, which is the good news.

I know you checked your wiring three times, but check again. Make sure the middle lug of the P1 pot is connected to B+3. That's where the 1K2 and 22k resistors connect. Also make sure the other two lugs connect to the 100K and 110K resistors. And make sure there are no accidental shorts in this area, or anywhere else on the board, or when the board got mounted to the chassis. Check for shorts at the tube sockets, too. Sometimes a stray bit of wire or solder is hard to see.

Also make sure the value of the pot was correct. The layout drawing says it's 5K. The schematic doesn't say what it is.

Were all the tubes in place when the pot fried?


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 Post subject: Re: Fried PI trim pot
PostPosted: Fri Oct 25, 2019 1:25 pm 
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mitch m wrote:
Hi Josh, welcome to the forum.

Sounds like your power supply is working, which is the good news.

I know you checked your wiring three times, but check again. Make sure the middle lug of the P1 pot is connected to B+3. That's where the 1K2 and 22k resistors connect. Also make sure the other two lugs connect to the 100K and 110K resistors. And make sure there are no accidental shorts in this area, or anywhere else on the board, or when the board got mounted to the chassis. Check for shorts at the tube sockets, too. Sometimes a stray bit of wire or solder is hard to see.

Also make sure the value of the pot was correct. The layout drawing says it's 5K. The schematic doesn't say what it is.

Were all the tubes in place when the pot fried?



Thanks Mitch,

No tubes were in. I was doing the initial testing as per the kit instruction manual. Everything seems to be connected correctly and I went back and tested if those plate resistors were accidentally grounded somehow and they are not. My meter shows no continuity to ground when I test them or the PI tube socket plates.


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 Post subject: Re: Fried PI trim pot
PostPosted: Fri Oct 25, 2019 2:30 pm 
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If V3 and no other tubes were in place there should have been no current through the trim pot and no reason for it to smoke. Check the two .1uF capacitors, C24 and C25, with an ohmmeter. If you removed the fried pot they should both read open circuit if you put an ohmmeter across them.

Maybe the pot was just defective.


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 Post subject: Re: Fried PI trim pot
PostPosted: Fri Oct 25, 2019 7:18 pm 
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Why would there be continuity across those caps? I didn't think caps ever had continuity across them.

When I went back today, I found the left lug of the pot at the 110K resistor WAS connected to ground somehow. I have no idea how. I have a picture of the underside of the board from before I put it in and I can't see anything amiss. No excessively long leads or excess solder or anything. Although there is a mounting screw nearby I don't see how it could be touching that far away.

Anyway, after removing the pot and pulling that resistor up slightly I get no more connection to ground. With the pot space jumpered I was able to power up again without other issues and seem to be getting appropriate voltages everywhere.


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 Post subject: Re: Fried PI trim pot
PostPosted: Fri Oct 25, 2019 8:34 pm 
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Glad you found the problem. It's strange that the 110K side of the pot should be shorted to ground. I have never seen an actual OSD amp, but looking at the layout drawing the closest ground to that area is the mounting screw. That's why I mentioned earlier to make sure nothing got accidentally grounded when the board got mounted to the chassis.

But as you said, the screw is well away from any terminals. Did you check to see if the resistor was still grounded when the board wasn't attached to the chassis?

You can run the amp without the trim pot with B+3 directly connected to the 110K and 100K resistors. You should eventually replace the pot since it's part of the OSD design, but you can run the amp without it for the rest of your testing. Most phase inverter circuits don't have a trim pot to set the balance.

Yes, you should not read continuity across a good capacitor. That's the usual way to see if they are good or not. The opposite of continuity is often called "open circuit," the same reading you get with an ohmmeter if the probes aren't touching anything.


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 Post subject: Re: Fried PI trim pot
PostPosted: Sat Oct 26, 2019 12:09 am 
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Ah, sorry about the misunderstanding about the open circuit on the capacitors.

Yes, I'm baffled how that eyelet was getting grounded. It isn't now but I don't see how it was in the first place.

Anyway, after jumpering where the trim pot was, I was able to power up and get what seemed initially like correct voltages. However, through a speaker it was squealing and I switched to a dummy load. Most of the voltages looked close to expected except V2b is way off. The plate voltage was only 4.5V (around 219 expected) and the cathode was almost non-existant (0.015V). I double checked the caps and resistors connected to these and they seem to be the correct values.

Would this V2b problem explain the squealing through the speaker? And what would cause the V2b voltage abnormalities? Is it a bad tube or do I have some other problem?

Thanks for your help!


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 Post subject: Re: Fried PI trim pot
PostPosted: Sat Oct 26, 2019 5:14 am 
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The low voltage at V2b wouldn't cause squealing. It would keep that gain stage from working, though. It could be a bad tube, but it's more likely to be a bad connection on R22, R23, or at the tube socket.

The squealing sounds like the phase is reversed at the output tubes. Try swapping the connections to the anodes (pin 3) of V4 and V5. It's probably easiest to do this at the middle terminals of the 6V6/6L6 toggle switch.

With amps that have negative feedback, there is a 50/50 chance of getting the output connected properly. If it's reversed instead of negative feedback it's positive and the amp becomes an oscillator. The output phase depends on how the transformer was wound. The easiest way to check the phase is just to connect it to the amp. One way the amp works, the other way it squeals.


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 Post subject: Re: Fried PI trim pot
PostPosted: Sat Oct 26, 2019 6:19 pm 
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That makes sense Mitch.

I swapped the middle terminals of the 6v6/6l6 toggle as you suggested and that did indeed get rid of the squealing! And now I get sound out the clean channel too! But nothing out overdrive and the voltages are still all messed up on V2. I tried swapping out tubes and that didn't change anything. I checked for continuity from the cathode to the cathode resister/cap pair on the board and from their other side to ground and that ll seems to be fine.

However, I also get very low voltage to pin 6 of V2 (4.7V), so I traced that back. I get high voltage at the B+4 junction where the 2 100k resistors meet on the board (282V). On the other end of the 100k going to pin 1 of V2 their is still high voltage (254V). But on the other end of the left 100k resistor (the one going to pin 6) I get very low voltage (4.65V).

It is the correct resistor in that position so why would there be such a huge difference in the voltage drop across those 2 resistors? Is there something wrong with that 100k resistor?


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 Post subject: Re: Fried PI trim pot
PostPosted: Sun Oct 27, 2019 2:14 am 
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You said the cathode resistor for V2b is ok and has good connections to pin 8 and ground. So it looks like pin 6 of V2 isn't connected properly to B+4. There is a chance the 100K resistor is bad. Have you checked it with an ohmmeter? Also the tube socket could be faulty and not grabbing the pins properly. But it's most likely just a bad solder connection at pin 6 or pin 8 on the tube socket or the 100K resistor.


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 Post subject: Re: Fried PI trim pot
PostPosted: Sun Oct 27, 2019 7:15 pm 
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I pulled up one leg of that resistor and tested it and it is ok 100k.

So I went back to scratching my head. I looked in the socket and the pins look fine and the tube slides in easily.

With a tube out I tested capacitance across that small cap across pin 6 and 8 and got nothing. I then switched it to resistance and got a low resistance across that cap? Does that mean that cap is bad and is leaking voltage from the plate to cathode or something like that? Would that explain my low voltages on that side of V2?


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 Post subject: Re: Fried PI trim pot
PostPosted: Sun Oct 27, 2019 7:51 pm 
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Actually I get similar readings on the other side of V2 which has appropriate voltages. At this point I'm at a loss. I have no idea why that pin 6 & 8 side has low voltage.


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 Post subject: Re: Fried PI trim pot
PostPosted: Sun Oct 27, 2019 9:03 pm 
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It still sounds like a connection problem at end of the 100K resistor that connects to B+4. Do you read high voltage on both ends of the resistor, on the actual leads of the resistor, not the eyelets it is soldered to?

Sometimes bad solder joints can look good but they don't conduct electricity.

Also try disconnecting one lead of C19 and check it for continuity. You shouldn't see any.


Last edited by mitch m on Sun Oct 27, 2019 9:14 pm, edited 1 time in total.

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 Post subject: Re: Fried PI trim pot
PostPosted: Sun Oct 27, 2019 9:12 pm 
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Thanks for your continued help Mitch!

I've reflowed the solder a couple times already. I get high voltages at the leads and the eyelet where both 100k resistors are attached to B+4. But on the other side of that left resistor, I get abnormally low voltages.


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 Post subject: Re: Fried PI trim pot
PostPosted: Sun Oct 27, 2019 9:18 pm 
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With V2 removed you should see the same high voltage at both ends of the 100k and at pin 6 of the socket.

What about C19? Does it test OK with one lead disconnected?


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 Post subject: Re: Fried PI trim pot
PostPosted: Sun Oct 27, 2019 9:37 pm 
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With V2 removed or not, I see a huge voltage drop across that left 100k resistor and downstream from there, both at its secondary lead, eyelet, and at pin 6 of V2.

I haven't disconnected C19 to test. That would be a PITA to desolder and remove with the heater wiring and all in the way. But I guess I might have to if you think that could be the culprit.


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 Post subject: Re: Fried PI trim pot
PostPosted: Sun Oct 27, 2019 11:46 pm 
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Looking at the layout drawing I see what you mean about C19 being hard to get at. But it's beginning to look like that's where the problem is.

First have a really good look at the tube socket to make sure no pins are accidentally shorted together. If that looks good, try removing everything at the eyelet where the 100k resistor, the .0047uF capacitor and the wire to pin 6 are joined. Now with the tube-side lead of the 100K free you should see the B+4 voltage on both ends of the resistor (no current being drawn through the resistor so no voltage drop).

Now tack the resistor back down to the eyelet and see if you still get B+4 at both ends. If you don't, you had some funky issue with the board earlier so that's what's happening again.

If you still get the same voltage reading you got when the 100K was free, tack the .0047uF back in and check the voltage again. Now if you don't still see the same reading check the .0047uF capacitor and its associated wiring. But if you still get the B+4 reading, then the problem is at the tube socket. It's either an accidental short between pins or the 270pF capacitor.


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 Post subject: Re: Fried PI trim pot
PostPosted: Mon Oct 28, 2019 1:38 am 
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I think I solved it!

I was inspecting it considering whether I should disconnect things as you suggested when I realized something. Just to the left of the eyelet where the 100k resistor in question attaches to the board and to the wire going to pin 6, there is a hole in the board for the blue wire coming from pin 8 on its way to the cathode cap/resistor pair. That hole had a metal eyelet inserted in it, even though nothing is supposed to be soldered there. The same was true for all the holes that are meant just for a wire to pass through under the board.

The ones in question were not only close, the metal eyelets were overlapping each other, connecting it to the next eyelet to the left. When I tested for continuity there was a clear connection from the 100k resistor eyelet to pin 6 to the cap/resistor pair eyelet that pin 3 connects to!

I used my wire clippers to shave off the sides of that middle unnecessary eyelet until there was no connection anymore and fired up again and the voltages look much more appropriate! Not only that, I now get sound on the overdrive channel!

I wasn’t able to play much more to make sure nothing else is wrong because my family is asleep already and I couldn’t turn the amp up! Lol I'll test more tomorrow.

But I’m excited that it seems to be working now! At least I’m getting sound from both channels. All I need is that replacement trim pot for the PI and I think I’m good to go besides final tweaking.

Although I couldn’t play it yet, I did take the opportunity to take some measurements. They aren’t exactly what is listed on the table in the instructions and on the layout but they are relatively close. To my untrained eye they look close enough but if anything looks suspect to you please let me know.

V1 pin 1 (192V) pin 3 (1.53V) pin 6 (193V) pin 8 (1.52V)
V2 pin 1 (197V) pin 3 (1.58V) pin 6 (202V) pin 8 (1.52V)
V3 pin 1 (311V) pin 3 (60V) pin 6 (309V) pin 8 (60V) (this is obviously without the PI trim pot, I know that for max bloom I’ll need to adjust for about 6.5V difference from pin 1 to 6)
V4 pin 3 (446V) pin 4 (445V)
V5 pin 3 (447V) pin 4 (445V)

B+1 450V
B+2 445V
B+3 436V
B+4 302V
B+5 294V

Thanks for all your help! I'm glad it seems to have been a relatively simple fix and didn't need to start taking apart the whole tube socket and such!

Cheers!


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 Post subject: Re: Fried PI trim pot
PostPosted: Mon Oct 28, 2019 3:35 am 
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Glad you found the problem. With those eyelets overlapping like that, V2b's anode was connected to V2a's cathode. There is enough resistance in that path so nothing smoked or got damaged, but it completely shut off V2b and killed the sound from the overdrive stage.

Good eye. But if the overlapping eyelets were more subtle and harder to spot, the strategy I mentioned would have pointed out the problem. The resistor would have passed voltage when disconnected but not when tacked back to the eyelet, indicating something was wrong with the board itself.

Your voltages are close to the expected values and close enough to not indicate any problems. The voltages on the schematic are average values and it's normal for them to vary by as much as 10% or more due to tolerances in the tubes and other parts. They also depend on the line voltage coming out of the wall.


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 Post subject: Re: Fried PI trim pot
PostPosted: Mon Oct 28, 2019 6:37 pm 
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Yeah, that was good advice. I'll have to remember that for future builds.

Thanks again for all your help Mitch! The amp already sounds pretty awesome! Just have to get that PI trimmer put back in and then I'll start rolling through some NOS tubes and such to find the best sound.

Cheers!


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