No. Next step I'm gonna unmount the board and start ringing it out from the bottom.
The VR is affected by the other components attached to it's leads.
It should clearly go to zero ohms when you turn it fully one way.
It may not go to full resistance when turned the other way.
Partially discharged capacitors will fool your meter when set to resistance. It is injecting a current and using that to generate a reading.
If the circuit is also injecting a current, that will appear as a lower resistance or a higher resistance. swapping the probes around may help prove this.
BTW, zero resistance of a VR may be around 0r5 for a lowish value VR.
It should clearly go to zero ohms when you turn it fully one way.
It may not go to full resistance when turned the other way.
Partially discharged capacitors will fool your meter when set to resistance. It is injecting a current and using that to generate a reading.
If the circuit is also injecting a current, that will appear as a lower resistance or a higher resistance. swapping the probes around may help prove this.
BTW, zero resistance of a VR may be around 0r5 for a lowish value VR.
I checked all the VR's. They all seem okay. When I start turning P1 and P2, I get absolutely nothing. I get 0VDC on both bias resistors and the offset resistor. I'm getting about 22 +/- volts to both sides of the board. The pots are 30-turn pots. I initially had the wipers centered, which may have been a bad thing?
I did notice something odd. When I kill power to the amp, the LED's on the PSU die instantly. I don't think that's normal.
I did notice something odd. When I kill power to the amp, the LED's on the PSU die instantly. I don't think that's normal.
I pulled P1 off the board and tested it. It's fine 0R5 to 500R.
No smoke. No smells. Just nothing. I turned P1 and P2 on one channel for a while and never got anything. My PSU is holding steady at about 22V. I get 22V on both sides of R10 and R11, so there's no voltage differential across them. It's almost like Q3 and Q4 are a dead short.
A couple questions:
Do I NEED to disconnect the output stages from the gain stage?
Do I NEED to short the inputs?
No smoke. No smells. Just nothing. I turned P1 and P2 on one channel for a while and never got anything. My PSU is holding steady at about 22V. I get 22V on both sides of R10 and R11, so there's no voltage differential across them. It's almost like Q3 and Q4 are a dead short.
A couple questions:
Do I NEED to disconnect the output stages from the gain stage?
Do I NEED to short the inputs?
I biased my front end first, just like I did for BA-3 FE as Preamp. Then hooked up the output stages. I, and others have found that there is a lot of turning of P1 and P2 before anything happens. ( on the FE board ) See the last few posts from Starbender and 6L6 on the BA-3 as preamp thread NOT the "Build" thread that 6L6 did. He was having no reaction, but when he kept turning, he finally got readings. Yes, short the inputs. Otherwise readings will tend to fluctuate more. Also, air blowing across the board will affect stuff too.
Russellc
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Those 25 turn pots are evil.
Its personal preference, I like the 25 turn pots better...especially when you have readings that are drifting around. Much finer adjustment, pots with just a few turns are difficult for me to set. Once the circuit is settled in, all that adjustment may not be necessary, but you are not going to need much then.
Russellc
Outputs are disconnected now. I just tried turning the pots all the way up: still nothing, on both sides. No DC bias whatsoever.
I guess it's possible maybe the FET's are blown. No physical evidence whatsoever. Would it damage anything if P1 and P2 were turned all the way up?
I have new FET's coming just in case.
I guess it's possible maybe the FET's are blown. No physical evidence whatsoever. Would it damage anything if P1 and P2 were turned all the way up?
I have new FET's coming just in case.
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Did they even get warm? By "Fets" you mean the mosfets, right? Not the Jfets?
Russellc
Yes, the output FETs. I'm using the FQP3N30 and FQP3P20. I don't think the jfets would take any abuse, since they're on the inputs, but I could be wrong. Never even got warm. Just never did anything from the start. I was measuring as much as I could while troubleshooting and found that I had 2.7VDC between the "D" output and ground, both sides.
OK, by output fets do you mean the 4 mosfets on the front end board? I thought you were trying to bias the FE board...or do you mean the 6 output mosfets on each side?
If you mean the 6 outputs, do you have the front end board biased up already? The outputs (6 devices) dont have an adjustible bias on the SE version right?
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OK, thats what I thought you meant. Did they ever get warm at all during this? I am assuming not. Something must be hooked up wrong...its possible the mosfets are dead, but I kind of doubt all four of them are already dead.
Sorry if this is obvious, but they arent soldered in backwards are they? ( the four mosfets)
Russellc
Yes, flat sides should be in. Again, sorry if this is obvious as well, but what about the pots? If they were oriented backwards, the other direction would be right...really sorry, just trying to think of any possibilities! If they are correct, Its going to take someone with more know-how than me to have you measure various places...if power is correct, next step would be to test the mosfets.
Russellc
Russellc
Totally appreciate the help thus far. Pots are correct. I checked the last 3 pages of the preamp build guide and they're the same. P3 is set to midpoint. When I fired up my amp for the first time with the FE connected, P1, P2, and P3 were all at their midpoints. This is how Digikey sent them to me and I forgot to turn them down. I think this may have led to me blowing something, but not sure.
Well, I think I did the same thing, not sure but I had a variac. Once i got some reading, could keep adjusting until I had full voltage up. I guess you could remove one and give it a test using the battery, resistor, multimeter method. If it is dead, I guess change them out. If alive, at least you only removed one! At this stage, I would get more feed back, I'm at the end of my knowledge! Wait for someone more knowledgeable to comment before heating up the gun.
I didnt do anything special, I followed 6L6 methods for both Pre and amp.
At this point, just trying to bias the front end I dont think there is really any difference between SE and complementary. All I can recommend is staring at it looking for a mis wire. One side of my BA-3 pre would not act right, I finally found where I had mis wired the power supply diodes hook up on one side. I was at a loss, then noticed wires going in a different place on the diode board than the other side. Moving to correct position fixed all.
Sometimes a mistake is hard to see, and just pulling away and coming back later helps. Someone elses eyes helps too. I may be wrong, but I would think 1/2way wouldnt kill them, but I dont know this for sure...I really had to turn awhile before anything happened. I paused, commented here, then twisted further and finally the readings started. Maybe I'm wrong, but I think I was at LEAST 1/2 of the way in.
Russellc
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