Bottlehead Forum
Bottlehead Kits => Mainline => Topic started by: bdog9 on December 07, 2023, 05:09:10 AM
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Need a little help getting pass the resistance check.
I have gone over wiring multiple times and feel confident that it is correct. I lingered however, on the soldering of connections 6 and 8 on the input switch. I am guessing I will need to replace it.
Terminal 1 and 5 are reading OL, 15 and 25 are reading 205.5 and 207.5 respectively. All others are in the 0.1 to 0.3 range.
Advice would be appreciated. Bill
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Could you post some build photos? Do you get a different resistance reading on 1 and 5 if you turn the fine switch all the way down?
Terminals 15 and 25 are supposed to read around 200 ohms, so I don't see an issue there.
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Inspecting the photos you sent me, could you let me know what meter you're using?
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Thanks for the help. I measured with a Klein CL390
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If you turn the fine control all the way up and the coarse control all the way up, what's the DC resistance reading on the center post of each 6C45 socket?
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130.0 for both
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Sorry, fat fingers. Should be 139.0 for both.
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I made an asumption that could be wrong. I measured from the ground at the power cord to the center of the outer tube sockets. If wrong by all means correct me.
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That doesn't make a lot of sense, was the coarse control turned all the way down by chance?
If the answer to that is yes, then that tells us that the coarse attenuator seems happy. The next resistance check would be to the lower two lugs on the fine switch with the fine switch turned all the way up.
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PB your patience is admirable. Thanks. Now I know full clockwise is up, and full counter clockwise is down.
The fine lug reading fully up was OL to either of the lower lugs.
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Can you check that resistance reading with the switches in those positions against the resistance reading of the center lug of each 6C45P jack?
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Measuring with fine and course all the way up, clockwise, the resistance reading between ground and the center lug of the 6c45p both right and left is OL.
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What I would do is to hold a probe on one center socket and turn the coarse switch back down and stop when you see a resistance reading. Let us know what that resistance is and what step you're on with the control.
I am somewhat confident that the meter you have just isn't properly reading large resistance values, though it is advertised as being able to do so. If you lose the DC resistance value on both center pins of both 6C45P sockets at exactly the same step on the coarse switch, I would tend to blame the meter. If the resistance reading drops out on one side but continues counting up on the other and then drops out, I would suspect flaky solder joints on the coarse switch itself (though doing a ton of resoldering on these switches is not at all recommended).
If the meter is the problem, the ~$20 meter at Harbor Freight can resolve these measurements.
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I started seeing resistance reading when I reached -36dB. It registered 394 both socket. At -45dB it dropped to 139.5 both sockets. I then switched to another multimeter and everything improved. 5 and 12 now register 39.5. I am reading resistance all along the coarse switch but the reading between center socket both sides and ground is still 394 at -36dB. It then drops to 139 when at -45dB and I wonder if I should question that or move on to voltage checks ?
Thankyou for your help.
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The resistance readings at the center pins of the 6C45 sockets should go from close to zero up to about 25K as you rotate the coarse switch assuming the fine switch is turned all the way up. Provided you see those resistances present, you can move onto the voltage checks.
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OK. Check resistance as suggested and range form 25 to 0 noted, so moved to voltage test. When I switched on the power the tubes did not glow and I turned it off. I could smell heat and on careful inspection the green resister on the power board had one leg with bubbling solder.
It has cooled now. Advice?
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Remove the red wire leaving the +275V pad and power up the amp again, measuring what's available on the +275V pad if you don't get the hot smell again.
If the resistor continues to get hot under these conditions, then you have a backwards 100uF/450V cap or you have swapped two of the caps on the power supply board on accident.
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Cleared both red wires from 275 connection, powered up and no smoke and 365 volts between 275 + and ground.
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When you put back one of the two red wires, you'll get that issue again, and that can be used to narrow down which half of the amp is misbehaving. It's also possible that either wire causes the same problem, in which case you've just made the same mistake on both channels.
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Thanks for your help ! Back to the build.
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Reporting back. Stepped away a day or two and came back to reinspect all connections. On removing the two C4S boards I embarrassingly found the black wire going from inboard bA to inboard bB going instead to a lead just to the inside of the transistor on both boards. The lead is on the inside of the transistors but in a direct line with IA.
I unsoldered them both cut new wires and positioned them properly.
I continued to inspect and found a few solder joints that I cleaned up.
I rechecked the resistance and all was good.
Inspired I installed the tubes and they did not light up in spite of a good fuse.
I rechecked my voltage at 275 and got no reading. Disconnected red wires at 275 and still no reading.
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Are you certain the fuse is in the correct half of the fuse holder?
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yes .
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I have power to the transformer from the switch. No power at the +125v lead, either one.
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OK. Went back to the manual and ran all voltage tests as described. All were successful. Redid my resistance checks and all were good. Stopping for the evening
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Removed the red leads to the power board and tested from +275DCV both links to ground and got nothing.
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Your last two posts are somewhat contradictory. If you passed all your voltage checks, then you have to have the correct DC voltages coming out of the power supply board.
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Sorry, I just retested using method on pg 49 and also the method you recommended in the the forum and did both get 361-389 vdc. My mistake.
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Checked resistance again, All check ok but some that should be o ohms are 1 ohm, I used the harbor fright dmm for these measurements, It did not want to measure high volt dc so I switched dmm to the Klein which worked well. I think this was this mornings problem. In any case I installed the tubes powered up and they lit up. Voltages varied from what they were supposed to be. +275vDC 302V +6.3vDC 59 -6.3vDC 52.8. The wording on the next measurements is confusing to me. The IA on one board was 302 on the other it was 301. I did not recognize an IA on the B side but measured Ib and got 215 one the first board and 300.5 on Ib on the second. The Breg was 215 and 300 The -reg was 215 and 300 and the Kreg was 2.7 and 300. I think something is wrong ?
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You have 300V on a Kreg terminal?
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I did. But now I do not. Solder joint had broken loose.After it was repaired everything sorted out on its own.Both sided biased to 145. Could not have done it without your help. Next time your in Fort Worth call me and I will make a steak dinner. Bill
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I do get to Texas from time to time...
One thing I would recommend in your instance is to plug in some cheap headphones and play music through your amp, then give the top panel a few hard whacks with your hand. If the music keeps playing uninterrupted, then it's rather unlikely that there are any other issues that may pop up. If the music drops or pops, then you may need to poke around a bit more to look for other loose/broken stuff.