Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: Colonl_Charisma on November 17, 2020, 09:39:09 PM
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Hi all,
Seems as if I have a new problem haha. My amp had been working great since installing the 33K and 75K resistors a few days ago. This morning and into the early afternoon my amp was working without issue and sounding great. I turned it off because I left my house for a few hours. When I got back home I turned the amp on and let it warm for about 15 min, but now when I play music I am only getting sound from the right channel.
Nothing odd occurred when or after I turned it on. The tubes are burning evenly, LEDs are all lit, and I checked my voltages and resistances and they all checked out. I even went back and reflowed my joints on the volume pot, but that did not seem to help. I can confirm that the 33K and 75K resistors are where they are supposed to be on the pot.
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I should also note that my amp was working fine prior to the volume pot modification. It was just loud haha.
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You should probably redo the resistance and voltage checks just to rule out a whole bunch of stuff.
If that all checks out, I'd trace the signal wiring from the rca jacks to the pot and on to (a) ground and (b) the first tube (12AU7), and reflow every connection along that chain.
I'd also scrutinize the wiring at the headphone jack.
And post pics.
cheers and good luck, Derek
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You could also post photos and we might be able to spot something.
It could be an issue with the additional resistors at the pot, or there could be something else that's just now causing problems.
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Thanks guys! I’ll post pics along with my resistance and voltage measurements a little later today.
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Resistance + Voltage check:
T1= *, 83.5 V
T2= *, 182.1 V
T3= 0 ohms, 0.5 V
T4= *, 182.2 V
T5= *, 81.9 V
T6= 0 ohms, 0.4 V
T7= 2.93K ohms, 109 V
T8= 0 ohms, 0.5 V
T9= 2.95K ohms, 108.6 V
T10= 0 ohms, 0.4 V
T12= 0 ohms,
T13= *,
T14= 0 ohms,
T20= 0 ohms
T22= 0 ohms
B3= 2.92K ohms,
B6= 2.95K ohms,
RCA Jack L= 107.3K ohms,
RCA Jack R= 99K ohms
Ground Tab = 0 ohms
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How about the 9 pin socket?
If you had said you had no sound from the right channel, I would say that the insulation is melted on the red wire going from the volume pot to the 9 pin socket, and it might be shorting out to where the black wires connect to the volume pot, but I don't see any issues there for the left channel.
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I'm having some issue uploading photos. I modified my previous message with my 9 pin socket pics.
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I would temporarily solder in a jumper between the upper and lower middle lugs of the volume pot, then let us know if that changes the behavior of the amp.
If it doesn't, remove that jumper and install it between 6U and 10U, then let us know if that changes anything.
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Will do! What kind of wire should use for the jumper?
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If you have anything leftover from the kit, that will work fine.
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I definitely do. Should it be the bare buss wire or can I use any of the insulated wire?
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I would use insulated wire. You really don't want the jumper going between 6 and 10 to touch anything other than those two terminals.
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Cool! Just installed the jumper between the U and L middle lugs of the volume pot. I’m going to do my resistance and voltage checks then let you know what happens.
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No, don't do the resistance and voltage checks, listen to the amp to see if you have both channels.
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Sorry, I did do the res and vol checks lol. It definitely worked! Both channels are working, and the amp sounds amazing again.
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Sorry, I did do the res and vol checks lol. It definitely worked! Both channels are working, and the amp sounds amazing again.
No, that's not to fix the amp, it just makes the amp into a mono amp.
Do both channels work with just the jumper on the volume pot? (this needs to be on without the jumper between 6 and 10)
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Gotcha! Yes, both channels work with the jumper on the U and L middle lugs of the volume pot. I have not installed any other jumper cables.
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That tells you that the problem is between the RCA jacks and the volume pot, which eliminates 90% of the amp.
It is actually possible to destroy the pot if lots of solder gets into it (and the flux with it). You can remove that jumper, then measure...
What's the DC resistance between the outer top lugs of the pot?
What's the DC resistance between center pin and shell of the left RCA jack?
What's the DC resistance between the left RCA jack center pin and the upper outer lug on the volume pot?
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I hope that I have not destroyed the volume pot haha.
Resistance of left upper lug of vol pot = 32.6 ohms
Resistance of right upper lug of vol pot = 00.1 ohms
Resistance between center pin and shell of left RCA jack = 107.2 ohms (I think I measured this correctly)
Resistance of left RCA jack center pin and upper outer lug of vol pot = 107.2 ohms (again, I believe I measured the correct thing)
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Resistance of left upper lug of vol pot = 32.6 ohms
Resistance between center pin and shell of left RCA jack = 107.2 ohms (I think I measured this correctly)
Resistance of left RCA jack center pin and upper outer lug of vol pot = 107.2 ohms (again, I believe I measured the correct thing)
32.6 ohms or 32.6K ohms?
I'm guessing those are all K ohms, can you put the pot in the middle of its rotation and tell me the resistance between the center lug on each level of the pot and ground?
The 107K between the left RCA jack center pin and the upper outer lug of the volume pot would be a problem. You have a 75K resistor between those places, you should see 75K for that resistance measurement.
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You are correct those are all in K. I forgot to add that in my post. Regarding the 75K resistor. I see a 75K resistance drop. Isn’t that what is supposed to happen or should it be 75K exactly? I think I may have measured the wrong spot.
I measured the left U lug of the volume pot = 32.6K
I then then measured where the white wire connects to the 75K resistor which measured the same as the RCA jack = 107K
Volume pot set to noon position:
U center lug= 11.58K
L center lug= 10.78K
U ground = 00.1
L ground = 0
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I measured the left U lug of the volume pot = 32.6K
That would indicate your pot is damaged.
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I uploaded a photo of the vol pot to make sure we are talking about the same places. There's a 75K resistor between those two spots.
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Yes, if you measure the bottom outer pot lug I bet it shows 24K.
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Yep, that is what it measures. What should I do? If I get a Alps Blue vol pot will that also give me better volume range like the vol pot mod?
Or is there a way for me to get the original volume pot? That resistor modification worked beautifully by increasing the volume range.
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You should replace the pot. An Alps Blue is a bit annoying because it requires a PC board mounted to it and that often requires cutting a little relief in the wood base.
You could get the TKD, PEC, or Noble pots and those would work nicely. You can use those same resistors on a new pot as well if you like the increased range you get.
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Thanks for your help PB!
I found a store online selling a TKD pot. Does this look like what I would need for a Crack?
https://www.partsconnexion.com/TKD-64187.html
One last question. I should not have to manipulate the chassis in any way to install this TKD potentiometer, correct?
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You may need to snip off the locating pin on the TKD, but otherwise it should fit properly.
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Thanks PB. If I need to would it be difficult to file the locating pin hole so that the TKD potentiometer would fit without snipping the pin? I think snipping the pin could cause me trouble down the road.
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I think you'd need to drill a new locating hole.
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Right on. I found this PEC pot that was linked in another BH forum post. I think I will pick this one up. It's much cheaper, and I may not have to do any manipulation of the chassis with it.
Linking below for anyone else interested:
https://www.digikey.com/en/products/detail/KKA1041S28/KKA1041S28-ND/770961
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A drill, jeweler's saw or tiny file will all do the job - which one's faster or cleaner will depend on whether the new hole overlaps with the existing one. If it is in a totally new spot or if it more or less perfectly overlaps the existing hole but just needs to be bigger, a metal drill bit (or even a wood one if you don't care about it getting dull) will do the job in a matter of seconds. But if the new hole needs to be centered on the edge of the existing one, a file or jeweler's saw is the better way to go.
cheers, Derek
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Thanks Derek! I really appreciate the tips. I should get the PEC pot in a couple of days. I'll know for sure then if I will need to tweak the chassis. In the meantime, I will disassemble my current volume pot.
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PB would it be okay if we kept this thread open until I install my new PEC pot? It should arrive by Tuesday.
I was able to easily desolder all wires from the old pot, and I removed the old pot from the chassis.
For clarification, What should my resistance checks show once I solder the wires on the PEC pot?
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Are you putting the resistors back on the PEC? Resistances should be consistent between the levels of the pot.
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Yes, I will put the resistors on the PEC pot. Haha the volume is a bit loud without them. I couldn't really turn the volume past 9 o'clock, and I was getting channel imbalance at the low levels
Theoretically the resistances on the new pot should be:
U and L left lugs = 75K (due to having the 75K resistors connecting the L and R channels to the PEC pot)
U and L middle lugs = ?
U and L right lugs = 0 (this should be 0 because it is the ground connection)
What should the resistance for the middle lugs be?
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If you have one probe on ground, the upper and lower outer lugs will be about 24K, the inner lugs 0, and the middle lugs variable depending on where you have the pot positioned.
I would not even bother to measure this, just install the new pot and listen to it.
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Will do haha. Do you think I used too much solder causing the old pot to burn out?
I looked at the old one after I removed it, and it looked rough with all the flux everywhere.
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The flux can get in and disrupt the connection the solder lug to the carbon track in the pot. That isn't really possible with the PEC.
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I totally see how that can occur, and after looking at the PEC design I see how it helps protect the pot.
I’ll update the thread once I install the PEC pot in a couple of days.
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Just received the PEC pot, and I'm about to install it. There does not seem to be any instructions, but the installation regarding where the wires go should be the same, correct?
I do have to enlarge the volume pot hole along with extending the hole for the locating pin. I will report back tomorrow after I visit my friend with a drill haha.
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Yes, just install it exactly like the stock pot.
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Perfect! This PEC pot is much larger, and should make for an easier time soldering. Thanks, PB.
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My friend successfully drilled a wide enough hole for the PEC pot to fit through along with slightly elongated the locating pin hole.
I've attached a picture of the PEC pot. Should I slightly bend the bottom lugs up? I potentially see some wires touching the chassis.
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Happy to report that we can mark this as resolved! PEC pot has been installed, and it works great. I once again have sound in both channels.
I really appreciate everyone's help!
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If they touch the chassis, then yes, bend them up.
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Thanks PB! I went ahead and bent them slightly up. Installing the new potentiometer was a lot easier than I expected it to be.