vipor3D wrote: ↑Thu May 18, 2023 3:51 pmI wasn't sure which you meant, but I tried reconnecting the wire on the jack sleeve lug that goes to the volume pot and instead connected it to the grounding lug for V1 pin 3 (which I believe is where the cathode is grounded). This actually made things worse. The buzz was still there with the volume on zero, which wasn't the case before. Although, the volume pot didn't seem scratchy like it had been. The buzz still cut out around 80% volume to max volume.
When I plugged the guitar in, the buzz got worse. I tried sweeping the volume pot with the guitar in and when the volume got near max, somewhat of a "stuttering" sound came through the speaker.
Great! Something changed! What I meant was to run a new wire from the jack sleeve lug to the ground lug where the R and C from the input stage are connected. I'm now wondering if all your front panel mounted parts have poor grounding.
I'll try to go back and file more paint away under each of those front mounted components and will add the new wire to the jack sleeve lug and will report back if there's any change
EDIT: I just added the wire, and haven't filed any extra paint away yet and noticed the buzz was as bad if not worse than before. Now I'm getting a "sputtering" sound from the speaker. The sound becomes amplified (no pun intended) when I plug my guitar in. I took a video of this:
As usual, youtube sucks the volume out of the original video so it doesn't sound as loud as it does here in person. You may have to turn the volume up a good bit to hear the sputter before I plug the guitar in.
It does. The reason the manual probably doesn't mention it is assuming the builder doesn't have probes that clip on, and putting both hands inside the chassis is a potentially fatal maneuver. You have to be very careful when you do this. It will give you the voltage the heater actually sees though and you can expect it to be lower than the highest voltage you measure when you measure pin to ground. Heaters will work okay when they see a voltage between 6.1 and 6.5V.
I do have clip on probes I can use. I'm still a little confused, though. Which two pins would I want to connect the probes to? Or are you saying I'd connect both probes to the same pin at once?
You can take this reading at the lugs or pins on the pilot light where the filament leads from the transformer connect, or from pins 4 and 5 on V2 or, pins 4/5 and pin 9 on V1. The pilot light lugs are safest and probably easiest. Clip one probe on before you switch it on. Then with it switched on use one hand to probe the other lug/pin and take your reading.
Just tested the AC voltage across the pilot light lugs as suggested and got 6.48vac, so that seems to be okay
vipor3D wrote: ↑Thu May 18, 2023 2:09 pm
I measured the AC voltage across the speaker jack and got a max value of about 3.5vac, but it seemed to hover around 2.0vac which is consistent with the kit guide/schematic.
That 2.0VAC reading is supposed to be under the conditions detailed at the upper left of the schematic. You set the Volume control on about 2 (50K from ground) and strum a low E on a Mexican Strat. That's way too high for hum, although it may be an ultrasonic oscillation. See if the reading seems to increase suddenly (out of proportion with loudness of the hum) as you turn up the Volume control.
vipor3D wrote: ↑Thu May 18, 2023 2:09 pm
I measured the AC voltage across the speaker jack and got a max value of about 3.5vac, but it seemed to hover around 2.0vac which is consistent with the kit guide/schematic.
That 2.0VAC reading is supposed to be under the conditions detailed at the upper left of the schematic. You set the Volume control on about 2 (50K from ground) and strum a low E on a Mexican Strat. That's way too high for hum, although it may be an ultrasonic oscillation. See if the reading seems to increase suddenly (out of proportion with loudness of the hum) as you turn up the Volume control.
I remeasured the AC again and measured the volume pot resistance to get it exactly at 50k to match the schematic. I also strummed only the low E string on my Tele in middle position. My results are below:
TP1: .110vac
TP2: .600vac
TP3: .080vac
TP4: 1.25vac
TP5: Still cannot get an accurate AC reading on this pin. I tried clipping the probe multiple times at different angles and only get minimal AC readings. Could that suggest a connection issue at that point?
TP6: 1.00vac
Also, I noticed my input jack is spotty again. I had to keep the cable in just the right spot or the signal would go out. I've tried tightening the jack as much as I can, but it doesn't seem to be fixing the issue. Could that jack need to be replaced?
Is the shorting contact making a good connection to the long tip contact? If that is not happening you will get hum since the input is open with a 1 Meg impedance to ground. Sometimes you have to bend the shorting contact a bit or clean the contact points. An intermittent contact with the plug is not common, there is usually plenty of pressure to maintain the connection.
vipor3D wrote: ↑Fri May 19, 2023 5:54 pm
TP5: Still cannot get an accurate AC reading on this pin. I tried clipping the probe multiple times at different angles and only get minimal AC readings. Could that suggest a connection issue at that point?
Forget about TP5. Sometimes even high-cost meters can't properly measure an AC signal on the output tube plate.
martin manning wrote: ↑Fri May 19, 2023 6:39 pm
Is the shorting contact making a good connection to the long tip contact? If that is not happening you will get hum since the input is open with a 1 Meg impedance to ground. Sometimes you have to bend the shorting contact a bit or clean the contact points. An intermittent contact with the plug is not common, there is usually plenty of pressure to maintain the connection.
I really think you may have figured it out for me...
The jack was mounted with the two contacts vertically aligned so I couldn't really see the contact between them. Took the jack out, and saw the two contacts connecting. But when I remounted the jack and rotated it so I could see the contacts, there was space in between them when mounted to the chassis. So I powered up the amp and used my chopstick to push the contacts together and sure enough the loud buzz went away!
My only problem now is that I've tried to bend the contacts together and I can't seem to get them to keep in contact while mounted to the chassis. Is there an easy fix for this?
If not, could a new jack solve my problem?
Edit: I kept trying and was able to get the contacts to bend to stay in contact with one another and keep a tight hold on the input cable. The nasty buzz is gone!
martin manning wrote: ↑Fri May 19, 2023 9:00 pm
Try bending the contacts with a chopstick or something that you can use to put pressure on them close to the jack body.
Some needle nose pliers seemed to do the trick (after draining the filter caps). I guess I'll just have to see how it holds up after using the jack a few times. But with that fix everything seems to be working/sounding like it should!
It will be nice getting to finally enjoy the amp I've built and really test it out. If everything holds up I may take a look at doing the MOD 102+ upgrades in a few days.
I can't express my appreciation enough for all of the help trying to sort out this issue. I learned a lot during this process and feel encouraged to try another build in the future (Just not another MOD 102 lol).
I don't think there was anything wrong with the kit. Switchcraft jacks are generally of good quality, but occasionally have this problem. I would fault the instructions for not being clear on the need to remove the paint from the contact surfaces, and the layout is not the best, IMO, and could be improved. You could get some more practice by rebuilding this amp to a better standard and clean up your lead dress.
martin manning wrote: ↑Fri May 19, 2023 9:25 pm
I don't think there was anything wrong with the kit. Switchcraft jacks are generally of good quality, but occasionally have this problem. I would fault the instructions for not being clear on the need to remove the paint from the contact surfaces, and the layout is not the best, IMO, and could be improved. You could get some more practice by rebuilding this amp to a better standard and clean up your lead dress.
I wasn't really knocking the kit as much as just joking about my experience with it due to something simple like the input jack connection being a little off. I've also read several comments here and elsewhere about it not being the best design so I'm just thinking if I do another build it would be something more like a 5F2.
I do plan to go back and fix a few things and shorten some wires. I also plan to redo the PT and switch wiring based on the suggestions I've seen to update it to modern standards
The layout could be modified to correct the issues that have been mentioned here. You could also play around with the operating points on the preamp tubes to get a feel for how that works.
Its nearly impossible to get those switch tags bent right again once the tip tang has been bent over so far so as to bend that switch tang to being open with no 1/4” cable end in place.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Stevem wrote: ↑Fri May 19, 2023 9:27 pm
So what’s the reason you can’t jump a scrap of wire across them, just tin the wire once you strip it.
you can even give up a spare cap or resistor and cut a lead off of one of those to span the two terminals.
I'm probably missing something, but aren't the two tips on that type of jack only supposed to touch with nothing plugged in? Wouldn't jumping a wire between the two keep them connected with a cable plugged in? Or is that not an issue?
