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Hello,
I'm having a problem with my newly built plexi mark II. In my case, the overdrive channel works great, but the normal channel (volume / tone only) gives me just a ghost of a sound. No hum or static. Can just barely hear the guitar with the vol on 10. Stephen noted that I've probably got problems with the shielded cable. Looked things over this AM - resoldered one joint - with no improvement. Nothing else looks particularly sketchy.

My voltage numbers:

N and L at IEC socket: 117.5VAC
Indicator terminals: 117 VAC
B+ 423VDC

V1 Pin No pre-amp/power tubes all tubes in place
1 421 VDC 344.8 VDC
6 420 VDC 344.1VDC
4/5 to 9: 6.91 VAC 6.3 VAC

V2 Pin No pre-amp / power tubes - all tubes in place
1 421 VDC - 344.4VDC
6 424 VDC - 347.8 VDC
4/5 to 9: 6.75VAC - 6.3 VAC

V3 Pin No pre-amp / power tubes - all tubes in place
1 419 VDC - 220.4 VDC
6 420 VDC - 214.0 VDC
4/5 to 9 6.86 VAC - 6.42 VAC

V4 Pin No pre-amp / power tubes - all tubes in place
7 424 VDC - 351.4 VDC
4 to 5 6.8 VAC - 6.5 VAC

V5 Pin No pre-amp / power tubes - all tubes in place
7 423 VDC - 349.4 VDC
4 to 5 6.83 VAC - 6.53 VAC

V6 Pin no pre-amp / power tubes - all tubes in place
1 to 7 615 VAC - 611 VAC
4 TO 5 6.86 VAC - 6.67 VAC

1st value in each row is rectifier tube only. Second is with all tubes in place.

Any help would be appreciated. No time today, but I'll likely just rewire the input jacks shielded cables tomorrow if I have time.

Thanks! Mike Keefe

Check that the coax core wire is not accidentally connected to the shield. This can be disconnected and tested with an ohm meter. Should be no continuity there.
Can you post a good picture of the preamp end of the build?

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

rewired the coaxial cables just to the normal channel. turned it back on and I've got rip roaring static on both channels. Maybe I've knocked something loose at V1. Didn't see your reply, Stephen, until afterwards. I'll check the continuity on the cable and post a couple pictures. Thanks for the guidance.

Mike

/Users/michaelkeefe/Desktop/IMG_1285.jpg
/Users/michaelkeefe/Desktop/IMG_1292.jpg

pictures of the pre-amp end and V1
Mike

Didn't realize that I need board administrator permission to place an attachment. How do I obtain the permission?
Mike

You shouldn't need board administrator permission to upload and attach a photo. If the photo is on your computer, just go to the "Upload attachment" section, use the "Browse" button to find your photo, then click the "Add the file" button. The upload section is below the message area when you are typing a message.

If the photo is on an external server you can add it to your message using the "Img" button in the formatting toolbar above the message area. Just type the URL to your photo between the [img] tags the button places in your message.

The "Quick Reply" section doesn't show the upload area or the formatting toolbar. You have to type you message from the full editor which you get to from the "Post Reply" button.

If the voltages at the anodes of V1 and V2 are around 344 volts with all tubes in place, as you posted earlier, there is a problem in that area. They should be around 155 volts except V2-6 which should be around 270. Looks like these tubes are not drawing as much current as they should. Check the cathode connections and their resistors and make sure they all have good grounds. It could possibly be trouble at R21, too. Make sure it's really a 2K2.

Mitch M,
attached the photos (with your help) that Stephen had asked for. I'll take another look at the items you noted in your note. Excuse my ignorance, but when you note "R-21" I'm lost. can you help me with a little description of this location.

Thanks,
Mike

Thanks for the photos.

The shielded cables look OK. You can plug a patch cord into each input jack and see if you have continuity from the tip of the free end of the cord right up to the corresponding 68K resistor. Measuring from there to ground you should see a very high resistance. If you see a short from there to ground with a cord plugged into the jack, there is a problem with the input wiring.

Some of the solder connections to the ground bus at the end of the board board nearest the input jacks look a bit iffy. These are associated with the cathodes of V1 and V2. Make sure there is good reliable contact from the bus to the leads on the resistors and capacitors. Measure continuity from the chassis to the lead right on each resistor and capacitor. Re-do the connections if necessary.

Referring to the schematic, R21 is a 2-watt 2200 ohm resistor that supplies power to V1 and V2. It's not visible in your photo. It's on the board over at the power supply end. According to the layout drawing I have, it's in the sixth row connected to a couple of 100K resistors and a couple of jumpers.

Also look at the leads of the capacitors around the tone and vol pots. they should not be touching each other or the pot bodies

took a picture of the power end of the turret board. The R-21 resister shows a resistance of 2166 ohms. Added a photo of the regular channel input jacks disconnected from the chassis. A moot point if there is a short.

I have solid solder connections of the elements on the turret board (chop stick probing shows nothing loose.) My continuity tester buzzes when I probe virtually everything on the board to ground. Once again, My ignorance of the basic procedures is exposed here. The manual for the tester says that buzzer sounds if the resistance of a circuit under test is less than 100 ohms.Does this buzzing with probing of the capacitors / resistors to ground indicate that there is no continuity? / no ground?

Stephen told me to check the coaxial cables for a potential connection between the shielded inner cable and the ground sheath. The continuity tester buzzes when probing the coaxial cables to the overdrive channel, but not the normal channel. Once again, can you help me interpret this. I took another look at his note, and he said to disconnect these to test them. I tested this in place. Will disconnect and test again.

virtually certain that the leads to the capacitors / resisters on the volume / tone pots are not touching each other, but will check this more closely.

With nothing plugged into the input jacks, you should read a short from the hot terminals to ground. You should see this at the 68K resistors, too (the ends not connected to the tubes). This is the switch on the jack connecting hot to ground to mute the channel if nothing is plugged in.

With a patch cord plugged into a jack, the switch opens and you should no longer see the short from hot to ground at that jack. You should see continuity from the tip of the free end of the cord right to the 68K resistor that's associated with that jack.

With your tester, beeping means continuity. If you are looking for shorts that shouldn't be there, you should hear no sound. If you are looking for something that should be connected, there should be a beep.

I prefer a good digital ohm meter when looking for continuity. If you see 0 or .1 ohms, you know for sure the connection is good. A cold solder joint that's gone reisistive might show good if the tester's threshold is 100 ohms.

my ohm meter is digital and will read 0 ohms. Didn't know how to interpret the beeping. This helps. I'm being dragged to New York by my wife for a long week-end. Will be back in town next week.
Thanks,
Mike