KMD XV100SD Tube Amp has FX Loop problem - please help

[CoopDaKill]

KMD xv100sd all tube guitar amp. This amp has two sets of FX loop in and out jacks on the front control panell. When I plug an effect pedal or processor in the left pair of jacks, there is a severe hum, and faint signal. When plugged into the far right pair, there's no hum, but no signal.

I've checked all resistors and caps. All test ok. I'm stumped! Maybe it's a transistor? Also, the v5 tube seems to not glow near as bright as the other 4 preamp tubes. This amp uses (5) 12ax7 tubes, and (4) 6550 output tubes. Schematic and pics below. Thanks in advance my friends!
Coop

http://music-electronics-forum.com/a...md-xv100sd.pdf

[Jazz P Bass]

Nope.
No transistors on either loop.

If the amp works without any FX inserts & doesn't work with an FX inserted, then I would look at the jacks.
You may have a bad ground connection.

[CoopDaKill]

Thanks Jazz P Bass.. I thought of the jacks and a grounding issue after my last post. I'll check those out when I get a chance, and let you know what I find. Thanks for your hep and insight. Much appreciated.

[The Dude]

I have a couple of thoughts.

1) Could you be supplying an instrument level pedal to a line level input? That might cause a low signal level and hum.
2) Since there are two loops and two channels, could the non-working loop be for a channel you're not using? In other words have you tried the non-working loop on both channels?

[g1]

The Dude is correct about the loops, one is for clean channel only, pre volume, the other is for both channels, post master.

[CoopDaKill]

Hey guys, I went over all the FX loop jacks and reflowed the solder. Everything looked good. I then used an Alesis Midiverb 4 into the fx loops and no change.

There's a 100 ohm resistor that goes to the line out jack on the back panel. It looks pretty burnt but tested 96.7 ohm. But when I run a cable out of that line out jack, into another amp, I get very little difference in volume level. Just seems to add a little treble. But the pot tested good too. ???

Also, V5 tube (12ax7) is barely lit. Nowhere as bright as the other 12ax7 tubes.
Pics included below.

[The Dude]

Different tube filaments glow differently. I wouldn't worry much about that.
In your second picture, I notice a few things. 1) The chassis seems to be corroded a bit. 2) There's a circuit ground wire tied to the pot body. 3) The solder connection to the pot body doesn't appear to be a solid connection.
So, give the wire a good tug and make sure it's well soldered to the pot. Make sure all of the nuts are tight on the pots and well grounded to the chassis (check with a meter). You may want to scrape off some of that corrosion to be sure that the pot bodies are making a good chassis ground connection. I'm betting you have a poor ground connection somewhere.

[CoopDaKill]

I scraped off all corrosion from the chassis, making sure there's solid ground connection, throughout the amp.

That wire The Dude referred to is not loose, but I gave it some soldering anyways. Still no change. I'm still stumped. Any other suggestions?

[g1]

Try the guitar straight into each of the return jacks. One of them should give a clean sound.

[CoopDaKill]

Ok g1, I plugged the guitar straight into fx loops. The clean guitar came through only one fx jack. And that was the fx out Jack located below the channel switch and foot switch jack.

[The Dude]

This is still not making any sense to me. Presumably the amp works fine when the loops are not in use, correct? Both loops simply interrupt the signal path that already exists. There's nothing fancy here- no real additional circuits added when the loop is used. In other words, if the amp works without using loops, it should work using loops unless there is a "user configuration problem" or a jack issue. Try inserting a standard guitar cable in each loop. Does the amp work this way?

[CoopDaKill]

Yes, the amp works fine as long as nothing is plugged in the FX loop.

I tried what you suggest about plugging just a cable into each fx loop Jack;
From facing L to R of the front of the amp:
Out and in jacks (ones below channel switch) cuts the signal and just hums.
Out and in jacks on far right, drops the signal, but NO hum.

Could a faulty C8 or C25 be a cause? One of these disc caps looks like it's been hot
Ya think I could have a bad Jack?

[mozwell]

Fairly straight forward send / return circuits. Line level on these, lets get these working with patch leads & then we can go to getting effects working

the first send / return has VR4 "effects gain" pot, if this is all the way down, you wont get signal through. Turn this pot to maximum, do you get signal through this effects loop now ?

checks grounding to the effect jacks is good
Plug a patch cable into the first send socket. measure resistance from R7 to the tip of the patch lead (should read 0 ohm if effects gain pot is at maximum).
Plug the other end of the patch cable into the first return socket, measure resistance from R7 to the top connection of VR5 "volume" (should read 0 ohm)

Plug a patch cable into the second send socket. measure resistance from R34 to the tip of the patch lead (should read 0 ohm).
Plug the other end of the patch cable into the second return socket, measure resistance from R34 to C30 (should read 0 ohm)

[CoopDaKill]

No signal through fx loop 1 with effects gain pot at maximum setting.

Plugged a patch cable into the first send socket. measured resistance from R7 to the tip of the patch lead is 1.2M , with FX gain pot at max setting.

Plugged the other end of the patch cable into the first return socket, measured resistance from R7 to the top connection of VR5 "volume" is 143k ohms

Plugged a patch cable into the second send socket. measured resistance from R34 to the tip of the patch lead is 0 ohms

Plugged the other end of the patch cable into the second return socket, measured resistance from R34 to C30 is 1.01M ohms.
None of the boards are marked for any components. So, this makes it more challenging for me. But I like a challenge! Haha. Hopefully I found all the right components that you instructed.

[mozwell]

Did you do this with the amp powered on ?
Resistance should be with the amp powered off.

All this is basic continuity checking, to see where we lose continuity

From wiper of treble pot VR1 to top lug of effects gain pot VR4 is 100K R7
From wiper of effects gain pot to top lug of effects gain pot (with pot on max) is 0 ohm
From wiper of effects gain pot to send jack is 0 ohm, and this should also be 0 ohm to the switched contact of the send socket(with nothing plugged in)
From bottom lug of effects gain pot to middle connections of the send socket should be 0 ohm
From 0V to the inside connections of the effects send socket should be 0 ohm
From the switched contact of the effects send socket to the switched contact of the effects return socket should be 0 ohm

etc.....

Follow the signal path & check all the connections with your meter.

When checking, don't forget to check connections from each pot lug to the wiper. A bad wiper connection could make you "lose" your send signal, and then its like the whole signal is lost....

Double check board soldering, and don't forget to measure the actual components themselves are ok (resistors & pots). Also check there are no stay bits of solder or wire lead from components near the edge of the board touching the metal front panel & shorting signal to ground.

A methodical approach should find where your signal gets lost.

edit - the schematic mentions send & return, but the panel is marked effect in & out, is there any chance you have input & output to the effect connected wrong ? As a quick check, try swapping the input & output leads to the effect.
If you had "send" connected to effect out, then you would get the symptoms you describe