Koch Studiotone 20 Combo fx loop 50Hz noise (hum)

[gitarzysta]

I recently got Koch ST 20 Combo - a great little amp, however...
....the FX loop is useless. It "hums" badly (seems to be 50 cycle hum - I am in Europe).

If I do not use the FX loop the amp is very quiet. As soon as I plug anything in the loop the humming becomes loud and clear.

I do not use anything that can cause ground loops, my signal path:

1. First nothing was plugged into FX return - only background noise could be heard, the amp was dead quiet
2. Then I plugged just a cable into FX return (nothing plugged on the side of the cable) - you can clearly hear the hum creeping in
3. Then I plugged an overdrive pedal (battery powered to eliminate bad power supply issues), as a preamp, directly into FX return, a guitar was plugged into the pedal IN. The overdrive level was at 0, the guitar volume was at 0, too. Loud hum can be heard.

NOTE #1: For the 3 tests above I only used FX return - nothing was plugged into/from the amp's SEND.

NOTE #2: various OD pedals were tested as well as cables.

I also tested (but not recorded) the fx loop with a delay pedal (amp send - delay in - delay out - amp return): loud humming noise was present as well.

I also tested an isolation box with ground lift between the overdrive and the amp RETURN - no change.

THE NOISE / HUMMING IS NOT PRESENT IF I PLUG DIRECTLY INTO THE FRONT OF THE AMP AND NOTHING IS PLUGGED IN THE LOOP.

Any ideas what can be wrong and how to fix it?

I recorded the noise here:

https://www.youtube.com/watch?v=TnPk9ch50dg

[Latole]

Brand new amp or second hand ?

Bad ground contact on the amp's  FX loop female jacks ?

If the amp purchased is used; oxidation on the contacts?
 1/4 socket contacts do not always last forever

Complete schematic here ;
https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/977458-koch-studiotone-20-combo-schematic

[Latole]

Or do the tests you've written about contain omissions or errors? 

Because a quick answer would be that the problem lies with your accessories, not the amp.

[shooter]

beside jacks mentioned, the environment you're in could be a buzz with noise, relocate amp n re-test.

[gitarzysta]

The amp was b-stock but new.
Yes, I still need to try a different location - thx.
As far as my "test" - I have been trying various pedals, cables, I checked the jacks from the outside (without opening the amp) - seems tight. But sure, there may be something inside/bad joint etc.

[BrianS]

Do you have a power amp/powered speaker or even another amp to run the the FX SEND into?  This test could eliminate/confirm any issue with that jack and the preamp. 

Test #2 I'd expect noise from just an open cable run into the RETURN.

Test #3 has too many variables to be a viable test.  When testing equipment, you have to test it in the manner that it will be used...or in a manner that the manufacturer expects it to be used.  It's unlikely that the amp manufacturer expects this amp to be run with the guitar plugged into a distortion pedal that is plugged directly into the FX RETURN.

Your delay pedal test is the most reliable indicator of a problem, as this is how an FX loop would typically be used.

Another test would be to run a cable from SEND to RETURN.  Let us know the results.  I would expect there to be no noise.

Having sold many B-Stock guitars in the past as part of my business, I can tell you that it is possible that a noisy FX loop could put an amp into a B-stock situation.  I don't know anything about Koch amps, but it's possible that for whatever reason it wasn't worth it for the manufacturer or reseller to repair the amp, instead marking it down as a B-stock with no warranty.  In my experience with retail customers, many, many guitar players never use the FX loop, and so would never even know this was a problem.
Good luck.

[gitarzysta]

Test #3 is the test reflecting the way I wanted to use it - amp in a box preamp into FX return. I tested Friedman IRX and Origin Effects Deluxe 61, as well as this Digitech overdrive. And this is a pretty common use of FX loops, I don't think I am trying something the manufacturer did not consider.
From the amp manual: "FROM FX (=RETURN): This jack provides an input for an external effects unit and must be connected to the output of the unit. When used, it disconnects the preamp of the STUDIOTONE from the power amp and therefore it also allows the STUDIOTONE to be used as a "satellite" for other preamps."

As far as B-Stock: according to Thomann, where I bought it from, reverb unit was noisy and it was repaired. The amp is covered by full 3 years warranty (as any new unit) and I can always send it for repair. However shipping the amp back and forth poses a risk on its own and I hate the idea, and I'd like to avoid it. But if push comes to shove.. ;)

In the meantime I opened the amp - the jacks are isolated (no connection to chassis), soldered to PCB and without removing the PCB I cannot check if there's anything wrong. I tried poking them and pressing to no avail.

As far as jumping Send with Return, on scale 1 to 10 the hum is as follows:
- nothing in the FX loop - 1
- send into return (cable "jumper") - 3
- just a cable in RETURN - 4
- full setup (be it a delay between SEND and RETURN or a preamp in the RETURN only) - 7

Therefore, yes, jumping send and return with a cable lowers the noise a little, but does not eliminate it

[gitarzysta]

Since I had the chassis removed I took some guts/PCB shots - there's not too many posts about this amp online. I hope someone can find it useful. This is revision 4.4 of the amp/PCB.

[jbrrrrr]

Have any of these tests been performed with a cable connected to the input jack of the amp?

[BrianS]

3 year warranty?  Send it back.  Hopefully you didn't void the warranty by opening it.  You will almost certainly void the warranty by doing any amount of work to it.

[gitarzysta]

Have any of these tests been performed with a cable connected to the input jack of the amp?

Plugging into the return jack disables the preamp. And if I want to use the amp as a power amp only, for my own preamps, I would not be plugging anything into the main input.

But on this amp it is physically disconnected once Return jack is inserted

[gitarzysta]

3 year warranty?  Send it back.  Hopefully you didn't void the warranty by opening it.  You will almost certainly void the warranty by doing any amount of work to it.

If they don't read the post here I'll be OK ;)
For now I dropped them an email about the issue and waiting for their feedback

[acheld]

If you ground your effects to the chassis, does that reduce your hum? 

[Latole]

Thank's for the pictures.

I hope you will not loose you amp for 3 months if you return on warranty

[jbrrrrr]

Not sure if this is the same model, but might help identify whether this is a design flaw, or just a flaw with your unit

[AlNewman]

Is the switch opening when you plug in the effects return?  If not, it won't be disconnecting the pre amp.

[Latole]

Not sure if this is the same model, but might help identify whether this is a design flaw, or just a flaw with your unit

Thank you.
Better link than mine at answer #1

[gitarzysta]

If you ground your effects to the chassis, does that reduce your hum?

No

[gitarzysta]

Is the switch opening when you plug in the effects return?  If not, it won't be disconnecting the pre amp.

If both send and return have a cable plugged in: the preamp is NOT disconnected
If only return port is used (nothing plugged in SEND): the preamp gets disconnected

[AlNewman]

If both send and return have a cable plugged in: the preamp is NOT disconnected
If only return port is used (nothing plugged in SEND): the preamp gets disconnected

Right, but in your tests you were plugged into the effects return only.  So if the switch wasn't disengaging, you'd be paralleling your preamp with whatever you had into the return jack, which may or may not cause hum.  It's something to consider.  If the switch works, then it's not the problem.