Fender Deluxe 112 fast ticking noise

[Jaap Stam]

Hi there,

Just subscribed to this forum because it seems there are a lot of knowlegeable people here!
I have soma basic electronics knowledge and know how to solder. In the past I have re-wired guitars, build my own drive pedal and fixed some loose joints in amps. But now I have a problem that I cannot get my head around....

A little while ago I got a broken Fender Deluxe 112 solid state amp from the nineties, with a defective drive channel and a loud hum on the reverb. These two problems were easily fixed; the broken drive channel was caused by a loose C6 and the reverb was hooked up the wrong way around. So I thought I had done a good job, until I noticed a fast ticking noise, kind of like an engine. It has a frequency of 5-10 Hz (just a guess) and varies in volume over time. Its not crazy loud, but loud enough to be very annoying.

Some observations:
- The ticking starts as soon as some kind of lead is either touching the common ground or one of the inputs.
- It happens when a guitar is connected and without (i.e. just a lead).
- It even happens when I touch the common ground or one of the inputs with a loose probing wire from my DMM. So it happens when you toch the common ground and/or the inputs!
- The volume of the ticking seems to be related to the length of the lead; a shorter lead gives a lower volume.
- If I touch the guitar strings or touch the chassis the volume gets less.
- It disappears as soon as the chassis is connected to the common grund
- The frequency is always the same
- The volume of the ticking does not depend on the clean or drive amp settings; it is always the same even with the volume turned completely down.
- So it seems to originate in the power amp. I connected the pre-amp out to another power amp and that combination sounds perfectly fine.
- But the funny thing is: if I plug in another pre-amp in the Fender (and effectively only use it power amp) the ticking noise is gone.... So it seems to be something in the combination of the pre-amp and power-amp.
- Other than this noise, the amp is working fine. No crackly pots, both channels and reverb work as they should.

So I opened up the amp again and checked for loose joints, bad caps etc. I found two loose connections on caps that I fixed. Other than that it looks clean.
I also found that some previous owner had replaced the two input jacks with non-standard parts. The two inputs were hooked up directly to R2 and R3 and the ground to R6. This means that the "muting" circuit no longer works, but that is OK. Also someone before me has replaced R4, just jumped it over the previous resistor which seems to be fried. But the resistance between the common ground and chassis is ±22 Ohms, which (according to a post I read here) seems to be normal.

I checked the DC Test points and they also seem to have normal readings:
TP3 = 40.3V dc
TP4 = -40.2V dc
TP5 = 16.7V dc
TP6 = -16.5V dc
TP16 = 26.8V dc
TP17 = -26.7V dc

The channel switching TP20 also seems to work fine: -14.8V in one setting and 15.0V in the other. Unfortunately I do not have a tone generator to supply the standard AC test signal and I also do not have a scope.

Is there anyone here that has had such a problem before or has a guess where it could originate from?
I am thinking of some kind of grounding issue, but cannot think of anything that could cause a regular ticking sound. Just to be sure I tried different spots in the house, but the effect remains. I have never before experienced any grounding problems in the house, nor with my other amps and equipment.
I could simply connect the chassis ground to the common ground permanently to make it go away, but I do not know if that is a good idea.

Bye!

[Enzo]

Is there a cell phone within three feet of this amp? Does it act this way at a friend's house or some other place?

[Jaap Stam]

Hi Enzo,
Good point, I checked that it is not caused by cells, i-pads etc.
I did not try it yet at someone else's house, but I never had any such problems at home with my other guitar or bass amps. The house wiring is only 10 years old and up to-date.
Bye!

[Enzo]

I don' think it is house wiring, but even if something has changed in your house, trying the amp somewhere else will either verify or eliminate an environmental cause. One of my favorite shop episodes was an amp that made a humming noise every minute or so for like 20 seconds. I spent a couple hours trying to track it down, and yes, I turned off everything in the building that could make noise. It wasn't until I stepped onto my porch with the amp running inside th shop that I noticed the hum neatly matched the traffic light at the end of my block. Whenever it was yellow one direction, I got the hum. SO a noisy triac in the traffic light was causing noise on my power lines.

[glebert]

Have any pots been replaced? Some of these fenders used pots with brackets that are grounded and are used as ground interconnects.

Enzo, that's a great story!

[Jaap Stam]

Hi Glebert, the pots seem to be all original. Indeed it seems as if the connection to the chassis is made via the pots. When the pcb is removed from the chassis, all pot brackets of the clean channel connect to the ground of R4, while all pot brackets of the drive channel connect to the ground of R5. So there are two paths from the common to the chassis ground, both via a 47ohm resistor.

I checked the amp in another room of the house and I am pretty sure it is not the wiring. Even if it was, then why would the ticking stop if I use another power amp? Or when I use my HX stomp as a pre amp? I suspected the power amp jack, which also activates the muting circuit
To make it even more mysterious: when I jumper the pre and power amp connections it does not go away. So it has nothing to do with the muting circuit that is part of the power amp jack.

[g1]

some previous owner had replaced the two input jacks with non-standard parts. The two inputs were hooked up directly to R2 and R3 and the ground to R6.

This could be your problem. Was the stock part insulated from ground where as the new is not? Suggest restoring to original ground scheme.

[Steve Blackdog]

I don' think it is house wiring, but even if something has changed in your house, trying the amp somewhere else will either verify or eliminate an environmental cause. One of my favorite shop episodes was an amp that made a humming noise every minute or so for like 20 seconds. I spent a couple hours trying to track it down, and yes, I turned off everything in the building that could make noise. It wasn't until I stepped onto my porch with the amp running inside th shop that I noticed the hum neatly matched the traffic light at the end of my block. Whenever it was yellow one direction, I got the hum. SO a noisy triac in the traffic light was causing noise on my power lines.

I am single sided deaf and rely on my high powered hearing aid. I have a built in RF receiver that auto-connects to a remote microphone. Unless the microphone is transmitting it stays in normal hearing aid mode.

I started to find that it kept switching on even with the external mic not connected. I searched the house for RF interference. Eventually I tracked it down to the extractor fan in the downstairs toilet. It comes on when the light is switched on and stays on for a minute or so on a timer. So every time somebody visited the WC, I got a blast of rf interference in my hearing aid.

So the lesson is, intermittent interference might be explained by WC use!

[Enzo]

No shit?

[The Dude]

....

[Delta362]

That's wild, Steve!

[The Dude]

I should clarify. My ROFLOL post wasn't in reference to the misfortune of having a hearing problem. It was to Enzo's pun intended "No shit?" comment.