Fixing Fender Hot Rod Deluxe PR-246 (solder cracking)

Fender Hot Rod Delux is a 40 Watt hybrid guitar amplifier. This particular one had a problem of a cracking noise. There was a normal sound coming out of the speaker but with annoying full volume cracks, while playing cracks happened more often then in a standstill. 

The owner told me that he wanted to by a new set of vacuum tubes because he noticed that he can cause this disturbance by touching the left most 12AX7A tube. I took a look at the tube and noticed that it is quite difficult to take it out because tube sockets are fitted very tight.

After opening a back of the implement I took a look at a PCB and noticed that there actually was a solder crack on the pin of tube socket. The crack is visible in the center of this picture:

I heated this broken solder joint with soldering iron, added some new solder and problem was fixed. As a precaution I reworked the same way all pins of this tube socket because if one solder joint was broken than another one was just waiting to crack.

Lesson learned:

1. Be careful with replacing tubes - they are fragile and rest of an amplifier is too.

2. PCB holes diameter for this tube socket are in my opinion a little bit to big giving pins possibility to cause mechanical tension while replacing tubes. Sockets have no mechanical support except for solder joints - shame on you Leo Fender!

Fixing Honeywell Rondostat

In a scrap parts box I found a Honeywell Rondostat - a radiator thermostats which allow you to define different time zones and different temperatures for them, it is even aware of a day of a week and can control temperature accordingly. The thermostat I found was where it was for a reason, actually two: after putting batteries it turned out that a metal spring which connects batteries in series is loose and doesn't provide right electrical contact. This shiny metal part here:

It was a bit tricky to get it out:

But it was possible without demolishing whole device:

I bent this metal part so it was a bit tighter put it back, plugged two AA batteries inside and the Rondostat started to work immediately. New problem was that a adaptation process in which the thermostat learns what is possible valve operation range ended with an error. As it turned out it was a problem with a built-in gear. I pulled out the gear by levering three snaps, for two snaps I used stripes of EC card which I sicked into holes which are actually there for detaching gear:

 Third snap is under control PCB which is easier accessible:

Root-cause of the gear adaptation problem was this little gear - metal axle was pushed into plastic part too deep:

With a pair of tweezers I pulled the axle out and put it back in but not that deep like it was before. Last step was to assembly it all back together. If you try it on your own, be aware that the gear with two metal parts has to be placed on top. This metal parts make gear visible for rotation sensor on PCB.

 Further steps for putting it back are fairly easy and Rondostat works now.

This is PCB with NEC microcontroller, red connector is for a DC motor connected to the gear.

Honeywell Rondostat doesn't look like a new one, from a layer of dust I suspect it is more than one year old but mechanics is still OK. I didn't see any wear-out, it makes reliable impression. One problem with axle moving into plastic part might be planned obsolescence but even if - It is comparatively easy to fix it. I know that this thermostat was in a scrap box more than two days and surprisingly its clock was almost correct, turns out to have some super-cap or battery backed up RTC.

Last point is that I suppose that Rondostat does not like rechargeable batteries because of their lower voltage, I will try to test it deeper maybe my rechargeable batteries were already flat.