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. 

Fixing Squier Champ 15 type: PR-408

My friends Squier Champ 15 had a problem - it didn't produce any sound. Red LED was illuminated but there wasn't even a hum from the speaker. My first guess was that a power stage was damaged. After opening up the Champ 15 reveled its uncomplicated construction:

 One-sided PCB with two double operational amplifiers and TDA2030A monolitic amplifier. I searched for any sight of thermal events and measured supply voltage which turned out to be correct -20V/+20V. After feeding input 1 with audio signal from a laptop I measured with an oscilloscope that there is a correct signal on pin 1 which is input of the TDA2030A (it means that a preamp is working fine). There was amplified signal on the output of the TDA2030A to, so I moved on to connection between amplifier, speaker, and two connectors for headphones and a external speaker.
Plugging in headphones should disconnect the built-in speaker to make the combo silent, in case of this amplifier the problem was that one terminal of the speaker was disconnected because of defective headphones connector. Pair of normally shorted pins which should be shorted when nothing is plugged in was disconnected. I hard-wired them to check if this was it. Amplifier started to work.

I didn't want to replace defective connector so I've just fixed it. I desoldered and took out an elastic metal part which wasn't switching reliable enough, bent it so it was tighter, put it back and voila! Squier Champ 15 is working now.

This is mentioned elastic metal part, I am holding it with tweezers.

Metal part is on its way to be fixed:

Metal spring after bending:

crude power supply with LM741 2N3055

Recently I had chance to look inside vintage power supply of my friend. It has got settable voltage and current limit so it's just typical, quite old workbench power supply (I don't dare to call it "laboratory")

After opening housing I've seen one-sided PCB with all components neatly described. The owner of the power supply told me that "current limitation doesn't work below 5V" and I was curious why it was so and if we could do anything with it. I could find damaged component quickly (there is not too much parts inside) but I started drawing circuit diagram to have clear overview.

As you can see this couldn't be simpler. According to circuit diagram current limitation should work just normal - when voltage across shunt resistor rises to around 0.7V, the transistor starts to conduct and pulls base of BC337 to lower voltage so it closes BC337 and in consequence closes 2N3055 to. This should be independent of voltage set-point.
I figured out that BC337 was damaged and replaced it with the new one. (OK, actually there were two transistors in Darlington-like configuration and I just omitted one). Power supply worked almost perfect, the only one problem was that current limitation (it worked below 5V to) smallest possible current limit setting was 1.4A when I expected it to be around 0.7V/1.5Ohm=0.46A. I just assumed that this two parallel power resistors which form shunt resistance (two white resistors below) are really 3 Ohms each like on the visible description on PCB. It turned out that they are 1Ohm each what gives 0.5Ohm and explains that current limitation can't be set to smaller value than around 0.7/0.5Ohm so around 1.4A. Never trust such a descriptions, measure it first!

By the way, here are some hints if you would like to build this power supply.
1. LM741 is not rail-to rail operational amplifier and this causes that the lowest possible to set voltage is around 5V, possible workaround would be to change operational amplifier to the one which can pull its output almost to the negative supply rail like LM321 for instance.
2. Current limit like on the circuit works actually from zero to around half of the potentiometer position, if you turn the knob further there will be no result. To eliminate that dead movement of potentiometer you could put another 470R in series with current pot, on top of it
3. By placing larger values of shunt resistor you get possibility to set lower current limit, I would prefer to put around 5.6Ohm as a shunt so I could set around 125mA as a lowest possible current limit.

Sachs Pentasport gearhub with two cables - fixing blocked extreme gears

If your Sachs Pentasport blocks extreme gears (you can switch only between 2 to 4, gear 1 and 5 can't be chosen because you are afraid of braking the lever) then:
1. Make sure that you've disassembled, cleaned with white spirit, lubricated and assembled gear correctly.
2. Make sure that you are adjusting the lever correctly (starting from gear 4 which means all cables are in their "longest" position and than adjusting length).
3. Check if you've placed sun gear correctly, in my case it was the root of all evil. Befor mounting a wheel to bicycle it seemed to work but after riding for a while gearhub resisted against switching gears 1 and 5, I didn't want to break plastic lever so I've started to investigae what might be the reason. Sun gear can be easily flipped (two side are different but it seems to make no difference but it does make a huge difference).

The part 25a on this drawing is the mentioned gear which can cause problems if flipped.

How to succesfully replace Opel/Vauxhall factory radio or why your new radio can't pick up much

Opel/Vauxhall calls its factory radios CDxx where xx is a number, in my case it's Opel CD30, originally produced by blaupunkt. CD30 does not play MP3s so I decided to fit new radio - Blaupunkt Melbourne 120. I removed factory fitted CD30, mounted plastic shelf to fit regular ISO sized radio and tried to connect ISO connectors, unfortunately it is impossible because CD30 radio has got it's own standard - to have it done neat and nice I bought adapter for this and used to connect speakers and power supply.
CD30 has got FAKRA connector for antena so I have bought FAKRA to ISO adapter and was naive enough to believe that it is everything what I need.
Radio played mp3 but it picked up only two stations on FM band. After short research I found out information that CD30 radio provides DC component to its antenna connector and this DC component is used to supply antenna preamplifier which is located on the roof right under mentioned antenna. FM preamplifier has to be used because there is not that efficient antenna (it is quite short) and quite long coaxial cable which introduces loses. My brand new fancy Blaupunkt Melbourne 120 is not able to provide DC component so I had to exchange regular FAKRA to ISO antenna adapter to adapter with a circuit similar to this(it is circuit diagram from http://www.electroschematics.com/488/fm-antenna-amplifier/):

MPPT solar panel controller

PIC16F1782 has got built in almost all components needed to build a switching regulator. I've already tested a possibility of building step down regulator utilizing a microcontroller with "advanced analog peripherals". This time I would like to build a true maximum power point tracking converter for a solar panel. 

My monocrystalline test solar panel is rated with:

Peak Power (Pm) 10W

Open Circuit Voltage (Voc) 21.67V

Short Circuit Current (Isc) 0.61A

Maximum Power Voltage (Vmp) 17.49V

Maximum Power Current (Imp) 0.57A

Maximum power voltage is lower than voltage needed to charge a 12V lead battery so I need a step-down converter:

I found it difficult to arrange which pins should be connected to which signals. There are some alternatives on which pins periferals are connected and it is rather difficult to notice the best arrangement looking only at schematic. It is much easier to notice every possibilities and clashes on a spreadshit like this:

	pin								1st	2nd
RA0	2	AN0	CxIN0-						COMP_I
RA1	3	AN1	CxIN1-	OPA1OUT					PV_I_SHUNT
RA2	4	AN2	CxIN0+						COMP_I
RA3	5	AN3	C1IN+						PV_I_FBK
RA4	6		C1OUT	OPA1IN+					PV_I_SHUNT
RA5	7	AN4	C2OUT	OPA1IN-					PV_I_SHUNT
RA6	10		C2OUT						COMP_I
RA7	9				PSMCxCLK
RB0	21	AN12	C2IN1+		PSMCxIN	CCP1(2)			STOP_SWITCHING
RB1	22	AN10	CxIN3-	OPA2OUT					COMP_V
RB2	23	AN8		OPA2IN-					PV_U_FBK
RB3	24	AN9	CxIN2-	OPA2IN+		CCP2(2)			OUT_V_FBK
RB4	25	AN11	C3IN+						COMP_V
RB5	26	AN13	C3OUT						COMP_V
RB6	27						TX(2)	ICSPCLK	PROG	TX
RB7	28						RX(2)	ICSPDAT	PROG	RX
RC0	11				PSMC1A
RC1	12				PSMC1B	CCP2(1)			PV_I_SET_PWM
RC2	13				PSMC1C	CCP1(1)			PV_V_SET_PWM
RC3	14				PSMC1D
RC4	15				PSMC1E				LED2
RC5	16				PSMC1F				LED1
RC6	17				PSMC2A		TX(1)		DRIVE_HS
RC7	18				PSMC2B		RX(1)		DRIVE_LS
RE3	1								PROG
VDD	20
VSS	8
VSS	19