Marshall JCM800 4211 & JCM900 1960A

An offer you can’t refuse sounds like a clishé, but every now then they come along. I got one of those offers when I was asking my friends for broken guitar equipment. One of my friends mentioned that he had an old Marshall JCM900 4*12 cabinet lying around. The price was very budget friendly since there seemed to be some defect that caused the sound quality to suffer. In later conversations it came up that he also had a faulty JCM800 2*12 combo that had served as a cabinet and the amplifier was just dead weight since it was unfunctional.

I got my hands on the JCM900 cabinet first and I took it apart right away. As I got a peek inside I noticed that almost all the screws holding the speakers in place had come undone. At most the gap between the chassis of the speakers and the cabinet was about 1/8”. This meant two things; the backpressure inside the cabinet was escaping from the gap and the speaker chassis were moving around absorbing the energy that was supposed to move the speaker diaphragms. I predicted that this did not improve the over all sound quality. I tightened the screws and put the back plate of the cab back in place. 

Since I didn’t have an amplifier at my disposal, I returned the cabinet to its previous owner and took the combo with me to have a look at that. Later he said that te sound quality of the cabinet was improved dramatically by my operation.

I took the combo home and plugged it in and turned it on. I was almost thrown up on the wall when I switched the amp on. Not because of an electric shock, but because of sound pressure. The speakers started to hum extremely loudly. I tried plugging into another outlet, this time a grounded one, but the result was the same. At this point I got orders from higher up inside the family to not do that again in our apartment. I didn’t really mind this since I had to take amp apart anyways. After I got the amp opened I stood there for a while observing without finding anything out of the ordinary that would cause a fault detected earlier.

While putting everything back together I discovered that one of the power amp tubes was discolored and stood out from the rest. On closer inspection I noticed a small crack in the glass envelope of the tube.  Considering that this is supposed to be a vacuum tube I figured that this check might have something to do with the less than optimal performance of the amp, so I placed an order for a set of new tubes for both pre- and power amp sections.

Power amp tubes being biased

I took the amplifier with me to my work place for the tube replacement and biased the power amp for optimal balance between sound and durability of the tubes. After this the amp worked just as it was supposed to so I sold for a nice profit. Some time later the new owner called me to tell that the capacitors on the rectifier circuitry were faulty and demanded me to return half of the money he had paid. He wasn’t willing to show me any kind of proof of the fault or let me take look at the amp and have chance to repair it so I’m guessing he was trying to swindle me out of the money. Especially since he had told me that he hadn’t told his wife about being new expensive gear and predicted that it would cause some turmoil at home.

For this project I had to learn how to bias a tube amp, which proved to be quite a profitable skill later on. The guys at our local music store heard about my ability to bias amps and after that they directed all their customers searching for such a service to me.

Sony Bravia KDL-40U-2000

Sometimes things break in a way that it’s no longer reasonable to fix them. That’s what happened to the projector I repaired earlier. Constantly powering on and off the device and the change in temperature caused by it made the mirror rods’ gluing loosen until it collapsed. This caused the whole image projected look skewed and warped.  I tried to glue it back together but it proved to be futile since in needed to be perfectly aligned, which was impossible to achieve with the tools at  my disposal. The only solution to this problem would have been to order a new mirror rod from the other side of the globe, which would have been unreasonably expensive. It was time to start considering other cheaper options to view the video from my game consoles. The simplest solution seemed to be trying to find a broken flat screen television that had a functioning LCD-screen. I spent about six weeks searching until one day I went for a walk and wandered to a nearby recycling center. From there I found exactly what I had been looking for. This was the first device that I fixed that wasn’t given to me as a donation or I had found discarded in a waste disposal unit.

I bought the TV and dragged it home. After plugging the power cord into a socket I switched it on. I was a bit baffled when the TV turned on without a problem. This had to mean that somewhere inside the television there was a component that either over heated or lost its charge. My diagnosis proved to be right in just a couple of minutes as the TV powered off and began to blink one red LED-light in Morse code kind of way. At this point the story was running out of pages. The next couple of nights I spent surfing the internet and Googling while trying to deduce where the problem could lie in. Every once in a while I took my multimeter and tested spots where other people had found their TV:s to have problems. One of those spots was the current that the power board fed to the inverter for the backlight. From this point it was easier to move on. I found out that the issues on the power board were caused by a malfunction on the inverter. All the measurements I made after that pointed once again unsurprisingly to a bunch of faulty capacitors on both the power board and the inverter. With fairly small effort I found schematics of the television which helped me to determine which of the caps needed replacement. Finally I got everything done and the whole TV put back together. Plug in the power cord and power on, right? Well… No. I was disappointed and got a little mad. Sisu kicked in. Franticly clicking the power button hoping that maybe the 78th time would be the charm. It was. Or it could have been the 83rd time but never the less, it powered on. Being afraid that I just got lucky I didn’t celebrate yet. I left the TV on for a while and let it warm up before turning it off. Another try and another success. I tried this a couple more times and felt like I had reached some stability. Apparently the capacitors needed the current to rise gradually in small increments until they were at level they needed to be for proper functioning. A couple more on-off-cycles everything was running smoothly so I put the TV up for a testing period. When I initially was ready with writing the blog entry the TV had been in use for two days with no problems. All in all I spent €19,10 on the whole project of which €10 was the television itself.

This was where the story was supposed to end, but it wasn’t. After using the TV for a couple of weeks the faults started to reappear. Backlights wouldn’t light up and the red LED was blinking again. As the first measure I checked all the soldering joints I had soldered before but found nothing wrong with them. I spent a couple of days pondering the issues while staring deep into the PCB:s. I finally decided to remove the signal processing board for a check. On the underside of the board right next to a leg of a capacitor there was a small lump of solder which I assumed was a consequence of a cold solder. I re-soldered the joint and once again put the TV back together. This caused the TV to work even less than before.

A couple of weeks went by and I couldn’t come up with a solution to this issue. It was only after I brought the television to the place I interned at and testing it out with an external power supply that I got proof that the power unit of the TV was functioning as it was supposed to. I directed my attention once again to all the places I had tinkered with. My predictions were right. It wasn’t an actual defect but instead my own fault. While putting the TV back together I had accidentally pressed the pins of one connector  out of place and they were causing a small shortcut. After bending the pins back I tried powering the TV on again. It turned on so I put everything back together and started using the TV. I had the TV for a year or two until it broke beyond my repairing skills so I sold it my more skilled friend who insisted on paying more than I had spent on whole project. I learnt a lot by messing around with this project. The best part was when my interning supervisor saw me staring at the circuit boards for 20 minutes and the picking up a soldering iron to re-solder that cold soldered joint he asked me if they could hire me. It was only for a couple of months but this TV made me a small profit and got me a job. I was pretty happy with this outcome.