Ace Bass Sub Nova

Swapping the caps

From the same pile headed for the heap where I got the amp in the previous post, I also picked up an active subwoofer with a 10” speaker. My dad was throwing it in the trash because it started making an annoying loud noise after being on for a while. The noise was at a steady frequency and unbearably loud. My initial thought was that the rectifier bridge lets the 50 Hz AC from the mains to the amplifier circuit, so I started off by replacing the capacitors in the bridge. This didn't solve the problem, but it was a good idea to swap those caps for new ones preemptively.

I let the subwoofer stand in the corner of my living room and kept pondering about the issue. While trying to figure out what the problem was, I cleaned the potentiometers for the volume and low-pass filter. At some point a thought of building a new amplifier circuit from a kit also crossed my mind.

One potential culprit was the coil in the speaker, so I measured it with a DMM by hooking the probes to each terminal of the voice coil and moving the membrane back and forth. The resistance values kept changing as they were supposed to, so the fault wasn't there. This was good news because a new speaker would have been way too expensive for this project.

At some point I realized the noise started after the subwoofer had been in use for a couple of minutes. From it was easy to deduce that this is a result of some component overheating. I reached behind the sub and felt the heat sink with my hand. It was completely cool. This meant that the heat was somewhere else, so I took the amplifier apart once again and removed the heat sink from the power amp transistors. All the silicone paste that was supposed to be between the transistors and the heat sink had found its way to the sides of the transistors and away from the heat exchange surface. Now instead of conducting heat, the paste acted as an added insulation for all the heat. I removed all the silicone paste and replaced it with silver paste that was left over from some previous project.

The subwoofer had been in daily use ever since for the past three years and is still working without any problems. I didn't really need a sub for my home theater, but this was a fun experience and I learned something new. In the end I got a quality sub for the price of the capacitors which was around 20 euros. 

Test drive

Sony STR-DB840

My dad has been a home theater fanatic since the 90’s when the first affordable Dolby Surround kits started to hit the stores. His first one was a ready-in-a-box set but it didn’t take long for him to start building his own vision by replacing the speakers, amp and subwoofer by better quality ones. Because this was a hobby, all the parts were chosen by their success in hi-fi magazines regardless of the price. Since then the technology has come a long way and newer models are more refined and better equipped while the prices have come down but they are still more expensive than ones saved from a trash pile. On top of that the ones bought from a store don’t give me that feeling of accomplishment that the recycled ones do once I get them to work. At this point it might be fair to mention that many of the devices on this blog have given out sooner or later in a way that fixing them wouldn’t be feasible or cost-effective any more.

Having served faithfully for more than a decade, the 5.1 channel amplifier had started to make weird clicking noise and my dad was afraid that it could be a fire hazard. He had already thrown the amplifier on top of a pile of junk headed for the junkyard when I intervened and rescued it. I decided to take the amp home for closer inspection because it had line level RCA-outputs that my own amplifier lacked. I had been using a converter to attach active speakers to speaker outputs and the sound quality was horrendous. Because I assumed the clicking noise was caused by some relays’ old age and dust and dirt accumulated on some contacts, I figured that there was no immediate danger in using the amp. I plugged the newly acquired amp into my existing set-up and just started using it. Because of the line level outputs the sound quality improved dramatically. It took me several months to grow tired of clicking relays.

Finally I detached all the cabels of the amp and opened it up to locate the faulty relays. They were easy to  find, so I started figuring out how get my hands on them from the opposite side of the PCB. To my surprise I realised that Sony had taken the serviceability into account in the design process. On the bottom of the amplifier there was removable hatch that allowed me to inspect the solder joints. A quick glance was enough to find the cause for the clicking. One of the solders on a relay was cracked and that caused it to open circuit seemingly randomly. The fix was fairly easy. I just removed the old solder and re-soldered it. After this the clicking ceased and I was happy for a while.

I would have been happy longer if I had just stuck with my 3.1 arrangement. I wanted to experience the  full surround sound immersion so I added rear speakers to the set-up. Since I didn’t want speaker cabels running across the living room floor, I decided to route them via the ceiling. With my girlfriends’ help I installed cabel trays and hid the cabels there. After two days of hard  work the trays were installed and the rear speakers plugged in. First thing I noticed when testing the new set-up was that the clicking had returned. This mighty have caused my blood pressure to rise slightly but luckily the fix to  this was the same as for the front speakers. When first fixing the relays I didn’t realise that there were dedicated relays for front, rear and  center speakers as well as for the subwoofer. I re-soldered all the relays I could find because those that weren’t cracked yet would crack eventually so I went over all the solders that I could find. The amp has been working flawlessly for well over a year now and I’m happy with it considering the cost and everything.

Bogner Alchemist Head

After making some new connections I started to acquire music equipment to repair on a more regular basis. This lead me to learn a lot more new stuff about electronics. I haven't had much time for my own projects, now I was handed an interesting problem.

A customer had brought a Bogner Alchemist head to the local music store because it had mysteriously stopped powering up. I took t

he amp in and after a while I had to admit that my skills weren’t up to the task. The power switch had no effect on the head itself but the lights on the foot-switch lit up. This meant that power was running somewhere so all hope was not lost yet.

Autopsy

I was baffled. I took the amp out of its casings and started to eye over the circuits. At some point I noticed that the secondary coil of the power transformer was connected to a wrong place on the circuit board. Moving these two leads to the correct spot brought some life to the nearly dead patient; the power amp tubes started to glow and warm up. Pre-amp was still flatlining and all the lights on the front panel were dark.

After pondering the situation for a while I decided to let the customer know that finding and repairing the fault would take so long that it wouldn’t make much sense since I was working on an hourly rate. I offered to buy the head from him for a  reasonable price knowing that I might be buying just a pile on spare parts. At least this way I wouldn't have to keep looking at the clock all the time while working.

I spent many nights searching for information and studying the circuit board. In terms of efficient working time it must have taken dozens of hours, but all that time I kept learning new things about the inner workings of a tube amplifiers. After a while of going through the amp with a DMM and studying the schematics I noticed the the pre-amp wasn't getting the voltage it needed. The pre-amp tubes require an AC voltage off +3.5V/-3.5V. My measurements showed only +1.6VAC/-1.6VAC going in.

At this point it was again time to take out the schematics and take a good look at where the current flows on the power board section. The circuit feeding the 12AX7’s had two resistors of equal values on both the positive and negative sides of the power feed. These two resistors were connected to each other via potentiometer that controls the voltage going to the tubes.

After this discovery I connected the DMM to one of the sides and started adjusting the potentiometer until it showed about 3.5 volts. As the voltage rose to the desired value, all the lights on the front panel turned on.

The head found its place in a new home

I re-assembled the head and plugged it into my trusty 4*10 bass cabinet the I had designed and built some time earlier. Everything worked just as the user manual said it was supposed to. I was ecstatic since now I had in my hands my very first own tube amp. I was also extremely tired since this project had kept me up for numerous nights in a row. Soon after getting the Bogner fired up, I got a 4*12  guitar cab to pair it with so I could actually hear what it sounded like. I kept this amp for a while as a part of my tool arsenal before eventually trading  it in for something getter suited for my type of music.

Suzuki Intruder VS750 1988

I have still hung on to my view that buying things that are intact is a waste of money. I sneakily planted that thought into my brother Jannes head when he started to talk about buying a motorcycle. I didn’t see the point in paying more than 3000 euros for a bike when we could buy a defective one for half of that and put in some work to get the same bike. We had been talking about buying a Suzuki Intruder if one would come up for a reasonable price. While I was determining what those models were going for, I spotted one that was only half the price of the average. The reason for this was that it was running on just one cylinder. This was apparently caused by rust on the inside of the gas tank. As the rust flakes got to the fuel lines they clogged up one of the carburetors. 

We ended up buying the bike to repair together. Since I was broke at the time my contribution to the project was my time and knowledge. Also the fact that I already had all the tools needed, excluding a few model specific ones, meant that the overall budget would stay considerably smaller. 

We rented a trailer to bring the bike from Jyväskylä to Mäntyharju. The trailer came from Jyväskylä and it had to be brought back the same day so I got my fair share of gripping the steering wheel for one day. 

The next day we got to work. My brother placed an order for a tank repairing kit containing acid to remove the rust and some kind of teflon compound for coating the inside of the tank to prevent it form rusting again. We also remove the carburetors and gave them to a friend of ours having more knowledge on how to maintain them. While the carbs were being overhauled we concentrated on changing fuel filter.

As we waited for the ordered stuff to arrive we also swapped the air filters for new ones as the bike was already taken so far apart and we had access to them. 

Once all of the parts and accessories required were within our reach, Janne started working on the gas tank. We decided to leave that job to be done by just one of us since it required attention every couple of hours and there was no point for the both of us staying up all through the night. This process took about 24 hours and after that there was the agonizing wait for the compound to set.

During this wait Janne had to leave for Jyväskylä as his vacation came to an end. A few days later I came back to Mäntyharju to put the bike back together. This simple task that I wasn’t even performing for the first time took me more than 10 hours. The biggest challenge was attaching the new air filters to the carburetors because the cone shaped rubber barrels were reluctant to fit over the intakes. This fight alone took several hours.

I finally got the bike put together and synchronized the carburetors. The day had been long and I was tired so I decided not ride the bike to Mikkeli that night. I rested the next day and planned a tactical attack at the bike for the day after that.

The adjustment for idle was completely missing. There were two options: the engine running at full throttle or shutting down completely. This combined with difficulties to start the engine lead me to suspect that the fault was in the air-to-fuel-mixture. This needed to be adjusted just by the feel of it because there was no meter to check it with. I turned the adjusting screws half a turn at a time on both carbs and went for a test ride in between. After three or four tried I got it right and after that I double checked the sync. When everything was in order I went to enjoy my last couple of days with the bike before Janne took it to his tender care. 

The bike was mostly with Janne and I got to ride it when he came to our neighborhood until he moved out of the country and gave me full custody of the bike. I had it for a couple of years and it had its fair share of problems which finally culminated in the bike catching on fire and burning beyond my repairing abilities so it had to be sold to someone who had the time and resources to fix everything that was destroyed by the flames.

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.

Topfield TF500PVRc

Sometimes  these fixes are so simple that when the device is back in working condition it’s almost annoying to realize that there’s nothing more to do than to put the device in use. This was one of those cases. The word of my hobby had started to spread and one day I received a message from a friend on Facebook about DVR that had taken a habit of losing all recordings from its hard drive. Once I got this top field TF500PVRc in my hands I started to look for a fault in hard drive itself since it seemed to the most logical place to start. I took the casing apart and removed the HDD for reformatting on a PC. Since the DVR was new to me and already broken I saw no point in trying to save any remainders of the pre-existing recordings. Before doing anything else I thought it would be easiest to let the DVR format its own hard drive. As a theory this was a brilliant one. Unfortunately that was all it ever amounted to; a theory. Each and every time I tried to format the drive it got stuck at 14%. After inspecting the hard drive a little more it became abundantly clear the hard drive had come to the end of its road. In my collection of spare parts I had a hard drive suitable for this device so I replaced the old one with it. The replacement was a little smaller in capacity but it was sufficient. When I proceeded to power the DVR on it asked me if I wanted it to format the hard drive. After the formatting I had myself a fully functioning DRV by pretty much no cost at all.