Buttkicker BKA1000-4A repair

Just curious whether any have tried repairing the BKA1000-4A? Mine died and it clearly has one blown Cap on the +70 line on the Amp board and a number of lytics that are domed on the top indicating they may be bad or near failing. I called Guitammer and they do not repair the amps but they did send me a set of schematics for the Amp so I will probably be tackling that in the not to distant future when my shipment of various parts comes in from DigiKey. My plan is to replace all of the Lytics and then see where things stand. Power supply voltages all look normal.

Any tips from those who have done any work on these amps would be appreciated. Mine is a Mar 2003 vintage amp that was sent to me by Guitammer when the first amp I had died. This one is clearly out of warranty so it is either fix it or buy a new Amp.

Lots of reads but no comments back. I figured I would report on the progress of my repair. In addition to the blown 100 uf Cap on the Amp board, every 100 volt capacitor on the board was domed indicating pressure building up in the capacitor and possible leakage and degradation of capacitance. All 6 of the 470 uf 25 volt caps on the power supply board were also domed and have been replaced. In all, 16 capacitors were domed and have been replaced. A key item here when replacing these caps is to be sure to get capacitors with the same or greater temperature rating. Most of these caps were -40C to 105 C caps. This is actually more important in terms of lifetime than buying a higher voltage rating cap as it is operating time at temperature that causes failures. This is based on what is called the Arrhenius Equation which is a standard for failure prediction in the semiconductor industry. Since an increase in temperature increases the speed of a chemical reaction, it also increases the failure rate in the capacitor chemistry. When choosing capacitors for long life look at the temperature rating and the predicted lifetime hours at that temperature. Most catalogs like DigiKey and Mouser give that information in there selection tables.

Next week we will be doing the Smoke Test on the Amp as I put it back together to see if we have fixed all of the problems or if there are possibly dead Semiconductor components.

Comments are still welcome.

Just another update. Was not able to work on this over the holidays.

After replacing the numerous bulging and blown electrolytics I found that one of the Bridge rectifiers had also departed life. I replaced that and all the supply voltages are now normal. I started tracing the Class D circuit and the small audio preamp board with the volume and frequency cutoff controls is working fine. The audio signal was arriving fine at the TL074CN op amp on the power Amp board, as was the sawtooth signal on its other input. However there was no output from that Op Amp that is essentially the Integrator that creates the Class D signal. I replaced that OP Amp this week which turned out to be a bit difficult to remove because of the very small plated through holes. Even with a commercial vacuum assist solder sucker I could not get the pins to release and had to use a dremel tool to cutoff each of the IC pins at the body of the IC and then heat and pull them one by one from the board. I then had to drill the remaining solder out the holes with a number 65 drill. (Not sure why they use such fine plated through holes on a 14 pin DIP IC). So once the TL074CN was replaced, that whole portion of the circuit is now working. I get a square wave from the TL074CN going into the IR2111 FET Drivers that varies in pulse width as the input audio frequency is changed. It now looks like the two IR2111 FET drivers could be bad so once again I need to remove the board, pull those two ICs and replace them. I should be able to get to that next week.

I have a fair amount of experience with removing ICs having worked in the Semiconductor industry but this board is a combination of surface mount and through hole with very fine geometries so all the normal tricks didn't work too well. Getting 7 pins hot on one side of a 14 pin DIP and trying to pry it up would pretty well fry some of these traces. In the end it was easier to cut the package from the leads and then heat and pull each lead individually. After that the holes were so fine that even the Hakko 808 vacuum desoldering gun could not clear the holes and I had to carefully drill the solder from the holes while viewing through a microscope and not damaging the plated through holes. First time I have worked on a Class D Amplifier so it is a bit challenging.

WooHoo!! The Buttkicker Amp is finally repaired!

The last items were two 15 volt Zeners that were bad. After getting the circuit to work all the way up to the input of the IR2111 Gate Drivers that drive the 30 amp MOSFETs in the output, I found that the bias voltage on the IR2111s was only 7 volts instead of 15 volts. So replacing the two 15 volt zeners in the power supply fixed that and the amp started working again. I had missed these in doing the initial power supply repair as I didn't realize that this single 15 volt supply was referenced off of the negative rail of -80 volts. In the end I had to replace about a dozen electrolytics that were bulging or blown, one Bridge rectifier, Three 15 volt Zeners and one quad Op Amp. I learned a lot over the last couple months as I was not familiar with Class D amplifiers going into this repair and only could work on this part time. If anyone reading this has questions about repairing their BK1000 amp, I would be happy to share what I can.

Looks like I can get all my Buttkickers working in the theater again this weekend.