Mackie Thump TH-15A Powered speaker Repair

 I was asked to repair a Mackie Thump TH-15A Powered speaker by a friend.  It powered on and everything looked good but there was no sound coming out.  I checked the speakers and the power supplies but everything was just fine.  However, the computer board was getting really hot.  It had 3.3 volts coming into a voltage regulator but only .5 volts coming out.  I tried easter egging the capicitors and it turned out to be the tiny little .1 uF capacitor that was shorting out the power supply.  The correct output is 1.2 volts.

This is what the guts look like, we are going to zero in on the logic board.

This is the logic board once the metal cover is removed.

These are the filter caps on the 1.2 volt power supply circled in red.

This is what the powered speaker looks like once it was back together.

Repair of Suzuki GP-3 Digital Piano

My brother dropped off a Suzuki GP-3 electric digital piano for repairs. Many of the keys were stuck in the down position.  So I googled how to fix it and here is what I found:

Here is a video telling how to take it apart:

https://www.youtube.com/watch?v=KE7A7I0mirg

Then I found this coment on how to fix it at: 

https://www.yamaha-keyboard-guide.com/suzuki-keyboard-repair.html

"I used the orange fibreglass driveway marker from Home Depot. It’s the perfect size. With a rubber mallet I used it to punch out the old rods and replace them at the same time."

The first step was to remove the keys from the piano.

Then I tried to remove some of the individual keys. BTW you do not need to number the keys they are already numbered in the indentation area.  

Eventually I decided to remove every other octave, then remove all of the rods, then carefully push an orange driveway marker through all of the octaves.  I left the angled tip on the rod, and still had to lift some keys slightly to get the rod to go through them.  The commentator above said that he drove the marker through pushing the rods out as he went, but my keyboard was so far gone that that was not a possibility.  I had to use some pliers to twist and pull out the rods, one at a time, as they were really stuck in place.

When I put it back together there were way too many screws left over.  So I took it back apart and put in a lot of missing screws.  Even so I still had about 16 screws left over!!  

Peavey Combo 115 repairs

My brother bought me a Peaver Combo 115 for repair.  They used controls with plastic shafts that broke off. They upgraded the design to use controls with metal shafts but they are out of stock everywhere unless you want to pay a fortune for them.

Here is a picture of the controls with their values and part numbers next to them.  You can see the broken controls in the picture.

This shows what the insides of the amplifier looks like. All of this could be 1/4 the size if it was made today.

The only critical controls that were broken were the 50K linear tone controls. The crossover and compressor can be ignored or bypassed.

Here is a possible source of parts.  The needed parts will cost you over $50 with shipping and may not be in stock.

As a solution I purchased 10 x 50K pots with knobs on eBay.  Short wires have to be run from each control to the circuit board.  I have seen this kind of solution before.

 

Another amplifer rebuild with a TDA7394 Power Amplifier board

Once again I am rebuilding another amplifer this time with a TDA7394 Power Amplifier board.  I picked up theis fried amplifier at a Hmfest from a friend for like $10 for the poweer transformer.  I tested the output transistors and all but one were shorted.  So I decided to rebuild it with a dual TDA7394 amplifier board for $22 on eBay.  The power transformer is 60 VCT so the rectified power is + and - 42 volts DC.  That is a little high for the IC but within its specs.  It should deliver 100 watts per channel into 8 ohms without any problems.

Here is the old circuit board.  The preamp seemed OK so I decided to try to save it.

So I cut the old circuit board in half with a hand saw.

This is the new power amplifier board, it is a lot smaller than what it replaced.

This is the new heatsink to go with the IC based power amplifier.

Here is the finished amplifier.  All it is missing is the power on LED and a shielded audio cable from the preamp output to the RCA jacks on the back panel.  The preamp had a slight buzz in it, I don't know why.  Perhaps better shielded audio cables would help.

JBL SB2 Subwofer Speaker Repair

 I recently repaired some JBL SB2 subwoofers.  They had been overpowered and there were boils in the voice coil form. I have never fixed a speaker with this problem before.  It was hard to get a file in there because of the magnetic field.  But once in place it turned it side to side to remove the bubbles.

Once the bubblew were cleaned up I glued the cover back on and tested the speaker and it worked!

Shure Wireless Microphone Repair

I recently had to replace the power switch in a Shure wireless microphone.  If your microphone will not turn on, try replacing the switch.  It might be bad and still "click" when you press the button!  When I tested the last one with a volt meter it was about 500 ohms when pressed.

You need to remove the two screws that hold the battery holder in place and remove the battery holder.  Then remove the three piece power switch panel as well.

Then you need to remove the "C" clip that holds the guts inside the shell.  Small needle nose pliers can sometimes get the clip out.

Then all the guts should come out of the microphone.  There is an additional part or two to remove and the piece that fits around the on off switch was missing.

Here is a picture of the new on off switch.  I soldered it using the head mounted magnification lenses.

This picture shows all of the guts.

Here is a comparison of some of the amplifiers found on eBay.

I have been comparing the specifications of some of the amplifiers that I have worked with:

L-10 Amplifier:

Size: 200x41.5x?? mm

8 Ohm 100 Watts

4 Ohm 200 Watts

2 Ohm 350 Watts +-55VDC

Gain: ?? 

Output Transistors - 3 EA: A1943 C5200

L-15 Amplifier

L-15 Amplifier:

Size: 150x40x?? mm

Size: 5.9x1.57x??"

8 Ohm 150 Watts

4 Ohm 300 Watts

2 Ohm 600 Watts +-55VDC

Gain: 36x

Output Transistors - 3 EA: IRFP240/IRFP9240

L-20 Amplifier

L-20/28 Amplifier:

Size: 110x65mm (L-20)

8 Ohm 200 Watts +-65V

4 Ohm 350 Watts +-60V

2 Ohm

Gain: 34x

Output Transistors - 4 ea D1047 B817

Note: L-20-SE has 2SA1943 2SC5200 transistors

Note: L-28 Includes capacitors and speaker protection

PR-800 Amplifier

PR-800 Amplifier:

Size: 245x65x30mm (or 195x90 with 4 Capacitors)

Size: 9.64x2.55x1.18"

8 Ohm 500 Watts +- 95VDC

4 Ohm 1000 Watts +-90VDC

2 Ohm 1000 Watts +-65VDC

Gain: 40x

Output Transistors - 4 ea TTA1943 TTC5200

Note: Includes Capacitors and Speaker protection

Note there are 2 sizes; one with 2 large filter capacitors and one with 4 large filter capacitors.

NJW0281/NJW0302 Amplifier

NJW0281/NJW0302 450W+450W HIF

Size: 259x73mm

8 Ohm

4 Ohm 450 Watts +-85VDC

2 Ohm

Gain: Very Low - needs preamplifier

Output Transistors - 4 ea NJW0281/NJW0302

Note: ATX power supply pins fit the connectors on the right side.

Rebuilding some old speakers

I was given some speakers that I had built back in the 70's and that had been in use at a churhc for about 20 years or more.  They were made to look nice, but I will rebuild them to look tough.  I also want to upgrade them from 25 watts to about 150 watts or more.

This is what I bought at a Hamfest to rebuild the speakers with.

This picture compares an old speaker with one of the new ones.

 Initially this is what initially had to work with.  I only had one of these bass speakers.

 Here is one cabinet stripped down and the old speakers sitting on top of it.

I painted the front plywood black and started reassembling the speakers. I traded something for some tweeters.  They are rated at 75 watts.  I also added some grills to protect the woofers.

Rebuilding LAB Series LS800 Amplifier with a PR-800 Amplifier

I had rebuilt this LS800 amplifier with a L-15 amplifier but I decided to upgrade to PR-800 amplifiers for much more power.  This is a picture of the old amplifier dissembled.

This is a picture the old power amplifier section.  It was really cobbled.

This is how the PR-800 amplifier boards arrived.  Note that the power output transistors will have to be soldered on.

Pr-800 as it arrives

Here I have added the output power transistors.  I also moved the driver transistors to the top side of the board for mounting on the heat sinks that I have.  Their leads barely reach the tops of their holes.

PR-800 with power transistors

This is what it looks like mounted on the heat sink.  I went back and added washers under the heads of the 3/4 inch 4-40 screws.  Note that little plastic washers must be installed in the two To-220 transistors (Q12 and Q15).  Make sure to check for shorts to the heatsink with a voltmeter!

PR-800 Mounted on heatsink

WARNING!!  Something was really wrong, it tripped the circuit breaker and shorted the output transistors!!!  I eventually found the problem.  The 2SD669 (Center transistor Q13) has to be soldered UPSIDE DOWN on the top board!

Upside Down Transistor on PR800

Here is the schematic of the output section.  Note that the 2SC5200 (Q16, Q17, Q18, Q19) transistors are connected to the + power source and the 2SA1943's (Q20, Q21, Q22, Q23) are connected to the negative power source.  Also not that the two boards are the opposite of each other.  The 2SA1943's will always be next to the power filter capacitors.

The schematic was derived from this schematic found on the internet.  The protection circuits do not match what I received.

Here is a labeled picture

I have both channels working fine now, but only one at a time.

PR-800's mounted on heat sinks

As you can see I cut off one side of the heatsink.  I also switched to silicone insulators because they are less messy.  One channel is missing two power transistors because they were fried when I installed the 2SD669 right side up.

This project is finally done!  The cooling fan was mounted to the front cover to make working on the amps easier.  The heatsinks were mounted to the bottom of the cabinet.  The LM3915 LED VU meters have the bottom LED always on and the top two red ones are in parallel thus giving 12 LED's per channel.

Niles ZR 4630 tear down and rebuild

A friend has a Niles ZR-4630 that he wants made into independent amplifiers.  There are six stereo amplifiers based on the LM3886 IC.  I have been reverse engineering it.  By default the mute is "on" for the amplifier modules.  Here are some pictures of the guts.

This shows the amplifier modules and the power transformer.  We could not get the power on by the on off switch so we hot wired power to the transformer.

This is another view of the transformer and amplifiers.

This is a view of the back side of the guts.

So far I have been able to confirm the pins of the amplifiers. There is more to come.

Here is a picture of one of the amplifiers running on the workbench.