I was gifted some broken audio gear from son's school.
A pair of 110v KRK Rokit 5 G3 powered monitors got plugged into the 240v local supply without step-down transformer = both transformers cooked. So I stripped the amps out and converted them as 2-ways for the son's bedroom.
They sound ok up against the wall with no crossover or tweeter attenuation. I've asked KRK service department for sensitivity info so I can at least put an L-pad in there. I'm not hopeful they will be forthcoming with that info and as the drivers are 4ohm the options for off-the-shelf adjustable L-pad are limited (or non-existent locally here in Laos). The product manual says they have a 2600hz crossover. I might get around to doing that at some point but don't think the speakers are really worth the cost - my son is happy enough with them as they are, even with no tweeter attenuation.
The powered section comprises 2x TDA7296 (one each for woofer and tweeter) and as far as I have read they are nothing special. In both the two 1500uf 25v caps on the psu for the chip serving the tweeter are obviously gone (bulging etc) so would need to be replaced and the over volts might have done other damage - certainly I don't think it worth the cost of new transformers. They'll go in the junk drawer for now and I will probably make use of the heatsinks elsewhere at least.
A pair of 110v KRK Rokit 5 G3 powered monitors got plugged into the 240v local supply without step-down transformer = both transformers cooked. So I stripped the amps out and converted them as 2-ways for the son's bedroom.
They sound ok up against the wall with no crossover or tweeter attenuation. I've asked KRK service department for sensitivity info so I can at least put an L-pad in there. I'm not hopeful they will be forthcoming with that info and as the drivers are 4ohm the options for off-the-shelf adjustable L-pad are limited (or non-existent locally here in Laos). The product manual says they have a 2600hz crossover. I might get around to doing that at some point but don't think the speakers are really worth the cost - my son is happy enough with them as they are, even with no tweeter attenuation.
The powered section comprises 2x TDA7296 (one each for woofer and tweeter) and as far as I have read they are nothing special. In both the two 1500uf 25v caps on the psu for the chip serving the tweeter are obviously gone (bulging etc) so would need to be replaced and the over volts might have done other damage - certainly I don't think it worth the cost of new transformers. They'll go in the junk drawer for now and I will probably make use of the heatsinks elsewhere at least.
Won't take long to cook the tweeter if you don't put in a non-polar capacitor in series. Something in the 10 to 15uF range should work.
Thanks. Ive got a couple of Monacor MKP 10uf 250v sitting here - I’ll pop them in there today.
I only know enough to be dangerous!!!
I only know enough to be dangerous!!!
Don't do as I do:
Electric food-chopper found at tag-sale, $3. No go. The lid safety interlock rarely lined-up right.
Screw it! I busted off some housing, drove a drywall screw behind the interlock, then wrapped the sharp edges with tape.
NOTE that I have no children here, ever.
Still not proud of it.
Electric food-chopper found at tag-sale, $3. No go. The lid safety interlock rarely lined-up right.
Screw it! I busted off some housing, drove a drywall screw behind the interlock, then wrapped the sharp edges with tape.
NOTE that I have no children here, ever.
Still not proud of it.
I did a similar repair for my fathers washing machine. The lid switch failed open so it wouldn't spin... It's a short now
Ya know, I did that about 15 years ago to my Maytag washer, jumpered out that damn lid switch.
It annoyed the heck outa me whenever I had to open the lid to toss something else in.
I don't have kids around to worry about.
Just think twice about the microwave, OK fellas?
Ahh, the microwave!
Mine only had the light on while running, opening the door it was dark inside.
An "extra" place for a miscroswitch inside (probably for an upscale model) was just the place to put in an over-ride switch.
Now, the light comes on whenever the door is opened, OR running, off otherwise.
And that damned non-socketed light - I replaced with a double-contact bayonet socket type bulb.
Easy-peasy to replace, and cheaper too.
Repaired a new design of a model railway dcc shuttle controller.
I trailed the internet for information on dcc before starting to try and get a working circuit and working software.
Bare pcb's turned up and I populated the pcb.
Couldnt get the loco to move.
SO went back to internet to get some clues.
Turns out it needs AC and not switched DC which i had used.
So a new fixed pcb made.
Managed to get it to move but only in one direction.
Turns out wit hdcc the loco has a acceleration and deceration routine.
What was happening was when I sent reverse command it was slowing down and stopping but before it could go backwards it got another command to go forward !
So increased cycle time and this time it went both ways.
Then after I added current overload circuit it kept overloading randomly.
Loads of noise on 12 volt power supply due to loco.
So decreased load sensing resistor by factor of 2 and now its fine.
I trailed the internet for information on dcc before starting to try and get a working circuit and working software.
Bare pcb's turned up and I populated the pcb.
Couldnt get the loco to move.
SO went back to internet to get some clues.
Turns out it needs AC and not switched DC which i had used.
So a new fixed pcb made.
Managed to get it to move but only in one direction.
Turns out wit hdcc the loco has a acceleration and deceration routine.
What was happening was when I sent reverse command it was slowing down and stopping but before it could go backwards it got another command to go forward !
So increased cycle time and this time it went both ways.
Then after I added current overload circuit it kept overloading randomly.
Loads of noise on 12 volt power supply due to loco.
So decreased load sensing resistor by factor of 2 and now its fine.
I finally got around to finishing up doing service on my neighbor's Technics RS-1500US reel tape deck.
The machine needed a thorough cleaning, re-lubing, and repair to the audio board - some lousy PUTZ with a screwdriver from hell apparently got inside and twiddled almost every adjustment pot - damaging several in the process.
Now, however, she sings sweetly again, up to spec, all the leaky caps are gone, and set up for Maxell and RTM tape bias.
If you don't know what you're doing, and don't have the proper tools and test equipment, stay the hell out of stuff, it makes my job 100X more difficult.
The machine needed a thorough cleaning, re-lubing, and repair to the audio board - some lousy PUTZ with a screwdriver from hell apparently got inside and twiddled almost every adjustment pot - damaging several in the process.
Now, however, she sings sweetly again, up to spec, all the leaky caps are gone, and set up for Maxell and RTM tape bias.
If you don't know what you're doing, and don't have the proper tools and test equipment, stay the hell out of stuff, it makes my job 100X more difficult.
Attachments
I bought a Rising brand AM/FM radio literally from a dumpster just because I like the long tuning dial. It must be over two decades old. Being so old, I did not power it with 220 VAC, instead used 48 volts AC for couple of days so that the capacitors get reformed. Next, I used 115 volts AC for a day. When I connected 230 VAC, it started working.
And the project which was supposed to be for a long time ended in 4-5 days.
gajanan phadte
And the project which was supposed to be for a long time ended in 4-5 days.
gajanan phadte
I have been working on a model railway digital command control shuttle system.
It sends out commands as +/-15vdc to the track.
I managed to get the comms working and can move train back and forth and change the speed.
I couldnt get the software right for changing trains ID number.
Trawled the internet relentlessly for information on data packet sequence.
Eventually I joined a pro model railway group who have designed all sorts of DCC equipment.
They passed on a pdf showing required data packets.
I input these to my software and it worked.
The comms software is quite interesting. To keep bits correct width I had write a serial shift register algorithm.
Just bit banging the outputs wasnt accurate enough.
I have to create a bit structure then send out bits one at a time on 2 phase pins.
Built a new JLH1996 class A amplifier.
On first power up output went to positive rail.
After a long visual inspection I spotted a driver transistor had a blob of solder between two pins. Serves me right for buying cheap Chinese solder.
It sends out commands as +/-15vdc to the track.
I managed to get the comms working and can move train back and forth and change the speed.
I couldnt get the software right for changing trains ID number.
Trawled the internet relentlessly for information on data packet sequence.
Eventually I joined a pro model railway group who have designed all sorts of DCC equipment.
They passed on a pdf showing required data packets.
I input these to my software and it worked.
The comms software is quite interesting. To keep bits correct width I had write a serial shift register algorithm.
Just bit banging the outputs wasnt accurate enough.
I have to create a bit structure then send out bits one at a time on 2 phase pins.
Built a new JLH1996 class A amplifier.
On first power up output went to positive rail.
After a long visual inspection I spotted a driver transistor had a blob of solder between two pins. Serves me right for buying cheap Chinese solder.
Last edited:
After realizing I had no sound because I had switched plate and cathode, I got to work on why I had LF instability where a loud peak (quickly crank volume from 0 to full to 0 again) would make it motorboat. As an aside, the 6F12P is a remarkable tube indeed. Vhk was over 600V, but the damned things still work!
I ended up making it stable by changing one of the RC poles (the PI to driver caps) from 1.4Hz to 0.31Hz (0.22uf/510k is now 1uF/510k) and that solved it. It still has an LF wiggle or two on those loud excursions, but it damps itself.
I ended up making it stable by changing one of the RC poles (the PI to driver caps) from 1.4Hz to 0.31Hz (0.22uf/510k is now 1uF/510k) and that solved it. It still has an LF wiggle or two on those loud excursions, but it damps itself.
Last edited:
Pass DIY Addict
Joined 2000
Paid Member
Just finished repairing a Marantz AV9000 processor from circa 1999 that was holding down the DD/DTS duty in my family room system. Bought it on ebay in the early 2000s for $150 because it had problems. The problem at the time was a broken connection between the RCA jack for the surround channel outputs and the PCB. Fixed these up and I had a nice preamp on the cheap.
Then, in 2009, it failed on me and I just repaired the same problem again this week. The symptom was blowing PSU fuses. Not right away, but over time (hours to days). At first I thought it was bad caps, but this turned out not to be the issue. Very careful inspection with a 10x loupe and a bright light revealed hairline cracks in the solder joints on the bottom of the main PSU board. The specific problem was with the ribbon cable headers that go through-hole and are soldered to the board. The solder joint on several of these connections cracked and was causing arcing and burning that was causing the circuit to draw extra current in an effort to maintain its power supply. This doubled the current flow through the slow-blow fuse and caused it to open.
After removing old solder with a wick and applying new solder and cleaning the flux from the board on each of the ribbon headers on the PSU board, it's up and running again! No more dead fuses or error messages on the display panel.
Hopefully, this can help someone else trouble shoot a piece of gear that really doesn't need to be replaced...
Then, in 2009, it failed on me and I just repaired the same problem again this week. The symptom was blowing PSU fuses. Not right away, but over time (hours to days). At first I thought it was bad caps, but this turned out not to be the issue. Very careful inspection with a 10x loupe and a bright light revealed hairline cracks in the solder joints on the bottom of the main PSU board. The specific problem was with the ribbon cable headers that go through-hole and are soldered to the board. The solder joint on several of these connections cracked and was causing arcing and burning that was causing the circuit to draw extra current in an effort to maintain its power supply. This doubled the current flow through the slow-blow fuse and caused it to open.
After removing old solder with a wick and applying new solder and cleaning the flux from the board on each of the ribbon headers on the PSU board, it's up and running again! No more dead fuses or error messages on the display panel.
Hopefully, this can help someone else trouble shoot a piece of gear that really doesn't need to be replaced...