I see that there is a soft start (inrush current limiter) on the primary of the transformer. In that case, 1.6AT or 2AT is used for the 300VA transformer. Without soft start, a twice as strong fuse must be placed in the primary, that is, 3AT or 4AT, so that it does not burn when switched on. A short circuit to the secondary will certainly blow the 4AT fuse. In this case, it burned 10AT without a problem. In case of long-term overload of the transformer, the thermal fuse in the primary will melt, if there is one. The transformer can usually be thrown away if that thermal fuse blows, that is, it must be rewound because that fuse is deep in the primary, where it is the warmest. Usually, that rewind is not worth it. I don't think that happened here, but it is not excluded.
Now the steps would be to first check everything, that means first the transformer with an ohmmeter, then at idle if it has the required voltages. Next is the replacement of the rectifier diodes. Then try the power supply and that at idle. Then everything else as mentioned here, new amplifier modules, checking with an ohmmeter that there are no short circuits, etc. Insert 22ohm 0.6W resistors into the plus and minus lines of the amplifier for the first switch-on. Measure the voltage drop on them, calculate the current. If it has a quiescent current setting, the trimmer pot must be in the minimum current position. Connect the amplifier module one by one, to reduce possible material damage if there are any problems again. An incandescent bulb in series with a primary for first switching on is a good recommendation. After each trial, discharge large electrolytic capacitors in PS with a 220ohm 10W resistor.
Now the steps would be to first check everything, that means first the transformer with an ohmmeter, then at idle if it has the required voltages. Next is the replacement of the rectifier diodes. Then try the power supply and that at idle. Then everything else as mentioned here, new amplifier modules, checking with an ohmmeter that there are no short circuits, etc. Insert 22ohm 0.6W resistors into the plus and minus lines of the amplifier for the first switch-on. Measure the voltage drop on them, calculate the current. If it has a quiescent current setting, the trimmer pot must be in the minimum current position. Connect the amplifier module one by one, to reduce possible material damage if there are any problems again. An incandescent bulb in series with a primary for first switching on is a good recommendation. After each trial, discharge large electrolytic capacitors in PS with a 220ohm 10W resistor.
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Yeah unfortunately there wasn’t much I could do about it. However I’m keeping the AC wiring up high and the signal wire is shielded and passes along the bottom of the case. I hope this is good enough.
When testing the transformer what is it I’m looking for? Where am I measuring from?
Measure the ohmic resistance of all windings, primary and both secondary. First, disconnect the transformer leads from the rest of the device. Then connect the transformer via fuse 3AT to the mains and measure the voltage on the secondaries. If you don't have any experience, consider a life insurance policy first. 🤣
Way too complicated and it seems unnecessary as you already verified it to be OK. It got a firm kick under its *** but survived. Inductive stuff is also next level material. The diodes are the ones that need preventive replacement as these had lightning currents.
I would replace those and carefully check that they are soldered in OK and then again test/measure the PSU and pray it will stay that way for years to come 🙂 IMHO it seems that your experience is (yet) insufficient but you learn fast 😀
The learning curve of heavy destruction is not steep, it only costs some money and after a while you laugh at it.
I would replace those and carefully check that they are soldered in OK and then again test/measure the PSU and pray it will stay that way for years to come 🙂 IMHO it seems that your experience is (yet) insufficient but you learn fast 😀
The learning curve of heavy destruction is not steep, it only costs some money and after a while you laugh at it.
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My view on the rail fuses and a pair for each channel are that they will protect your speakers (not the amp) provided ofc that their value is chosen with this in mind.
The mains fuse will need to cope with both channels and the startup current which depending on your speakers might not be enough to save your speakers if one channels fails in a bad way.
The mains fuse will need to cope with both channels and the startup current which depending on your speakers might not be enough to save your speakers if one channels fails in a bad way.
I don't know exactly, let's say for that transformer, the primary is about 3 ohm, each secondary maybe 0.7ohm
Hi, I'm building my dim bulb tester at the moment but I'm not sure what wattage of halogen bulb to use. Some say to start with a low wattage but then some say that the amp will not appreciate that. Someone else said to go for the same wattage as the amp is expected to output so 150W in my case. What is common practice?
Common practice is to screw in whatever wattage bulb you need for the particular job. Sometimes you might want a 40W bulb. Other times multiple 100W in parallel. I’ve run as much as a kW off a 100W bulb, but they are low bias PA units and I also use a variac to get supply voltage all the way to 120 despite a dim red/orange glow. More fragile units could still sustain damage. High bias amps might not ever develop full voltage and end up snapping to the rail. Some stuff runs fine at reduced voltage. There is no one universal wattage or even rule of thumb.
That’s why a lot of professionals don’t like the practice of using them. Lack of precision, a lot of guess work.
That’s why a lot of professionals don’t like the practice of using them. Lack of precision, a lot of guess work.
Hi, I’ve now built a dim bulb tester and replaced the diodes and everything else on the list. However my soft start doesn’t seem to appreciate my dim bulb tester. It will not turn on the amp when the dim bulb is connected. Anyone know what the problem could be. I’m using a 50w bulb in the tester
Is it a bad idea to go ahead and plug in the amplifier units without the dim bulb. Will I then just be making the same mistake as previously
The first thing i do when i believe everything is correct is to check that there is no contact between + . and ground on the pcb to the cooler.
The isolation of the power transistors is the single thing i miss mostly.
And then i contact + - ground and everything to the PCB.
If you are going to build more - why not - maybe you will find as exciting as i do.
The buying list is like this:
Multimeter, A good soldering tool, Oscilloscope, Tone generator (diy), Distortion analyser and last a variac.
The isolation of the power transistors is the single thing i miss mostly.
And then i contact + - ground and everything to the PCB.
If you are going to build more - why not - maybe you will find as exciting as i do.
The buying list is like this:
Multimeter, A good soldering tool, Oscilloscope, Tone generator (diy), Distortion analyser and last a variac.
It's because the purpose of the soft start is to limit the in-rush current, which is the same thing the DBT is doing, but doing it for the entire time you have it in series with the power supply, the soft start should be in circuit for less than 1-second and then a relay takes it out of circuit. So they are fighting against eac other.
Two options: no DBT and you may well do this repair again, take out the soft-start and put in the DBT and you will be able to visually "see" if there is excess current (i.e. a problem) but it will also limit the current flow and may avoid the same problem you just repaired.
most likely the same mistake as before - but it's DIY, so it's up to you how good you want to become at fixing amps before you get to listen tot them...LOL
As already mentioned:
- power-up one channel at a time, bring it up fully and let it idle for 10-20 mins., only then play some music at low volume
- then unhook prior channel and hook up other channel (by itself) and power-up, test the same way as above
- only after both channels test good separately, should you combine everything, power up and test both (much harder to "see" an issue when you have to focus on two channels, unless you have very flexible eyes...
Additional checks before powering up - use your DMM to test for continuity on all transistors to the heatsink (no legs should be shorted to the heatsink or GND), then check resistance from positive to GND (with the PSU hooked up but not powered on), then Negative to GND, they should be not be below a few hundred ohms, if any of this oes not check out good - stop, take a picture, make some notes and post it here to get help to avoid any further damage.
Hopw you get it running and have some music playing by the weekend!
I got it working before the weekend and it sounds really good thanks! 🙂
If one rail blows it will test how well engineered the whole system is. There should be safeguards in place to make sure that Nothing Else Bad Happens.
In my experience, the speaker protection stuff that comes with the typical LJM works fine and triggers on any significant DC bias
It would be more of a risk to put rail fuses. Trust the speaker protection chips than to potentially lose voltage on one rail
Imagine if you have shorted transistors on one rail and not have speaker protection systems and relying only on rail fuses, the problem is just multiplied infinitely
It would be more of a risk to put rail fuses. Trust the speaker protection chips than to potentially lose voltage on one rail
Imagine if you have shorted transistors on one rail and not have speaker protection systems and relying only on rail fuses, the problem is just multiplied infinitely