Old majestic amp blows chassis fuse at turn on.where do I start looking for problems?

[spooney]

I have an old 4 channel majestic amp (don't laugh I know it sucks) laying around the garage. A buddy gave it to me years ago because it did nothing but blow fuses and said If I fix it its mine.Well five years later I'm finally getting around to messing with it. All wiring to the amp was good so the blown fuses are being caused by something internal.I originally suspected the power supply section but everything seems in order.Where do I go from here?I'm a newb at this and even though the amp is cheap it would make a decent addition to my audio system.I don't have a lot of test equipment on hand....just my DMM. Will the DMM alone be enough to help me fix this thing?I suspect a bad transistor.what would cause a bad transistor or do they sometimes just die on their own?

[Perry Babin]

Transistors rarely fail without a bit of help. The exceptions would be old equipment. After ~20 years, you'll see random failures.

If you can post a good quality image (no phone cam), it would be easier to help.

If you set your meter to ohms, you should not read anything less than 500 ohms between the center pin of any power transistor (those clamped to the sink) and either of its other legs. If, for example, you get 5 ohms from pin 2 to pin 3, that component (or a component connected in parallel) is defective. If you read something near 0 ohms when you connect the meter across the B+ and ground terminals (no power applied), the amp likely has either defective FETs or a shorted reverse-protection diode.

If the amp blows the fuse as soon as B+ and ground are connected (no remote voltage applied), the problem is typically in the power supply. If the fuse only blows after remote is applied, the problem is in the power supply secondary or in the audio section (generally blown output transistors).

[dB-r]

Have you taken the amp apart yet? I have seen on the bottoms of the circuit boards where the power input connections are soldered into the circuit board, the amp will draw enough current to heat up the solder connections and actually melt the solder from the connection, where the fuses connect, causing it to drip down on the circuit board and short with the ground or positive. Especially if the amp was mounted on it's side rather than flat.

I know this is elementary, but if you haven't completely pulled the board from the heatsink you wouldn't be able to see that.

Normally this would only blow an external fuse, not the builtin fuses, but if the ground is the one that melted and shorted to another strip on the PCB that was positive then it will blow the builtin fuses.

I would think if a transistor failure was to blame it would go into protect mode (if so equipped). But there are possibilities that it could blow fuses when a power supply FET goes bad. Power Supply FET failure is about 75% of what I have seen when amplifiers fail. So it is likely.

Also, on an older amp, that has been used for some time, the FET's pins can break loose the solder holding them to the PCB. This connect-disconnect of the FET in the circuit, caused by the loose connection will certainly make the FET go bad. Will not smoke that FET in most cases, just cause it to short internally. So it won't necessarilly be a visibly bad FET.

[spooney]

is it possible that shorted speaker leads could have caused the transistor failure?I know that the guys car who this amp came from used to have a problem with his subwoofers tinsel leads snapping.Maybe one snapped off and shorted to the other?

[dB-r]

Not sure on this particular amp, but I would say it is possible if the amp was playing at medium to high volume when the shorting occured.

I personally have blown the output stages of amplifiers by accidentally shorting the speaker outputs during testing, so I know it is possible at least on some amplifiers. I don't know if that amp has shortcircuit protection on the speaker outputs.

It does not seem likely that many amps would have that kind of protection, so yeah I would say entirely possible if in fact it was being shorted out during high volumes. Most amps I have worked on have little protection built in. You can run them at 8 volts and they will still play, run them into .25 ohms and they will still play. Most amps have thermal protection, and I think that is about it, but it doesn't seem like the thermal protection works very well on the cheap amps. They just keep going until they burn up the power supply, despite the thermal protection.

[spooney]

well i finally got the parts in the other day.I couldn't wait to solder them up. I was pleasently surprised when I powered up the amp with its replacement transistor and it did not blow a fuse.I was even more excited when I proceeded to test all 4 channels and all 4 played great. It even worked well in bridged mode.The real test will come when I get it in my car and drop a 4 ohm load on the bridged channels which I was unable to do in my preliminary testing.I'll beat on it pretty good and if it survives it will be pretty encouraging for me as I'm getting ready to make an attempt at fixing my Orion 2150 sx.

[dB-r]

Glad you got it going, was it a bad power supply or output transistor? What was the part number on the transistor?

[spooney]

just one bad power output transistor. It was a KTD998.those things are pretty hard to come by.took nearly a month to finally get them in.

[dB-r]

Yeah, I tried to get Mouser to carry those KEC parts but so far with no luck yet. Would be nice seeing as how Mouser has online inventory status and all that, very nice web site.

[spooney]

well so far with the amp bridged down into two channels with a four ohm load its holding up fine.I pounded on it pretty good for a few minutes to see how she held up.If it makes it a week I'm officially calling it good.