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Started by Jasz2, September 25, 2016, 02:49:09 PM
Quote from: J M Fahey on September 25, 2016, 08:12:26 PMDefine "horrible".Does hum respond to ny control?It hums strongly even with nothing plugged in the input?Touch the speajer cone with a finger (I guess it´s open back), does the cone jump forward or backward and stay there?If the latter, *disconnect* the speaker, set multimeterto 200V DC scale and check whether you have a DC voltage across speaker out terminals.
Quote from: J M Fahey on September 29, 2016, 12:45:58 PMI couldn´t find a schematic for it.Just in case, write them, quote amp serial number, mention you have a repair shop (where you live) , that a customer brought one for repair and that you need a schematic for that.Try to sound like a Tech with a working shop and not like "a musician with a soldering gun". Worst case they will not answer or say "no way" ... which won´t change todays situation ... and they *might* send a copy.I)n any case, I guess they use a couple chipamps , LM3886 or TDA7294 come to mind, take pictures showing them and also the power supply area, you *might* have bd or poorly soldered filter caps, always a cause for constant humYou might also have poor ground/cracked solder around input jacks or pots, but we can´t trust plain guess, but do some testing.Ok, go ask for that schematic and post some pictures
Quote from: phatt on October 02, 2016, 06:21:15 AMGood pics :tu:Q, Was the hum always present or has it slowly got worse?Phil.
Quote from: phatt on October 02, 2016, 09:14:14 AMOK, Then plug in a lead and with the other end gently contact the bare *Sleeve* to the metal case. Does the hum change in any way? (i.e. louder, softer or disappear)Obviously Keeping away from the live mains wires.You may have to scrape it a bit to make full contact with the case.If no change then you will need a meter to test other stuff.Phil.
Quote from: phatt on October 03, 2016, 07:51:51 AMI hope you used the Sleeve part of the plug and not the Tip which is signal Hot?The Sleeve is at ground (circuit common) and in most cases it should also be connected to the Case Ground.The Sleeve test should bridge the whole circuit common to the Case and "IF" there is a grounding issue the hum "Should" be much less. Sorry if I was not clear. Maybe do that test again With Volume at Zero then Sleeve to Case *Not the Tip*. With ground path problems you have to find that break in the ground path which can be quite tricky. Without a schematic tiss very hard to point you to a possible solution. I did find a PDF of the Power Engine manual for larger models and they have a Ground lift switch built in while your model has that silly plugin to ground the circuit which makes me think it's a ground path patch-up after thought.If not a ground path problem then it could be power supply filtering and you will need to test voltages.Q, Do you have a DMM? Phil.
Quote from: phatt on October 04, 2016, 08:18:42 AMHi Jim,, Power up and measure and post the DC voltages;First Supply rails , at the two big Electro's.Expect to see maybe +/-35VDC with respect to common, or even +/-40VDC.You can pickup circuit common from the con block next to the big Electrolytic's,, The *Black* lead in the center of the two Yellows. There is usually just enough room to slip the probe down the back of the connector. That black wire is circuit common and should also be case common. If not it could be the hum problem. Remember the power is on and you don't want the probes to fall out and short the circuit.Then there will be a preamp supply to run the small chips.Those 2 large resistors on the edge of PCB in *image 0100* are likely the voltage drop resistors.They are banded Brown Black and Red which is 1k (1,000 Ohm)One side will read the same as the main supply while the other side should read +/-12VDC or maybe up to +/-16VDC. Don't worry if they don't read exact but they should be close to the same on each side.Also with the power off; Set DMM to lowest Ohms, bridge the two probes together and note the reading.If not zero then note the reading. you subtract that from the final reading. Now check the resistance from that black wire in the conn block to the case. You may have to scrape the probe on the case a bit to make good electrical contact before you get a correct reading. It may jump around a bit before you get a stable reading,,go with the lowest number you get. Phil.