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I hope this is not too off subject. I purchased a few old McIntosh pieces a couple of nights ago and used a varic to bring up the tube items. I have yet to turn on a solid state amp in the package and was wondering if it it desireable or necessary to use the varic on it also. Thanks for the responses. Robin
Always use a variac to initially power up ANYTHING that you are uncertain about.
Just monitor the amperage draw as you are bringing up the voltage on the variac.
Check the fuses rating in what you are testing first.Then variac it up.
Make sure that the unit being tested does not draw more amperage thru the variac than what the fuses rating is inside the tested unit.
If the meter reads higher than the fuses capacity(STOP IMMEADENTLY) you got a problem within the unit being tested.
If your variac does not have a meter to monitor amperage draw-Get one that does.
Oldmics
No, mine doesn't have a meter. I've just been slopping through and getting lucky. I'll get one that does immediately. Any sugguestion on how slowly they should be brought up? Or how long at each level or something.?? I've been using about 20 volt increments for about 30 mins each up to 110.Quote:
Originally Posted by Oldmics
You also want to check DC offset if it is a solid state amplifier (Tube stuff usually uses transformers in the output so DC is not a problem).
DC is a huge enemy to your woofers ! DC offset is the ever present voltage at the speaker output of an amplifier, preamps too; though not nearly as high as power amps.
Use a DC volt meter capable of reading milli-volts. Hook up a dummy load (8 ohm 5 watt resistor) and test each channel one at a time. Slowly bring up the Variac watching that the current doesn't go beyond an amp or so. Large filter caps will cause large peaks in current if you bring the voltage up too quickly. This may trick you into thinking there is a problem.
Now, there are two complications you can run into here: Relays inside the amp.
Your unit may have a relay for both power on/off and for speaker output. If you unit has a main on/off relay, it won't power up untill about 75% of operating voltage. All remote control stuff has relay of some sort. If there is a problem such as shorted output transistors, you won't know untill the relay kicks in. At that point the current on your AC input will go sky high (if the rail fuses aren't blown). If your unit has a regular switch to turn on/off, this scenario doesn't apply.
Second complication, most solid state amps use a relay between the output stage and the speaker jacks on the rear of your unit. This relay also will not kick in untill about 75% of line voltage. But it WON"T kick in at all if there is a sizeable DC offset. Usually about 1V or more.
Keep in mind that most solid state amps are calibrated to run with less than 50mv (millivolts) on the output. You can troubleshoot just by listening. If you hear a click at the exact second when you turn the amp on, that is your power on/off relay. The speaker relay usually has about a 2-5 second delay. If you hear that second click, the output stage is probably ok.
Multi channel stuff will draw more quiescent (idling) current than two channel gear, with the exception of "Class A" stuff which draws constant current.
IME, most amps (120V) idle drawing less than 1 amp of ac current.
I hope this is helpful
Thank you. How slowly should I bring up the power through the variac on tube or solid state?
Bring it up to full voltage in about 30 - 60 seconds for solid state. Maybe 2 - 4 minutes for tubes, allow the tubes to be fully warm.
I agree with bringing the unit up to full power within the timeframe that LE15 Thumper suggests.
UNLESS
You are trying to reform caps in a tube system.(Which does work but is only a temporary fix)
If you are reforming caps the time frames that you are originally using (20 volt incriments every 30 minutes or so) should be applied.
Oldmics
Anything with a switch-mode power supply will also not start at voltages below specification. Usually around 170VAC for 220/240V regions and 85VAC for 110/120V regions.
Cheers, Ralph.
There are so many different technichs and procedures in electronics repair that it should have a forum for itself.
At least hopefully these few suggestions will be of help in getting you started.
Oldmics
Also, some SS amps with brick-wall power supply regulation can have their voltage regulators damaged by too little input voltage. Unless you're SURE that you DON'T have regulators or switch mode components, I'd be careful about running a SS amp at lower-than-spec voltage for more than a few moments...
Regards,
Gordon.