Good day
I have an Advent 300 receiver that I'm hoping to use for a shop amp. I bought it not knowing its condition. I plugged it into the mains via a dim bulb and was greeted with a serious hum and the bulb didn't dim. My first thought was power supply and I replaced the filter caps and the bridge rectifier as it looked as if one of the leads was fractured. This didn't reduce the hum which persists at the same loudness whether or not the volume control is up or down. I tried removing the bridges between the pre amp and main amp - no difference which leads me to believe the main amp is the problem. I can't leave the amp on for long as it'll blow the speakers (I'm waiting on some 8 ohm resistors to load the speaker terminals) so that I can more easily test thoroughly. I get a reading of 157 mV at the speaker terminals. Should I be looking at coupling caps?
Regards John
I have an Advent 300 receiver that I'm hoping to use for a shop amp. I bought it not knowing its condition. I plugged it into the mains via a dim bulb and was greeted with a serious hum and the bulb didn't dim. My first thought was power supply and I replaced the filter caps and the bridge rectifier as it looked as if one of the leads was fractured. This didn't reduce the hum which persists at the same loudness whether or not the volume control is up or down. I tried removing the bridges between the pre amp and main amp - no difference which leads me to believe the main amp is the problem. I can't leave the amp on for long as it'll blow the speakers (I'm waiting on some 8 ohm resistors to load the speaker terminals) so that I can more easily test thoroughly. I get a reading of 157 mV at the speaker terminals. Should I be looking at coupling caps?
Regards John
Not much to the power supply, remove the fuses and do the bulb test again.
If still bad, check the rest of the parts, including the unloaded transformer.
If still bad, check the rest of the parts, including the unloaded transformer.
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I have some experience with this unit, especially the version they made to work off 12VDC (i.e., vans, RVs, mobile homes). There are many service bulletins on this product. And the 12V version was perfectly happy to burn up its circuit board while otherwise healthy. Good frikkin luck with bringing it back to full performance.
I'm sure it did, this was years ago.
What I'm saying is even when it was running normally the 12V version had such crappy thermal design that it happily charred the power supply part of the board all the way to unrepairable a good percentage pf the time.
What I'm saying is even when it was running normally the 12V version had such crappy thermal design that it happily charred the power supply part of the board all the way to unrepairable a good percentage pf the time.
Thanks folks. I do have a schematic that I’ll post when I get home. The unit is 110v not the 12v version.
Pursuing rayma suggestions seems like a good initial attack. Should be very few volts ripple on the bulls filter caps.
J300/J301 are some sort of links to open the signal paths? Try removing the links and shorting amp inputs to ground to see how hum is affected.
Am I guessing correctly that speaker select switches allow you to turn off the speakers? If so, that could be a useful test case. Adding resistor loads may or may not change hum amplitude.
Let us know what you observe.
Good luck.
J300/J301 are some sort of links to open the signal paths? Try removing the links and shorting amp inputs to ground to see how hum is affected.
Am I guessing correctly that speaker select switches allow you to turn off the speakers? If so, that could be a useful test case. Adding resistor loads may or may not change hum amplitude.
Let us know what you observe.
Good luck.
Limited time availability today, but I did a quick check of the rail voltages which are supposed to be +22 & -22. I get +24 on the positive rail and -16.5 on the negative. That seems a little weird so I'll test again later. Even so, the dim bulb indicates a short somewhere. The fault is still there when the jumpers are removed between the pre and main amps.
Regards John
Regards John
The jumpers are just for the audio signal, and will not affect a severe fault in the power supply, etc.
Yes true, but I needed to check. I've replaced the PS caps, bridge rectifier and the resistor feeding the "on" light. I'll check all my solder connections later today.
At this point I would disconnect and tape off all of the transformer's secondary wires, and try the bulb tester again.
If still there is a problem, the transformer could be bad.
Also there is the circuit breaker, the accessory AC output, the power switch, and the suppression capacitor across it.
Any of these shorting to ground, or to another component, could cause similar problems.
If still there is a problem, the transformer could be bad.
Also there is the circuit breaker, the accessory AC output, the power switch, and the suppression capacitor across it.
Any of these shorting to ground, or to another component, could cause similar problems.
I just checked the plus/minus rail voltages at the fuse holders as suggested by the service manual and once again I get +24v on the 4 amp fuse but only 16,5v at the 3 amp fuse. I think I'll undo the work I did on the PS and re-solder. I don't believe the other PS components such as the reset switch can be at fault or I wouldn't get any readings. One thing though, is it possible to have a bridge rectifier that is too big? The original wasn't available so I bought some alternatives capable of handling 7 amps. I can't see it being a problem, but I'm not 100% sure. Anyway, I'll check my solder joints. I'll also check out the tranny while the secondaries are exposed.
Regards John
Regards John
Not really, the rectifier can be oversized and often is.
You said that the bulb tester lights up, which means there is excessive AC line current draw.
If you are measuring the DUT internal voltages with the bulb tester connected, that does not work.
The bulb tester is solely to prevent AC line fault currents from doing severe damage to the DUT.
Once the faults are corrected and the bulb tester does not light up, it must be removed prior to any further testing.
You said that the bulb tester lights up, which means there is excessive AC line current draw.
If you are measuring the DUT internal voltages with the bulb tester connected, that does not work.
The bulb tester is solely to prevent AC line fault currents from doing severe damage to the DUT.
Once the faults are corrected and the bulb tester does not light up, it must be removed prior to any further testing.
Given the strong hum and lightbulb glowing in principle I suspect shorted power amps.
To confirm/reject that load problem, remove main rail fuses F400/F401, 4A and 3 A respectively, and try again.
IF bulb blinks and glows dark orange, problem is what I suspect; if still glowing bright, THEN suspect supply diodes - capacitors - power transformer, in that order, which can be individually tested by removing one by one from supply.
To confirm/reject that load problem, remove main rail fuses F400/F401, 4A and 3 A respectively, and try again.
IF bulb blinks and glows dark orange, problem is what I suspect; if still glowing bright, THEN suspect supply diodes - capacitors - power transformer, in that order, which can be individually tested by removing one by one from supply.