General diagnosing of audio equipment

[Perplexed]

Electronic knowledge - 4 out of 10 but moving up!
Skills - Soldering - 7 out of 10, understanding schematics - 5, following troubleshooting guides - 8
Understanding basic electronic components and concepts - 5-6
Understanding datasheets - 6
Tools - really good variable temp soldering iron and soldering tools, a good to very good multimeter, a Variable DC power supply, a 4 chnl used old oscilloscope ( I have no idea how to use yet)

Working on a Denon AVR3313CI AV receiver. After 2 days of intensive diagnosis I determined 2 fuses are blown. The spouse was extremely impressed. The fuses are on a 10vac circuit - which feeds the 8V audio. If I replace the fuses ( 3.15A fast blow glass ) and turn the unit on they pop immediately. If I follow the circuit the next components are a set of 7808 and 7908 voltage regulators (+8V -8V out) followed by a bunch of other stuff (my component knowledge)

I'm about to take the whole unit apart to get to the base board where the audio circuit is located.
Any general help greatly appreciated and especially about these questions:

Lots of capacitors and diodes and some resistors in the circuit. I don't think there is a real way to test these while still in circuit but I do know that caps sometimes bulge if bad. Is there an order of priority that you follow when trying to isolate a problem? (capacitors first, resistors, diodes etc)

Once I remove the board could I power it bypassing the 8V regulator and testing the circuit using a lower than fuse Amp setting?

 

[Nigel Goodwin]

Working on a Denon AVR3313CI AV receiver. After 2 days of intensive diagnosis I determined 2 fuses are blown. The spouse was extremely impressed. The fuses are on a 10vac circuit - which feeds the 8V audio. If I replace the fuses ( 3.15A fast blow glass ) and turn the unit on they pop immediately. If I follow the circuit the next components are a set of 7808 and 7908 voltage regulators (+8V -8V out) followed by a bunch of other stuff (my component knowledge)

First off, I'm dubious about the 3.15A fuses feeding 1A regulators - assuming they are on that circuit?, then it's highly likely they feed something else as well.

Fuses blowing instantly usually points towards S/C rectifiers or output transistors - do you have a schematic of the unit?.

 

[MikeMl]

I create a "power Ohmmeter" by using my lab supply. I set the open-circuit voltage to ~5V, set the short-circuit current limit to <50mA. Suppose you want to test a diode. If you connect supply+ to cathode, and supply- to anode, then the diode blocks, no current flows, and the supply output stays at 5V. If you reverse the supply leads, then the diode conducts, and the supply goes into current limiting, and the output voltage is dragged down to ~0.8V. You can do this in-circuit in an unpowered board being tested. Easy to tell between a shorted diode, and a working one.

To test an electrolytic capacitor, observe the capacitor polarity, but you can see the difference between a shorted, and non-shorted capacitor. When testing in-circuit, components around a capacitor might make it look leaky, but if you see what looks like a dead-short (supply pulled all the way to zero V), that bears further investigation.

 

[Perplexed]

First off, I'm dubious about the 3.15A fuses feeding 1A regulators - assuming they are on that circuit?, then it's highly likely they feed something else as well.

Fuses blowing instantly usually points towards S/C rectifiers or output transistors - do you have a schematic of the unit?.

Thank you Nigel - I've attached the portion of the schematic I am focusing on. start at F9034 and F9035 the 2 fuses that blow are there.

 

[Perplexed]

I create a "power Ohmmeter" by using my lab supply. I set the open-circuit voltage to ~5V, set the short-circuit current limit to <50mA. Suppose you want to test a diode. If you connect supply+ to cathode, and supply- to anode, then the diode blocks, no current flows, and the supply output stays at 5V. If you reverse the supply leads, then the diode conducts, and the supply goes into current limiting, and the output voltage is dragged down to ~0.8V. You can do this in-circuit in an unpowered board being tested. Easy to tell between a shorted diode, and a working one.

To test an electrolytic capacitor, observe the capacitor polarity, but you can see the difference between a shorted, and non-shorted capacitor. When testing in-circuit, components around a capacitor might make it look leaky, but if you see what looks like a dead-short (supply pulled all the way to zero V), that bears further investigation.

Hey Mike - appreciate the help.

 

[Les Jones]

I suggest starting by unplugging the BN932 connector to the transformer and And the TOG connector (lower right of schematic.) This feeds the + Ve rail to some other part of the circuit. Now test the diodes using the diode test range on your multimeter. I agree with Nigel and Mike that the fault is unlikely to be the regulators. Report back with the result of this test.

Les.

 

[Nigel Goodwin]

Thank you Nigel - I've attached the portion of the schematic I am focusing on. start at F9034 and F9035 the 2 fuses that blow are there.

Check the rectifiers, D9325 to D9332 - you should be able to test them in-circuit using the diode test function on your meter - one or more of them may be S/C.

Unplugging the lead from the fuses would isolate the transformer, and prevent any false readings in that direction.

From a service engineers point of view those rectifier diodes are by far the most likely culprits.

 

[Perplexed]

Les Nigel thanks. I'll report back as soon as I can get back to the bench

 

[Perplexed]

Hi Guys,

Thanks to all of you for the advice, help and guidance. Nigel - you hit it on the head - two of the rectifiers in the group were DOA. Practised my de-soldering/soldering skills and voila! - all is right with the sound system. Once again thanks!

 

[Nigel Goodwin]

Nigel - you hit it on the head - two of the rectifiers in the group were DOA.

Like I said, it's a VERY common failure