Hi Solid state folks,
A Niles Si-275 has come into my possession. Its a great 2 channel amp that goes overlooked in used a lot of times. Sadly, this one powers on but does not have any sound output. Could anyone provide a brief checklist of things to go over with my DMM to try to locate the issue? I did a visual inspection and nothing looks burnt. The capacitors have some brown staining around some the bases, could be old glue?
I'm not an engineer but I am pretty capable, and know enough to not get myself electrocuted. I can post some photos of the inside if anyone's interested, its all discrete components, very nicely spaced out.
A Niles Si-275 has come into my possession. Its a great 2 channel amp that goes overlooked in used a lot of times. Sadly, this one powers on but does not have any sound output. Could anyone provide a brief checklist of things to go over with my DMM to try to locate the issue? I did a visual inspection and nothing looks burnt. The capacitors have some brown staining around some the bases, could be old glue?
I'm not an engineer but I am pretty capable, and know enough to not get myself electrocuted. I can post some photos of the inside if anyone's interested, its all discrete components, very nicely spaced out.
Fuses all tested good w/continuity tester. There are a few relays, I'll have to look into how to check the function.
If the power supplies have nominal DC voltages, output disconnect relays are the most likely.
But they could be triggering due to a circuit fault. Check the DC output voltage prior to the relay.
But they could be triggering due to a circuit fault. Check the DC output voltage prior to the relay.
See the input side as well, dirty or corroded selector can block or reduce signal.
What is your input signal?
Can you change the level of that?
What is your input signal?
Can you change the level of that?
I’m going to plug it in and test the outputs for shorts, etc tonight. Only did a quick check of the fuses and resistors with continuity a few days ago.
Don't do that, power can be fatal.
Check the outputs with a meter, with the power off.
Also output transistors.
If you can, connect the relay coil to an external source, and measure contact resistance as well.
Before that, check the relay coils are getting enough voltage to work, small variations are normal in older sets.
Check the outputs with a meter, with the power off.
Also output transistors.
If you can, connect the relay coil to an external source, and measure contact resistance as well.
Before that, check the relay coils are getting enough voltage to work, small variations are normal in older sets.
Well I was really excited bc I thought I had found the fault: the rca input header connector had the ground wire not making contact with its header pin on the board. How this could have happened is beyond me. Plugged all the chassis connections back together, turned it on and no sound. Ok. Went to check the front trim pots, and cycled the power off to reposition everything so I could adjust those while feeding a signal. Cycled it back on and the input fuse blew right away. Double checked everything is connected properly. Ac in/out was correct. For now I’m waiting on a new fuse to come from Amazon. Ugh.
Thanks for the link. I have a copy, any particular thing you're suggesting I reference?
It's possible but very unlikely to repair an amplifier without a good understanding of the circuit details, which requires a schematic. I don't see a service manual on-line that includes a schematic. Some people have simply tested the output devices with an Ohmmeter and replace the bad transistors. But it is rare that a directly coupled audio amplifier fails without also destroying the driver transistors, associated resistors and bias network. The first step is to check the bias and output offset voltages. Typically, one channel will have failed leaving a large DC on the output which causes the output relay to not close. It is often useful to disconnect the failed output devices (and any other obviously failed parts) and see if the drivers alone still produce a DC output, without any load. Once you have an amplifier operating correctly without power transistors, it is probably safe to replace the power transistors.
Thanks. I’ll give it another try once I get the replacement fuses tomorrow. My fingers are crossed the rca header ground was the issue. The protection circuit wasn’t engaging, and my checks on the transistors, resistors and relays all looked ok (checked with dmm in either continuity or diode mode). Nothing looks out of the ordinary by visual inspection either. If it’s still giving no output with a new fuse then I’ll move on.
You have to buy fuses on Amazon?
Here we pay 2-5 Rupees at industrial electric or electronic parts shops for a glass fuse!
I wonder how much extra they charged for sending a simple fuse.
Here we pay 2-5 Rupees at industrial electric or electronic parts shops for a glass fuse!
I wonder how much extra they charged for sending a simple fuse.
I think it’s 20pcs for $7.
There isn’t a shop like that anywhere that I know of.
I’m sure I could have gotten 100pcs for $2 from China but it would take 6mo to get here.
Can’t complain about 2 clicks and it’s in your mailbox in 24h, worth the $6.95 extra markup.
I got back to working on this amp. now I have some more info. I replaced the fuse for the AC input (yellow box). Powered on and promptly blew the fuse pictured in the green box. Replaced it with new one, blows right away again. I checked the voltages from the toroid and things look OK, nothing unexpected. Checked the other fuses and they all are ok.
Can someone help me determine what the next step to look at would be? This specific fuse should point to the faulty area, where I can dig some more.
Can someone help me determine what the next step to look at would be? This specific fuse should point to the faulty area, where I can dig some more.
F5 (together with its partner beyond him) is the fuse for some auxillary stuff. Maybe/Likely some regulators for opamps etc. Not for mains. The one in the yellow marking may be the mains fuse.
Check D30 (bridge after the two fuses) for shorts, also the two caps directly after it. D31/D32 (Zeners?) are looking suspect, too (they got warm - see marks on the pcb).
Check D30 (bridge after the two fuses) for shorts, also the two caps directly after it. D31/D32 (Zeners?) are looking suspect, too (they got warm - see marks on the pcb).
Basically a big fault, caused by as yet unknown component.
Difficult without schematic.
Tell us output device and supply rails, might give a clue.
Most amps are generic that way, rare to see specific circuits.
Difficult without schematic.
Tell us output device and supply rails, might give a clue.
Most amps are generic that way, rare to see specific circuits.
Niles never gave out schematics, so I’m out of luck there. I’ll check power rail voltages and output devices and annotate the picture.
the fuse in the yellow box is the fuse for the mains.
Iirc the power rail supplying the green box was lower voltage than the other two- they went to the bulk capacitors.
so I’m also thinking the F5 fuses are for accessories/op amps etc.
I’ll check on D30 and the parts around there. They do look suspicious like maybe they had some heat issues.
the fuse in the yellow box is the fuse for the mains.
Iirc the power rail supplying the green box was lower voltage than the other two- they went to the bulk capacitors.
so I’m also thinking the F5 fuses are for accessories/op amps etc.
I’ll check on D30 and the parts around there. They do look suspicious like maybe they had some heat issues.
ericwestpheling Did you get your Niles amp problem solved? I am curious about what you learned about D31, D32 diodes. Are they Zeners? I pulled C66, 67, 68, and 69 and all 4 capacitors were bad. Resistors R113 and R114 have seen some heat but they measure ok at 215 ohms whereas the banding indicates they are 200ohm resistors. SO I will replace the 4 caps, and if I can find out what diode is needed I will replace D31, D32 also.