Harman Kardon SR-300b repair/schematic hunt - Page 1
Forum section: electronics / repair

author: Arty30
author_rank: Regular Contributor
timestamp: December 17, 2023, 05:19:30 pm
content: Hello everyone, I've just recently found a Harman Kardon SR-300B FM receiver. I haven't gotten to testing it yet but I got it labeled as non-working. It appears the service manual/schematic is unobtanium on the internet, so I wanted to see if there's a small chance someone might know a solution. This is a quite early solid state unit, reminding me of the construction of tube radios I've worked on. without a schematic I'm not sure where to start troubleshooting, but I'll keep this page updated as I go.

author: Arty30
author_rank: Regular Contributor
timestamp: December 17, 2023, 10:11:50 pm
content: I've tested it now, and both channels make a loud buzzing, like angry bees, one channel also shows -4.7v DC at the terminals. I've taken note of all the caps as well since that's probably where I would start.

author: Arty30
author_rank: Regular Contributor
timestamp: December 18, 2023, 03:40:00 pm
content: Even though it seems no one is interested I'm going to keep going for the sake of recording my progress. I have done a more thorough signal tracing and I have discovered that the unit does in fact work as in music is making it to the speaker terminals, but it is drowned out by the very loud humming, not quite "angry bees" as I described earlier. the signal path was clear until I got to the tone controls, where it uses a pair of flat red square devices as filter modules. beyond that it's hard to trace anything without a schematic and loud hum. I have managed to find a paper schematic on the internet's favorite auction site. I might try to scan it for future people wanting to fix their SR 300b or 400b.

author: eetechTom
author_rank: Contributor
timestamp: December 18, 2023, 07:36:17 pm
content: If that orange filter cap is bad, it would cause some 120 Hz buzzing. Might want to look closely for a broken ground connection in the audio output section. If both channels are doing it, the power supply is the first place to look.

author: Arty30
author_rank: Regular Contributor
timestamp: December 18, 2023, 07:40:52 pm
content: Thanks for the reply. I've taken note of all the caps. haven't tested any yet, but I'm waiting on a schematic to arrive, so I'll probably poke around some more then.

author: eetechTom
author_rank: Contributor
timestamp: December 18, 2023, 08:54:15 pm
content: You could measure the ripple on the power rails to verify the filter caps functionality. Ripple will show on a meter, as an ac voltage measured on a dc power line. There should be (at most) less than 10% of ripple (ac), compared to the dc value. Due to the age of the orange capacitor, most would just replace it without measuring.

author: Arty30
author_rank: Regular Contributor
timestamp: December 19, 2023, 04:48:04 pm
content: I'm not sure exactly where the power rails are, but measuring across the filters to chassis doesn't show any ac voltages, it does show up on the output, though.

author: Arty30
author_rank: Regular Contributor
timestamp: December 19, 2023, 09:02:43 pm
content: I retract my prior statement, The orange cylinder filter pack is bad, two electronic meters either went blank or just shut off when testing. I popped in come used caps and the unit now plays well enough to hear over the noise, but it is still one sick puppy. one channel is lower than the other, the other makes a loud 120hz hum irrespective of volume, the FM radio barely picks up a channel broadcast a mile away with a long antenna, and if I move the volume knob the speakers sway in and out 1/4 inch. With my experience I would chock this up to more bad caps before I go looking for other issues, if anyone has other pointers I would gladly hear them, Thanks.

author: Arty30
author_rank: Regular Contributor
timestamp: December 29, 2023, 08:31:19 pm
content: Update: The seller of the schematic supposedly "lost it" so I haven't been able to secure one yet. on the other hand I have now re-capped the unit and it works much better now. It has a but of hum still, independent of volume, and the FM tuner is still not working, just a bunch of hum, not even static. until I locate a schematic I probably can't do much more to it. The amplifier itself is pretty decent, the hum isn't audible while playing. I have to say, point to point is kind of a boar to work on especially with radial caps, I wasn't spending 4 times the price for axial, but I'm satisfied for now.