Hi everyone,
I have an h/k 430 "twin power" receiver which has a strange problem I've been trying to track down. When the receiver is initially switched on, after about 5 seconds, a hum is audible in both speakers for a few seconds. It seems like 120 cycle hum, but more dirty with harmonics in it, and it swells and fades about 5 times, with each swell quieter than the previous one. After the noise fades out, the amp sounds great. Here is a link to a recording of the noise - you can hear the click of the amp power switch, about 5 seconds of silence, and then a series of about 4 swells of the humming noise, after which the amp quiets down:
https://soundcloud.com/user872357133...-powerup-noise
In trying to solve this, I have replaced all the electrolytic caps on the main amp board and all three power supply/filtering boards. I also replaced the rectifier diodes in the power amp power supply section because they seemed suspect, however that did not solve it. I recently realized that if I disconnect the signal wire between the pre-amp and the amp boards, the powerup noise will not occur. This led me to suspect that the problem may be in the pre-amp section or the power supply for the pre-amp. I have gone through and tested all of the electrolytic caps and the semiconductors (mostly diodes) in those parts of the circuit. All the parts I checked seemed fine, but I only have a DMM for testing, so I can really only check for open circuits or shorts and basic capacitor charging. I have worked on and fixed a lot of problems with electronic equipment before, but I am stumped on this one. Do I need a scope to figure this out? Any thoughts? Thanks! Let me know if you need any more information and I will try to provide it. BTW the service manual for this amp with schematics is available at Harman Kardon 430 | Owners/Service Manuals, Schematics, Free Download, Reviews | HiFi Engine
-Serge
I have an h/k 430 "twin power" receiver which has a strange problem I've been trying to track down. When the receiver is initially switched on, after about 5 seconds, a hum is audible in both speakers for a few seconds. It seems like 120 cycle hum, but more dirty with harmonics in it, and it swells and fades about 5 times, with each swell quieter than the previous one. After the noise fades out, the amp sounds great. Here is a link to a recording of the noise - you can hear the click of the amp power switch, about 5 seconds of silence, and then a series of about 4 swells of the humming noise, after which the amp quiets down:
https://soundcloud.com/user872357133...-powerup-noise
In trying to solve this, I have replaced all the electrolytic caps on the main amp board and all three power supply/filtering boards. I also replaced the rectifier diodes in the power amp power supply section because they seemed suspect, however that did not solve it. I recently realized that if I disconnect the signal wire between the pre-amp and the amp boards, the powerup noise will not occur. This led me to suspect that the problem may be in the pre-amp section or the power supply for the pre-amp. I have gone through and tested all of the electrolytic caps and the semiconductors (mostly diodes) in those parts of the circuit. All the parts I checked seemed fine, but I only have a DMM for testing, so I can really only check for open circuits or shorts and basic capacitor charging. I have worked on and fixed a lot of problems with electronic equipment before, but I am stumped on this one. Do I need a scope to figure this out? Any thoughts? Thanks! Let me know if you need any more information and I will try to provide it. BTW the service manual for this amp with schematics is available at Harman Kardon 430 | Owners/Service Manuals, Schematics, Free Download, Reviews | HiFi Engine
-Serge
'Scopes make life easier to identify noise and problems but if you don't have or aren't likely to have one with some input on how to use it, it's not going to help.
You may not have exhausted your test options anyway. The preamp is spread over the tone/preamp and equalizer boards so you could disconnect them in sequence to see how far back the problem goes - identify which channel or both and then look at the wiring from, lets say the auxiliary inputs or tape inputs, where you might connect a source like your disc player, media player or DAC.
Check input switching to see if that has an influence on the noise but I imagine that since the capacitors have been checked and they really are OK even though it's doubtful in a product this old, then the only remaining likely culprit is a signal transistor there in the preamp. There's not that many of them and often a little heat, freeze spray or gentle finger pressure (mind any higher voltages nearby) can induce the problem and identify the offending component(s).
There are reasons why bursts of oscillation (usually with temperature change) might occur on startup or even shutdown and periodically in use. It even happens with leaky caps. It may also cause a transistor to fail because it may be exposed to intermittent high voltage/current. Sometimes the silicon becomes leaky on its own too.
This isn't the best plan but it's a simple start to digging in a logical direction. Don't count small electrolytics, tantalums or ceramics caps as being good just because they charge or measure no leaks though. They can still go through brief failure cycles as do some film caps. On startup, voltages are not yet stabilised so unless you have an analog meter or scope, you probably won't see any peak voltage spikes at the inputs that can bring on noise and faults, particularly apparent in amps without muting or speaker relays.
You may not have exhausted your test options anyway. The preamp is spread over the tone/preamp and equalizer boards so you could disconnect them in sequence to see how far back the problem goes - identify which channel or both and then look at the wiring from, lets say the auxiliary inputs or tape inputs, where you might connect a source like your disc player, media player or DAC.
Check input switching to see if that has an influence on the noise but I imagine that since the capacitors have been checked and they really are OK even though it's doubtful in a product this old, then the only remaining likely culprit is a signal transistor there in the preamp. There's not that many of them and often a little heat, freeze spray or gentle finger pressure (mind any higher voltages nearby) can induce the problem and identify the offending component(s).
There are reasons why bursts of oscillation (usually with temperature change) might occur on startup or even shutdown and periodically in use. It even happens with leaky caps. It may also cause a transistor to fail because it may be exposed to intermittent high voltage/current. Sometimes the silicon becomes leaky on its own too.
This isn't the best plan but it's a simple start to digging in a logical direction. Don't count small electrolytics, tantalums or ceramics caps as being good just because they charge or measure no leaks though. They can still go through brief failure cycles as do some film caps. On startup, voltages are not yet stabilised so unless you have an analog meter or scope, you probably won't see any peak voltage spikes at the inputs that can bring on noise and faults, particularly apparent in amps without muting or speaker relays.
Thanks everyone for the suggestions. I had some time to continue troubleshooting - no dice so far, though. I disconnected the "equalizer" board, and the noise was still present. From what I can tell, that board is actually just the phono pre-amp, and it doesn't do anything unless you're on phono mode. Is that correct? Also, I forgot to specify - the noise happens on all settings and in both channels.
I looked for orange caps. There are no orange caps on the pre-amp, equalizer, or main amp board. There are two orange caps on the tuner board. I haven't dug into that board yet, because I assumed it is only the tuner section. However I'm just noticing that the board is labeled "Tuner, IF, MPX" in the service manual. Does that board do more than just the tuner - i.e. - is it doing anything to the signal when you're not on any of the tuner modes? The orange caps are labeled "CF___" - the service manual says they're ceramic filters. They have three leads. Do you think those could be causing the problem?
Any other suggestions? Thanks!!
I looked for orange caps. There are no orange caps on the pre-amp, equalizer, or main amp board. There are two orange caps on the tuner board. I haven't dug into that board yet, because I assumed it is only the tuner section. However I'm just noticing that the board is labeled "Tuner, IF, MPX" in the service manual. Does that board do more than just the tuner - i.e. - is it doing anything to the signal when you're not on any of the tuner modes? The orange caps are labeled "CF___" - the service manual says they're ceramic filters. They have three leads. Do you think those could be causing the problem?
Any other suggestions? Thanks!!
Ceramic filters are what they say. They are dual ceramic capacitors forming a high frequency (10.7 MHz) bandpass filter for the IF amplifier stage of the FM tuner. When the tuner is turned off, nothing happens here - it is a source, an input like plugging in your CD player to AUX or TAPE IN. Deselect it and you get nothing, just like any unselected input to the input selector switch.
If the tuner works and pulls in stations, those filters work well enough in the tuner but but none of the stuff you are worrying about has anything to do with the problem if you can switch it out, like the tuner, phono or other inputs. The fixed stuff that's connected all the time and so affects your audio is right on the schematic at the 2 power amplifier boards, plus the tone/preamp and equalizer boards.
As the other guys have said, this is likely a cap problem but specifically the electrolytic caps (polarized) which are probably long past their use-by date. Even if this not the only reason, it will likely be well overdue now (almost 40 years old) for a proper service in any case.
Use the manual to help specify the right sort of values and quality replacement caps. for just those boards and the power supply first. There are lots of good quality Japanese electrolytic brands, each in in several grades. I use well established industry standards like Panasonic FM, FR and the TSHA types for the big reservoir caps. Others like equivalents from manufacturers like Rubycon, Nichicon, Nippon Chemicon etc. for whatever reason. It's not critical if they are fresh stock from a reliable national vendor like Mouser, Digi key, Newark etc.
Just don't buy caps on Ebay if you want genuine, fresh product and if you want this to work and to last, I assume you will.
If the tuner works and pulls in stations, those filters work well enough in the tuner but but none of the stuff you are worrying about has anything to do with the problem if you can switch it out, like the tuner, phono or other inputs. The fixed stuff that's connected all the time and so affects your audio is right on the schematic at the 2 power amplifier boards, plus the tone/preamp and equalizer boards.
As the other guys have said, this is likely a cap problem but specifically the electrolytic caps (polarized) which are probably long past their use-by date. Even if this not the only reason, it will likely be well overdue now (almost 40 years old) for a proper service in any case.
Use the manual to help specify the right sort of values and quality replacement caps. for just those boards and the power supply first. There are lots of good quality Japanese electrolytic brands, each in in several grades. I use well established industry standards like Panasonic FM, FR and the TSHA types for the big reservoir caps. Others like equivalents from manufacturers like Rubycon, Nichicon, Nippon Chemicon etc. for whatever reason. It's not critical if they are fresh stock from a reliable national vendor like Mouser, Digi key, Newark etc.
Just don't buy caps on Ebay if you want genuine, fresh product and if you want this to work and to last, I assume you will.
Orange caps were designated as "low leakage", and would almost always sit in the signal path.
They tend to be really small and really dried up. Some of them have epoxy seals (not rubber, like the others).
Oh and they also come in light blue.
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