Hello, a friend gifted me a harman/kardon AVR255 7.1 amplifier. Front left speaker output is outputting DC Voltage between 1.73-1.93 V depending on the volume level. On the front right speaker could not detect dc volt output(0.03V).
Is that 1.73-1.93V DC normal? Why is it happening? Is there any chance that it could damage my speakers?
There is also a hiss on the front left speaker that you can hear if you go close to the loudspeaker. The hiss level remains the same regardless of the volume knob position
Is that 1.73-1.93V DC normal? Why is it happening? Is there any chance that it could damage my speakers?
There is also a hiss on the front left speaker that you can hear if you go close to the loudspeaker. The hiss level remains the same regardless of the volume knob position
Any DC on the output should be just a few mV, not that high. It will displace the woofer cone from neutral.
Seems there is a bad electrolytic capacitor somewhere in the left front amplifier circuit.
Seems there is a bad electrolytic capacitor somewhere in the left front amplifier circuit.
All those class AB channels cooking away on barely adequate heatsinks. Looks like the Sony AV systems - chip drivers running TO-3P darlingtons.
All the digital circuitry is "cooking away" with the AB channels.
OS
Welcome to the forum!
I found the service manual on line, but it was too large for upload to the DIY server--- so link is here:
https://www.manualslib.com/download/1183978/Harman-Kardon-Avr-255.html
To elaborate on @rayma 's comment, I suggest observing the DC voltage on R508 (page 141 of the SM pdf) as the volume control is manipulated. Ideally, it will be only a few mV and shouldn't change with control position. Any variation is suggestive of leakage through C503 and will likely contribute to output offset. Additionally, look for DC drop across R563. Any drop across R563 indicates leakage through C563, and any drop observed will be amplified by the ratio of R558/R563 (15 or 16.875, depending upon value of R563) and contribute to output offset.
If you don't find a suspicious cap, other diagnostic measurements that may be insightful will be to record the voltages at bases of Q518 and Q513, C563, and output at L_CH, all measured with respect to ground.
The hiss may be due to a noisy transistor, but finding the bias culprit should come first.
I found the service manual on line, but it was too large for upload to the DIY server--- so link is here:
https://www.manualslib.com/download/1183978/Harman-Kardon-Avr-255.html
To elaborate on @rayma 's comment, I suggest observing the DC voltage on R508 (page 141 of the SM pdf) as the volume control is manipulated. Ideally, it will be only a few mV and shouldn't change with control position. Any variation is suggestive of leakage through C503 and will likely contribute to output offset. Additionally, look for DC drop across R563. Any drop across R563 indicates leakage through C563, and any drop observed will be amplified by the ratio of R558/R563 (15 or 16.875, depending upon value of R563) and contribute to output offset.
If you don't find a suspicious cap, other diagnostic measurements that may be insightful will be to record the voltages at bases of Q518 and Q513, C563, and output at L_CH, all measured with respect to ground.
The hiss may be due to a noisy transistor, but finding the bias culprit should come first.
Hello, what value capacitor could be put on an amplifier's output to block DC but allow 20-20KHZ sound to pass without distortion?
I know that the right way to fix the issue is to disassemble the amp and find what is causing the DC on the output(s) then fix it.
But let's say that the owner of the amp doesn't have the technical knowledge to fix it on his own neither the budget to send the amp for repair.
Thank you so much for any input
I know that the right way to fix the issue is to disassemble the amp and find what is causing the DC on the output(s) then fix it.
But let's say that the owner of the amp doesn't have the technical knowledge to fix it on his own neither the budget to send the amp for repair.
Thank you so much for any input
If 8 ohms speakers, 1.04 millifarad. Could be non-polar electrolytic which might last 5 years, or polyprophylene which would last forever. If made of 5 parallel 200 uf polypro caps https://www.parts-express.com/Solen-200uF-400V-Polypropylene-Capacitor-027-630
they are only $73.58 each or $367.90 per channel.
For 4 ohm speakers 2.08 millifarad so double the # of 200 uf caps for $735.80 per channel.
Caps that large the inductance might block 10000 to 20000 hz, so parallel in one 3.9 uf cap https://www.parts-express.com/Audyn-Cap-Q4-3.9uF-400V-MKP-Foil-Capacitor-027-113 for $3.77.
I do not know anybody that has ever done that.
The HK AVR255 is likely made of surface mount parts, so probably unrepairable without a $400 solder station and perfect eyesight. Since you are in Greece, look for a used Peavey M-2600 or PV-4c amp for parts or repair. All through hole parts and schematic is available. I bought my M-2600 working for $120, then replaced the electrolytic caps and cleaned the dust out. I scored a dumpster reject PV-4c for $25+$40 freight with smashed speaker connectors and a bent rack flange. Since many $11 transistors in the PV-4c need replaced, the M-2600 is a better deal, especially in my home where no fan is a plus.
they are only $73.58 each or $367.90 per channel.
For 4 ohm speakers 2.08 millifarad so double the # of 200 uf caps for $735.80 per channel.
Caps that large the inductance might block 10000 to 20000 hz, so parallel in one 3.9 uf cap https://www.parts-express.com/Audyn-Cap-Q4-3.9uF-400V-MKP-Foil-Capacitor-027-113 for $3.77.
I do not know anybody that has ever done that.
The HK AVR255 is likely made of surface mount parts, so probably unrepairable without a $400 solder station and perfect eyesight. Since you are in Greece, look for a used Peavey M-2600 or PV-4c amp for parts or repair. All through hole parts and schematic is available. I bought my M-2600 working for $120, then replaced the electrolytic caps and cleaned the dust out. I scored a dumpster reject PV-4c for $25+$40 freight with smashed speaker connectors and a bent rack flange. Since many $11 transistors in the PV-4c need replaced, the M-2600 is a better deal, especially in my home where no fan is a plus.
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A 1 uf 250v poly cap works forever just fine.
The 3.9 uF cap from parts express is also good. Depends on the voltage and value you need.
The 3.9 uF cap from parts express is also good. Depends on the voltage and value you need.
I believe this one should be good to pass 20Hz - 20Khz and block unwanted DC:
UPW1C222MHD
It's low ESR too and Nichicon, so I would expect it to last a lot
UPW1C222MHD
It's low ESR too and Nichicon, so I would expect it to last a lot
So, I bought the capacitors kstgr mentioned because i already had the model on my hands and it was easily accessible to to me because i live on a greek island, connected them to the output of the amp. Everything seems perfect! 0.00 vdc on the multimeter. Had to use a second capacitor for the other channel because I had a feeling that the soundstage wasn't the same as before. The first time It turned on with the capacitor on there was a small delay in getting the same volume compared to the right channel which didn't have the capacitor. Bass remains the same and also the rest of the frequencies. Thank you all for your help and the effort you put in it. I appreciate it! I hope I can help somebody in the future with the small knowledge i currently have!!
Since that is a polar capacitor rated for 35 v, expect the capacitor on the channel with no DC to blow the top and fail soon. May happen to the DC channel too unless the voltage on it never goes negative. No free lunch. Non-polar capacitors are available, but not in long life versions. Life of e-capacitors not rated for long life, 3-5 years or 2000 hours service. .
Actually they are 16V 2200uF Nichicon PW low ESR capacitors. They cost here in Greece almost 2€ per item
Blocking a volt or two of DC on a direct coupled power amp that is supposed to have low (millivolts) of offset is band aiding it. There could be something else wrong. If it does go stupid on you and head for the rail a 16V cap isn’t going to like it very much. Find out why it’s running high and go from there. There could be excess offset coming into the power stage, and it would be better to block it there. If it’s out of adjustment, adjust it. If it’s oscillating ultrasonically, it could result in offset if that frequency has high 2HD. If that’s happening you’re sitting on a ticking bomb.