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Should I worry ?
Should I worry ?
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Old 14th June 2018, 10:04 PM   #1
Dinolobe is offline Dinolobe  Canada
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Default Should I worry ?

Hi everyone. I've been given a h/k 930 receiver recently. Lifted up the cover to discover some pretty beefy internals, so i decided to give it a try. I like the sound and the look and there is some emotional value added to it so it's a keeper. My father bought it new back in the days, along with a td-165 thorens record player and EV 12TRX speakers that he made cabinets for.

I'd like to take it back to it's original shape, even if it's in pretty good shape for its age. I checked and adjusted bias to the recomended specs in the manual and tested dc offset. Turns out the left and right channels have different values, but the manual does not include specs or adjustment procedure.

After the bias have been adjused to the recommended 15mV and the unit played for an hour, it had 7mV of dc in the left channel and 30mV in the right. With the rear bypass removed to isolate the power amp part of the receiver, it tested at 5mV for the left channel and 71mV for the right.

From what i'va read, those receiver have a tendecy to have a strong and a weak channel, but I don't know what kind of value is safe since i dont have acess to any specs... This is my first post here and i'm pretty new to this scene so be a bit forgiving please

Last edited by Dinolobe; 15th June 2018 at 12:05 PM.
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Old 14th June 2018, 11:11 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by Dinolobe View Post
I checked and adjusted bias to the recomended specs in the manual and tested dc offset. Turns out the left and right channels have different values, but the manual does not include specs or adjustment procedure.
Bias is 15mV across each 0.5R emitter resistor. Here's the service manual.
Harman Kardon 930 - Manual - Stereo FM/AM Solid State Receiver - HiFi Engine

Last edited by rayma; 14th June 2018 at 11:17 PM.
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Old 14th June 2018, 11:15 PM   #3
indianajo is offline indianajo  United States
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Those DC on speaker values are very low, I would ignore them.
Output transistor idle current of 15 ma is very low for TO3 transistors, I suppose these are TO-66 or something? That should be conservative. Remember this value can only be measured with the channel silent, unless you have a subtraction scope function and two probes for the two inputs.
More to the point on strong and weak channels, what is the AC voltage on speaker ( or 8 ohm or 4 ohm load resistor) at maximum volume? I find AC voltage readings of common DVM to be random numbers on music. The RMS fluke at $180 will make a valid measurement up to 7 khz but it won't detect ultrasonic oscillation- I won't be buying one. I use a $25 analog VOM for AC voltage measurement.
P=(V^2)/Z where Z is speaker impedance. Both channels should have similar, and near specification value, power output. Most 30 year old consumer amplifiers do not have anywhere near full power out. But do the measurement before attempting any repairs.
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Old 15th June 2018, 12:26 AM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
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If its of an age then it might need the electrolytics changing.
They tend to go first.
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Old 15th June 2018, 03:02 AM   #5
dotneck335 is offline dotneck335  United States
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Old 15th June 2018, 12:59 PM   #6
Dinolobe is offline Dinolobe  Canada
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Quote:
Originally Posted by indianajo View Post
Those DC on speaker values are very low, I would ignore them.
Output transistor idle current of 15 ma is very low for TO3 transistors, I suppose these are TO-66 or something? That should be conservative. Remember this value can only be measured with the channel silent, unless you have a subtraction scope function and two probes for the two inputs.
More to the point on strong and weak channels, what is the AC voltage on speaker ( or 8 ohm or 4 ohm load resistor) at maximum volume? I find AC voltage readings of common DVM to be random numbers on music. The RMS fluke at $180 will make a valid measurement up to 7 khz but it won't detect ultrasonic oscillation- I won't be buying one. I use a $25 analog VOM for AC voltage measurement.
P=(V^2)/Z where Z is speaker impedance. Both channels should have similar, and near specification value, power output. Most 30 year old consumer amplifiers do not have anywhere near full power out. But do the measurement before attempting any repairs.


The output transistors are Hitachi 2SC897. The bias spec recomended by h/k is in mV. It is not the idle current spec, but the voltage across a resistor (R617 for the left channel if I remember). Since h/k did not provide any way to adjust DC offset from the factory, I guess i'll just leave it like that.


About the AC voltage on the speaker terminals, can you tell me more precisely how you take those measurements and what they would mean ? I'm not shure I understand why I would need those numbers...


I appreciate your answers guys !
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Old 15th June 2018, 02:14 PM   #7
xXBrunoXx is offline xXBrunoXx
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To see if both channels output same power.
Use a signal generator (set it on 500-1000hz, measure with your speakers connected to the amplifier (preferably use some big 30-40w resistors instead of the speakers but at the same resistance, if your speakers are 4ohms impedance use 4ohm resistor) use your multimeter on ac on the output of each channel of your amp and compare the results.
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Old 15th June 2018, 03:08 PM   #8
Dinolobe is offline Dinolobe  Canada
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Quote:
Originally Posted by xXBrunoXx View Post
To see if both channels output same power.
Use a signal generator (set it on 500-1000hz, measure with your speakers connected to the amplifier (preferably use some big 30-40w resistors instead of the speakers but at the same resistance, if your speakers are 4ohms impedance use 4ohm resistor) use your multimeter on ac on the output of each channel of your amp and compare the results.


I don't have a signal generator. I guess a stable signal is required to have accurate measurements ? I have two big 50W 8ohm resistors I used to set the bias, as recommended by H/K. I'll do it with those even if my speakers are 4 ohm since the rated output power is at 8 ohm anyway.


Just to make shure I understand correctly : I should worry if the difference between the two channel output power is too big ?


Could I use a recorded signal of the correct value and play it through the receiver instead of a signal generator ?
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Old 15th June 2018, 03:12 PM   #9
Dinolobe is offline Dinolobe  Canada
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Just realized there is free signal generator apps for iPhone...
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Old 15th June 2018, 04:37 PM   #10
indianajo is offline indianajo  United States
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I use the local classic rock FM station for a signal generator, which compresses unashamedly and has about the same volume all the time, except when the announcer in commercials takes a breath.
You put the plus probe of the VOM on the red speaker terminal and the minus probe of the VOM on the black speaker terminal. Leave speaker connected. Set scale to 50 vac or something close to there. Turn radio up until is sounds a little funny (clipping) then back off. Measure steady voltage values. Put into equation above.
Z in the equation above is speaker impedance. If not marked, measure the speaker resistance with the meter and add about 20%. Use a calibration resistor from a store, meters are notoriously inaccurate measuring 2 to 10 ohms. Fresh battery in the meter helps.
If the power of the receiver is greater than the power handling capability of the speaker, then use resistors of the appropriate power rating instead. I've never had that problem. Probably cheaper, junk car radio speakers, usually have a power rating printed on the back.
I'm not looking up 2SC897 to see what the package is. Do it yourself. Datasheets on datasheetcatalog.com The idle current across the resistor you measure is V/R from ohms law.
Per suggestions above, one source of unbalanced channels is bad electrolytic capacitors. A consumer product that was stored in good repair decades ago can work great 4-8 weeks until the rubber seals in the capacitors break and let the water vapor out. Other common source of channel imbalance in geriatric products is dirty or worn out volume pot, or oxidized contacts in various switches in the signal path. Don't replace any e-caps unless you have a full power wattage problem. If so, replace them all, the ones coupling the stages together don't go first, but usually do soon after the rail caps do. Two at a time, with tests in between to prove you didn't make a problem worse with your newby soldering technique.
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