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HK 590i: broken and I broke it more

EdBacon

EdBacon

Active Member
Hey all,

I picked up an HK590i from someone who did... something to it and was giving it away for repair or parts. I am trying to repair because it's a pretty nice looking receiver plus I like learning from these projects.

This receiver powered on, lights up, but only one channel works. The other was dead. I opened the machine up to find one pair of output power transistors shorted with some nice black charring in evidence of some arcing. I replaced these: did not fix the issue. I inspected closer and found a number of BJT transistors in the circuit of the dead channel before the main power transistors were also shorted. Likewise some adjacent resistors had been burned. Replaced these, as well as some basic maintenance on electrolytic caps (some were leaking, others like he filter caps I replaced out of caution since they were covered in old circuit glue).

Now I have the issue that when I power on, speakers selected off, everything powers on normally, lights up, etc. When I switch speakers on (either 1 or 2) suddenly the dim bulb I am running off of lights up, and the display lights dim. Hum is apparent on output audio and this is in both channels, not affected by volume, input selection.

I've gone over my soldering and I'm not concerned there. I've also only changed transistors on one channel, so why am I getting this short suddenly on both channels ONLY when I select speakers on?

Any input is appreciated. Thank you.
 
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Maybe you have an odd short in the spkr selector switch or around the spkr connectors. With unit unplugged select sprk set A and measure the resistance between +(red?) and -(blk) left spkr posts, repeat for right set.

edit measure TP1 to GND then TP2 to chassis/GND with speakers set to A and not set
 
Ok so here's what I've found so far:

Unplugging the speakers and switching on the speaker select switch seems fine. The short is only when speakers are hooked up. I checked between the terminals and ground and seems OK (33kohm). Likewise I checked all the transistors I replaced to see if any of those were shorted. No dice. Cycled the speaker select. Again no short to ground there.

I plugged headphones in. Audio output is out in one channel with the loud hum. The hum is again not effected by volume or input select.

I'm a little confused. Wouldn't a short be evident without speakers plugged in? Is it somewhere between the output power transistor and the speaker/headphone?
 
The resistance checks were about eliminating odd faults like a frayed/broken piece of speaker wire shorting the speaker connector to chassis.

Seems like whenever you load the amp down it doesn't like it. Spkrs are nominal 8ohms where as headphones are generally about 300 (Grando 50-80???).

Stay on DBT.
What wattage bulb are you using? 60-75W?

The unit does not have a speaker protection relay/circuit so prefer not to connect spkrs.
If you have a dummy load to protect spkrs, headphones, 8ohms/10W would be ok since only idle

Have you set the bias? While on DBT set both channels to about 5mV so it's not a factor, spec is 33mV.

With headphones/dummy load connected, DBT somewhat bright, measure dc voltage at TP3 to chassis/GND, repeat for TP4,5,6 to GND, should be less than +/-10mV.

Q7,8 perform over current sensing, these should be diode tested.
 
Buck Rogers said:
What is the DC voltage on the positive output terminals with no speaker connected?

As measured to ground?
Click to expand...

30V straight DC acrossthe bad channel speaker posts. 130mV on the other channel.

Likewise for the respective test points on each channel.

Q7 and Q8 look good after diode test
 
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The correct procedure when you find blown output transistors is to remove them and test the transistors on that channel starting at the output and working back toward the input until you stop finding bad transistors. If you don't do that you will fail and likely blow the new output transistors you put in. Also, always set the bias current and check for voltage on the output before connecting speakers.
 
dr*audio said:
The correct procedure when you find blown output transistors is to remove them and test the transistors on that channel starting at the output and working back toward the input until you stop finding bad transistors. If you don't do that you will fail and likely blow the new output transistors you put in. Also, always set the bias current and check for voltage on the output before connecting speakers.
Click to expand...

I tested everything back to the input with the diode mode on my meter after swapping the output transistors did not fix the issue the first time. I should have been more thorough, yes.

I have gone back to those output transistors and tested them on diode mode and they test fine...
 
Kindly tell us which is the bad channel, needed for parts identification.

SM has unregulated supply at +/-39V, unit is on DBT and +(?) 30Vdc at speaker terminals.

Fairly sure your tester does not test at 30Vdc and 20-30mA

Assuming problem with left channel then candidates would include Q411, 417, 425, 421

You should also check all fusible resistors, they drift high when stressed.
Especially R439, R457, R483, R489.
Measure the voltage at Q421base, expect abouts +0.6Vdc, repeat for Q417b (+1.2V)
+31.2V at Q417b would point to Q411 or...
+0.6V at Q421b would point to Q421c-e short/failing under load.
 
I'll have a look at the schematic tonight. Meanwhile, if you have a positive voltage on the output it either means that you are missing drive on the negative half of the amp or something is shorted on the positive half of the amp, or the differential pair of transistors on the input are bad.
Sometimes a resistor is open, sometimes a transistor tests ok on the diode test but isn't working like a transistor; it isn't passing current from the collector to the emitter. You can find that by testing voltages on transistors. If the transistor is being turned on but the collector voltage is low, it should be removed and tested for Hfe.
 
Do Not connect a speaker to the bad channel until that voltage is in the Millivolt range.

You need to test every resistor, Capacitor, Diode and Transistor for starters [Again] Every transistor!

Find R454 or R459 on the bad channel and turn it down to about 500 ohms, Confirm with ohm meter this is your "Idle Current" adjustment.

With transistors a "Diode" check is Not sufficient you also need to test for Hfe or gain. An in meter or 30$ tester will work fine.

When damaged the "Gain" of a transistor can be altered beyond specs.

Audio amplifiers are a finely tuned "Balancing" act between positive and negative DC supplies.

Hope that helps.

Good luck

Buck
 
Checking voltage with headphones plugged in shows all transistors on the R (bad) channel with -30V on each leg: B C and E.

That is every transistor on that side of the amp.
 
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