I replaced the capacitors and transistors on this driver board and would like to set the bias. Can it be done with the information given in the schematic? If yes, what would the test points be?
* newbie alert ! * Please be specific regarding placement of positive & negative leads of the DMM as well as AC or DC voltage. Thank you.
.
* newbie alert ! * Please be specific regarding placement of positive & negative leads of the DMM as well as AC or DC voltage. Thank you.
.
The output stage bias current is always measured by a volt meter reading the DC voltage drop
across one of the high power emitter resistors, here 0.3 ohms, 7W. It doesn't matter which one of the two
in the channel, but they should be close in their readings. The channels may differ in the exact bias voltage.
It's also a good idea to measure their resistances (in circuit, unpowered) in case they have drifted or are open circuit.
Notice the bias voltage is given on the schematic as 0.02V DC (20mV DC). You will need a decent DVM for this.
The lead polarity isn't important, and the bias voltage polarity will inherently be correct if the amp is working normally.
You just want the magnitude of the voltage to be correct. Be careful to not slip and short something, as this can be very bad.
Sometimes the bias adjust pot goes bad and causes problems. You may want to replace that as well,
since this is a very old amplifier.
across one of the high power emitter resistors, here 0.3 ohms, 7W. It doesn't matter which one of the two
in the channel, but they should be close in their readings. The channels may differ in the exact bias voltage.
It's also a good idea to measure their resistances (in circuit, unpowered) in case they have drifted or are open circuit.
Notice the bias voltage is given on the schematic as 0.02V DC (20mV DC). You will need a decent DVM for this.
The lead polarity isn't important, and the bias voltage polarity will inherently be correct if the amp is working normally.
You just want the magnitude of the voltage to be correct. Be careful to not slip and short something, as this can be very bad.
Sometimes the bias adjust pot goes bad and causes problems. You may want to replace that as well,
since this is a very old amplifier.
At this point, I have only replaced the transistors and capacitors on the driver board and replaced the output transistors.
This may be a dumb question but before I start pulling things apart again, would it be useful to test the voltages supplying the driver board? There are several points labeled "4" in a square "+50v".
I only ask because when I decided to restore the amp, I started to take measurements and fried a portion of the left channel of the driver board.
If memory serves, I connected the POS lead of the dmm to the emitter of Q702 and the NEG lead to the chassis
This may be a dumb question but before I start pulling things apart again, would it be useful to test the voltages supplying the driver board? There are several points labeled "4" in a square "+50v".
I only ask because when I decided to restore the amp, I started to take measurements and fried a portion of the left channel of the driver board.
If memory serves, I connected the POS lead of the dmm to the emitter of Q702 and the NEG lead to the chassis
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Just to be sure: did you replace them with the original types of transistors?
A weakness of this design is that the quiescent current goes through the roof when the wiper of the trimming potmeter loses contact. You can solve that by adding a wire between the wiper and the upper terminal (upper in the schematic, that is) of VR701, increasing R720 to 2700 ohm and decreasing R722 to 820 ohm.
By the way, when debugging power amplifiers, people often connect a light bulb (ordinary incandescent light bulb or halogen light bulb) in series with the amplifier as a crude current limiter. It limits the amount of damage you get if something should go wrong. It's usually called a dim bulb tester.
A weakness of this design is that the quiescent current goes through the roof when the wiper of the trimming potmeter loses contact. You can solve that by adding a wire between the wiper and the upper terminal (upper in the schematic, that is) of VR701, increasing R720 to 2700 ohm and decreasing R722 to 820 ohm.
By the way, when debugging power amplifiers, people often connect a light bulb (ordinary incandescent light bulb or halogen light bulb) in series with the amplifier as a crude current limiter. It limits the amount of damage you get if something should go wrong. It's usually called a dim bulb tester.
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The amp is almost 50 years old so I used substitutes that were recommended by those with much more knowledge and experience that me. I hope they're right.
After rebuilding the driver board, etc. I powered the amp up through a "dim bulb tester", first a 75 watt bulb, then a 150 watt bulb (incandescent)
After rebuilding the driver board, etc. I powered the amp up through a "dim bulb tester", first a 75 watt bulb, then a 150 watt bulb (incandescent)
Bulb tester is only for the very first power up, then if no smoke or fire,
remove it and use AC line directly (or Variac if you have one).
Your bias may become nominal if you remove the bulb, if there are no other problems.
Every amp design is different, and experience counts less unless with that particular amplifier circuit.
remove it and use AC line directly (or Variac if you have one).
Your bias may become nominal if you remove the bulb, if there are no other problems.
Every amp design is different, and experience counts less unless with that particular amplifier circuit.
I was wondering whether the fact that the bias stays too low might just be due to different transistor types, if you now use a Q710 that has a substantially lower base-emitter voltage than the original and/or Q711 and Q712 with a substantially larger base-emitter voltage than the originals. You need a pretty big deviation, though.
Making progress
I pulled Q710/810 (small transistors attached to the heatsinks) and Q710 (left channel) was shorted, probably cooked it when I fried the left channel of the driver board.
I have a spare amp that I bought for parts and was able to match Q710 & Q810 so now both bias pots work but the voltage is limited to .9 mV to 2.1 mV, far from the 20 mV ideal.
All other voltages of the driver board and outputs are very close to what the schematic shows, some a little high, 1.1 V where it should be 1 V, 48.5 V where it should be 50 V, etc.
I pulled Q710/810 (small transistors attached to the heatsinks) and Q710 (left channel) was shorted, probably cooked it when I fried the left channel of the driver board.
I have a spare amp that I bought for parts and was able to match Q710 & Q810 so now both bias pots work but the voltage is limited to .9 mV to 2.1 mV, far from the 20 mV ideal.
All other voltages of the driver board and outputs are very close to what the schematic shows, some a little high, 1.1 V where it should be 1 V, 48.5 V where it should be 50 V, etc.
Checked voltages in the power supply per the schematic and they are very close as well.
DC offset is settling in below 25 mV per channel which is much better than the other day.
With 8 ohm 250 watt dummy loads connected, will inject sine waves into the aux ports and see how they look on the scope.
DC offset is settling in below 25 mV per channel which is much better than the other day.
With 8 ohm 250 watt dummy loads connected, will inject sine waves into the aux ports and see how they look on the scope.
Sine waves looked fine, right channel showed slightly less amplitude and right channel heat sink was
6 degrees warmer than the left. I didn't calculate the output because I didn't want to push it too hard until I get the bias issue solved.
I hooked up some speakers and a cd player and it sounded good but I'm going to set it aside until I learn more about how audio electronics work. It's a beautifully designed integrated amp which will be part of my collection.
6 degrees warmer than the left. I didn't calculate the output because I didn't want to push it too hard until I get the bias issue solved.
I hooked up some speakers and a cd player and it sounded good but I'm going to set it aside until I learn more about how audio electronics work. It's a beautifully designed integrated amp which will be part of my collection.
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I'm wondering if the bias issue I am experiencing could be due to the new driver transistors. The originals have a much lower hfe as measured with a Peak Atlas tester. I've had no luck finding data sheets for the original outputs but I do have one for the RCA1A10 & RCA1A09 which I will upload.
Original transistors in bold
Q701/Q801....N2484 (NPN)..... hfe=175 & 185
.....replaced with: KSC1845..... hfe=414 & 414
Q702/Q802....RCA1A10 (PNP)..... hfe=40 & 46
.....replaced with: TTA004B..... hfe=179 & 183
Q703/Q803....RCA1A09 (NPN)..... hfe=29 & 60
.....replaced with: TTC004B..... hfe=180 & 184
Original transistors in bold
Q701/Q801....N2484 (NPN)..... hfe=175 & 185
.....replaced with: KSC1845..... hfe=414 & 414
Q702/Q802....RCA1A10 (PNP)..... hfe=40 & 46
.....replaced with: TTA004B..... hfe=179 & 183
Q703/Q803....RCA1A09 (NPN)..... hfe=29 & 60
.....replaced with: TTC004B..... hfe=180 & 184
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