I finally got my parts order from Mouser to get my DH-110 back in play.
Original symptoms: loud, dangerous crackling when I used the volume control. Teardown revealed complicated ALPS volume control. Visual inspection of the board revealed "pillowing" in the tops of the power supply caps. Also revealed was that two of the heat-sinked BJTs on the board were missing their heat sinks.
I purchased Nichicon audio grade parts to replace the supply caps. The new caps are practically tiny. The filter caps the size of the old bypass caps, and the new bypass parts require magnification to work with. And thats after bumping up to 105c from the original 85c parts. Every kit I see online for the preamps use caps the same physical size of my original parts.
Thats the first sign I did something wrong. But typical of me, I forged ahead.
I installed the new caps, making sure I had the polarity right. I replaced the missing heat sinks. I also tore down the ALPS rotary volume and treated it and the other pots with D5.
With everything installed, I plugged the preamp in and hit the power switch. Two red lights appeared on the front panel. Success? That doesn't seem right.
I reassembled the preamp and took it upstairs to try out on a system I'd just thrown together for kicks. With it plugged in to everything, I hit power and.... nothing. No red lights. The preamp is dead. Ah, thats more like it! If I actually fixed something the world might start rotating backwards.
Interestingly, if I crank up the volume knob I got a very faint crackle out of the left channel, and I could get glimpses of the music coming out of the CD player.
DMM shows 125v at the transformer and at the volume switch. What do I look at next?
Original symptoms: loud, dangerous crackling when I used the volume control. Teardown revealed complicated ALPS volume control. Visual inspection of the board revealed "pillowing" in the tops of the power supply caps. Also revealed was that two of the heat-sinked BJTs on the board were missing their heat sinks.
I purchased Nichicon audio grade parts to replace the supply caps. The new caps are practically tiny. The filter caps the size of the old bypass caps, and the new bypass parts require magnification to work with. And thats after bumping up to 105c from the original 85c parts. Every kit I see online for the preamps use caps the same physical size of my original parts.
Thats the first sign I did something wrong. But typical of me, I forged ahead.
I installed the new caps, making sure I had the polarity right. I replaced the missing heat sinks. I also tore down the ALPS rotary volume and treated it and the other pots with D5.
With everything installed, I plugged the preamp in and hit the power switch. Two red lights appeared on the front panel. Success? That doesn't seem right.
I reassembled the preamp and took it upstairs to try out on a system I'd just thrown together for kicks. With it plugged in to everything, I hit power and.... nothing. No red lights. The preamp is dead. Ah, thats more like it! If I actually fixed something the world might start rotating backwards.
Interestingly, if I crank up the volume knob I got a very faint crackle out of the left channel, and I could get glimpses of the music coming out of the CD player.
DMM shows 125v at the transformer and at the volume switch. What do I look at next?
I would start measuring the PSU outputs. Start with the basics. Follow the circuit.
Are there regulators 78X LM3XX ? correct Voltages coming out ? BJT's insulated from the heatsinks if the same or bolted to chassis ? Rail Voltages at the op-amps.
Are there regulators 78X LM3XX ? correct Voltages coming out ? BJT's insulated from the heatsinks if the same or bolted to chassis ? Rail Voltages at the op-amps.
The power supply has two regulators: LM337 and LM317. Both of them come off the filter caps with pin 1, it looks like pin 2 has some Zeners to ground, and pin 3 exits to a voltage divider, a diode, and then out past a polycarbonate cap. Since pin 3 goes to the rail on each side, I guess I should check that pin to ground?
On the off chance that I might figure that part out for myself, I just checked those regulators with the DMM. I figured I'd check to the other pins first, and then to a convenient ground. Since there wasn't one listed, I used the phono ground. Probably a bad idea, but thats how I learn.
LM317 (POS):
pin 1-3 48.59vDC
pin 2-3 1.237vDC
pin 1-2 49.83vDC
LM337 (NEG):
pin 1-3 24.36vDC
pin 2-3 1.233vDC
pin 1-2 25.59vDC
Apparently something isn't right on that positive side, because the rail voltage should be at 23.2 according to the schematic, or I would assume somewhere near what I have on the NEG rail. Would that excessive DC voltage be the result of a failed resistor?
The only op I found was listed as IC3, an LM 393 "dual comparator". I haven't studied opamps yet. There's 8 pins on that DIP, and the pinouts on the data sheet are meaningless to me so where do I test?
I have no idea what you're talking about re: the heatsinks. why would transistors be insulated from heat sinks? Doesn't that go against the purpose?
Most power devices must be electrically insulated from the heat sink with a special pad made of ceramic,
mica, kapton, or other suitable material. The metal tab, or back plate, of the device is frequently internally
connected to one of the electrically hot terminals, so the heat sink must either be isolated from ground
and left floating, or else grounded and the device isolated instead. The latter is best for noise and
mechanical concerns.
These bipolar are TO-92 devices, just a 3 lead package floating over the board. I used these heat sinks, because they're the closest I can get to what was already there:
An externally hosted image should be here but it was not working when we last tested it.
Can You disconnect at + - ? X Z It would be easier to test the supply.
Your ground is A or P. So, one DMM probe on it for measuring voltages. It seems You have both secondary voltages on the positive side. Is the center ground connected ? Are there shorts on the PCB after the recapping ? Capacitor inverted ? Bad trafo connections or shorted ? Are You sure the caps are correct ? I mean should be something in the 35-50V range & 470 to 2000 uF
Measure A-B & A-C on AC scale should be identical. Now set DC scale & measure after bridge at each filter capacitor. Then at each output pin of the regulators at C35 & C37.
R51 & 53 set the voltage with / stabilized by the zeners. Usual arrangement is a pot to ground.
Check D5 & D6 back protection diodes (remove one leg power off)
Measuring the zeners while powered should give You also the same 22V reading on both.
Between Pin 8 , 4 & ground of IC3 should give same voltage reading.
Replace with regular TO220 regs, since they are easier to find.
Check for C37 or D7 or even C35 shorted with power off.
Your ground is A or P. So, one DMM probe on it for measuring voltages. It seems You have both secondary voltages on the positive side. Is the center ground connected ? Are there shorts on the PCB after the recapping ? Capacitor inverted ? Bad trafo connections or shorted ? Are You sure the caps are correct ? I mean should be something in the 35-50V range & 470 to 2000 uF
Measure A-B & A-C on AC scale should be identical. Now set DC scale & measure after bridge at each filter capacitor. Then at each output pin of the regulators at C35 & C37.
R51 & 53 set the voltage with / stabilized by the zeners. Usual arrangement is a pot to ground.
Check D5 & D6 back protection diodes (remove one leg power off)
Measuring the zeners while powered should give You also the same 22V reading on both.
Between Pin 8 , 4 & ground of IC3 should give same voltage reading.
Replace with regular TO220 regs, since they are easier to find.
Check for C37 or D7 or even C35 shorted with power off.
Unfortunately the project took a turn for the worse...
One of the first things I was told when I was getting set up was buy grippers for the DM probes, and use them. I had them pulled off to test something else and in the heat of battle I forgot to put them back on and ended up shorting out the neg regulator. It now reads fractions of a volt instead of the perfect reading I got before.
So when you were talking about replacing with TO220, were you talking about the IC1/2 or about that dual op IC3?