Dear all
I would really appreciate your help.
I just purchased a cheap 3020i.
I started by measuring and trying to adjust bias and DC offset. I de-soldered the short across R455 and R456 before adjusting bias to 30 mV.
All 4 trim-pots measure ok and are adjustable. Right channel OK (adjustable to 0 mV DC and plays).
Left channel no sound and DC >600 mV.
The preamp is fine and plays in both channels.
Could you help me fault find? Where do I start? I have a mulimeter and an ESR meter. And I can kinda follow the schematic with some help 🙂
Thank you - Morten / Denmark
I would really appreciate your help.
I just purchased a cheap 3020i.
I started by measuring and trying to adjust bias and DC offset. I de-soldered the short across R455 and R456 before adjusting bias to 30 mV.
All 4 trim-pots measure ok and are adjustable. Right channel OK (adjustable to 0 mV DC and plays).
Left channel no sound and DC >600 mV.
The preamp is fine and plays in both channels.
Could you help me fault find? Where do I start? I have a mulimeter and an ESR meter. And I can kinda follow the schematic with some help 🙂
Thank you - Morten / Denmark
Use junk speakers while working on this to avoid damaging your main speakers.
It's easy to compare the good and bad channels. Be careful to not let the probes slip.
Is the bad channel completely dead, or is there some signal at a low volume?
Try headphones, maybe the switch in the jack is damaged.
It's easy to compare the good and bad channels. Be careful to not let the probes slip.
Is the bad channel completely dead, or is there some signal at a low volume?
Try headphones, maybe the switch in the jack is damaged.
Good point about the junk speakers.
I attached a trash speaker to left channel (after checking that it works on the right one). There is very low sound (sounds distorted also) when I crank up the volume pot.
Normally I would look for relays, but cannot see any.
Headphone switch? As in it might have switched off left channel? Would that give so much DC on the terminals?
I attached a trash speaker to left channel (after checking that it works on the right one). There is very low sound (sounds distorted also) when I crank up the volume pot.
Normally I would look for relays, but cannot see any.
Headphone switch? As in it might have switched off left channel? Would that give so much DC on the terminals?
Output DC may be a separate, distinct problem.
The amp output goes through a switched headphone jack. Sometimes theses switches get dirty or broken.
What happens when you plug in headphones?
402: 0.2 Ohm
401: 2.2 KOhm
Should these be equal? I can see they are breakers?
EDIT: with power off
I will check this out
Update: headphones play in both channels.
When I put in headphone jack sound in speaker dissappears completely.
Last edited:
Ok, since the headphones are connected directly to the amp output,
then the headphone jack or related solder joints seem to be the problem for no sound in the speaker
when phones are unplugged.
Power off, unplug AC power, and inspect carefully. Post some photos.
Take some resistance measurements across the normally closed switch contacts to the speaker.
Try cleaning them if they are reachable. If a standard part you can buy, I would replace the jack.
This seems to be designed performance - plugging in the headphones jack disconnects the speakers.
I will look further at the headphone switch and report back.
Meanwhile: the two thermal breakers referenced above: the one in the left channel has a resistance of about 2,2 KOhm without power and circling around 30-60 Ohm when power is on. Something is wrong here, no? The right channel breaker is 0.2 Ohm with and without power on.
The thermal breakers must be near 0 Ohm for the amp to work properly. Note that bias to Q401 flows via R429.
Yes that was my assumption as well - do I replace the "bad" one? Or is it continuosly tripping because something is wrong somewhere else?
I put my headphones back in. Sound is distorted in them as well. When I unplug left pre->main jumper, there is still sound in both left and right (distorted). When I unplug the right pre->main jumpe, sound is still present in both channels but lower.
As long as you are using expendable speakers, I think you can safely short the defective thermal breaker as a test. With luck, you'll find the 600mV closer to nominal.
Yes - I already tried that as a test earlier. Sound very distorted in speaker. But much more volume.
Now; I could not figure out why headphone sound was now distorted as well (it was not at first test). When I measure BIAS on the TP's there is now 0 mV in both channels (I assume this is why sound is now distorted everywhere)? What happened? I had fine bias earlier (25-30 mV) and pots reacted fine. The pots are still ok.
I'm guessing a bit here--- with breaker shorted and headphones applied, the amp sees a relatively light load, that is the phones + R459. The light load may allow a clean listen.
So there's likely an additional problem. Thermal breaker is definitely failed as well. I suggest an 8 ohm test load and bypass the phone jack's switch to speaker so that you load the amp and phones at the same time. Then probe with your meter, comparing with the good channel.
Good luck. Keep us posted.
So there's likely an additional problem. Thermal breaker is definitely failed as well. I suggest an 8 ohm test load and bypass the phone jack's switch to speaker so that you load the amp and phones at the same time. Then probe with your meter, comparing with the good channel.
Good luck. Keep us posted.
I was musing on the circuit, then I reread your latest post more carefully.
If I interpret correctly, you now have the thermal breaker shorted for test expediency, but now both channels misbehave? If you remove the short across the thermal breaker, do the previous symptoms restore?
Do the regulators (+43, +/-28) behave nominally in both scenarios? Are you able to recover to the previous state?
If I interpret correctly, you now have the thermal breaker shorted for test expediency, but now both channels misbehave? If you remove the short across the thermal breaker, do the previous symptoms restore?
Do the regulators (+43, +/-28) behave nominally in both scenarios? Are you able to recover to the previous state?