I have just completed building the K8020, and after figuring how to switch the unit on, I find that there is no output from the right channel. The left channel plays just fine. I’ve tried swapping channels on my power amplifiers to rule them out, but it’s the same deal. Likewise, I have also tried different inputs to no avail.
The curious thing is, when there is no music playing, I can detect a slight hum in the left loudspeaker, but the right one is absolutely silent. Both valves’ heater filament light up (but I didn’t try swapping them). Could I have a faulty relay component? Where do I start trouble shooting this problem?
As both channels are identical, it is easy to follow the signal from input to output and compare levels and signals with the working channel. Be careful when you follow the signal. Sometimes, it can ride on a 300V+ DC voltage !
If you cannot find any obvious reason for the problem such as a defective component or assembly error, examine the board closely. Maybe there is a small soldering bridge somewhere or even an etching error which shorts a track somewhere. Use a multimeter to check for continuity.
Thanks for your initial comments. I swapped valves around this morning, and it’s still the right channel that is dead. So the valves are fine.
I don’t really understand electronics, and I have a natural fear of lethal voltages, so I think I’ll do the passive check for solder bridges first. Taking the built K8020 apart will be a pain but I guess that is unavoidable if I want it to work properly.
If there are no soldering or assembly errors, and it is likely to be a component fault, which one is it likely to be? Something like the output relay? And if it is a component fault (after I have mustered enough courage to prod it with my multimeter), would Velleman send me the replacements or do I have to go to my supplier (Qkits)?
I can tell you I’m not looking forward to the troubleshoot. But thanks for your help so far.
I have disassembled the K8020 to check for errors, but could find no apparent shorts. Looking against a bright light, the tracks and solder joints do not touch each other except where they are supposed to. I even swapped the output relays (between monitor and line-out) on the right channel, but it is still dead. I would trace the input and check for voltages if I knew where to prod. I’ve only just figured out V2 is the right channel, but the circuit board is not symmetrical
I bought my kit from Canada (Qkits) but I live in New Zealand. What is the best way of getting this fixed? Can I send it back to Belgium for you guys to look at it?
You can return it to Belgium (address in manual)
If you do so, don’t return the enclosure or tubes, to make sure they do not get damaged and to reduce the cost.
Before returning it, are you sure you’ve checked everything ?
Can you get local help from a friend who is into electronics?
Did you check if the correct relays get energized ? This is easy to check with a multimeter.
[quote=“VEL417”]You can return it to Belgium (address in manual)
If you do so, don’t return the enclosure or tubes, to make sure they do not get damaged and to reduce the cost.[/quote]
Good idea. Mind you, the heavy part is the transformer. I’ll find out what regular air mail postage costs first. Will you send it back to me at no cost to me?
[quote]Before returning it, are you sure you’ve checked everything ?
Can you get local help from a friend who is into electronics?
Did you check if the correct relays get energized ? This is easy to check with a multimeter.[/quote]
I’ve checked everything physically, but I haven’t dared prod a multimeter into the live circuit yet. How do I check if the relays are getting energised? Can I use the multimeter without taking the PCB out of the lower case? I have a friend who knows a bit about electronics, but I don’t think he knows that much. The local electronics repair shops would probably charge me more than the kit cost me. Let me sleep on it tonight (now past midnight) and I’ll take a final fresh look tomorrow to see what I might have done wrong.
I have the impression that your experience is limited. This is fine, but I cannot recommend to start probing on a live circuit.
Maybe it is better to return the circuit for inspection/ repair.
Warranty only covers parts and labor. Shipping and handling will be charged to your account. At this time, we cannot tell what the cost will be. You will receive an invoice as soon as your kit has been fixed.
Yes, my understanding of active electronics is somewhat limited, but I managed to test the various VDC values and they all came up to spec, so from a power supply perspective, everything was working fine.
I then traced the input into the ALPS volume control. Comparing the resistance between the left and right channel yielded identical results, but the right channel was shorting to ground, which would well explain why there was no output. I tried to see where there was a bridge, but even under a magnifying glass, I could see no obvious shorts. The track leading to the right channel (middle pin) volume control was stripped of its jumper and I removed the volume control to rule out a defective one. So there was no components fixed to this track, yet it was shorting to ground. The left channel was not shorted at all, which I presume is correct as the left channel works fine.
In frustration, I cut the track at the point where the ALPS R-Ch centre pin is soldered (and confirmed that eliminated the short), and ran a hardwire connection to the correct valve pin. Yet the right channel is still dead, which is so frustrating. It’s late now, but I’ll have to keep tracing further downstream for other shorts and see if I can’t resolve the problem. However, I would appreciate any other helpful tips before I throw my K8020 out the window. Have I been unlucky with a dud PCB?
Further to the first etching short I discovered last night, I’ve found another one tonight, which has fixed the problem. The funny thing is, even when looking against a bright light to spot any darkened areas where tracks are not fully etched did not show the short. Light was clearly visible between tracks, so I imagine there was a microscopic bridge. I had to use a multimeter to check each track and I’m now all the wiser not to rely on my eyes to determine discontinuity. Thanks for your help.
I must say that a fully operational K8020 is a very special sounding piece of kit, and much MUCH more fun than an Audio Research LS16 Mk2 I had home recently, which sounded boring in comparison. Keep up the good work and excellent support.
Happy to read you have found the cause of troubles.
We apologize for any inconvenience this etching error might have caused.
This is really very exeptional. The quality of the PCB’s is usually outstanding. Happy listening !
The probelm had me stumped for a while, but when I checked each track for continuity with ground for each channel, it quickly became clear where I had a short. Thankfully it wasn’t a VDC track that was shorting to ground.
Now that the K8020 has been in service for a few weeks, I’m very VERY impressed by its performance. The only issue I have is a slight hum. I have isolated the preamp as far as I can get it from my other electronics, but that doesn’t seem to make a difference. It’s not really bothering me, but I’d just like to know if a little residual hum is normal for the design.
If you look at the specs, the signal/noise ratio is quite high. I do not recall having heard of a hum problem with this pre-amp. Basically, it sounds like a piece of wire. How did you determine that the hum was coming from the pre-amp ? Try the following: disconnect all sources from the pre-amp and listen. If the hum is gone, it is related to a source. Next, disconnect the pre-amp output and short the cable. Hum still present -> power amp is the cause.
Check if the grounding is the cause of troubles. Try lifting power amp, pre-amp or source ground. Maybe there is a groundloop. Solving hum from a distance is difficult, so you will have to experiment a little.