Concept 2.0 receiver weak left channel

[birchoak]

Just when I thought I couldn't be impressed by another humble-watt receiver, in walks a Concept 2.0 receiver. It is both well-designed and well-made, and despite being near the bottom of the Concept line, no corners were cut in this fantastic machine.

It pulls in perhaps the strongest, richest FM I've yet to hear from a receiver, and that is in my basement without an antenna hooked up! Sound is clean and strong, but in the right channel only. This holds true whether it's FM or TAPE (this guy does not have AUX). I did some common sense troubleshooting and some initial steps to find/solve the problem:

1) Thoroughly cleaned the switches and pots (as best I could--I am reluctant to open up any switches but feel confident I got the pots cleaned good).
2) Tested whether problem goes away in mono. It does not go away in mono.
3) Tried the other speaker terminals. Nope, left channel still very weak and distorted. Problem not in my test speakers, either, as they check out with other equipment.
4) Looked for anything obvious-- burned out, loose, cracked, bulging, etc. Everything looks good--it is a very clean unit and I don't think anyone's messed with it.
5) Tried using it as a preamp and it's perfectly ok as such, so I'm reasonably sure that the power supply, filter caps, and preamp sections are good.

And here's the impressive thing. This humble receiver has so much, um, presence, that it's hard to tell it's only firing on one channel. Only by shifting balance to the left channel can you hear the bad channel. It is very, very weak and distorted, like when you aren't properly locked onto a radio station for example, and does not vary no matter what knobs or switches I manipulate.

It is such a cool receiver that I started recapping it (I know, I know--caps very likely not the problem and certainly NOT the right way to approach repair, but I had a bunch of caps on hand). Replaced the big filter caps, part of the power supply, and every single cap on the R and L power amps. Recapped the teeny PRE/MAIN board clinging underneath the chassis. All caps same capacitance; voltage bumped up maybe 20% on a few caps. Nothing seems amiss and I have yet to find a solder joint that doesn't look good. And oh yeah, it passes a dim bulb test--no problems there.

Any ideas, oh masters of the electron? It is a honey of a receiver and I don't want to see it gather dust in my closet of shame.

 

[birchoak]

I thought it might be either the output transistors or one or more transistors on the power amplifier board. The output transistors are smallish metal cased type (like flying saucers). I'd say the left ones either aren't getting enough power or went bad. But this is an amateur's guess.

 

[birchoak]

Really? No one? It's not that rare a beast, surely!

 

[allan1832]

Have you tried measuring voltages? I can't recall if the Concept schematics show voltages, but even if they don't you could compare left and right for various places in the circuits. That may narrow down your search.

Concepts are awesome, as you've obviously discovered. I wouldnt ever get rid of mine.

Allan

 

[zebulon1]

Can you check the amp settings?
Got manual?
Swap the outputs.

 

[birchoak]

Have you tried measuring voltages? I can't recall if the Concept schematics show voltages, but even if they don't you could compare left and right for various places in the circuits. That may narrow down your search.

Concepts are awesome, as you've obviously discovered. I wouldnt ever get rid of mine.

Allan

I will do that now. Pacific Stereo over at Tapeheads gave me a schematic. Great idea, thank you.

 

[birchoak]

Can you check the amp settings?
Got manual?
Swap the outputs.

Schematic only, no service manual. When you say swap the outputs, do you mean switch speakers?

 

[The Fuxtor]

I had a 2.0 and it always had a problem with a dirty tape monitor switch, took many cleanings to get it sorted. Try fiddling with that and see if the sound cuts back n forth?

 

[birchoak]

I had a 2.0 and it always had a problem with a dirty tape monitor switch, took many cleanings to get it sorted. Try fiddling with that and see if the sound cuts back n forth?

Thanks for the suggestion. I fiddled with it, to no avail. The left channel is barely there!

 

[birchoak]

Have you tried measuring voltages? I can't recall if the Concept schematics show voltages, but even if they don't you could compare left and right for various places in the circuits. That may narrow down your search.

Concepts are awesome, as you've obviously discovered. I wouldnt ever get rid of mine.

Allan

No voltages on schematic, sadly. I poked around under there as best I could with my DVM but I'll be damned if I can find the problem. I do know that the distortion starts before it gets to the left trimmer. Must be a bad transistor? This thing is so well made; it breaks my heart to let it gather dust.

Aforementioned, using it as a preamp = all good. Does this mean the power supply/caps are ok? I'm assuming the problem would show up in the preamp section, too, if that were the case.

 

[The Fuxtor]

Check with headphones?

 

[birchoak]

Check with headphones?

I think I did. I can't remember now. I did check the DC offset and the right channel was -25mV or something and the naughty left channel fluctuates, wildly, from 25mV to 139mV and just about every value in between. I thought maybe the trimmer was oxidized so I blasted it with CRC a bunch of times, worked the trimmer back and forth, repeated the steps. Let it dry. Nope. Not it. I can solder but am not good diagnosing faults like this. Preamp, power supply, filter caps seem good; output transistors measure the same for each channel, signal gets distorted before it reaches the trimmer (I made an audible signal tracer). Has to be a bad transistor to be so weak and so distorted, right?

 

[The Fuxtor]

I think I did. I can't remember now. I did check the DC offset and the right channel was -25mV or something and the naughty left channel fluctuates, wildly, from 25mV to 139mV and just about every value in between. I thought maybe the trimmer was oxidized so I blasted it with CRC a bunch of times, worked the trimmer back and forth, repeated the steps. Let it dry. Nope. Not it. I can solder but am not good diagnosing faults like this. Preamp, power supply, filter caps seem good; output transistors measure the same for each channel, signal gets distorted before it reaches the trimmer (I made an audible signal tracer). Has to be a bad transistor to be so weak and so distorted, right?

If you have both channels using headphones, I would think the drivers are not the problem, I would look to testing outputs and checking emitter resistors ... Check the fuse holder and fuse in that channel, I had a busted fuse holder in my 2.0.
Edit: I see outputs are good.... Check further back , transistors should be all checked

 

[birchoak]

If you have both channels using headphones, I would think the drivers are not the problem, I would look to testing outputs and checking emitter resistors ... Check the fuse holder and fuse in that channel, I had a busted fuse holder in my 2.0.
Edit: I see outputs are good.... Check further back , transistors should be all checked

Thank you for your help. I wish I could slap it and get it working again, like we used to do with old tube television sets. I'll start tracking down the transistors. The good news is that the right and left channels are clearly marked on the amp board, and there's plenty of space to get at things. A beautifully designed receiver, for sure, but there is virtually no service data to be had (aside from the schematic, which has no voltages specified on it).

 

[The Fuxtor]

Compare good channel to bad... Should eventually bring up the bad parts!

 

[birchoak]

Compare good channel to bad... Should eventually bring up the bad parts!

Ok, I have spent quite a few hours with this thing. Pulled a bunch of transistors and they checked operational except one B605 transistor, which would pass current from B to E but not from B to C. So, how do I find a replacement for this little bugger? It says B605 on the front, then L 257 below that. It is a flattened rectangle, not a semicircle case. I'm guessing there are many kinds of B605s out there and I don't want to install the wrong one.

Other things I did: checked the diodes glued to the heat sink (good god, what kind of glue IS that? It's like the Superman of rubber glues!); they're functioning. Re-soldered all joints on the bad side of the amp. Replaced all caps both sides of amp.

 

[The Fuxtor]

Not sure about the available subs for that transistor? It's a pnp , so I would try using a ksa1220...
Look for subs for a 2sb605 transistor and maybe you will find a better match?

 

[birchoak]

Not sure about the available subs for that transistor? It's a pnp , so I would try using a ksa1220...
Look for subs for a 2sb605 transistor and maybe you will find a better match?

Ok, so I'm looking for a 2sb605; that helps. At this point I'm willing to take risks because the receiver is not functioning properly so the stakes are low. Either I fix it = win or I make it worse (not really a big deal because it's not usable right now anyway). Thank you so very much for your advice and not laughing at me (at least not out loud!).

 

[The Fuxtor]

This is no laughing matter!!!

 

[ron wa9rhu]

The concept 2.0 is a straight forward stereo receiver i would bet the tape monitor switches are dirty or the tone defeat button interment. on the back are pre-out /main in jumpers that supply signal to the main amplifiers left and right. a scope would help tracing the loss of signal from the low channel. don't toss this receiver you will be amazed once you get it working. check these things first.you can input a Ipod with a audio cord/rca jacks into the main inputs and see if both amplifiers work turn down the audio on the ipod first. let us know.