Scott 340b no right channel

James Cox

James Cox

AK Subscriber
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So I've been working on a Scott 340b receiever that came with a bundle of tube gear i purchased about a year ago. Ive managed to restore all the other gear but I keep running into problems with the Scott. After recapping (except the cans) it a few months ago everything worked well except i had to hook up the speakers in a weird way to bypass the phase switch. After hooking it back up all the inputs worked well except the phono input had a crazy loud hum. Well after ordering replacement can caps i realized that the screws i used for the bottom cover were just a bit too long and were causing the hum. Figured the cans still needed replaced so i chamged them out and now i have no right channel. Ive checked my work and everything seems correct. No loose connections and all the connections match up with my before pictures. Im stumped and not sure were to go from here. Ive spent the last few days tracing all the wiring from the right speaker output and cant find anything loose. Any suggestions on what my next steps should be? Ive attatched some pictures of my work if anyone wants to take a look.
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Oh, also the green and green/white wires coming from the balance knob, can someone verify that they are hooked up right? When i was recapping one of those wores broke off and im pretty sure i reconnected it properly but i could be off.
 
Man i was really hoping someone would have an idea of where to go from here. I have a question. Say i change a multi cap that is 40/40/40/40. As long as i connect the wires in the same way as before does it matter what tabs i use? Ive went through and cleaned all the switches and pots but still no right channel...
 
the tabs aren't critical, just group the connections how they were. Nothing special about it being a square or a circle or whatever if all the sections are the same value and voltage.

can't help on the specific wiring questions, never worked on one of these but if it worked before and you didn't change anything, I doubt thats the problem.

some random thoughts in no special order that may help narrow down the problem area

Usually when I have a dead channel, the first thing I do is swap tubes side to side just to rule that out.

Would be very possible for the speaker switch to be messed up, and wiring around it would easily prove it out. Try it from the 4 to the 16 ohm tap and see what you get.

try messing with the tape monitor switch if you have not. If thats messed up it can cause no signal on one or both channels. Quick and dirty way of taking that out of the picture is to simply run an RCA cable from tape out to tape in. At that point it won't make any difference what the switch is doing or not doing.


Do the output tubes on the non-working channel bias normally? Not sure what the specs are, but you want to see something at the bias test point (pin 5 on the output tubes), ideally something that is basically the same as what you read at the working channel's test point.

If none of that leads anywhere, jumper the two grids of the first tube in the power amp section together. 340b should be pin 2 on V3 and V103. If you get signal on both channels, the problem is before the power amp. If not, you have something going on in the power amp.
 
I figured on the tabs but wouldnt to double check. I kept all of the wires grouped thesame as they were before changing the cans. I cleaned the tape monitor switch but ill try these trouble shooting tips and see where it gets me. Ill post back with my findings in a couple of hrs. Thanks for the help gadget!
 
James Cox said:
Oh, also the green and green/white wires coming from the balance knob, can someone verify that they are hooked up right? When i was recapping one of those wores broke off and im pretty sure i reconnected it properly but i could be off.
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Which one broke off? Which end broke off? Is the one that broke off the one that is associated with the channel that isn’t working now? Are those shielded cables? If so, are you sure you reconnected the conductor, rather than the shield?
 
Heres my update so far, ive dounle checked the wores from the balance pot and they are shielded. The sheilded out wires are connected to the center of the balance pot and the center wires are connected on each side. Swapping tubes had no effect on the sound. I noticed that if i turn the volume almost all the way up i do get some sound on the right side but faint and distorted. Possibly bleed over from the working channel.
I also bypassed the phase switch and tried all 3 taps with no change. Bias is steady on both channels. I also tried bypassing the tape monitor switch wit no effect
 
As far as hooking up a jumper wire between the first tubes in the power amp let me just make sure im right. Just run a wire from pin 2 pn each of the 6u8 driver tubes? If both channels work then the problem is in the preamp section, otherwise its the power amp right?
 
yup, just a jumper wire. That will force the signal at both channels to be the same. If both channels start making sound then the problem is the dead channel isn't getting anything to amplify. The problem is somewhere before the 6U8 in the preamp section.

If you still get nothing, then the issue is between the 6U8 and the speaker jacks.

if both channels go dead, then most likely the grid is grounded, either at the volume control or at the tube itself but basically whatever it does it will tell us something.

basically trying to break the amp into "blocks" to figure out where to go.
 
Alright so i tried the jumper and the right channel may be just a bit louder but its still very low and distorted. Have to turn it up about halfway before i hear anything at all. So that points to the preamp section then?
 
0k so after some more experimenting the left channel sounds great but the right channel is about 1/4 the volume of the good channel. Also extremely distorted if you turn it up to the point of being able to actually hear it. No bass on the weak/distorted channel. It sounds like a clock radio or something.
 
I swapped the wires for the balance pot to rule that out as well and the problem channel moved to left side on the balance knob but its still in the right speaker at the terminals so that tells me the balance controls are good.
 
James Cox said:
Alright so i tried the jumper and the right channel may be just a bit louder but its still very low and distorted. Have to turn it up about halfway before i hear anything at all. So that points to the preamp section then?
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I'm guessing actually the opposite, that it is somewhere downstream of the jumper wire. if I'm understanding this correctly, that jumper should feed the same signal to the input of both the left and right amp sections as you're basically summing the whole preamp section to mono. (If I'm misunderstanding, someone please jump in and correct me.)

You would THINK that your messing with the balance pot with the results you gave would say I'm wrong, but looking at the schematic, the balance pot is actually in the power amp section, not the preamp section like you'd expect.

Now I really wish that I'd done something that I thought of doing years ago, which was going to be to mark up the schematic with the wire colors, that might actually help you. I only got as far as doing it for the output transformers, however, and then I got busy and stayed busy for several years until last spring I gave up and dumped it on a professional. Who did, in fact, revive it and get it working well so there's that.

Have you tried bypassing the speaker switch and triple checked your bypassing of the phase switch? I'm thinking the latter seriously could be your issue as the phase switch only affects the right channel (and it's also basically right before the speaker terminals/headphone jack - that is, it's switching speaker level audio, not line level). A quick 'n' dirty check would be to simply quickly crank the volume and then reduce it again, that sometimes gets a signal to "punch through" a poor or oxidized connection, most commonly a speaker relay or protection relay, but you don't have those here. If that works, then you know it's a pretty good chance it's a Deoxit fix. If that *doesn't* work, you really haven't learned anything other than you're just going to have to do more troubleshooting...

If you posted previously, I missed it and I apologize, is the issue occurring on headphones too, or have you not tried that? If you did I completely missed it on my skim through the old posts.

Following with interest as I have one of these receivers myself and quite like it, so I'm hoping that you get yours working and find the same.
 
yeah if there was no real change with the jumper, more than likely the signal getting to the grid from the preamp is OK, and the issue is somewhere from the 6U8 to the speaker jacks.

The channel balance pot is in the preamp. The balance control in the power amp is DC balance, which has to do with output tube bias. Completely different thing. Normally this is set for minimum hum, but for now just park it in the middle of travel until the amp works more or less normally.

Do the output tubes bias correctly? Thats the voltage measured from ground to pin 5 of the output tubes. Don't know if this has a top side test point, but the schematic indicates "TP" at that spot. I don't know what Scott specifies for voltage here, but if you read about 1/4 volt (0.25) I expect that would be very much in the ballpark of correct. If its not somewhere in that area, see if adjusting the bias control sorts it.
 
So just a quick update, ive hit all the switches including the phase switch with deoxit a couple of times so i dont think thats causing the issue. The hias is staying pretty steady on both channels. There 2 test points on top of the chasis that i used to set the bias. I wont be able to do any further testing until tuesday but ill report back and let you guys know what i come up with
 
ok so we have current flow through the output tubes, which means they have voltage where it should be. Now we're down to checking voltages on the 6U8. If I had to guess at it, possibly the screen resistor is pooched. Thats the 220k on pin 3, but if the cap at pin 3 is shorted or backwards you'd also get no voltage there. No screen volts means the voltage amp doesn't function and no signal passes. If you don't get about 46 volts to ground at pin 3, look at those parts. Also worth confirming you get reasonable voltages at pins 1, 6, and 7 while you're at it.
 
Thanks for the info! Ill try to run some voltage checks tomorrow and see what i find. The holiday weekend kept me pretty busy
 
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