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SSE left channel dead....

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Didn't really expect any problems with my recently finished SSE (KT88Triode), but this one has got me stumped. I have no output from the left channel. The right channel plays loud and clear.

This is what I have done so far for troubleshooting:

1. Measured: B+ @424, Plate @ V21=418v, R27=40v, R20=1.76v
2. RCA inputs and speaker terminals are correctly wired with no shorts or opens. Audio source (CD player) has L & R signals.
3. Tested both Hammond 125 ESE OPT's on right (T2-PRI) channel, and both work. Neither one will work on the left (T3-PRI) channel.
4. Exchanged KT88's l-r. r-l; no change. Both tubes are known to be good.
5. 12AT7 is NOS Phillips. I have no tube tester, but both sides light. I assumed finding 1.76v at R20 means things are ok on the front end. I don't know how to run any other tests on the CCS.
6. All resistors and capacitors are spec'd value and installed correctly.
7. All joints on PCB checked, especially around V21.
8. The circuit is basic triode connected; no UL, no NFB, no volume control, no extra switches, wiring etc. Grounding as per schematic. Have aux. motor run cap and Triad choke.

What have I missed? This is not my first tube project and I expected it to work right off. At least it didn't smoke or make funny noises or blow up!

I searched the archives, and other builder's problems were more basic than this one appears to be. So I'm sending out my SOS to the experts, and thanking you for any help.


Right channel only (for now)....
"The Cat"
 
1.76V is about right. I would say it is most likely that you have a short at the RCA jack or at the speaker terminal. When you swapped OPTs, did you leave them on their respective speaker jacks? If so, then you can rule that out. What happens when you measure the resistance at the RCA jacks (with the amp off)? Another possible spot for a short is at or near R25/R26.
 
1.76V is about right. I would say it is most likely that you have a short at the RCA jack or at the speaker terminal. When you swapped OPTs, did you leave them on their respective speaker jacks? If so, then you can rule that out. What happens when you measure the resistance at the RCA jacks (with the amp off)? Another possible spot for a short is at or near R25/R26.

Russ,
Thanks for the reply.
Triple checked the RCA's. They are insulated from the chassis and have the outer shells going to ground on board. The "hot" sides are not shorted or open. Each RCA measures 220K to ground. Speaker jacks have also been checked and rechecked. Nothing wrong with them. R25/26 check out ok.
Frustrating!
 
You said you checked the tube pin solder joints. Did you measure the resistance from the socket pin to the relevant components on the PCB? Sometimes a cold solder joint is not very obvious.

One user a while back had a really weird issue with the coupling caps. I can't quite remember the details, but he sent the board back to George and he found the issue. I think the wires were too large for the holes, or something like that, and the solder joint wasn't complete through the PCB.

Do you own a scope? It would be a easier to find the issue that way.
 
mine too

SSE completed this past Saturday. Pretty much stock parts list. Iron - 6K7VG and Edcor XSE15-8-5K OPT's Had a day and a half of glorious listening. Spent all day at work today looking forward to more. Come home, power up, no left channel. No sparks. No smoke. No noise. Just silence. Swap in known good power tubes, no joy.

One clue. On power up a couple of times yesterday, a low "scratching" noise could be heard, on both channels (I think), for a few seconds. Then everything was nominal.

Bad 12AT7 tube?
 
Could be the 'AT7,or perhaps a crummy solder joint. The solder joints between the sockets and board might get stressed a bit from thermal cycling,re-flowing them is worth a shot.

Way ahead of you. :)
Even though I never make cold joints ;), I went ahead and reworked all the left channel stuff.
Still no joy.

Anyway, ordered a replacement 12AT7 (and a spare). While I wait, anything else I can do to narrow down the list of suspects?

Thanks in advance.
 
I am assuming that with the voltages equal on each channel, nothing is amiss there. I have inspected every connection through that channel and reworked anything that is questionable. Still silent on the left. Lacking the resources to evaluate further, I'm still leaning toward the 12AT7, or perhaps the IC, going south. Replacements are on the way.

Any other thoughts?
 
I'm still leaning toward the 12AT7, or perhaps the IC, going south. Replacements are on the way.

If the IC croaks, it's either going to go open or short. When it's functioning normally, there should be roughly 8~10 mA through each half of the 12AT7. Your check across R23 indicates things are normal. I'd expect it should be about 2.8~3.0 volts DC, but you're close enough. Also check the voltage across R20. It should be ~1.7 volts DC.

Your voltages verify the 12AT7, the IC, and the power tube are conducting normally. I'd guess you've got a short in the RCA input jack, a faulty connection across the coupling cap (C21), or a wiring error somewhere around the output transformer. I suppose you could also have an inadvertent short to ground near R21 or R25.

You state that it used to work once upon a time. Have you tried swapping the output transformers (left/right)? Maybe something horrible happened inside the left one. Swap your speakers while you're at it. Could also be a bad socket, though that seems kinda unlikely.

Did you say you had a 'scope?
 
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Still silent on the left. Lacking the resources to evaluate further, I'm still leaning toward the 12AT7, or perhaps the IC, going south. Replacements are on the way.

OK, if the voltages are identical (or close enough) it is unlikely that the 12AT7 or the CCS chip are bad. Usually a failure in either one will make the plate voltage and the cathode voltage very wrong.


One user a while back had a really weird issue with the coupling caps. I can't quite remember the details, but he sent the board back to George and he found the issue. I think the wires were too large for the holes, or something like that, and the solder joint wasn't complete through the PCB.

Yes, there have been two boards in the history of Tubelab that we couldn't fix here on the forums. In this case the builder used coupling capacitors that had leads too big for the holes in the PC board. The builder drilled out the holes to accept the larger leads. This PC board (and all other non - budget 2 sided boards) uses plated through holes. This means that there is circuit plating inside the holes to connect the circuitry on the bottom side of the PC board to the circuitry on the top side of the board. Drilling out the holes breaks this connection. Is this the case here?

Assuming no board modifications were performed and there is a wiring error, or defective component, we need a plan to find it. The first attack will be divide and conquer. The board has 4 main circuits. There are 2 drivers, and 2 output stages. Lets find out which of the 4 circuits are bad.

Looking at the board from the top side with it oriented so that the "Tubelab Simple SE" printing is right side up, run a temporary jumper wire (solder a wire, or use a clip lead) from the left side of one coupling cap to the other (C11 and C21). It can be on either side of the board, but it must be on output side of the caps. This allows either driver circuit to feed both output circuits.

Connect the board up in the usual manner and test it. Is there sound from both speakers? If yes, the problem is in the input section. If there is still sound from only one channel the problem must be in the output section. Verify this by unplugging each input cable one at a time. The sound should continue from one speaker with either input connected.

If the output section is at fault, the most likely place for a failure is the wiring on the secondary (speaker) side of the output transformer. This is about the only place that won't affect the DC voltage readings. The most likely place for a short is where the speaker connectors go through the chassis.

If the input section is at fault, the usual place is the wiring from the input connections to the volume pot and input connectors. Temporarilly connect the two outer terminals of the input connector on the PC board together. If both channels work, or neither work the input wiring is at fault.

I hope this helps. I will check back as soon as I can to see what happens. I have been stuck at work until 8 PM for the last 2 weeks, and for the next 8 days, then I am out of town for 10 days.
 
Looking at the board from the top side with it oriented so that the "Tubelab Simple SE" printing is right side up, run a temporary jumper wire (solder a wire, or use a clip lead) from the left side of one coupling cap to the other (C11 and C21). It can be on either side of the board, but it must be on output side of the caps. This allows either driver circuit to feed both output circuits.

Neat trick. Wish I had thought of that. I guess I learn something new here every day...
 
Neat trick. Wish I had thought of that. I guess I learn something new here every day...

You can do this on most stereo amps if you include a capacitor in series with your clip lead. You don't want to upset the bias on an amp with adjustable bias if they happen to be set to different voltages. In this case and most cathode biased amps the grids of the output tubes operate at ground potential and can be jumpered together.

This is an effective method for paralleling two tubes to make a PSE mono block. Remove the coupling cap from one tube, and jumper the grid of that tube to the grid of the tube that still has a cap. Presto instant PSE mono block with the Simple SE board. It also works on the Simple P-P, but I have found a better way here. I jumper the two phase splitters together to a common input tube.

Do not jumper the grids together on a Tubelab SE board. The PowerDrive circuits will fight each other. You can remove one 5842 and tie both coupling caps to one driver tube plate.

In all cases you tie the output tube plates together through an OPT that has half the impedance that would be used with a single tube. You can also use two OPT's with their secondaries tied together. You need to run 8 ohm speakers on the 16 ohm tap for a proper match.
 
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