Heyall,
Came across this old thread,
http://www.ssguitar.com/index.php?topic=3575.0 (http://www.ssguitar.com/index.php?topic=3575.0)
While trying to repair an '81 JC-120 I purchased as-is and seems folks in these forums can probably help.
The schematic is here,
http://music-electronics-forum.com/attachments/30030d1407958634-sm_jc-120_11016.pdf (http://music-electronics-forum.com/attachments/30030d1407958634-sm_jc-120_11016.pdf)
CH1 speaker is dead, when I connect a good speaker to CH1 I get a tone but no signal, seems like around 100hz. CH2 works fine, so, everything works except for CH1 and thus Chorus because it requires CH1. Any ideas what I should be checking? I've got some experience with electronics, amps and pedals, built amps from kits and small ones from scratch. Have an older oscilloscope, multimeter and such. Thanks.
Update: Okay, found a couple of bad resistors (R72, R73, R74) and a bad diode (D1) on CH1 and swapped those out, now I get some crackling from that channel but no signal.
Update: Seems like R75 and R76 are bad too, which are fusing resistors -- I suppose I could replace those with non-fusing ones for now?
Update: Replaced R75 and R76, still just crackling from CH1, but softer.
Poweramp hums,,, then likely you have DC on speaker,, :o
if so then remove that speaker before it burns out the Voice Coil like the original one probably did.
Now set DMM to DCV and note the *voltage at speaker output terminals* it should be very close to Zero,, If not the power stage is faulty. (100mV either side of Zero is normal)
These poweramps are Direct Coupled and generally if one part blows then it can destroy others.
you need to establish the fault before replacing parts which may just burn again.
Wait for the teck chaps to post with more details. :tu:
Phil.
Quote from: phatt on September 13, 2016, 03:19:12 AM
Now set DMM to DCV and note the *voltage at speaker output terminals* it should be very close to Zero,, If not the power stage is faulty. (100mV either side of Zero is normal)
Speaker output leads of the the bad channel, CH1, read 45 VDC. Outputs on the good channel, CH2, read 0.1 VDC.
Ouch, yes it's latched up so now we wait until one of the more qualified folks see your post and you obviously have the right tools so with some help I have no doubt it's not hard to fix. :tu:
I'm just the hobby geek around here but I'd guess that at least one transistor has died.
Meantime you might need a light bulb limiter.
http://www.ssguitar.com/index.php?topic=2093.0
You will need that to test the replaced parts are in fact working.
These are high current DC circuits and the light bulb (Current limiter) will stop instant destruction at turn on if something is not right.
Phil.
Quote from: phatt on September 13, 2016, 08:55:20 AM
Meantime you might need a light bulb limiter.
http://www.ssguitar.com/index.php?topic=2093.0
You will need that to test the replaced parts are in fact working.
These are high current DC circuits and the light bulb (Current limiter) will stop instant destruction at turn on if something is not right.
Phil.
Yeah, I'll totally whip up one of these, I'd heard of them but never realized how simple it is until now.
A bit hazy on transistor testing, but the closest transistors seem like they've bought the farm as well.
(Q14) 2SC1624
(Q15) 2SA814
The A814 shows OL on all combinations with the DMM and the C1624 reads a variety of low resistances (200ohms and less) across all leads back and forth. Neither of these results seem right for a transistor, could someone confirm that? From what I understand from this thread,
http://music-electronics-forum.com/t16431/ (http://music-electronics-forum.com/t16431/)
Those are the driver transistors. A lot of that thread is over my head, could someone with a bit more understanding of transistor specs suggest available replacements for these? Cheers.
Ok the tecks are busy with other stuff,,,well here's what an amateur hobby guy with limited skill and no Scope would do. :lmao:
Fair chance Q11 & Q12 are ok so I'd leave em for now. I'd just replace the other 5 Q's and be done with it.
Checking D1,2&3 are ok then power up through the lightbulb limiter (No Speaker yet)
If the light Stays bright then something is pulling too much current.
If the light goes bright then dims then you are getting close,,,check voltages.
The basic idea with these circuits is to keep the bases of Q11 & Q12 and the Output at close to Zero Volts DC. It's never perfect but for guitar amps it's all you really need.
Remember you have a working amp right beside this one to cross reference voltages so should be easy to workout if something is amiss. :tu:
As long as the replacements are the same ratings or better then it will work.
i.e. that 2SC1624 transistor data reads; 120Volt/15Watt/1Amp So something at least those ratings will work.
Try these pages ; http://alltransistors.com/transistor.php?transistor=12170
I think there are also cross reference pages,, others might know better links.
I use Addblock so I can't see all the other stuff.
Hope it helps,, Phil.
Replaced Q14, Q15, made a light bulb limiter. Powered it back on and saw some smoke coming off, I think, the large inflammable resistors (R77, R78, R83) saw a spark, along with audible snap from one of the output transistors and measured 25 VDC at the CH1 speaker output. Bulb in the limiter is 60 W, it lit up, stayed lit up, but didn't blow. That blueish/white glow at the output transistor in the lower portion of the photo is the spark.
The bulb should never "blow".
Unless the filament has reached end of life and simply burns out.
Then nothing works, bulb or amp.
Remember, all power to the amp passes thru the bulb filament.
At most a dead short in the amp causes the bulb to glow with it's "normal" brightness.
Think of it like this, the bulb is normally plugged directly into the mains.
By putting it in series with the amp you have simply added the amp into the circuit with the bulb.
So if the bulb can normally withstand full mains voltage, why would it blow with the amp in series with it, even if the amp has a fault?
I normally start with a 25W bulb for SS amps.
For the rest of your post I'll let the pro's speak. :)
Quote from: galaxiex on September 15, 2016, 09:34:27 PM
The bulb should never "blow".
Unless the filament has reached end of life and simply burns out.
Then nothing works, bulb or amp.
Remember, all power to the amp passes thru the bulb filament.
At most a dead short in the amp causes the bulb to glow with it's "normal" brightness.
Think of it like this, the bulb is normally plugged directly into the mains.
By putting it in series with the amp you have simply added the amp into the circuit with the bulb.
So if the bulb can normally withstand full mains voltage, why would it blow with the amp in series with it, even if the amp has a fault?
I normally start with a 25W bulb for SS amps.
For the rest of your post I'll let the pro's speak. :)
Not sure exactly what's wrong with the amp, so I wasn't sure what to expect from the bulb. The amp being a 120W and the bulb being 60W I thought blowing was a possibility.
I cleaned the power transistor that sparked, tested it, seems fine. Put it back in and now with the bulb limiter the amp won't turn on, not even the indicator light, though the bulb lights up and stays lit. When tried without the limited (yes, dumb I know) it blows its fuses. So it seems cleaning the corrosion off of the power transistor leads has exacerbated the short.
From that picture; If that arc is constant then your molex plug has corroded and is now Arcing inside the plug. very bad!!! Cleaning won't bring the transistor back to life,,,
Washing a blood stained shirt won't fix a bullet wound :crazy2:
As I said Before ,, Your output transistors are likely dead and if you are unsure how to test them then just replace all 5 actives and save yourself the fussin. 8)
Then retest with light bulb first, and no speaker until it's stable.
Oh and the molex plug will need to be replaced as the arcing will have destroyed the contact inside by now.
That may have been what killed it in the first place. 8|
Phil.
Quote from: phatt on September 16, 2016, 08:37:37 AM
From that picture; If that arc is constant then your molex plug has corroded and is now Arcing inside the plug. very bad!!! Cleaning won't bring the transistor back to life,,,
Washing a blood stained shirt won't fix a bullet wound :crazy2:
As I said Before ,, Your output transistors are likely dead and if you are unsure how to test them then just replace all 5 actives and save yourself the fussin. 8)
Then retest with light bulb first, and no speaker until it's stable.
Oh and the molex plug will need to be replaced as the arcing will have destroyed the contact inside by now.
That may have been what killed it in the first place. 8|
Phil.
The arc is not constant, I think since I replaced some components it's changed where the short it occuring. I didn't clean the transistor to fix the transistor, only to get rid of debris and corrosion that may have contributed to the short. There was some sort of corrosion on the case around that molex, and one of the leads on the transistor seems a bit corroded, that's why I took it off and cleaned the case and the transistor. The transistor tests fine, as far as I can tell.
I'll test the rest of the output transistors, by output you mean Q1, Q2, Q13, Q14, Q15? Trying to squeeze in this troubleshooting after work, after the kid goes to bed doesn't get many transistors changed out nightly ;)
It seems indicative that now that I've replaced D1, R72, R73, R74, Q14 and Q15 the fuses pop right away. Not sure what that's indicative of, but seems telling?
Yes all the transistors except the 2 input devices. :tu:
Q 13,14 & 15 may Not have survived this last attempt and they may now also be stressed. up to your testing.
I'd bet my last dollar at least one of the output transistors is dead shorted.
If it were me; I'd start again and replace all 5 Q,s.
Yeah must be hard to focus if you have family to attend to as well. :(
Phil.
It is possible the output transistors have been or were originally damaged but look at the DC voltages at the base, emitter and collector of Q1-4 (wouldn't hurt to look at the same junctions of Q14-15. See this link if you need some help in understanding how to trouble shoot transistor circuits: http://www.allaboutcircuits.com/worksheets/bjt-amplifier-troubleshooting/ (http://www.allaboutcircuits.com/worksheets/bjt-amplifier-troubleshooting/)
I sometimes use the LBL to help establish what is getting hot in the circuit: a cheap IR thermometer is a good tool to use and scanning the board before and after power is applied sometime gives a clue as to what is not happy on your board.
Quote from: gbono on September 16, 2016, 04:01:28 PM
It is possible the output transistors have been or were originally damaged but look at the DC voltages at the base, emitter and collector of Q1-4 (wouldn't hurt to look at the same junctions of Q14-15. See this link if you need some help in understanding how to trouble shoot transistor circuits: http://www.allaboutcircuits.com/worksheets/bjt-amplifier-troubleshooting/ (http://www.allaboutcircuits.com/worksheets/bjt-amplifier-troubleshooting/)
I sometimes use the LBL to help establish what is getting hot in the circuit: a cheap IR thermometer is a good tool to use and scanning the board before and after power is applied sometime gives a clue as to what is not happy on your board.
I have a cheap IR thermometer -- I'll give that a go :)
Replaced Q1, Q2.
Q14 is bad, but it's 2S1628-Y, any chance I could replace that with one of the 2sC1624's I picked up? At least temporarily?
http://alltransistors.com/transistor.php?transistor=12170 (http://alltransistors.com/transistor.php?transistor=12170)
http://alltransistors.com/transistor.php?transistor=12177 (http://alltransistors.com/transistor.php?transistor=12177)
Otherwise I have to order some parts. The local shop didn't have any 2S1628-Y.
I swapped in a 2SC1624 for the 2SC1628-Y temporarily. Looks like Q1, 2SD845, an output transistor? is completely shorted. Any suggestions for a replacement? Does that mean I have use a new matching pair? Once I pulled Q1 the amp powers up with the LBL and CH2 plays, I heard another snap, I'm guessing from Q2 again, but it still tests fine.
Update: Seems like a Sanken 2SC2921-ND will work for the 2SD845, though a little pricey at $9 per. Still trying to find a replacement for the C1628-Y, NTE190 is supposedly equivalent but the only source I've found wants $21, yeesh.
Because you have fly leads to the power transistors you are not limited to using the same case. 8|
You can simply remount any flat pack power device that has the *same or higher ratings*.
You may need to drill a couple of holes to accommodate the new devices but not a problem as you have plenty of space. TO264 or similar flat devices should not be hard to find.
Your 2SD845 is an NPN, 120Watt, 150Volt, 12Amp device.
Might be overkill but 2SC5200 is an NPN flat pack 150Watt, 250Volt, 17Amp device but I have no doubt it would work. Mouser stock them for $3~$5 depending on brand.
an Yes Wise to replace *Both* Output transistors.
You may have to cut those molex connectors in the process but they are obviously failing anyway. Revert to the old block connectors if you can't find anything suitable.
re the other Tr's;
If you can't get 2SC1628 then use 2SC1624, *AFAIK* it will make little difference.
Q13 is the *Voltage Amplifier section* (VAS) it swings the largest signal then the rest just do the heavy lifting and convert that into current so it can drive the speaker.
Yes *gbono* is onto the thermal issues,, a good manual feedback with ye old finger burn test tells you if something is running too hot,, :lmao:
Of course making sure you are not touching anything that has dangerous voltage lurking. 8|
If something runs very hot at idle then it's bound to blow up at full power. :(
If it all goes south you can always throw in a LM3886 power amp chip,, just another option for you. :tu:
Phil.
Replaced both output transistors, Q1, Q2 with Sanken 2SC2921-ND's. Things were still being wonky. Pulled all the transistors off the board and tested them. They all tested good so I soldered them back in place. Now CH2 plays fine, the LBL bulb lights up then dims, voltage at CH1 output is 0.01 VDC or so -- in other words everything seems fine but I'm getting no sound out of CH1. I have to test that speaker, will report back when that's done. Feels like I'm getting close to cracking this nut.
Speaker seems alright. When I measure resistance across the good CH2 speaker outputs where they connect to the board I get an expected 7 ohms and change, when I measure resistance across the problematic CH1 speaker outputs where they connect to the board I get a slowly climbing and falling resistance that climbs and falls, 100 ohm, 200, 70, 50. Does that mean anything to anyone?
You have a dead speaker. :'(
Ideally you need to disconnect the speaker from the amp and Then measure the resistance.
With your meter connected by clips on the speaker terminals wiggle those braided wires that go from the speaker terminals across to the Cone. They sometimes fray and give intermittent signal.
The braids tend to break right next to the solder join and I've saved a few otherwise dead speakers by simply resoldering.
If the coil is burnt out which is probably what has happened (DC on the output) then I'd say the coil has burnt out in which case you need a recone or a new speaker.
Meantime you can use the working speaker to test if the repair passes signal,, as long as there is no DC on the output.
Phil.
The speaker works fine when used on the working channel and measures 7.5 ohms, so I don't think it's that.
Though I just tried it again and I get DC hum on the good channel when a speaker is plugged in on the bad channel.
It's changed again, now the bad channel is measuring 20 VDC at the speaker outputs with no speaker.
Oh dear :o
I think we need better minds to look at this one.
Enzo, JMF, Teemu, anyone care to comment? xP
As I'm sure you know I don't wish to turn substatica's Amp into landfill with wrong advice. ;)
Phil.
Quote from: phatt on September 21, 2016, 08:40:30 AM
Oh dear :o
I think we need better minds to look at this one.
Enzo, JMF, Teemu, anyone care to comment? xP
As I'm sure you know I don't wish to turn substatica's Amp into landfill with wrong advice. ;)
Phil.
Thanks for all your help. I'm pretty confident that the transistors are all good now. I guess I'll go over all the resistors and caps on that board again.
Is the problem in the preamp or the power amp section? It appears that the problem was ch2 and now it's ch1??
If you don't have a scope you can make a "signal chaser" https://www.youtube.com/watch?v=IEcyDzYAsuU (https://www.youtube.com/watch?v=IEcyDzYAsuU)
Quote from: gbono on September 21, 2016, 03:37:02 PM
Is the problem in the preamp or the power amp section? It appears that the problem was ch2 and now it's ch1??
If you don't have a scope you can make a "signal chaser" https://www.youtube.com/watch?v=IEcyDzYAsuU (https://www.youtube.com/watch?v=IEcyDzYAsuU)
Pretty sure the problem is with CH1, it only affects CH2 when the CH1 speaker is plugged in and the short continues over to CH2 -- if that makes sense. If there's no speaker plugged into CH1 then CH2 works perfectly fine. I have a used a signal probe/chaser on effect pedals but I was wary of using it with such high voltages around, I don't want to fry another amp trying to test this one. I do have a scope, an old Tektronix, but I'm not fully proficient at it.
Using a signal generator I ran a square wave through and it seems to lose the signal (compared to CH2) at R63, in other words, past Q11 and Q12, which I guess I'm going to test again.
Replaced Q11 and Q12 and I still lose the signal at R63, it may be lost before that, but I definitely have it at R62. See attached portion of the circuit. Any thoughts?
Update: Seems I lose the signal at the emitter of Q11, R64 tests alright, C34 was OL so I replaced it with a 12pf I had on hand, no change, D3 tests alright.
What signal do you have at the collector of Q13?
I'd Check that R61 (68k) is in fact going to ground.
The Base of Q11 needs a common (ground) reference, otherwise the circuit won't work.
Verify Common at R68 also.
Phil.
Quote from: phatt on September 22, 2016, 07:12:35 AM
I'd Check that R61 (68k) is in fact going to ground.
The Base of Q11 needs a common (ground) reference, otherwise the circuit won't work.
Verify Common at R68 also.
Phil.
R61 measures 67k with 0.2 ohm to ground, R68 measures only 500 ohms when it's a 1k resistor so I'll swap that out -- again 0.2 ohm to ground on the ground side. I swapped out R68, no change.
Quote from: gbono on September 22, 2016, 02:38:02 AM
What signal do you have at the collector of Q13?
No signal at Q13, the scope loses trigger and the line drops off.
Seems like it may indeed be a grounding issue. When the pots are secured to the chassis the behavior changes, and that strikes me as something that shouldn't happen if the board is properly grounded.
Quote from: substatica on September 22, 2016, 09:52:04 AM
Seems like it may indeed be a grounding issue. When the pots are secured to the chassis the behavior changes, and that strikes me as something that shouldn't happen if the board is properly grounded.
I agree it is likely part of the issue,,,When I said check ground/common at R61 & R68 Check they are all connected to each other. You should have continuity between all points that go to common.
Then,,, check DC voltages at points shown.
As you have had bad connections, drifting resistors then check resistors at those nodes for drift or open circuit.
R63 & R65 supply power to the emitters of Q11 & Q12.
Obviously if no voltage at emitters then the amp won't go.
Use the working amp as cross reference.
Quote from: phatt on September 22, 2016, 09:56:58 AM
I agree it is likely part of the issue,,,When I said check ground/common at R61 & R68 Check they are all connected to each other. You should have continuity between all points that go to common.
Then,,, check DC voltages at points shown.
As you have had bad connections, drifting resistors then check resistors at those nodes for drift or open circuit.
R63 & R65 supply power to the emitters of Q11 & Q12.
Obviously if no voltage at emitters then the amp won't go.
Use the working amp as cross reference.
I'll do that tonight, yes, I've been comparing with the working channel which is a great resource.
I get 0.6 VDC coming out of R1, 16 VDC out of Q1 and 25 VDC coming out of Q2, not the output Q1/Q2, the 2SK117's near the inputs, that's very early in the signal path no?
Replaced all the electrolytic caps on that board, C32 was definitely burnt, but replacing it (and the others) hasn't changed the behavior.
R61 and R68 are connected and have 0.5 ohms to ground, same on the good channel.
Please forget the input section as it obviously works.
Just fix the power amp *First*,,, don't mess with other stuff until you have that pwr amp working otherwise it's all just way too hard to follow. xP
The idea is to find the problem ,,Then replace as needed, just replacing parts will just complicate the situation.
Use a radio output or Signal inject into the *Main in* socket thereby bypassing any other issues that may cause confusion.
If C32 (power amp) is burnt then check the voltage at ***R65, C32 and R63. Then check you have voltage at the emitters Q11 & Q12.
Cross reference them with the working amp.
If voltages are ok and still no signal at base of Q11 then check the pinout of the transistors,, you may have replaced them without checking the pinout and that will stop it dead. 8|
All done with light bulb until you have it stable.
Please don't change anything until you establish what voltages are present in the circuit. :trouble
Phil.
R65 reads 30 VDC, C32 reads 0 VDC on the negative side and 30 VDC on the positive, R63 reads 6 VDC on one side and 30 VDC on the other.
I've also verified that the pinouts are the same Q11 and Q12, (replaced 2SA970GR with 2SA970GR) they're in the correct orientation and their connections to the next components in the circuit are good. Could it be that Q11 went bad as soon as I replaced it due to the bad cap on C32?
Yeah,,, BUT How do those readings compare with the working amp voltages?
I assume you mean that the 6 Volt side of R63 is the Emitters of Q11 & Q12?
6VDC on the emitters seems high,, I'm guessing it should be less that 1Volt.
If Q11&12 are working you should have about ~100mV on the bases of those two transistors.
Phil.
Quote from: phatt on September 23, 2016, 08:33:32 AM
Yeah,,, BUT How do those readings compare with the working amp voltages?
I assume you mean that the 6 Volt side of R63 is the Emitters of Q11 & Q12?
6VDC on the emitters seems high,, I'm guessing it should be less that 1Volt.
If Q11&12 are working you should have about ~100mV on the bases of those two transistors.
Phil.
CH1 (bad)R65 30 VDC
C32 0 / 30 VDC
R63 6 / 30 VDC
CH2 (good)R65 31 VDC
C32 .001 / 32 VDC
R63 .8 / 30 VDC
Then Q11 & Q12 are likely the problem. Check the pinout, the replacements might be different to the originals.
Phil.
Quote from: phatt on September 23, 2016, 08:59:49 AM
Then Q11 & Q12 are likely the problem. Check the pinout, the replacements might be different to the originals.
Phil.
Unfortunately the pinout is the same and the transistors are all good (old ones and new ones). Haven't found any additional failed components on that board.
Then print out that power amp schematic you posted and write down all the voltages from the working circuit and then cross reference those voltages with the faulty circuit. Somewhere in that first section you have a failed part or broken track or even a cold solder joint.
Hairline cracks in the copper tracks can be right next to the solder and are hard to see. Bright sunlight and a magnifying glass will help.
Could even be a tiny fray of wire bridging a track or solder point which can't be seen by eye, I've had that happen before.
A good gentle scrub of the track side with a tooth brush can dislodge those offending little critters. :tu:
Phil.
Quote from: phatt on September 24, 2016, 07:12:15 AM
Then print out that power amp schematic you posted and write down all the voltages from the working circuit and then cross reference those voltages with the faulty circuit. Somewhere in that first section you have a failed part or broken track or even a cold solder joint.
Hairline cracks in the copper tracks can be right next to the solder and are hard to see. Bright sunlight and a magnifying glass will help.
Could even be a tiny fray of wire bridging a track or solder point which can't be seen by eye, I've had that happen before.
A good gentle scrub of the track side with a tooth brush can dislodge those offending little critters. :tu:
Phil.
I've done the toothbrush thing, but yeah there's been a lot of debris with all the de-soldering. I'll try more voltage comparisons as you suggest.
Here are the voltages in the area, the bad channel in red, the good channel in green.
Update: Added more measurements.
Ouch,, If all actives have been replaced then a failed Feedback resistor R66 (47k) can cause that problem. If it reads ok then check for broken tracks which connect to the nodes either side of R66.
Phil.
Quote from: phatt on September 25, 2016, 08:37:27 AM
Ouch,, If all actives have been replaced then a failed Feedback resistor R66 (47k) can cause that problem. If it reads ok then check for broken tracks which connect to the nodes either side of R66.
Phil.
R66 tests fine and its connections to adjacent components are good. Attaching a photo of the board as it stands in case there's something obvious that I'm missing.
One of the things I've noticed is that when I test D1 on the bad CH1 board I get 1.8 M voltage drop the wrong way when using the diode test, the correct way is .56, off the board I get OL for the wrong way. On the good CH2 board I get OL the wrong way and .56 the right way. I've substituted a 1N4148 for the IS2473 on the bad board. I've checked the traces around D1 and they all seem fine. Getting a reverse drop on a good diode mean anything to anyone? Apologies if I'm using the wrong wording for the situation.
You need to test diodes out of circuit otherwise other components are in series with the diode giving flawed results.
Well I'm fast running out of possibilities.
You asked earlier if you could use a 2SC1624 for Q13 instead of 2SC1626.
Well now that I look at the actual picture Q13 is not the right package for a 2SC1626. The closest I can find that resembles Q13 in your picture is 2SC1628 which has a different pinout.
The schematic is hard to read,, the 6 could be 8.
So I'm Wondering what is that part? Also what transistor have you used to replace Q13?
The way they screen print transistor placement is usually the flat edge as the front face and I notice Q13 would be back to front,,, Which makes me think it's not right.
I've simulated the circuit with some fault conditions trying to recreate the voltages you posted and found that if Q13 was wired up wrong then it would give similar fault readings.
If all passive parts are ok then it can only be the 7 transistors so at least one has to be faulty.
Phil.
I did test D1 out of the circuit and it's fine. Just in the circuit it's readings differ from the good channel's D1.
As for Q13, the diagram calls for 2SC1628 with EBC pinout, I had asked earlier in the thread (not noticing the pinout) if I could use a 2SC1624, which is BCE pinout. I had hoped a couple days back that this was my issue, however after testing the original and swapping back to it the problem persists. Its orientation matches that of the good channel Q13. Attached in an image of the good channel, CH2.
For now I've ordered some KSC3503 DSTUFS-ND to replace Q13's 2SC1628Y. Different pinout but should be able to make it fit.
Replaced Q13 with the KSC3503, had to put it sideways due to the different pinout -- and CH1 came back to life! So I guess Q13 was bad after all, even though it tested alright. Thanks for all the help everybody, especially phatt!
https://youtu.be/vbaU8HK2h1I (https://youtu.be/vbaU8HK2h1I)
It's got some noise oscillating, but I'll sort that later -- it's got chorus!
Glad to hear you got it sorted out. :dbtu: :dbtu:
Phil.