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1981 Roland Jazz Chorus 120 CH1 Problems - Solved

Started by substatica, September 12, 2016, 06:17:45 PM

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substatica

Heyall,

Came across this old thread,

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

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.

phatt

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.

substatica

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.

phatt

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.

substatica

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.

substatica

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/

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.

phatt

#6
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.

substatica

#7
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.


galaxiex

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.  :)
If it ain't broke I'll fix it until it is.

substatica

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.

substatica

#10
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.

phatt



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.

substatica

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 ;)


substatica

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?

phatt

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.