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Author Topic: Roland JC-120: no effects working, any tips?  (Read 746 times)

Cpt. FixIt

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Re: Roland JC-120: no effects working, any tips?
« Reply #15 on: November 06, 2018, 03:37:04 PM »

Reverb: 7-8 measures around 700 ohms when connected, but I guess it has to be disconnected to know for sure?
Take a look at the schematic. When measurung DC resistance, any cap can be considered "no connection". There is no DC path between 7 and 8, so you don't have to disconnect it. 700 ohms is fine, the reverb tank seems to be ok.
Pin 10 does hum, but I could not hear if the hum went into the tank.
My bad, I overlooked that that junction is also the input of the gain stage around Q4, so this is certainly inconclusive.
Tapping the tank with something does give an echo to the signal, but not the guitar signal.
This means that the reverb pickup and recovery amp(Q16/17) are fine.
So there is not that much left,  either driver amp TA7200P does not work or there is a broken trace/solder joint around its output/C49/R96.
15VDC at pin 3 are present?
Chorus: R137 does not give significant hum when touched, some faint crackling can be heard though when touching it.
The connection from R188 to point 20 of the channel 1 PCB looks fine and measures fine.
Ok, so you should check the return path of the chorus first. Does touching one of the legs of R31 give any hum? It is the input of Q19, the switching FET. Touching it and switching the chorus or vibrato in and out should make an audible difference.
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rens

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Re: Roland JC-120: no effects working, any tips?
« Reply #16 on: November 07, 2018, 04:57:33 AM »

Reverb: Pin 3 of IC1 gets 0,6V. Traced that supply back to R152 (through a jump wire); no +15V there either. R152 leads to IC4; leg 13 and 14 should provide +15V but give +0,6V.
That said, I can't trace back +15V to C144 in the supply section (if it's called that way ;) ); the + side of C146 gives 0,6V, - side 0. -15V and -43V and +43v are still present. On the negative terminal of C144/cathode of D14 I measure 0V.
The supply of +15V might be the issue, also disabling the other effects; as far as I can see only the effects paths use +/-15V.

Chorus: Yes, the return path seems fine. R137 hums only when effects are engaged.
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phatt

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Re: Roland JC-120: no effects working, any tips?
« Reply #17 on: November 07, 2018, 07:45:22 AM »

Sounds like you have lost +15 rail which is regulated via 2SC-1624Y.
Then I'd suspect that 2SC-1624Y transistor is fried.
Phil.
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rens

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Re: Roland JC-120: no effects working, any tips?
« Reply #18 on: November 08, 2018, 03:08:03 AM »

I tested the 2SC-1624Y: no signal passes between either of the legs. I guess this means it's fried, although the connections are not 'open' but closed (don't know if this is a common fail, you usually read about blown transistors that pass signal both ways).

Will look for replacement/substitute.
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rens

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Re: Roland JC-120: no effects working, any tips?
« Reply #19 on: November 08, 2018, 09:36:24 AM »

The original transistor is more or less obsolete. Original Toshiba is available on ebay from China, but relatively expensive and of course a long shipping time. What would be suitable alternatives for the 2SC-1624Y?

I found a lot of possibilities on alltransistors.com but because I don't understand what this transistor does exactly, I don't know which values should be exactly the same as the original. Comparison of alternatives with the values I took from the datasheet: https://alltransistors.com/crsearch.php?mat=Si&struct=NPN&pc=15&ucb=120&uce=120&ueb=5&ic=1&tj=150&ft=10&cc=60&hfe=70&caps=TO220&r1=0&r2=0&r1r2=0.

Can anyone help me with this?
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phatt

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Re: Roland JC-120: no effects working, any tips?
« Reply #20 on: November 08, 2018, 08:42:47 PM »


The 2SC-1624 data sheet reads;
Vcbo = 120Volts    (Vcbo = Collector Base Voltage MAX)
Ic = 1Amp            (Ic = Collector Current MAX)
Pc = 15Watts        ( Pc = Collector Power Dissipation MAX)
Don't worry too much about all the other data those 3 are generally all you need to know.


So any NPN TO220 package that reads those numbers or higher will likely do the trick But do remember to check they have the same pinout. 8| You may need to replace the Zener as well.

The +15 rail pulls more current than -15 rail, hence the 2SC-1624 circuit is added as a current booster.
Phil.
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g1

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Re: Roland JC-120: no effects working, any tips?
« Reply #21 on: November 08, 2018, 11:30:38 PM »

Phil, are you using the schematic from the first post?
I'm a little lost as to the regulator you are discussing?
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rens

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Re: Roland JC-120: no effects working, any tips?
« Reply #22 on: November 09, 2018, 03:24:50 AM »

g1, part of the schematic with this transistor on it is attached to this post.

Thanks Phil, I will look for a replacement with those values.
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phatt

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Re: Roland JC-120: no effects working, any tips?
« Reply #23 on: November 09, 2018, 06:09:00 AM »

Hi *rens*, Yes that is the transistor I'm speaking about. :tu:
It's likely mounted on the heat sink.

Hi *G1* yep the first post is the schematic,, And that transistor has no part number but D6 zener connects to its base.
I'm also a little confused as to how it all comes together as the Collector of 2SC1624 is drawn connected to the 43V rail (Hollow Arrow) while emitter is drawn to the +15 rail (Solid Arrow). Emitter also connects to pin3 of IC1 (the reverb driver chip) and also C50 (100uF/25V)

Then cast your eye down to the main secondary were you will find C144 (1000uF/16V) but note the + side of C144 is also connected to the +15 rail (solid arrow). I note *rens* mentioned he could not find a path on the pcb that connects those 2 points so maybe a drawing mistake, Who knows?

So mistake or not, I figure the +15 volt rail comes from the Emitter of 2SC-1624 so as there is no DC voltage at the Emitter it's a good bet that Tr 2SC1624Y is dead.
But hey I could be missing something lol.
Phil.
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rens

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Re: Roland JC-120: no effects working, any tips?
« Reply #24 on: November 09, 2018, 06:36:34 AM »

Hey Phil, by saying "can't trace +15V back to C144" I meant I did not measure +15V at C144 which it should originate from. The positive terminal of C144 is jumped to point 6 on the PCB, which leads to the emitter of 2SC-1624Y. (see page 2 of the schematic, it's jump 6(point 6) to X, a point on the PCB that's directly connected to C144 + terminal). The jump is directly connected to point 6 (so the transistor emitter and C50) and IC1 leg 3.

Help me understand: the +15V rail is provided by the ps section C144, right? the +15V supply is then amplified by 2SC-1624Y which uses the +43V for this? (I don't know any semiconductor theory, sorry for that). So if the transistor isn't working, there should be +15V but not enough for all the current in the circuit that uses +15V? Because I only measured +0,6V at the points that should read +15V (including point 6 and the + leg of C144).

Could you help me pick an easily available transistor substitute for 2SC-1624? I'm a bit overwhelmed by all the options I find on alltransistors.com and the fact that lots of them are obsolete. Any help in picking the right Zener diode after that step would be greatly appreciated too.
I have an old SS bass amp at home that has some TO-220 transistors, I will check if any of those can be reused.
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phatt

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Re: Roland JC-120: no effects working, any tips?
« Reply #25 on: November 09, 2018, 07:13:26 AM »

Whoops,,Thanks for the correction. :-[

OK,, I'll try to explain and better minds can pick me up here if I'm wrong but the best I can make out is thus;
                               *C144 does not supply the +15 rail*
C144 is Drawn in a confusing place,, It's NOT connected to R193.
If it WAS it would then be connected directly to the 43Volt Rail. That would blow C144 as it's only a 16Volt Cap.  :o
Notice how there is no DOT as the line up from C144 crosses over the 43 volt rail.

Your Neg 15Volt rail is easy, it comes from R195 (2k7) which is Regulated there by Zener D14 and filtered by C145.

BUT the Pos 15volt rail originates at the Emitter of 2SC1624Y

There is NO AC signal at 2SC1624,, It's ALL part of the DC supply to run the Amp.
That 2SC1624Y is needed because that reverb driver (IC1) takes a fair bit of current from the +15 rail so the Zener needs a booster.

Re replacement Tr,, err I'll have to get back on that latter but yes if you have some electronic waste at hand see if they are close to those ratings.
hope that helps,, Phil.

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rens

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Re: Roland JC-120: no effects working, any tips?
« Reply #26 on: November 09, 2018, 04:29:58 PM »

I have one similar-ish NPN transistor that's labeled D726 in the power section of an old bass amp. If this is the same as a 2SD726 that I found on alltransistors, the values are:
Vcb: 100V
Vce: 80V
Ic max: 4A
Pc: 60W
datasheet: https://alltransistors.com/pdfview.php?doc=2sd726.pdf&dire=_inchange_semiconductor
Pinout is the same as 2SC1624.

Vcb is lower than 120, is this within a safe margin to try it, or is it too low?
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Jazz P Bass

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Re: Roland JC-120: no effects working, any tips?
« Reply #27 on: November 10, 2018, 01:06:54 AM »

I would give it a go.
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g1

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Re: Roland JC-120: no effects working, any tips?
« Reply #28 on: November 10, 2018, 01:55:56 PM »

the best I can make out is thus;
                               *C144 does not supply the +15 rail*
C144 is Drawn in a confusing place,, It's NOT connected to R193.
If it WAS it would then be connected directly to the 43Volt Rail. That would blow C144 as it's only a 16Volt Cap.  :o
Notice how there is no DOT as the line up from C144 crosses over the 43 volt rail.
Agree.  I think C144 should be drawn up where C50 is shown (emitter of 2SC1624).  If C50 even exists, I doubt it is directly connected in parallel to C144 like that.  Maybe Rens can verify what C50 connects to.
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rens

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Re: Roland JC-120: no effects working, any tips?
« Reply #29 on: November 13, 2018, 12:09:54 PM »

Maybe Rens can verify what C50 connects to.
It's definitely there, and connects to point 6 (so to the + terminal of C144 through a jump wire, and the transistor).

I tried to fit a 2SD726 and it didn't work. Still no +15V at its emitter. +43V is present though at the collector. Maybe it's broken and I should try another one (I've got two).
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