Roland Blues Cube 30 question

[Garydean]

http://www.schematicheaven.com/newam..._cube30_60.pdf

above is the link to the schematic.

Basically it had about 2% output when I got it. I replaced the output transistor and still I had no change. I replaced Q12 with a NTE123-AP . and now I have no output. Prior to that I replaced Q1,Q2,Q8,Q9,Q10,Q11 with a
2SK184. I replaced Q15,Q16 with a 2SK363. I'm getting - and + 28v off the power supply which is about 4V too high. I injected a signal at the output board at D21/D20 and the signal came thru my bench speaker. However no place on the power/preamp board did a signal come through. I have continuity from the one board to the other. If I need to supply further info please request it. All diodes test good.

FYI
Q19
E 9.7v
B 7.9v
C 17.37v

Q17
E -7.49v
B -28.9v
C -8.5v

Q18
E 19.15v
B 28.73v
C 19.75v

Q12
E 1v
B 714mv
C 1.13v

I appreciate the assistance.

Gary

[Enzo]

Q12 is a 2SC1815 - an asian type. Looking at the flat face, left to right the legs are Emitter - Collector - Base. That NTE thing is I do believe a US type transistor with the legs Emitter - Base - Collector. So unless you put it in sideways, the legs won't be right. And since Q12 controls the mute for the output stage, I don't doubt the amp is quite.


Why replace all those transistors, was the whole row of them fried? or were you on an easter egg hunt hoping one was bad?


May I suggest getting away from that NTE stuff? The 2SC1815 is one of my favorite transistors. It is a great general use NPN part and it is low in noise as well. Rated 60v, it will work most anywhere an NPN signal transistor is needed with an Asian footprint. Next time you order anything from Mouser, tack 10 of those onto the order and have them. Mouser stocks them for 22 cents, if you buy 10 of them they drop to 17 cents. Start building a parts collection. The PNP complement would be the 2SA1015. Those two alone would cover most signal transistors in the Asian types. get a few American types too, and in one little row of drawers you have everything covered.

[Garydean]

thanks for the reply Enzo,
when I first got the amp, and it was quiet, I was taking some measurements at the ribbon cable, on the main board and I shorted something. That's when all the transistors on the board were off. I then figured while I had the desoldering gun out, might as well pop as many as I could in..... as I probably fried most of them. I ordered a bunch but didn't order 1815s. Thanks for the stocking tip by the way. Interesting about the pinouts between NTE and the Asian transistors. That's probably why I've lost the signal completely. I agree NtE has been a pain and I've lost faith in them, plus they are way too expensive at my local joint...and I'm being nice now.

[Enzo]

The NTE part probably more or less covers the 2SC1815, but it mainly covers many US types. It maybe covers the 1815 as long as you adjust the pinout. Remember, NTE is not making one for one replacements for everything out there, they try to cover a million types with 10,000 of theirs.

Now that I look, they cross 2SC1815 with their NTE85 in my guide. And that one SHOULD have the correct pinout for you. The NTE123AP is a 40v 600ma part, not a very good match for the 2SC1815 with its 60v and 150ma rating, but should work if in the holes right. This mute circuit is not very demanding.

One of the most common asian transistors is the 2SC945 - it also crosses to NTE85. I happen to stock them, but what if I didn't? If I needed a 2SC945, I could look up an NTE sub, or I could look to my collection and discover the 2SC945 is rated at 60v and 100ma, and... there, the 2SC1815 is rated at 60v and 150ma - it will work fine. I made my own substitution.

[Garydean]

Great stuff. I'm making a shopping list as we speak. Are there any other
"go to" transistors I need to add to my cart?

[Garydean]

FOLLOW-UP

I correctly reinstalled Q12. I've got my signal back yet still very weak with everything on 10. Ch 2 is distorted as the vol increases as I would expect. Chan 1 remains clean all the way to 10.

D10 is at 19.6V
D9 is reading -8v on the anode .04 on the cathode side

Q17
E -7.4
C -28.1
B -8.04

so is my neg rail low? My -D should be -19ish right?

C32 or C33 is kaput or is it further down the line? That's where my knowledge ends.

[52 Bill]

so is my neg rail low? My -D should be -19ish right?

The negative supply is only used to turn on/off the fet's for channel switching, etc. The schematic shows sn 8.2 volt Zener (D9), so your -D voltage is correct.

I'd try injecting a signal to the top of the master level pot, and then work forward till the signal stops.

[Garydean]

Thanks Bill. I get no signal at the emitters of Q5 and Q4. Just found an interesting observation. If I touch the shaft of the PRE VOL pot on the lead chan I hear a hum thru the speaker. I applied a signal on the shaft of that same pot and I get a signal thru the speakers. This only happens when I'm switched to chan 2. I get a good signal at the master and the presence.

[52 Bill]

...I applied a signal on the shaft of that same pot and I get a signal thru the speakers. This only happens when I'm switched to chan 2. I get a good signal at the master and the presence.

Do you mean to the wiper of the pot or the actual shaft?

Q4 and Q5 are part of the channel switching circuit so there should be no audio there.

Inject the signal at the top of the volume control with the amp set to clean mode, any signal?

Inject the signal at the top of the pre-volume control with the amp set to distortion mode, any signal?

[Garydean]

Yes the actual shaft when touched hums.

Top of vol contol good signal.
Top of pre vol control good signal when chan 2 is selected.

[52 Bill]

Yes the actual shaft when touched hums.

Top of vol contol good signal.
Top of pre vol control good signal when chan 2 is selected.

Well, you've eliminated everything but fets, Q1 and Q2. When you replaced them did you check correct lead placement? Did you check all of the usual stuff, like loose parts, broken pc traces, bad input jack, etc.?

[Garydean]

Q1 and Q2 are both 2sk184s. I put exact replacements in.

Ive been thru the board several times, reworked he input jack as well as the foot-switch jack. I'm wondering if the new output IC bad.

When I apply a signal from my generator I could be getting the same output that I'm getting with a guitar plugged in, so yes a signal is going thru but at what stage am I being robbed of power?

I've got about 15-18 of those silver low uf coupling caps. what is the procedure with testing those. I have an ESR meter but it's useless at low value caps such as those.

thanks

[Enzo]

Testing coupling caps? Put or find a signal on one end and see if it comes out the other.

[Garydean]

ok, so I can hook up my generator at the input and scope each side of the cap, but only one side should show a wave?

thank you

[52 Bill]

When I apply a signal from my generator I could be getting the same output that I'm getting with a guitar plugged in, so yes a signal is going thru but at what stage am I being robbed of power?

Sorry, when you said that there was good signal at various points in the amp, I assumed you meant that the signal was full volume output.

If it isn't, then start at the power amp chip and inject a signal at pin 7. Can you get a full volume signal here? If not check the chip and the mute circuit centered around Q12.

If you have full output there, start working forward through the pre-amp till you find out where the signal dies.

[Garydean]

The signal at the output transistor is full volume. when I work forward thru the connecting ribbon and on to the main board the signal was at low volume just where it connects to the main board. Continuity is good thru the ribbon. Can I remove the output board that houses the output transistor, which is mounted to the chassis, in order to take measurements? Or does it need the chassis to help dissipate heat? There are 9 small components on this gum sized board.

[52 Bill]

The signal at the output transistor is full volume. when I work forward thru the connecting ribbon and on to the main board the signal was at low volume just where it connects to the main board. Continuity is good thru the ribbon.

Is it possible that the ribbon cable opens up under stress at either end? Try flexing the ends while checking continuity. Try bypassing the ribbon cable one trace a a time with a wire jumper.

Can I remove the output board that houses the output transistor, which is mounted to the chassis, in order to take measurements? Or does it need the chassis to help dissipate heat? There are 9 small components on this gum sized board.

I don't know, if you turn on the amp with the chip unheatsinked does it start to get hot? They often do, leading to overheating and burn out. To be safe either leave it on the chassis or screw it down to a piece of aluminum sheet as a temporary heatsink.

[Garydean]

thanks again Bill,
I put the output board back on the chassis and turned on the amp. Works fine.... jiggled the ribbon and got an intermittent signal. You were right the ribbon or solder points need t be looked at. I guess my best bet is to rework both sides just in case.

[Enzo]

GAry - coupling caps are there to connect to stages of a circuit together. The caop blocks any DC difference but passes signal through. SO to test one for signal, if the signal is on one end, it should show up on hte other end too. The DC on one side should stay where it is though.

[Garydean]

So I'm only gonna see those low value, orange drop, lemon drop, dog doo drop and gum drop caps in the signal path blocking DC? Higher value polar caps will be seen in the supply line filtering and leveling DC? Generally speaking? How about ceramic, are they just tone shapers, different values block certain frequencies, but I see them in supply lines as well? and tantalum? Any general rule to follow, besides "there are no rules"?

thank you.

[52 Bill]

So I'm only gonna see those low value, orange drop, lemon drop, dog doo drop and gum drop caps in the signal path blocking DC? Higher value polar caps will be seen in the supply line filtering and leveling DC? Generally speaking?

Generally speaking yes, but the values of the caps relate to the impedance of the circuit. In tube amps coupling caps will rarely get above .22uF, while in a solid state amp they can easliy be in the 10uF range.

How about ceramic, are they just tone shapers, different values block certain frequencies, but I see them in supply lines as well?

Small value caps used on supply lines are "decoupling" caps. They basically serve the same purpose as the larger filters but work in frequency ranges that aren't efficently covered by the larger caps.

...and tantalum? Any general rule to follow, besides "there are no rules"?

Caps are caps, some types work better at certain applications, but they all serve the same basic function. Sometimes a designer will chose the type of cap based upon size, or cost, or temperature range, or sometimes on their audio qualities (most of which have been debated here already).

[Garydean]

good answers Bill.... I appreciate your knowledge.