Roland K500 issue

[andyjazz]

As I posted before, this amp makes an electrical "hum" at start up. Kind of like an open ground. In fact when I plug anything into any of the inputs (Like the 1/4 plug from my freq generator that is grounded) the hum goes away. This however may be a secondary issue. Here's a link to a previous thread where there is a schematic for this amp posted:

http://music-electronics-forum.com/t3056/

After probing around some I found what looks to me like a problem. The V on either side of the big 2W R's (R201, R202) is not what the schem indicates it should be. These R's are dropping the V to feed Q201 and Q202. From what I'm getting, R202 is not dropping the appropriate V. It is -52V or thereabouts on both side of the R. I'll do some more checking on the transistors (Q201, Q2020) that are fed from these R's. Thanks for your patience.

[Jazz P Bass]

Check R202 & D202.
They will drop the voltage even if Q202 is open circuit.
That does not not prove Q202 is good or bad, so you must also check it.

[andyjazz]

I had some time last night so I decided to do some digging. I pulled the big Caps and the 2W R's off the board so I could test them better. They all tested fine. I checked the Zener diodes Jazz P Bass mentioned and they seem to be acting they way they're supposed to. I re-assemble the amp. fire it up. Got the same behavior. I'm back to where I'm started. There are these two transistor amplifiers Q201 and Q202 that were my first suspects - the V readings are not right.

Well, I've already been through the hell of trying to order these goddamned things. They've been out of production for years so I have to find an equivalent. I tried. They didn't work. I go to the Digikey website. I go to Mouser, I go to Allied. They all have different ways of entering in the specs for the part you need and I'm just not very good at figuring this stuff out. It's time consuming, tedious there are all kinds of parameters like package size and pinouts and shit, and it's driving me crazy. I can see why alot of amps just end up in a landfill or whatever because figuring out the problem is only the first part. Then you have to spend hours figuring out how to actually aquire the correct replacement part.......sigh.....I'm going to give Digikey another try...........

[Jazz P Bass]

2SB1017 Transistor.
Type: PNP
Case: TO220
80 volt, 4 amp, 25 watt.
Any of the following will work.
TIP42B
TIP121
TIP126.
find chips.com will find them for you.
My only question is how the 2SB1017 is mounted.
Does the transistor sit up in the air or is it mounted to a heatsink?
Is it a full plastic case or does it have a bare metal tab at the back?

[Jazz P Bass]

You "checked" the zener diodes.
As I stated before, you should have a voltage drop if the diode is zening (that is an Enzo term)
What voltage do you have at the base of Q202?
Should be -16 volts. The value of the zener diode.
You need to troubleshoot this problem a little more presicely.

[andyjazz]

Why is the # on the part not the # on the schematic?

I'll go over this again. The # on the part is 2SB1642. I google 2SB1642. I get a spec sheet.
I type the # into Digikey, Mouser. "There are no parts that match your inquiry"
I call the people on the phone, he tells me that chip is long out of production and I have to look for an equivalent.
I go through the ordering parameter-spec-dealio on the web site looking for one that matches my spec sheet.
I could not find one that matched my spec sheet so I found one that I thought was close. I bought one for the other rail too, same specs. After paying way too much in shipping and waiting for a week. They come. I install.
They don't work. Fucking typical.

I put the old chips back in.

So now today I put 2SB1017Y into "find chip" and got "no matches." God this is frustrating. And yes there is nothing wrong with the Zeners. They are "zenering."

So how are you supposed to know that a TIP42G, TIP121, TIP126 will do the same thing? After spending an hour last week at Allied I couldn't find a chip that was rated 60V 4A 25W (That's what the spec sheet for the 2SD2531 says, BTW thats the # on the part, the schematic says 2SB1017Y, I thought maybe it would be better to search for the number on the actual part, think again I guess.) Near as I could get in a PNP was 100V 3A 40W. Yeah well that didn't fucking work. Today at Digikey I found an 80V 4A 25W. Guess I'm getting better at searching through the "filters" dealio. But how am I supposed to know that an 80V will work? The 100V's didn't work. It seems like I can never actually FIND the part that has the actual specs. Am I supposed to weed through all these tech sheets and find something thats close? Is this typical? Also, shouldn't I replace both of these Transistors? If I have this 80v on the neg rail and the original 60v on the positive rail won't that screw things up? Sorry for the venting but you know....BTW the base of Q202 is -5V, it's totally screwy.

I could just bring back to the guy and shrug my shoulders but....just trying to save landfill space.

[Jazz P Bass]

As I stated on the above post, any of the transistors that I posted will work.
The 100V, 3 amp transistor was incorrect in that it could only handle 3 amps.
Again, the minimum specs to shoot for are 80V, 4A, 25W.
The TIP42B meets all of these conditions.
Is there any way to disconnect where the 15 volts is going.
It may be loaded cown.
That -5V on the base is not good.
I would measure that point with Q202 removed.

[g1]

Did the original transistors that you removed test bad? When you say that the sub. transistors "didn't work", do you mean they are just not solving the problem? It's hard to tell if they are not working as subs. or if you mean the amp still doesn't work.

[Jazz P Bass]

My lucky day!
I received a Roland KC-500 for repair today.

[bkahuna]

To the OP: This is a shot in the dark, but there is a grounding screw in the power amp pcb (near the bridge rectifier) that has to be there, and tight, for proper operation. I've seen numerous KC500s with this as a problem, and it presents itself as hum, if missing, and crackling, if loose. It's worth taking a look.

[Ronsonic]

As I posted before, this amp makes an electrical "hum" at start up. Kind of like an open ground. In fact when I plug anything into any of the inputs (Like the 1/4 plug from my freq generator that is grounded) the hum goes away. This however may be a secondary issue. Here's a link to a previous thread where there is a schematic for this amp posted:

Okay, maybe I'm sorta dense here. You've got an amp that shows symptoms of having a bad ground or open conx in the input circuit. So what's this stuff about transistors and resistors? If there is a problem in the power amp, plugging something into an input jack won't silence it.

Why not just fix or at least rule out bad solders or whatever in the input circuit?

[km6xz]

I have not been following this thread and tend not to want to get involved when parts start getting replaced for the hell of it.
Like Ronsonic asked, what does all this have to do with an amp that works, when the input has a ground return? Nothing would work if the -15V is putting out 50 volts as mentioned in a prior post, so the likelihood is that the ground point the test lead is using is not at ground potential.
The symptom sounds like an open ground at the input. One post suggested high voltage was input to the input jack of some kind. That would, if that is true, point to a burned trace at the input. The traces as you can see are very light gauge on the jack board, and even if the inputs did not see 50 volts, broken traces on those Roland jack boards are imfamous.
I think you should step back and put all the parts removed, back in as carefully as possible and start trouble shooting based on symptoms rather than on illogical observed voltages that would not cause hum but a dead preamp section. If Q202 really was not regulating and 50 something volts were impressed upon the -15v rail, can you think of ANY component that would not likely fail in the low voltage section. The caps in the low level section, IC in all sections and a few dozen resistors would have been up in smoke.
1. Does the amp work when grounds are established at the input after returning the parts removed to their rightful places? If yes, forget all about the power amp and 90% of the preamp.
2. Does the ground trace of the jack sleeve pins of the input jack board have a low resistance continuity to the central ground point between the main filter caps C201 and C202?
If not, you found the cause of hum, now find the broken trace.
If there is no low resistance continuity between the jack board(with nothing plugged into the jacks) and the filter caps, try between the jack board common ground and the ribbon connector wire #5 on CN3 and CN2.
Good luck, .....but do not replace any parts until you can prove it is bad or else you will bury yourself in uncontrolled variables that make trouble shooting harder.

[oc disorder]

I aggree with the above.
Try plugging a signal in to the stereo link in , this should bypass the pre-amp and enable you to see if the power amp is working ok without hum.
I suspect they can be used as trolleys by resting keyboards on top as the ones I saw are on wheels.
I have seen some where the knobs have been pushed in from the top which has forced the pots (volume controls) to start to disintergrate.
Its possible to remove the board and carefully squeeze the pots back together and bend the little tabs firmly around the pot body.
Another way to try to isolate the fault between "pre" and "pwr" would be to take the line out and feed it to an external working amp.
That way you can hear if the basic mixer/pre is working without hum although I suppose the external amp may ground it.

[Ronsonic]

To go a bit further in this direction, there is a tech bulletin from Roland called "Smoke Problem" actually it addresses a lot of problems and it all goes to resoldering connections.

Another thing to look at. Roland used the yellow assembly glue that you find in a lot of Japanese products. Over time that gummy yellow glue turns brown, hard, corrosive and conductive. See that component leads and ribbon cables aren't compromised by it.