Roland KC-300 low preamp power supply

[DrGonz78]

So I have a KC-300 in for repair. The amp does put out a signal but it is really too quiet. Did not take long to find the faults behavior as there is only 3.6v on what should be 15.4v preamp power supply. So, I have checked the zener diodes D1 and D2. They check good on diode setting on my meter and don't appear to be the culprits. I am really thinking that these voltage regulators are the culprits. The drop down resistors are getting really hot so I have been careful not to run the unit very long during testing.

Voltage readings on each side of dropping resistors and other related components in the circuit.
R92 49.2v / 4v
R93 -49.2v / -4v

R94 -49.2v / -4.7v
R91 49.2v / 4.65v

Q1 Base= -4.6 Collector= -4v Emitter= -3.92
Q2 Base= 4.4 Collector= 3.7v Emitter= 3.68

op amps pin8=3.63v pin4=-3.69v

The person I am repairing the amp for is moving in about a week, so I need to rush this repair. I am considering NTE291 (NPN) and NTE292 (PNP) for the replacements as that is all I might be able to find on such a short notice. I know that using NTE parts is really for general repairs and not the preferred manner. Would they work okay though? That is if they are the problem and it is not something else...

Also does this look like these regulators are the culprit? Could a bad IC be loading down the preamp voltages too? The amp does pass signal pretty good and functions like it should, albeit being rather quiet.

[Enzo]

I hate using NTE. I don't claim their parts are bad or that they don't work. The main problems I have are the cost - your $4 power transistor becomes a $15 "equivalent". And compatibility. If a part stands alone, it will sub easily. A driver transistor for example. But when parts must work together, they need to be the same part, so current can share. The six parallel output transistors in some power amp as example.

Your +/-15v supplies have collapsed. When you see hoofprints in the dirt, think horses, not zebras. In my mind, and I may be wrong, having both supplies fail in the exact same manner at the same time is unlikely. Why would both zeners fail? And from 15v to 4v both? Same thing the transistors. before replacing them all, look at their loads. I see the op amp, and the headphones amp. Are those phones transistors getting hot? How about the op amp.


But you left out the best part, where is the preamp? Don;t those two supplies power the whole preamp? If so, disconnect the preamp from the power amp, do those supplies come back up? Is anything on the preamp getting hot?

[Mick Bailey]

My thought is with the drop being roughly equal on both supply rails the fault is more likely to be downstream of the PSU - something that sits across both rails. Are any of the op-amps getting hot? This is where one of those Fluke thermal imaging cameras would be nice!

[Mick Bailey]

Ah, Enzo beat me to it while I was typing. Should have left that cup of tea alone for 10 minutes......

[Enzo]

Ah, suck up that tea. If you had got there first, I wouldn;t need to. My fingers used to be a lot longer, but after all this typing and typing and typing...

[DrGonz78]

But you left out the best part, where is the preamp? Don;t those two supplies power the whole preamp? If so, disconnect the preamp from the power amp, do those supplies come back up? Is anything on the preamp getting hot?

Yeah great advice Enzo and Mick! I was wondering if the problem could be downstream as well but needed some experience to tell me why. I am not sure how to completely disconnect the preamp from it's power supply in this case. I guess I can figure out a couple of components to remove a leg out of circuit. I wish it was easier to pull the preamp circuit to see if the power supply was restored, as I thought of that too. I will study the schematic some more and figure out a way. Thanks.

[Mick Bailey]

I'd check IC10 first. If the rest of the circuit is working as it should (but very quiet) then the culprit could be the headphone driver circuit.

[DrGonz78]

So thanks again guys for the great advice as always. I figured out that fault was in IC9 the M5218AL op amp. I finally got to use my temperature function on my old multimeter and tested to see which IC was getting hot. Could not really detect it with my fingers, but measuring the IC's told me that IC9 was heating up. So I pulled it out of circuit and that restored my 15v rails. At that point I thought of jimmy rigging a 4558 chip some how to the SIP type pin configuration on the board. Then I remember I had an old junk pedal that had some BA15218N op amps and did a bit of research to see how close it would match. The match was pretty darn close to the other IC so I just went ahead with the component replacement. It did the trick and now the amp is in good working condition again. This was a good trouble shooting experience where I really felt that I isolated the fault and defeated the enemy. So without using the temperature measuring tool this repair would have taken a lot longer and I did not have time on my side. The community here always helps me figure how to use trouble shooting skills with greater accuracy.

[Mick Bailey]

Can you post a schematic of the preamp? The only one I have for this amp is the power amp sheet.

[DrGonz78]

Oh sorry I forgot about posting the preamp-- thanks, here ya go.