Roland KC-500 voltage problem

[soundinvst]

Good morning/evening, whatever the case may be,

Initially, I found these problems on this amp and fixed them:

D201 and D202 were shorted
Q202 and Q204 were blown, I replaced Q201 and Q202 with newer complimentary since originals are obsolete
trace from CN1 pin 3 to JP 15 burned into

HV at R201 and R202 is good but voltage drop is bad low. By the time it gets to +/-15v there is nothing. R201 and R201 both check at 470 ohms. So, I unplug CN201 and I get a voltage drop to only about +/-49v and the +/-15v is running 29v now. The extra voltage takes out C205 and C206 16v so I replaced them with 35v. To recap (no pun intended) voltage is low with the preamp and high without it.

I checked it with and without the speaker load but there was no change. Any suggestions?

RM

[nickb]

Have you checked R204 and R205 - they may be open. BUT, you have another problem in that it's not limiting to 15.5V out.
The bases of Q201 and Q202 should be about +16V and -16V respectively at all times.

What exactly did you use to replace Q201, Q202, D201 and D202 with?
Are you sure you wired them in correctly?

[52 Bill]

Yes, there must be a large current draw from the preamp that has done the damage to the low voltage power supply. Fix the two supplies first and then move on to the preamp.

Did you have the preamp connected when you tested and got 29 volts at the LV supply outputs?

[soundinvst]

I used a KSD1408Y for Q201 and KSB1017Y for Q202. I double-checked the pin-out and they should be drop right in. The zeners are some I had that regulate 15.5v rails in a Kustom PA head. The RD16EB2 that diodes that were in there have a zener voltage of between 15.33 and 15.96v. I figured at worst the ones from the Kustom would blow since it was just a 60w head. The pcbs are pretty idiot-proof with the component symbols and outlines drawn on them.

I halfway expected that unplugging the preamp would either return the voltage to normal or it would stay low. But, since it goes high is has me stumped. C203 and C204 look good, meaning they don't appear to have been stressed, so I have not checked or changed them. I

[nickb]

I used a KSD1408Y for Q201 and KSB1017Y for Q202. I double-checked the pin-out and they should be drop right in. The zeners are some I had that regulate 15.5v rails in a Kustom PA head. The RD16EB2 that diodes that were in there have a zener voltage of between 15.33 and 15.96v. I figured at worst the ones from the Kustom would blow since it was just a 60w head. The pcbs are pretty idiot-proof with the component symbols and outlines drawn on them.

I halfway expected that unplugging the preamp would either return the voltage to normal or it would stay low. But, since it goes high is has me stumped. C203 and C204 look good, meaning they don't appear to have been stressed, so I have not checked or changed them. I

In that case test Q210 and Q202 again. The excessive load of the preamp may have destroyed them by overheating as Bill said above in post #3. Replace if necessary. Leave the preamp off and report the voltage on C, B, & E of each of Q201 and Q202.

[soundinvst]

Here are the results of the latest poll:

Q210 good
Q202 also good

Voltage on Q201:
Base 29.4
Collector 50.4
Emitter 28.6

Voltage on Q202:
Base -29.3
Collector -50.9
Emitter -28.8

All readings are with the preamp unplugged. It's only dropping 4v across R201 without the preamp. But, with the preamp it's dropping close to 48v across R201. Based on the reading of R201 I did some figuring and the current is going down with the preamp plugged in from 107mA to about 14mA. Hmmm...

[g1]

You can't get those base voltages if the zeners are working. Are the resistors R203 & R204 ok? If so, those zeners must be bad, or not really zeners.

[nickb]

Here are the results of the latest poll:

Q210 good
Q202 also good

Voltage on Q201:
Base 29.4
Collector 50.4
Emitter 28.6

Voltage on Q202:
Base -29.3
Collector -50.9
Emitter -28.8

All readings are with the preamp unplugged. It's only dropping 4v across R201 without the preamp. But, with the preamp it's dropping close to 48v across R201. Based on the reading of R201 I did some figuring and the current is going down with the preamp plugged in from 107mA to about 14mA. Hmmm...

1) The zeners are not working. I note you reported the zener voltages earlier as 15.33 and 15.96V so these do not jive.
2) Because the zeners were shorted originally, R203 and R204 should be checked see post #2. Based on the voltages these are probably OK, but check them visually and for value.
One you have that fixed we can look at the preamp but don't plug it in else you risk causing more damage.

Afterthought: C203 and C204 have had excessive voltage on them and probably should be replaced. The fault current would have been quite small but why take a chance? Remove them but don't replace until you have the correct voltages on the bases.

[soundinvst]

I didn't make myself clear on the RD16E "15.33-15.96v" - those were the original parts that I removed, and the rating is from the data sheet, not a measurement.

I have two new candidates for consideration: 1N4744 and Z15, both of which have a 15v zener rating. From the looks of them they probably have a little higher current rating.

The ones in there now that are "not working" could possible be a switching diode since they were on rails with ICs (I hate components with virtually no identification).

[soundinvst]

You guys are my heros! I swapped D201 and D202 with the Z15s, checked R203 and R204 (good), went ahead and replaced C203 and C204 and now I've got 14.9v and -14.1v on Q201 and Q202, respectively. A wee bit low but much closer than I was. I figure I'll take a look at Q203 and Q204 before plugging the preamp back up. Thanks!

[g1]

Do some resistance checks on the preamp board before you connect it to the supply.
Measure from +15 to ground, -15 to ground, and +15 to -15. If you find any low resistances, fix first before connecting to the supply.

[soundinvst]

Here's what I know about the preamp:

The feller what belongs to this amp has been using the the headphone jack as a direct out. He has it labeled as such.
There is some soot at the feet of C13, between it and R13, but neither component appears to be the cause of it.
The a fore mentioned burnt trace that was connecting pin 3 of CN1 to J5.
Pin 3 is the ground on CN1 but it's pin 2 on CN201 even though it's the same cable. How confusing is that?
There are 119 jumpers on the preamp board and an additional 6 jumper wires.
Resistance is quite low pin 3 and 4 of CN1, as low as 4ohms at times but is constantly fluctuating (-15v rail I think).
There has been no indication of any short by my dim-bulb, or any blown fuses that I'm aware of.

I'm guessing if there is a short it must have opened something somewhere. I have the jack board XLR cable unplugged but the two ribbon cables are still hooked up. I guess I could take them a-loose and out of the picture.

[52 Bill]

I see a lot of problems with line outs due to differential voltages between the amp and whatever it is being plugged into, so that may be the burnt trace source. And I guess that it could have been the problem that took out the power supply.

I guess that I would start by checking the outputs from IC6 that directly feed the headphone jack.

If you are using a limiter, then I'd plug it all in and see if lights the lamp, while monitoring the 15 volts supplies.

[nickb]

Here's what I know about the preamp:

The feller what belongs to this amp has been using the the headphone jack as a direct out. He has it labeled as such.
There is some soot at the feet of C13, between it and R13, but neither component appears to be the cause of it.
The a fore mentioned burnt trace that was connecting pin 3 of CN1 to J5.
Pin 3 is the ground on CN1 but it's pin 2 on CN201 even though it's the same cable. How confusing is that?
There are 119 jumpers on the preamp board and an additional 6 jumper wires.
Resistance is quite low pin 3 and 4 of CN1, as low as 4ohms at times but is constantly fluctuating (-15v rail I think).
There has been no indication of any short by my dim-bulb, or any blown fuses that I'm aware of.

I'm guessing if there is a short it must have opened something somewhere. I have the jack board XLR cable unplugged but the two ribbon cables are still hooked up. I guess I could take them a-loose and out of the picture.

.
Four ohms is too low. Remember to hold your probes on for some time (10 secs at least) until caps charge up and the reading stabilizes. Finding a low resistance path on a power line is always a PITA as they go to so many places. Unless you get lucky and see a scorched component as a clue. Since C58 and C56 got zapped by excess voltage, albeit at low current, and also because caps do fail, I'd be inclined to yank at least C58 and test it for resistance as a first step.

My preference in such a situation would be to run the preamp from a lab power supply so I can set the current limit and monitor the current as I bring the volts up but that option may not be open to you. Often the offender shows up as a component that is unusually warm. You could approximate this if you used a variac. I estimate the preamp will use around 50mA normally on each + and - rail. You can check the current by measuring the voltage drop across R201 and R202. The key is limiting the current to safe value.

Having hard time finding J5. I see C13 & R13 and a ground tracks between them goes to J122. Do you think the track is burned? Maybe a picture might help.

[soundinvst]

J5 is basically a node connected to the neg. lead of C58. I agree it probably could use replacing. C13 and R13 are in channel 2 circuit. I had I time finding them on the schematic. I don't know what heated the board in that area, unless it was a prior failure and unrelated to the present problems.

The dim-bulb just gives a flicker when I first power up, with or without the preamp plugged in. And, I get a speaker bump. The preamp is still bringing the voltage low on Q204, but I hear grounding noise through the speaker when I touch the base with my probe, like when you touch the plug of a guitar cable that's plugged in.

The voltage on the ICs is on pins 4 and 8 and pin 1 and 7 is ground, correct?