Balder lightning guitar amp schematic please!

[bigdrums925]

I replaced both mj11013's and mj11014's randomly based on what was blown and what was available. At one point both the 13's were bad and I ordered 7 of them, replacing the 2 bad one's. Then one of the 14's was bad and I only had 1 so replaced that. They're expensive!

None of the low transistors (TO92's) have checked bad throughout this whole time so I have not replaced any of them, I got it this way in that regard. Never even unsoldered any except the BC182 next to the big heatsink (part of the bias I believe)

The 3 disk caps: left (0.2uF) & center (33nF) are original. The green on right needs replacement with another 0.2uF. I don't have one to replace it with so I had paralleled 2ea 0.1uF together and since then removed 1 leaving the green at 0.1uF. (Hoping to find one so I don't have to order just 1 small disk cap.)

The Zener is labeled "C3 V6" Motorola. Or perhaps "V6 C3". Not sure but could be 3.6v Zener? Checks good

4ea resistors on top left of board are 68 ohms ea. I mistakenly labeled them (67 ohms 4ea)


The resistor 2nd column, third from top measures 33k. (Pic is of previous configuration.) I think this one and it's pair are bias trim pot resistors I was asked to replace with 330 & 5.6k, then told I should change them back to 33k. That is the current configuration in spite of the resistor in the picture.

Yes I replaced various resistors, all within the picture. The replacements were all left above the board. The famous 220 ohm, once replaced with 3k6, is now back to 220 ohm, like when I got the amp.

No hidden resistors below BD419/BD420, They are the driver transistors I believe.

Last check on power rails were +/- 46.6 Do you need me to check again as this would require removing the outputs?

Yes the 33k in the bias were changed and changed back. (see above)

Hope this helps

[nickb]

OK guys here is where we're at. We still need part numbers for the rest of the TO92's.


Schematic:





DC Simulation showing expected voltages. Ignore the component references as they do not match the schematic above. Note I tweaked R26 to give a sensible idle current with the pot about midway and not to have the trimmer be too sensitive.

[bigdrums925]

Nick: I placed the numbers for the remaining TO92's far left and right of the devices in the latest picture because of space in the middle by the trim pots.
So verify please but I think Q5 then is a MPSA56, Q1 & Q3 are BC182's, Q7 MPSA06, Q2 & Q4, BC560's

[nickb]

Nick: I placed the numbers for the remaining TO92's far left and right of the devices in the latest picture because of space in the middle by the trim pots.
So verify please but I think Q5 then is a MPSA56, Q1 & Q3 are BC182's, Q7 MPSA06, Q2 & Q4, BC560's

So you did! I missed it I've edited the post above with the TO92 part numbers .


I see a problem with the BC560's and MPSA56/06. The max voltage for a BC560 is 45V. There parts have 46.6+0.6= 47.2V on them so they will be operating out of spec. Even the BC182 at 50v rating is marginal. MPSA06/56 are 60V and they could have nearly the full supply of 90V on them. Could the power supply voltages reported at +46.6V and -46.6V be wrong? If they are right then this amp has been brutalized at some point in the past, beyond what we already knew.

[bigdrums925]

For what it's worth the transformer says it's rated at 2 x 31.8 V at the secondary's. Not sure what that is once rectified. I know I measured the rails several times previously and got +/- 46.6, and the 15v rails were +/- 12.3. Someone here in a previous post said I should check the output of the transformer disconnected and check for current capacity with a load but I wasn't sure how to do that or what the load should be. Nonetheless if I need to do that I will with some directions.

I should note here that while getting the part values and numbers last night and replacing wrong value and incorrect replaced parts, I got what appeared to be a short measuring across 2 of the output transistors MJ11013's. I removed 1 of them but can't verify a short in the transistor or the board, but my cheap tester says the transistor is bad. That may not mean much since it has given false readings before with these Darlington's
At the end of the night I decided to see if the sudden current draw at 15% variac was still there. It was not, but started to smoke the famous 220 ohm resistor located between the drivers.
So I'm not sure how to check the rails at this point without some problems. Please let me know how to proceed.

[nickb]

Well 31.8VAC x 1.4 = 45.5VDC so that confirms the 46.6V measurement. We'll need to find substitutes for the driver trannies before too long. I had mentioned one or both of the the MJ11013's were bad way back in post 51.

Test Q8 and Q9, if they are OK, then test Q5, Q5 & Q7. If that is inconclusive remove Q9, power up and measure the voltages around Q5, Q6 and Q7

[MarkusBass]

I see a problem with the BC560's and MPSA56/06. The max voltage for a BC560 is 45V. There parts have 46.6+0.6= 47.2V on them so they will be operating out of spec. Even the BC182 at 50v rating is marginal. MPSA06/56 are 60V and they could have nearly the full supply of 90V on them. Could the power supply voltages reported at +46.6V and -46.6V be wrong? If they are right then this amp has been brutalized at some point in the past, beyond what we already knew.

It looks like the transistors were there originally. Also BC182 is rated for 50V only. Somehow the amp worked. The capacitors in the power supply are rated for 50V so power supply rails between 40 and 45V are expected. Bigdrums, can you make a photo of the capacitors (showing how they are connected to the board)? Are they original, or they were replaced?
Nick, I see few minor problems with the schematic and the simulation. The Q9 transistor on the schematic is connected incorrectly (it should be collector to V- rail). There are two 100k resistors on the board, which on the schematic are connected to the output. In the simulation they are missing. Are they connected to the output, or to the ground (I don't see it on the photos)?
There should be also 33n capacitor between collector and emitter of the bias transistor. Input capacitor is 220nF.
I'm not sure whether the amp could ever worked with two 33k resistors in the bias circuit. I have to check it but it seems to me that such values made setting the bias impossible. Even if they worked, the bias circuit is very sensitive and you shouldn't even touch it without undrstanding how it works.
Also, the 220R resistor in the feedback circuit (in series with 220uF) looks also suspected to me. It sets very high gain of the amp. I would expect there something like 820R, or 1k resistor. Bigdrums, did you replace any of these resistors previously?

Further steps depends on Bigdrums soldering skills. Can you desolder BD419/420 without destroying the board? I'm asking because the 220R resistor (replaced with 3k6) was desoldered in the way that PC board tracks were destroyed. We don't want this.

Mark

[nickb]

It's nice to have more than one pair of eyes

I've fixed Q9 and updated (post 62 above).

I left out the 100K's in the sim on purpose - they seem to have no discernable effect, especially on the DC conditions. I changed the Vbe multiplier lower res (R26 in the schematic) in the sim to 8.2K get a sensible idle current at mid piont and make the trimmer is less sensitive (see post 62).
.
PS: I set the gain to x100 using 22k /220 ohm in the feedback. Was stable in sim and 0.9Vpp in for full out seems reasonable

[MarkusBass]

PS: I set the gain to x100 using 22k /220 ohm in the feedback. Was stable in sim and 0.9Vpp in for full out seems reasonable

0.9Vpp is 0.32RMS. A little bit high for me but maybe the amp is designed like this.
I think that we should wait for a photo of the power supply (capacitors) and information about soldering skills of the OP.
I was looking for replacement transistors and ZTX753 and ZTX653 look good. But this is for the future.
I've just noticed two additional resistors on the schematic R31 and R32. Are they used in the amp?

Mark

[bigdrums925]

I can solder and repair well. I can't always prevent damage from occurring though.
This is an overview of the PS:

And a closeup:

A sideview:

And the bottom:

[nickb]

I've just noticed two additional resistors on the schematic R31 and R32. Are they used in the amp?

Mark

They are under the heatsinks of the drivers. Can't really see to be sure.

[dmeek]

They are visible in the photo in post #59 - 470 ohms. Also the 100k resistors are soldered to the ground plane.

[bigdrums925]

Removed and tested Q8, Q9, Q1, Q3, Q5, Q2, Q4, Q7. All checked good. I checked Q6 recently.
With the 4 output transistors in I powered up slowly with the variac and the current draw rises sharply at about 15%
I removed Q9 but still have the excessive current draw so until I remove at least some of the outputs I won't be able to measure around Q5, Q6, Q7.
Any suggestions beyond removing the outputs?
Still not showing bad outputs though....

[bigdrums925]

Is there a updated schematic available?

[bigdrums925]

OK I just put everything back in after finding and replacing Q13, which was shorted.
I fired it up with low (normal) current draw, no oscillation, no smoke.
However the output is very low and channel 2 doesn't work.
I'll check the outputs later or tomorrow and post the results.