Balder lightning guitar amp schematic please!

[MarkusBass]

What worries me is the resistors that are in series with the BIAS trimmer are both 33k and they both were replaced. 33k seems to much for me. But it look like the OP lost interest of fixing the amp

[J M Fahey]

Hey, maybe we have good news in a week or two

[dmeek]

Obviously I have too much time on my hands, but I do love a puzzle. The 220 ohm feedback resistor
probably should be 2.2k -looks like it has been replaced with the wrong value, might even solve the
oscillation problem. It's the 1W resistor on the left

Doug



http://music-electronics-forum.com/a...1&d=1506712143



(I've edited this post to make a few corrections)

[nickb]

Way to go Doug! I think there are some errors a so I'll try a redraw in due course...


Agree with other suggestions plus I'd check the small TO92 transistor next to the heatsink. I suspect it's dead.


... this is my interpretation, with suggested value corrections based on Doug's hard work
Balder Lightning Power amp- not verifed V1 .pdf

[MarkusBass]

Nice . I wonder whether some resistors that we think are 33k, are actually 2k2 (the stripes are red instead of orange). But if someone replaced the resistors in series with the bias trimmer with 33k, such high values make it impossible for the bias transistor to work correctly. The OP could measure some of the 33k resistors.

Mark

[J M Fahey]

Hey!!! thanks to you both, dmeek and nickB

following the latest fad, Iīll also suggest a small correction and a couple tests,

1) I like this input pair connection better



2) after checking that schematic and actual PCB match, Iīd suggest measuring voltage difference between points 1 and 2 ; 3 and 4 ; 5 and 6.
I know that amp does not currently stand getting full voltage , so please rise Variac slowly, until idle or mains current starts to dramatically rise, then back down a little into a safe zone, we donīt want to overheat or damage anything, and *then* measure suggested pairs.

I bet we will find overbiasing signs there ( and that at way less than mains voltage)

Once we turned a suspect into a culprit, weīll try to solve that, but letīs go one step at a time

EDIT: suggest using a plain graphic format, such as gif or jpg, to make editing and reposting easier

[dmeek]

[QUOTE=nickb;466678]Way to go Doug! I think theee are some errors a so I'll try a redraw in due course...


Thanks for sorting out the input stage, it had me confused.
btw the zener at lower left is backwards

[bigdrums925]

No, it was working some years ago, friend gave it to me. At that time it needed some minor repairs that were more or less obvious. I don't remember what that was though. It was working but not used much until about 6 months ago I found out that some gtr players really like it and had some value. I had a buyer coming to see it and I checked it the night before. The input jack was making noise so I assumed it was like many others just cracked solder joints on the jack at the board. I resoldered them but became curious about the 2 trim pots. I don't remember exactly but after marking their positions and adjusting them slightly, the resistor between the drivers smoked in the center. I could see the 2 red stripes but not the multiplier so measured it at 220 ohms. I replaced that and then started blowing the output transistors. Got it to settle down but then started getting the oscillation.

[bigdrums925]

No not at all losing interest. Just had a few obligations to deal with this week. I started to draw a schematic of the PS and amp section but I have never drawn one before so it will take some time before I have anything I can post.. I know one of the trimmers is for DC offset, the other seems to have some effect on the bias based on the emitter resistor voltage drop changing between 3.x volts and 6.x volts last time I measured it.
How can you tell from just the pics the 33k resistors raised off the board are in series with the trim pot? You are correct of course, I checked, one to each fixed leg.
From much input here it seems the idle current is not being controlled properly, from what little I know it should be down at about 10-20 mA
I have been trying to get that down by changing a few of the resistors that were suspect, out of spec etc.
So how can I reduce the idle current to the point where the trim pot is in range?

[nickb]

Mark - I had changed the Vbe multiplier resistor values to 330 and 5.6K based on a requirement of 6 Vbe's and to give the trimmer a decent range. With 33K's that Vbe multiplier transistor would likely be current starved and could cause high idle current as you say. But with two equal resistors the Vbe multiplier would only be x2 which is not enough.

Hmm..just read post 38. Kinda blows that idea out of the water. Time to measure volts.

Juan: Yes, I forgot the check the phase

Doug: Yes. Thx I see on the schematic I got one of the zeners the wrong way round on the input offset trim.

Updated (jpeg):

[bigdrums925]

OMG Doug! You drew this from the 2 pictures alone? You all are incredible.
If the 1watt 220 ohm resistor you are referring to is the carbon, that value was because its pair is 220 ohm.
Much of that section seems to be laid out symmetrically. That's all I had to go on.
Please forgive any dumb remarks or observations on my part because I know very little. That's why I'm here

[bigdrums925]

Bear with me please. I may sound quite dumb here. Disconnected from the board with no load I measure 2 pair that are 34.x VAC.
Not sure what amount of load to use though or how to determine the current when loaded.

[bigdrums925]

The TO92 next to the heat sink checks good. Its a BC182 BTW. I believe its the one near the "idle" on your PDF? Originally it was pushed down to touch the heat sink with some heat sink compound.
Once again I'm blown away at the help here, now a proper PDF with labels. Wow Thank you!
I noticed the transistors are mostly labeled Q?. Would it help if I provide the transistor numbers?
Or to provide any other part values?

[J M Fahey]

Would it help if I provide the transistor numbers?
Or to provide any other part values?

Oh, please DO.

I suggest you leave schematic as is so we donīt lose track, but add actual PCB numbers, labels, or whatīs written (or painted) on components by them, either in another colour (say, violet) or between brackets or similar, such as (R4 100k) or whatever.

Amplifier and sound strongly remind me of Pearce amplifiers (evolution of Gibson/Moog LAB L5 and family).

[bigdrums925]

I have changed the 33k trim pot resistors to 330 Ohms and 5k6 as per the new drawings, I changed resistor that was burning up back to 220 Ohms.
And changed the resistor that was in the speaker output path parallel with the inductor near the big filter caps.
I still see a fairly sharp rise in current ramping up the variac.
Will check this later tonight.
Thanks everyone