Odd 80s Amp Combo - "Starmaker" any info?

[Maddus]

I bought this "Starmaker 8015" combo to use the cabinet for my first diy build. I got it for 10€ and assumed it is just some asian made department store crap - although the cab looked very well made. Now I opened it up and see that there are almost only british made components inside; the jacks are original cliff, the switch is a swann 51 (jcm800 style) and the speaker a Celestion G10-20!
The pots are RadiOhm and the electrolytic caps are made in Japan.
It also sounds very nice, if you don't dial in too much bass, this is a clean only amp, there is no distortion circuit.
So has anyone ever heard about this amps? And where they were offered in the 80s? Google only shows me some classifieds where these amps were offered, but mostly in (eastern-) european countries.

[Maddus]

Oh man, what an odyssey I've been through...
I wanted to know what can I improve on this design and ordered new caps, opamps (NE5534) and power transistors.
The amp has a sweet sound, but all of the tone controls, the more they were turned up, added some background hiss on notes, reminiscent of a noisegate. I guess that's because of the CA741 opamps. And then there was also a 100Hz buzz (230V/50Hz wall supply) which increased exponentially when volume knob was above 50%.
So my first step was to renew all electrolytic caps. Noise problems were still there, but I got a noticeable increase in maximum bass and volume.
Then I swapped the opamps from CA741 to the NE5534s. On the first test the amp was dead quiet, no signal, only the mid shift/gain circuit could self oscillate with fart-like sounds. Then I checked Vcc on the opamps (the CA741's before had 22VDC), and I only measured 3VDC!
Then I analyzed the opamp power supply, and spotted a series 2.2k resistor in front. There is 40VDC coming from the rectifier, and 37VDC were falling on that resistor, leaving 3 volts for the opamps (there are also a 24V zener diode and a 220uF cap involved).
With crocodile clamps I tried different resistors in parallel, and with finally around 800Ohms series resistance I got 22 volts at the opamps again. So I soldered a 820Ohms 2W resistor in place of the 2.2k old one.
Now the amp worked and finally lost the background hiss but I still got that 100Hz buzz at higher volume. It was also there when nothing was plugged in.
I checked all supply circuits and there was no excessive ripple or something like that.
Desperate I also changed the output transistors (one of them had burn marks anyway), but this didn't solve it.
Long story short, in the end it were the input jacks, the switching contacts to ground on both were dirty and had high resistance...
Tomorrow I can finally do a meaningful test run on this little amp.

[g1]

The amp was working before with the 2K2 resistor, so changing it is just a 'band-aid' solution to some fault in the low voltage powered circuits.
You need to repair it rather than modify.
37V across 2K2 was 16mA of current, where was it going?

[Maddus]

The amp was working with the 2k2 resistor when the CA741 opamps were in there. When I changed them to NE5534 there was more current needed.
Min-Max Currents:
CA741: 1.7 - 2.4mA
NE5534: 4 - 8mA
So current draw is approx 4 times higher with the new opamps and the series resistor was limiting the current.

[g1]

Yes, I see now.
From the voltage drop (across the 2K2) you had with the 741's, assuming you are again at 22V with the 820R, you have increased from 8mA to 22mA.

So there is no defect, just an imperfect part substitution.  TL071 would have been a direct drop-in replacement with no mods.

[dmeek]

The 5534 often needs a small compensation capacitor between pins 5 and 8, usually 22pF.
Without that it might be oscillating thus drawing more current.

[Maddus]

Ok, thanks. Would I also hear that oscillations if they happen?
I hear some background fuzz underlying the notes.

[joecool85]

Ok, thanks. Would I also hear that oscillations if they happen?
I hear some background fuzz underlying the notes.

Typically opamp oscillation is above the hearing spectrum, nothing you would be able to reproduce through a speaker or hear.

[Maddus]

What I also noticed is that I get an unpleasant clipping when the input gets too high, for example when I hit the strings a bit harder on a humbucker guitar. I tested with a sine wave generator, and the clipping begins at 0.5 volts, regardless of set amp volume. Is this normal for old solid state designs? And can I modify something to prevent input clipping?

[phatt]

without a schematic it's hard to know but I'd guess the preamp section is causing the harsh distortion, not so much the power stage but it is a budget amp design and high chance there was little to no R&D before it went on sale.
Phil.

[Maddus]

I think I am going to trace it this week, just need to make photos from both sides and then lay them over each other half transparent in some imaging software.
Just did some quick measurements with an o-scope and it showed at the first opamp output that the clipping is asymmetric and in the negative half wave. At the input I had 0.7Vpp and on the output around 14Vpp when clipping starts.
Seems that also the bias voltage isn't exactly Vs/2, I have Vs=22.2VDC but 10.4VDC at the input. The single supply bias resistors are 33kOhms.

[g1]

If it is running the chips with single supply, it will have blocking caps at input and output of IC.  Check that they are not leaky.
Also check for protection diodes at IC input.

[Maddus]

Tracing is in progress, see attached pdf. Took me 3 evenings to get this far. The preamp part is hard to understand for me, but it seems that the tone controls will turn out to be all active.
I also played a little with the values of R16 and R17, 100k made the input clipping worse, 22k better than the stock 33k and finally 10k gave me the most headroom. I still can get it to clip when I hit the strings real hard, but now that's unlikely in a normal play situation.

[Maddus]

So, here's the final schematic.
Are there any obvious design flaws?

[phatt]

Hi Maddus,
          Good on you for having a crack at this 
but sorry but I doubt it's correct.

You will have to go over this quite a few times to get it right.
My advice is use paper and pencil as you may well end up having to draw it out many times.

Use Nodes to label each wire, then count how many components connect to each node or wire.

I started out doing this well over 30 years ago, no internet just some books and a few schematics. I used to end up with a bin full of scrap paper before I got it right.

Also check pinouts on the active components,wrong pinout on transistors is a common mistake.
Oh ed1; if you can up load some pics of both sides of the PCB we maybe able to help you verify the drawing.

Phil.