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Peavey Blaze 158

Started by gbono, June 26, 2016, 11:20:51 PM

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gbono

I'm unable to fix this little beast - symptom is that the clean channel distorts at almost any volume setting and the maximum output level is about 3-4VRMS into 4 ohms. If I put in a 1kHz sinewave at J1- I get a nice clean signal until the node between R9 and R10  (positive half of cycle is clipped). I assume these resistors are part of a feedback loop for the darlington pair of Q3/4 - is this right?

  DC voltages on Q3/4 look good and all the supply voltages are correct. If I inject a signal into the power IC U1 (lift C23) I get a nice clean signal that meets the rated power into a resistive 4 ohm load. There is a nice clean signal (undistorted) at R14/C9 - reverb send. Checked and repaired any broken solder joints too.

I'm baffled.. ???

J M Fahey

Quote from: gbono on June 26, 2016, 11:20:51 PM
I'm unable to fix this little beast - symptom is that the clean channel distorts at almost any volume setting and the maximum output level is about 3-4VRMS into 4 ohms. If I put in a 1kHz sinewave at J1- I get a nice clean signal until the node between R9 and R10  (positive half of cycle is clipped). I assume these resistors are part of a feedback loop for the darlington pair of Q3/4 - is this right?
That´s done on purpose, it´s part of the Transtube design.
D4 will clip any signal positive peak going higher than +1.5V + 1 diode drop (~700mV) so anything above, say, +2.2V peak gets the axe.
But signal may get lost between that point and the chipamp input so check along that path.
Lower signal so D4 does not clip it, in any case we are talking a very healthy 1V RMS or so , Q4 collector should be clipping with about 8 or 10 VAC.
Then lower signal so you have, say, clean 2V RMS there , follow the path , S2 might be making poor contact, you should have around 1V RMS on top of tone stack R17/C11 and around 200mV RMS on top of VR6 with tone controls on 10.
Then it goes straight to the chipamp through R22, you already checked from there on it works.
I suspect a bad switch or some cracked track/solder in between.
Unless Q4 is misbehaving and signal stops there.
Vr6 might be open.

"The Scope is your Friend" [tm]  ;)

QuoteDC voltages on Q3/4 look good and all the supply voltages are correct. If I inject a signal into the power IC U1 (lift C23) I get a nice clean signal that meets the rated power into a resistive 4 ohm load. There is a nice clean signal (undistorted) at R14/C9 - reverb send. Checked and repaired any broken solder joints too.

I'm baffled.. ???

gbono

#2
I'm back on this little trouble maker and I've started at the input (J1) and worked back using a scope to trace a 1kHz signal through the clean channel.

VRMS voltages:

at J1 = 211mV
V@C4= 1.36V symmetric sinewave
V@C5= 1.36V      "                "
V@C6= 1.04V      "                "
V@C6 and R8= 41mV positive cycle of sinewave distorted

Something is loading down or distorting the signal after C6 - Checked VR1 with a DMM and I can't find an issue.
I've checked all the above capacitors/resistors and even replaced C6, resoldered all pads and verified that the switch (S1) is working. Can't find the issue. :grr  :grr



Enzo

Your title says Blazer 158, but your schematic is for Blazer III.

What does it say on your serial number plate for a model name?  I have a number of Blazer schematics.  But I will assume for now yours matches what you have.

Q3,4 feedback loop??  The only local feedback I see is R14.

Your clean channel signal is distorted and diminished, right?  Measuring at C6/R8 you get little signal.  So put the amp into distortion channel but turn that channel's controls to zero so we don't have to hear it.  Now still scope the signal at C6/R8.  Is it the same or has it come back to life there.  By switching channels we have opened the relay contacts at S1.  This will tell us if something after S1 is loading us down, or if it is right in the volume control.  Leave that volume control all the way up.  C6/R8 is on the wiper of that pot, so if the pot is turned down, so will be your signal.

I don't know how you checked VR1, the volume control, but what matters is the circuit, so what resistance to ground do you read at the junction C6/R8?  I get that it should vary from zero up to about 320k as the control turns.  Do you get about 320k at max volume?

Have we checked out the rest?  Inject a signal into the reverb return connector, turn up the reverb control.  Does that come out strong?

Run a good line level signal into the CD/tape input jack, how does that come out?

gbono

#4
The Blazer III schematic matches the amp - thx.

I just noticed that if I switch S1 to "lead" channel the signal clears up nicely when scoping node C6/R8. So the issue is downstream between C7 and C23. I'll try your suggestion regarding the reverb send and try and isolate the issue.


If I inject a signal into C23 via J2 I get nice clean strong output.


gbono

#5
I need a sanity check here. This amp was driving me crazy   :loco Though the output level was "normal" (in spec with load) the clean channel was really distorted and really sounded terrible. I isolated the issue to be between R9 and R15. All components checked out good and DC voltages on the transistors and power supply rails were spot on.

I simulated the preamp from J1 to R15 using TI SPICE. Using the oscope utility, I thought I'd start experimenting with the feedback network around Q3/4 and found that the waveform (using Oscope utility in the program) at node at C9/R14 was distorted just like what I was seeing on the bench. The stock components were were used in the simulation - no values had been changed. ???  :o

I'll change out R14 when I get chance and see if simulated matches measured.

Any comments?

Enzo

First this:  if the amp has a problem, we need to repair it, not re-engineer the circuits.  If the design is flawed then EVERY Blazer 158 will sound this way.

If the output level is normal, that means most likely the power amp section is working.

Does injecting a clean signal into the reverb return result in clear sound?

The only feedback on Q3,4 is R14.  I imagine you could remove it and the circuit still work.  Did you remove Q3,4 from the board to test them?  Wired together as they are, it is difficult to test them in circuit.

You isolated the problem between R9 and R15?  OK, so what about in between those?  How about at the collectors of Q3,4?  And at the base of Q3 and at the base of Q4?

gbono

PA section works fine.
Injecting a signal into reverb return gives nice clean un-distorted output signal at output of amp.
Didn't remove R14 from circuit yet
Q3/4 removed and tested - I even replaced these transistors with KSC2331 - no change in distortion.
All nodes of active devices are connected and DC voltages (Vcc measured at Q3/4 collectors and bases are one diode drop more positive than emitter voltage.

I've spent several hours trying to find out what's wrong with the components, traces, etc between R9-R15 and can not find any issues.  :grr Am I just hearing the "transtube" tone/design - I have no reference since this is the only Blazer I've every had to play with.



Enzo

I of course cannot hear your amp, but they should sound reasonably clean on the clean channel.  I suspect the dirt channel is also affected, but you may not notice because of that channel's inherent distortion.

phatt

I think you will find these amps are aimed at a particular market and unlikely to get all clean signal even on the clean,,,Basically working like a dirt box with a poweramp on the end. :-X

Now I'm not a pro at circuit design but my guess is D4 would be clamping the signal via that 1.5V offset.
If you rewire the base of Q3 with a voltage divider then a simulation reveals a much bigger clean swing.

Remove R10 and R14, then connect 2M2 from Collector to base of Q3. then say 220k from Q3 base to ground.

Even more swing can be squeezed if you lower the value of R9,, try 100k. Don't go too low with R9.

See Pic,, Blue trace is the stock wave out Yellow is the mod.
Input voltage is 2Volts so that would be extreme and unlikely from a real PU so the output should hardly ever hit the rail.
I have never tested this but Certainly worth a try.  8)

Phil.

gbono

Phil thanks for doing the simulation - I believe you are right about the clean channel is a dirt box with a chip amp in the output. I going to mess around with the feedback until I get a nice clean signal through Q3/4. What SW tool are you using?

So if I haven't forgotten Fourier transforms  :-\ the non-symmetric signal, that this amp generates (distorts), will allow very rich third order harmonics - which gives a warmer sound  ;) I'll try and do an FFT on some sinusoidal input waveforms, into the simulation, to see if the math works out.

phatt

#11
No worries, it was bugging me and I had to know. Besides I need to understand the peavey circuit as a mate has a similar amp and he is asking my advice. :tu:

I use Circuitmaker student edition "free up to 50 components" http://my.ece.ucsb.edu/York/Bobsclass/2C/Simulation/circuit_maker.htm

Once you do a few simulations and then cross refference results with real circuits you soon get an idea of what is going to happen.
The best part of sims for me was "Seeing" just how the "Tone responce" changes as it passes through different parts of an amplifier. That put me way ahead in a very short time. Distortion comes in many Flavors but "Tone shaping is the critical factor."
Be aware that simulation can give you a false idea of what you might hear. :'(
Take the "Duncan tone stack Calculator" that is often used by DIY's to help tweak their circuits. It might look impressive "At the tone stack output" but that curve can be trashed by the following stages if the user is not aware of the tricks of the trade. 8|
You really need to sim a fair bit of the circuit to get the overall curve.

Ok I'm crap at maths and I'm the first to admit that simulation helps me to Cheat :-[ as it were and saves me doing equations. But Hey I'm a muso first and at 60 I don't have enough time left to go back to school now as well as play music live.
Lots of reading and hundreds of simulations and 4 worn out breadboards as well as some really wonderful people here who have helped me to save $$$ by building my own gear. I now use 4 basic circuits 2 of which are my own ideas and I'm very happy with the outcome. OK It's taken a number of years to perfect but it's worth the effort.
Be ready to build a few failures before you get what you want.
There are much better programs around than Cirmaker but for most guitar circuits this is likely all you need.
My advice don't go chasing for the perfect circuit like trying to analyze the 17th harmonic at the 49th fret :loco,,you don't need too much fancy stuff to get great guitar sound. :dbtu:
Same as things like speaker breakup is raved about in some circles,
sure it happens and may reap a small advantage but that alone will hardly be noticed in the real world.

My circuit needs are simply a good basic rock sound clean but with a little edge (triode rattle, think classic Stones sound)
Then an OD which does not colour the tone much.
All this has to come together with a thick bottom end response (but not farting flabb when cranked)
A clean boost as well as an extra pedal for the over the top when needed (But not with rip your ears off treble)
I play and sing so I can't be wasting time stepping on too many knobs so getting tone shaping in the right place along with strict level control is important to be able to do all that.
By luck I now have a full version of Cmaker but most of what I've achieved was all done on the cut down version. I've got hundreds of files from that program of all sorts of circuits but only a few were successful.
Phil.

gbono

Replaced R14 with a 300K resistor and this amp sounds amazing. Clean channel is what it should be, "lead" channel sounds fantastic and the amp' output level is as designed. SPICE simulation agrees with what's measured on the bench............ ;) ;)

J M Fahey

Thanks Phatt  :dbtu: :dbtu: :dbtu: :dbtu:

phatt

Quote from: J M Fahey on August 11, 2016, 12:11:02 PM
Thanks Phatt  :dbtu: :dbtu: :dbtu: :dbtu:

Tar mate,, yes there are occasions when the brain puts the fingers on the right keys :)
Phil.