Diagnose a cheapo amp?

[Valvehead]

Anyone want to throw me some ideas on this ?

ill try to attach the scem.

Fender squier 15 .

Amp works but its just a "farting" out type distortion - you cant dial in a clean sound.

The master vol works, but the gain , volume , and tone controls dont seem to do anything.

Th input jack is new , ive tested all the resistors, I ve tested all the caps on a cap meter up to C14 .

The IC has its +15v / -15V supply.


The supply section is working , seems like all the transistors have their 21V or 15v - see scem.

cant find the culprit ?

[Enzo]

It is one IC and a power amp.

And a speaker. Have you connected it to a different speaker?

Is there DC offset on either output pin of the IC?

Dual op amps cost 40 cents, I might just swap it.

I'd scope the thing with a signal applied, where does the signal degrade? Onject a signal at the master control, what comes out the speaker?

[Valvehead]

well, i dont have a scope.

i can try a speaker--but its a known sound--something wrong in the chain--like if you have your overdrive on 10--- this sounds like its got 2 distortion pedals in top of that-- just a "wrong " sounding distortion tone........over the top clipping and weirdness.

and the tone controls dont work....




Is there DC offset on either output pin of the IC?"

how do i test that (with a meter) ?


yes- i can go get another opamp and try that , its a M 5218

thanks enzo

[Enzo]

use a voltmeter, and probe the output pins of the IC.

[Valvehead]

1 .002 mv

2 same

3 same

4 -15V

5 41mv

6 41mv

7 31 mv

8 +15V

[J M Fahey]

Signal tracing.

Hi Valvehead.
The Op amp voltages are normal.
The only suspects remaining are Q1 and the power amp itself.
Q1 should have around 5 to 7 V on its collector.
All power amp transistors should have around 0.65V base to emitter; positive on NPNs and negative on PNPs.
Be *very* careful when measuring because if you short pins with your probe you *will* burn it instantly, no second chance.
A remaining possibility is that some track joining preamp ground and main ground is cracked , that would cause all kinds of wild symptoms.
Also check that you have no DC voltage across R17.
Get a 1kHz (or 440 Hz or whatever you can) MP3, load it on an MP3 and send it to the input jack.
Most MP3 players can provide up to 200mV clean sinewave, you set it to 100mV and start tracing it along the amp.
EDIT: set the MP3 Player to "repeat one" so it provides that signal all day long. It will interrupt periodically (one second silences) when it restarts, so don't worry about that.
If you can't get a sample signal, I'll post one for you. If impossible here (never tried before) I'll used Rapidshare or something.
Your multimeter *must* provide a 200mV AC scale, or better. Those available anywhere cheapos with two scales: 200 and 750V AC don't cut it.
Start with all controls on 0 (imagine a 0>10 scale).
Set volume on 10, you must have around 600mV on pin 7, you must be able to vary its level smoothly from 0 to 600mV rotating volume.
Then set TMB controls on 5, you should have around the same signal on pin 1 or you should be able to get it by moving TMB controls.
The amp is basically a loose copy of cheap Vox SS amps of that era.
Then start raising gain, you should easily get around 6 or 7 VAC on Pin 1.
You should get around 500mV AC across R17 (the diodes clip the signal) which will become around 5V AC on Q1's collector.
That clipped audio should get to the top pin of R22 (master) and rising it you should drive the power amp to saturation, providing around 8V AC on the node R33-R34-C22 .
You should measure the same on the speaker + pin, and 0V AC on the - pin.
All AC voltages are approximate, I am estimating gains along the way.
Post whatever abnormal readings you get.
Good luck.

[Valvehead]

thanks, i will try to get to it soon

[Valvehead]

Hi Valvehead.
The Op amp voltages are normal.
The only suspects remaining are Q1 and the power amp itself.
Q1 should have around 5 to 7 V on its collector.


HAS 1V


All power amp transistors should have around 0.65V base to emitter; positive on NPNs and negative on PNPs.


THE OUTPUT TRANS ? OR THE SMALLER ONES?

Be *very* careful when measuring because if you short pins with your probe you *will* burn it instantly, no second chance.
A remaining possibility is that some track joining preamp ground and main ground is cracked , that would cause all kinds of wild symptoms.


Also check that you have no DC voltage across R17.


NO DC




Get a 1kHz (or 440 Hz or whatever you can) MP3, load it on an MP3 and send it to the input jack.
Most MP3 players can provide up to 200mV clean sinewave, you set it to 100mV and start tracing it along the amp.
EDIT: set the MP3 Player to "repeat one" so it provides that signal all day long. It will interrupt periodically (one second silences) when it restarts, so don't worry about that.
If you can't get a sample signal, I'll post one for you. If impossible here (never tried before) I'll used Rapidshare or something.


I HAVE A 1K TONE GEN. I USED


Your multimeter *must* provide a 200mV AC scale, or better. Those available anywhere cheapos with two scales: 200 and 750V AC don't cut it.
Start with all controls on 0 (imagine a 0>10 scale).

Set volume on 10, you must have around 600mV on pin 7, you must be able to vary its level smoothly from 0 to 600mV rotating volume.


1V AC


Then set TMB controls on 5, you should have around the same signal on pin 1 or you should be able to get it by moving TMB controls.
The amp is basically a loose copy of cheap Vox SS amps of that era.
Then start raising gain, you should easily get around 6 or 7 VAC on Pin 1.



1.8V AC


You should get around 500mV AC across R17 (the diodes clip the signal) which will become around 5V AC on Q1's collector.




700MV AC




That clipped audio should get to the top pin of R22 (master) and rising it you should drive the power amp to saturation, providing around 8V AC on the node R33-R34-C22 .
You should measure the same on the speaker + pin, and 0V AC on the - pin.
All AC voltages are approximate, I am estimating gains along the way.



6V AC ON SPEAKER +
0V ON SPEAKER -




Post whatever abnormal readings you get.
Good luck.

heres some readings i got today

[J M Fahey]

The only suspects remaining are Q1 and the power amp itself.
Q1 should have around 5 to 7 V on its collector.
HAS 1V BAD
To begin with, Q1 is shorted.
That typically gives farty, stuttering distortion.
I'm sure you have that same 1V on its emitter, and probably in its base too.
To be 100% sure, if you pull it you should have around 19V on its collector pad, a little less on its base pad and 0V on its emitter pad.
Replace it with any general purpose NPN transistor, TO92 case.
Don't know what's the most common one in US, we use European types: BC547 family.
Check the pinout because it may differ.

All power amp transistors should have around 0.65V base to emitter; positive on NPNs and negative on PNPs.
THE OUTPUT TRANS ? OR THE SMALLER ONES? <- in general, *any* transistor, and if not, there must be a very good reason for that.

Also check that you have no DC voltage across R17.
NO DC <- Good

Get a 1kHz MP3, send it to the input jack.
Set it to 100mV and start tracing it along the amp.
I HAVE A 1K TONE GEN. I USED <- Good.

Your multimeter *must* provide a 200mV AC scale, or better.
Start with all controls on 0 (imagine a 0>10 scale).
Set volume on 10, you must have around 600mV on pin 7, you must be able to vary its level smoothly from 0 to 600mV rotating volume.

1V AC Good.

Then set TMB controls on 5, you should have around the same signal on pin 1 or you should be able to get it by moving TMB controls.
The amp is basically a loose copy of cheap Vox SS amps of that era.
Then start raising gain, you should easily get around 6 or 7 VAC on Pin 1.

1.8V AC <- Reasonable


You should get around 500mV AC across R17 (the diodes clip the signal)
700MV AC <- Good

which will become around 5V AC on Q1's collector. <- Obviously this did not happen, Q1 is dead. You *might* have some audio signal through it, but farty, chopped, horrible.

That clipped audio should get to the top pin of R22 (master) and rising it you should drive the power amp to saturation, providing around 8V AC on the node R33-R34-C22 .
You should measure the same on the speaker + pin, and 0V AC on the - pin.
All AC voltages are approximate, I am estimating gains along the way.

6V AC ON SPEAKER +
0V ON SPEAKER - <- The power amp seems to be doing its job, the speaker is receiving audio power, its - pin is grounded (if it were open you should also have 6VAC there) so the big suspect is Q1.
Replace it.
As a final check, bypass it, that's to say, with Q1 out of the way, hook a short piece of wire from C15 to C16, joining the pads that were formerly connected to Q1, the amp should work properly, only with much lower volume.

Good luck.

[Valvehead]

thanks for your time J .

I replaced Q1 . no change.

i replaced the Volume pot--it now works.

the tone controls do not.

all solder joints look great


The amp basically sounds like there 3 distortion pedals plugged into it.

[J M Fahey]

Hi Valvehead.
Get a guitar cable with one plug, one end free.
Mount a crocodile on the hot end, solder a foot long wire to the screen , ground it.
Lift the left end of R21, clip your guitar there and play.
You will have to strum loud with a pick, because you are using no preamp (unless you have EMGs , he he)
You should hear your guitar clearly, soft but clean.
If all well, reset R21 , lift the left end of C15 and repeat, you should be able to play clean and quite loud.
Depending on your answers we go on.

[Valvehead]

R21 - very faint signal and distorted


C15 - much louder signal , distorted



replaced C16 for the hell of it.


at Q1 i have 12V , 1V , 1.6V

[J M Fahey]

R21 - very faint signal and distorted<-I guess it's the power amp then
C15 - much louder signal , distorted <- "much louder"=Q1 good ; "distorted"=power amp bad
at Q1 i have 12V , 1V , 1.6V<- if those apply to Q1 collector, emitter and base respectively, Q1 is healthy, confirmed by its amplifying strongly the guitar signal.
So we concentrate now on the power amp.
I'll give you some voltages you *should* have in the transistors.
Names: NP= negative probe ; BP= positive probe; G= ground; +21 and -21= power rails; E/B/C=Emitter/Base/Collector; SP= speaker rail: the union of R33 and R34.
1)NP to Ground, PP measures:
Q1 Q2 Base= 0V or only a few millivolts (< 100mV)
Q1 Q2 Emitters= -550 to -650 mV
Q1 Q2 Collectors=+21V
2) NP to +21V, PP measures:
Q4 Base= -600mV
Q4 collector= around -21V
3) Are you sure R33 and R34 are not open? Check again.
Any of them open will cause an amplifier that measures almost normal, has reasonable DC (<100mV) on the output, drives a speaker, yet distorts horribly.
If Q7 or Q8 fail open instead or short (sometimes happens) causes the same symptom.
It's more common on TO3 transistors, because there is a thin wire joining the real on-chip emitter to emitter pin; not so in plastic cased ones where everything is encased on solid Epoxy, there it usually explodes cracking the plastic.
If R33/34 check ok, measure:
4) NP to SpkRail ; PP to Q7 base (around 500mV), then to Q5 Base= (around 1100 mV)
5) NP to -21 ; PP to Q8 base= around 500mV
6) NP to SpkRail; PP to Q6 base= around 600mV
7) across Bias Net:
NP to union R29/R30, PP to union Q4/R28= around 1300mV
8) As a last resort, consider replacing Q5/6/7/8 , there's nothing else there !!!!
As always, good luck.

[Valvehead]

#3. r33/r34 ok

#4. Q7 legs , left to right : 500mv / 23v / 0v

and Q5 base 22v . the scem shows it should be + 21v at base -- yes?


#5. Q8 , 0v / 24v / 0v


#6. neg 24v


#7. 9.4v

for the speaker rail connection / tests , i just clipped to the pos. speaker wire -- thats ok ?

[J M Fahey]

Please answer #1 and specially #2, because that will confirm or not the strange Q 6 voltages , Q8's connectivity issues and impossible bias string voltage.

#3. r33/r34 ok <- good

#4. Q7 legs , left to right : 500mv / 23v / 0v <-Base, Collector, Emitter = good.

and Q5 base 22v . the scem shows it should be + 21v at base -- yes? <- No, you should have "NP to SpkRail ; PP ... to Q5 Base= (around 1100 mV)".
Yet you may have mis-identified the pinout, label them with thin point Sharpie , identify them by what are they connected to; vg.: Q5 E to Q7 B and R31, Q5 B to Q4c, C20 and R28, Q5 C to +21. We have 2 possibilities here: either you confused Q5 B with Q5 C, which would explain those +21 there, or Q5 is open, which would explain the horrible sound.
I can not know which one is true because you did not answer #2. Q4's collector voltage is the same as Q5 base voltage, the first "strange" voltage we are finding.

#5. Q8 , 0v / 24v / 0v <- another bad value, we should have 500mV/24V/0V

#6. neg 24v <- another bad value, only possible with R34 open,
*or* Q6 open, measure it outside the board, all 6 diode permutations, just in case replace it.
#7. 9.4v <- It's impossible to have that across the bias net , unless it's open somewhere or R28 should be smoking (1.5W into a 0.5W resistor). Arrrrrrgggghhhhhhhh !! you did not give me the important #2, which would have confirmed or rejected this.
for the speaker rail connection / tests , i just clipped to the pos. speaker wire -- thats ok ?
In principle, yes, although please re-check that that point is *really* connected to SpkRail onboard.
When answering do what I did, quote the question and answer it, or I have to switch up and down between 3 screens (Reply, Schematic and original post) to see what your numbers refer to.
Anyway I am seeing two outcomes here: either some transistor failed open, or some track is open.
I do not even want to think that some earlier technician replaced some transistor and confused Base with Emitter or something like that.
Good luck.
EDIT: forget it, we are not communicating smoothly.
Tonight I'll post the power amp schematic with all voltages and probe positions on it, color coded to avoid confusions.
Please measure and write actual measured voltages by "expected" ones.
When you have all, re-post corrected schematic.
Good luck.