Solid State Acoustic Guitar Amp - Hum Problem

[robdean]

I'd really sincerely appreciate help - I've just bought a lovely seldom-seen amp model (Crafter DSP1) - about 10 or 15 years old - bought it in a noisy environment, discovered very conspicuous hum when used at home.

I managed to get a schematic which is attached here. I am very new to electronics, although have long experience with passive guitar circuits.

The hum is irrespective of the volume control and starts before the relay switches on. With the amp loud, the hum is drowned out and the amp sounds very good, but quiet playing is spoiled by it. I estimate the hum at 100Hz-200Hz, though will check next time it's reassembled.

I have replaced the two big 4700Uf power supply caps and the two adjacent 470uf caps - no improvement :-( [boxed in yellow on the schematic]

I have ordered new rectifier diodes.

I have begun checking earths - I've checked the ones boxed in blue on the diagram so far. The only odd thing I have found so far is that C64 and C66 (orange box on schematic) appear to be polarity reversed compared to the schematic. Can anyone tell me if this is significant?

Please does anyone have any other suggestions (or any encouragement!). I'll really be very very grateful... I love this amp, my first one specially for acoustic guitar, but the hum is really disappointing. I have multimeters to hand if there is anything I can test to help diagnosis!

Thank you,
Rob


Crafter DSP forum.pdf

[loudthud]

I have begun checking earths - I've checked the ones boxed in blue on the diagram so far. The only odd thing I have found so far is that C64 and C66 (orange box on schematic) appear to be polarity reversed compared to the schematic. Can anyone tell me if this is significant?

I suspect it's just an error on the schematic. C43 and C44 are also drawn with polarity reversed. If there is hum with the Master Volume turned all the way down, that would seem to indicate hum is entering the signal chain at the power amp (lower left of the schematic) or it is on the ground at the Master Vol. Try inserting a phone plug in the Return Jack. That will interupt the signal from the preamp. Ant change?

[robdean]

I suspect it's just an error on the schematic. C43 and C44 are also drawn with polarity reversed. If there is hum with the Master Volume turned all the way down, that would seem to indicate hum is entering the signal chain at the power amp (lower left of the schematic) or it is on the ground at the Master Vol. Try inserting a phone plug in the Return Jack. That will interupt the signal from the preamp. Ant change?

Thank you, loudthud. It's reassuring you think the cap polarity discrepancy is an error in the schematic, it was my guess but very much a guess!

I can't retest immediately, but I did plug music into the return as a general test two days ago and I'm sure I'd have noticed if the hum had stopped! Also since the volume control has no effect on the volume of the hum, I have been working on the assumption that it is being introduced after it (and thus after the preamp), so I do think you are right there...

[robdean]

I have now changed the four rectifier diodes: the amp still hums.

The hum sounds like 100Hz (comparing it by ear with a tone generator). UK mains is 50Hz.

Plugging into the fx return makes no difference to the hum. None of the amp controls makes any difference.

Having changed the four power supply caps and the four diodes, I'm stuck, as I don't have the expertise or experience to figure what to try next: I've done no such fault finding before. I really like this amp, aside from the hum, and won't easily find an identical one, so would love to fix it.

I'd be very very grateful for any suggestions. The schematic is in my first post!

[mozwell]

OK, lets try to narrow down the problem
100Hz hum, main power supply electros replaced, no controls affect the hum, plugin into FX return doesn't stop it. Sounds like power amp hum only.
No need to hook up a speaker until we finish getting rid of the hum

1. measure the speaker output voltage to 0V, use both AC and DC ranges on the multimeter, report measurements here
2. short out R60, on the first transistor of the power amp, does this fix the hum (this will check if the fets FET2 & 3 are the cause of the noise)
3. measure the power amp & pre amp power supply rails (A, B, C & D on the sch). measure from each rail to 0V, measure on both AC & DC volts ranges on your multimeter. Report measurements here.
4. replace C60 & C49 in the power amp
5. with power off, measure from the main 0V power supply point (junction of C57 & C58 to EVERY 0V point in the amp, including the transformer center tap, these should all be 0 ohms (or close)
6. connect from the junction of C57 & C58 (or other suitable 0V point) to the chassis of the amp through a 10 ohm resistor (temporary check only, no need to permanently attach this resistor)
7. Double check you got the polarity correct on any electros you replaced

These checks will give us a good idea as to where to go next.

[Jazz P Bass]

I for one do not trust "I changed the filter capacitors" and that's that.

Check the Vac ripple on the caps just to be sure.

[robdean]

Mozwell, thank you so much! I ran out of time today so did not get everything done. Here's what I do have, which I will double check and expand on tomorrow. Do let me know of any additional tests or of any results that look too crazy to be correct.

1. measure the speaker output voltage to 0V, use both AC and DC ranges on the multimeter, report measurements here

0.2v DC
0.4v AC


2. short out R60, on the first transistor of the power amp, does this fix the hum (this will check if the fets FET2 & 3 are the cause of the noise)

Hum becomes somewhat quieter, maybe up to 50%, but still very much there.


3. measure the power amp & pre amp power supply rails (A, B, C & D on the sch). measure from each rail to 0V, measure on both AC & DC volts ranges on your multimeter. Report measurements here.

I'll double check these tomorrow. A couple of components seems to have their labels swapped schematic vs pcb!

A: 11.6v DC, 25.0v AC (!)
C: 10.8v DC, 23.2v AC (!)
B: -25.8v DC, 0v AC
D: -24.8v DC, 0v AC


4. replace C60 & C49 in the power amp

I have dug out the replacements but have not had time to do this yet, because 5 below took a long while


5. with power off, measure from the main 0V power supply point (junction of C57 & C58 to EVERY 0V point in the amp, including the transformer center tap, these should all be 0 ohms (or close)

All good EXCEPT: R73 at the end of the power amp circuit seems ungrounded BUT also appears to differ circuitwise to the schematic. I wonder if this is another component label swapped, though it's the right value resistor. I ran out of time tonight but will take a good long look tomorrow.
[UPDATE NEXT MORNING: it seems that the PCB swaps the order of R73 and C50. It is indeed circa 0 ohms from the corresponding leg of C50 to the C57-C58 junction]


6. connect from the junction of C57 & C58 (or other suitable 0V point) to the chassis of the amp through a 10 ohm resistor (temporary check only, no need to permanently attach this resistor)

Will do this when things next reconnected, but the connection from that junction to the chassis is very direct and substantial and metered as no resistance. Do you mean I should replace this link with a resistor?


7. Double check you got the polarity correct on any electros you replaced

I was very careful about this, and triple checked! Polarity is marked on the PCB, and I checked this corresponded with both the components I removed and the schematic before fitting tested new components.


Thank you again, I really really appreciate your help and expertise. Tomorrow I should have time to complete the work and to double check everything: but I thought it might be worthwhile to show the results I have, as you might be able to add to my tasklist for tomorrow based on what I have...

[robdean]

Remeasured rail voltages,pretty much confirm yesterday's measurements.

HOWEVER: it has been suggested to me that my DMM likely cannot reliably quantify AC superimposed on DC, since it is a pretty basic model and, I'm told, likely simply diode-rectifies AC to DC in order to measure it?

A: 12.0v DC, 25.8v AC (!)
C: 11.2v DC, 24.1v AC (!)
B: -26.6v DC, 0v AC
D: -25.5v DC, 0v AC

additionally:

VCC- : -33.1v DC, 0v AC
VCC+ : 32.9v DC, 72.8v AC (!)

I hope that despite the metering issue, someone can suggest what I might be able to do to fix this amp! Are there other tests I can do or components I can swap?

Thank you again.

[robdean]

I for one do not trust "I changed the filter capacitors" and that's that.

Check the Vac ripple on the caps just to be sure.

Seems like a very wise thing to do, but also seems I don't currently have the gear to measure it effectively: see posts below.

Any suggestions gratefully received as to any alternative way to narrow this down! In the medium term I can budget for and order some moderately better test equipment, but I'd be elated if I could make progress with this amp on a shorter timescale than will be dictated by finding money for additional test gear. It seems there is some significant ripple on VCC+: I'm totally inexperienced in such power problems, but I'm guessing (really guessing) that there might be some worthwhile indications in this data as to where the solution might lie?

[Jazz P Bass]

Quote: "A: 12.0v DC, 25.8v AC (!)
C: 11.2v DC, 24.1v AC (!)
B: -26.6v DC, 0v AC
D: -25.5v DC, 0v AC

additionally:

VCC- : -33.1v DC, 0v AC
VCC+ : 32.9v DC, 72.8v AC (!) "

There is your hum.

The positive VDC rail has something going on.
There should not be that much Vac riding on it.
Open resistor, bad ground track, bad (even though it is new) capacitor, bad solder connection......

[g1]

it has been suggested to me that my DMM likely cannot reliably quantify AC superimposed on DC, since it is a pretty basic model and, I'm told, likely simply diode-rectifies AC to DC in order to measure it?

I've been feeling a little odd about some gaps and redundancies and finally figured out I've been following the same unit on 2 separate forums .

See the info on the other forum about using a cap in series with your + probe to block DC when on AC ranges.

[robdean]

I've been feeling a little odd about some gaps and redundancies and finally figured out I've been following the same unit on 2 separate forums .

See the info on the other forum about using a cap in series with your + probe to block DC when on AC ranges.

Being so new to this I threw two bottles in the ocean! Mozwell really got me started here, and the probe cap suggestion is much appreciated and awaits only my picking up some high voltage caps tomorrow. All my stock is geared towards battery circuits, as is my very limited experience! I really appreciate the help, and seem to be enjoying myself. In a tearing-my-hair-out kind of way...

[Jazz P Bass]

Try to keep in mind that the power supply, as designed , is nothing out of the ordinary.
(be thankful that it is not a SMPS).

So the volts ac on the +Vcc rail is an anomaly.
The supply, as designed, should filter that out.

So your job is to find out why it is there.

[g1]

awaits only my picking up some high voltage caps tomorrow. All my stock is geared towards battery circuits

This is not a high voltage circuit (like tube amps would be) so you can probably get away with a 100V cap if you have any.

[robdean]

OK, lets try to narrow down the problem
100Hz hum, main power supply electros replaced, no controls affect the hum, plugin into FX return doesn't stop it. Sounds like power amp hum only.
No need to hook up a speaker until we finish getting rid of the hum

1. measure the speaker output voltage to 0V, use both AC and DC ranges on the multimeter, report measurements here
2. short out R60, on the first transistor of the power amp, does this fix the hum (this will check if the fets FET2 & 3 are the cause of the noise)
3. measure the power amp & pre amp power supply rails (A, B, C & D on the sch). measure from each rail to 0V, measure on both AC & DC volts ranges on your multimeter. Report measurements here.
4. replace C60 & C49 in the power amp
5. with power off, measure from the main 0V power supply point (junction of C57 & C58 to EVERY 0V point in the amp, including the transformer center tap, these should all be 0 ohms (or close)
6. connect from the junction of C57 & C58 (or other suitable 0V point) to the chassis of the amp through a 10 ohm resistor (temporary check only, no need to permanently attach this resistor)
7. Double check you got the polarity correct on any electros you replaced

These checks will give us a good idea as to where to go next.

I have now upgraded from a budget DMM to a £50 / $75 model which offers true RMS AC measurement. It took a few days to figure out what I could afford and what to get.

1. measure the speaker output voltage to 0V, use both AC and DC ranges on the multimeter, report measurements here

Cheap DMM: 0.2v DC - 0.4v AC
RMS AC DMM: 165mv DV - negligible AC

2. short out R60, on the first transistor of the power amp, does this fix the hum (this will check if the fets FET2 & 3 are the cause of the noise)

Hum becomes somewhat quieter, maybe up to 50%, but still very much there.


3. measure the power amp & pre amp power supply rails (A, B, C & D on the sch). measure from each rail to 0V, measure on both AC & DC volts ranges on your multimeter. Report measurements here.

I'll double check these tomorrow. A couple of components seems to have their labels swapped schematic vs pcb!

A: 11.6v DC, 25.0v AC (!)
C: 10.8v DC, 23.2v AC (!)
B: -25.8v DC, 0v AC
D: -24.8v DC, 0v AC

RMS DMM:
-VCC: -32v DC, under 0.1v AC
B : -25.7v DC, under 0.1v AC
[D] : -24.7v DC, under 0.1v AC
+VCC: +32v DC, under 0.1v AC
[A] : +11.5 vDC, under 0.1v AC
[C] : +10.8v DC, under 0.1v AC

Simple DMM:
-VCC: -32v DC, negligible AC
B : -25.8v, negligible AC
[D] : -24.8v DC, negligible AC
+VCC: +32v DC, 71.1v AC
[A] : +11.6 vDC, 25.5 AC
[C] : +10.8v DC, 23.6 AC

Analog Meter:
-VCC: -32v DC, negligible AC
B : -25.5v, negligible AC
[D] : -24.5v DC, negligible AC
+VCC: +32v DC, 90v AC
[A] : +11.5 vDC, 26v AC
[C] : +10.6v DC, 24v AC


4. replace C60 & C49 in the power amp

Done - no improvement


5. with power off, measure from the main 0V power supply point (junction of C57 & C58 to EVERY 0V point in the amp, including the transformer center tap, these should all be 0 ohms (or close)

All good EXCEPT: R73 at the end of the power amp circuit seems ungrounded BUT it seems that the PCB swaps the order of R73 and C50. It is indeed circa 0 ohms from the corresponding leg of C50 to the C57-C58 junction.


6. connect from the junction of C57 & C58 (or other suitable 0V point) to the chassis of the amp through a 10 ohm resistor (temporary check only, no need to permanently attach this resistor)

No effect. The connection from that junction to the chassis is very direct and substantial and metered as no resistance. Do you mean I should replace this link with a resistor?


7. Double check you got the polarity correct on any electros you replaced

Definitely correct. Triple checked by independent means: pre/post photos, polarity markings on PCB, circuit trace vs schematic.

ALSO

I used an iPad app and USB audio interface to capture the hum waveform - attached...