Menu

Show posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.

Show posts Menu

Messages - robdean

#1
What I found educational when I needed to add EQ to something was to build this simple add-on circuit:
http://tagboardeffects.blogspot.co.uk/2013/08/bmp-tonestack-w-lpb1.html

It has only a single tone control, but it can be scooped (well, there is a mid-scoop by default) and offers active boost to treble and bass as well as cut (albeit not independent of each other: boosting treble is coupled to cutting bass and vice versa).

Here's the best bit: this free software simulates the circuit (amongst others) so you can see what effect changing component values will have:
http://www.duncanamps.com/tsc/

I guess you could even add switches to 're-voice' the EQ with different component values. I found it a great intro to active EQ, and built it with an 'unscooped' response for use in an active acoustic guitar circuit, where due to careful choice of component values it does pretty much exactly what I needed.

Of course there are more sophisticated tone-stacks, but I found this an ideal first step...
#2
Thank you again! Just checking in to say my lovely, much used top-of-the-range butane soldering iron has stopped working and is going back to the (excellent) manufacturer (Antex) in the hope they can revive it. I'm asking them to send me a cheaper model as a spare, but I'm out of action for a couple of days :-(
#3
I don't have an oscilloscope, only an app that can plot an audio waveform
:-/

I hear tell of kludges using voltage dividers, I don't know if anyone here would recommend it... ?

I've changed the two 4000uf, and two 470uf caps plus C60 and C49. Any other suggestions?

Thank you for the thought y'all have been putting into this, I really appreciate (and need!) the help.
#4
1. The mod to take the relay out of circuit left the hum unaffected. I have left it in place in case of any suggested checks/tests. Thank you so much Phatt for marking it out for me.

2. Using an iPad app and a USB audio dongle I grabbed a trace of the hum waveform via the headphone output - image attached

3. I bought a more sophisticated DMM: UNI-T UT60E http://uni-trend.com/UT60E.html - true RMS AC
This meter does not report AC on the + rails VCC+, [A] and [C], although both my basic DMM and Analog meter do - none of the meters show AC on the -Rails

RESULTS:

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 (due to diode bias?)
B   :  -25.8v, negligible AC (due to diode bias?)
[D]   : -24.8v DC, negligible AC (due to diode bias?)
+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 (due to diode bias?)
B   :  -25.5v, negligible AC (due to diode bias?)
[D]   : -24.5v DC, negligible AC (due to diode bias?)
+VCC: +32v DC, 90v AC
[A]   : +11.5 vDC, 26v AC
[C]   : +10.6v DC, 24v AC

Incidentally the Hz range on the meter reads 50Hz against speaker/headphone output.
#5
Thank you! I'll take a run at it tomorrow...
#6
Thank you! Grounding that end of C46 increases hum. Short recording attached.

As yet I only have the measurements I posted earlier in the thread which I have been warned by JMF are untrustworthy since I have a cheap DMM which probably cannot measure ripple. I'm puzzled that attempting to use a capacitor in series with the probe simply yields 0v on all scales and measurements. I hope I haven't invented a whole new kind of stupid.

I'll order a better DMM today if i can find a worthwhile one I can afford which will offer better ripple measurements.

Would anyone encourage me to buy, or warn against, a UNI-T UT39C http://www.ebay.co.uk/itm/291301645135 or a Mastech MS8268 http://www.amazon.co.uk/dp/B000JQ4O2U ? I could spend a bit more if need be, but can't go 'pro'!

I sincerely appreciate your help with this amp...
#7
Phatt,

I think I read that myself regarding the DSP introducing noise! Thank you for thinking of it.

In this case I'm not so sure: there is no hum in the send of the send/return loop, which I believe is post-fx. There is hum before the signal-path relay is triggered at power on. The power amp still hums when the coupling capacitor of its signal input has a leg lifted (C46 on the schematic).

What is interesting is that there does seem to be unrectified AC tapped into the FX, relay and FET2/3 subcircuits, so maybe something there could be leaking. Just for my education, can anyone explain why there is a power connection from both unrectified (pre-D6) and post-rectified power (after R75)? All 3 subcircuits seem to be part of that power loop. As I mentioned earlier, there seems to be lots of ripple within the relay circuit power, the FETS seem to get clean DC via D3. I'll measure the fx circuit power in situ when I get the chance.

So the anomalies are hum in the power amp and a nasty ripple on the +ive power rails: if the hum has a cause in one of the above subcircuits it seems it is being induced in the power amp via the power rails rather than noise in the input signal.

I've retraced the rectifier and smoothing capacitor circuit today and they appear to be in agreement with the schematic. The key components seem to meter as one would expect.
#8
Thank you Phatt.

The design seems to have been quite thoughtful as regards earthing. When I tested all the earth connections in the schematic for resistance, they all connected to the same single earth point on the board, a soldered wire between the rectifier diodes and the large electrolytics, which connects to the mains input earth and the chassis. There are two mounting holes like the one you point out: neither has PCB track under it. The bodies of the potentiometers are earthed through their mounting to the chassis, but are isolated from the circuit.

This issue is beyond my skill level to resolve without expert help, the main clue is evidence of substantial ripple on the positive DC rails. This seems to induce hum in the power amp stage directly, as the hum starts before the relay triggers at power up and is unaffected by lifting a leg of the coupling capacitor bringing signal into the power amp.

I'd be delighted to be wrong about everything - I'm just longing to fix the darn thing (rehearsed without it last night!) and eager to learn as much as possible along the way. There's no way I can fix this without some guidance: I'm a hard worker but am much more familiar with a 9v battery by way of power supply :-/

#9
JMF (& G1),

Thank you, I really appreciate your help.

Lifting the top leg of C46 (top as on schematic, the leg towards R83) does not seem to change the hum at all.

Bridging R60 made some difference whilst C46 still connected, maybe 50% less hum. Seemed not to make a difference with C46 top leg disconnected, although not entirely easy to be sure. There is still plenty hum in each configuration.

I've tried using a DMM with a series cap (metallised polypropylene) and also to build the probe with cap and diode. I'm embarrassed to say I'm then metering 0v in both cases DC and AC.

I'm curious about the transformer output between the secondary coil and D6. Following that, I find D3 seems to leave clean DC in the FET2 / FET3 circuit, but the relay circuit meters some AC either side of all its diodes. This being at all significant is pure idiot speculation on my part, given my lack of expertise and of equipment, but I'm trying!

I will be very very grateful for more guidance.


#10
I don't have any high voltage caps but will pick a couple up tomorrow, and try re-measuring.

I sincerely appreciate the very educational advice.

I'm still very eager for any suggestions as to what might remedy this issue: beyond the four power supply caps and four rectifier diodes which I've replaced, I'm at a loss. Of course I'll re-re-check what I've done...
#11
JMF,

Thank you for that information: I had no idea. I looked out an old analogue meter that has 4 ranges (10v AC and 50v AC as well as higher ones) but it reads the same so guess it's working in the same manner as my (yes, cheap) DMM.

I will very seriously consider buying a better DMM, although since very little of my (limited) electronics work involves mains circuits I have not come across this issue before and perhaps seldom will again: that said, my enjoyment of this field has grown a great deal since I originally turned to it out of neccessity, and I now take on challenges (such as this one) that I would not have touched before.

Is there any possibility, despite these readings being quantatively incorrect, that someone here could offer a suggestion as to what failure might be giving rise to these readings? Are there useful tests I can do with my existing meters or by swapping out components? I certainly can't get another meter for a week or so and am longing to fix this amp if at all possible...
#12
Remeasured rail voltages,pretty much confirm yesterday's measurements:

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 (!)

The caps C54-C58 and the diodes D6 to D9 are oriented as they were originally (I took photos!) and in keeping with markings on the PCB.

Centre tap is grounded.

In case it is useful I've attached an image of the front and back of the actual circuitboard superimposed!

Sincere thanks.

#13
gbono,

Thank you for the guidance: here are answers and more...

1. Speaker output:
0.2v DC
0.4v AC

2. Send/Return
There is no hum on the fx send. Signal via return has hum. Nothing affects hum level.

3. Hum in headphone output?
Yes

Also

Power amp & pre amp power supply rails (A, B, C & D on the sch):
(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

I'd be very very grateful for advice!
Thank you again...

#14
I'd really sincerely appreciate help - I've just bought a lovely seldom-seen amp model (Crafter DSP1) - about 10 or 15 years old. I bought it in a noisy environment, and discovered very conspicuous hum once using it at home.

I managed to find a schematic, which is attached here.

I have long experience with passive guitar circuits, and some with simple stomp/active circuits, but very little with mains audio electronics.

The hum is irrespective of the volume control and starts before the output 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 (comparing it by ear with a tone generator). UK mains is 50Hz.

As a best-guess before looking for help I speculatively replaced the two big 4700Uf power supply caps and the two additional 470uf caps - no improvement, nor from changing the four rectifier diodes :-(

Plugging a jack into the fx return makes no difference to the hum. None of the amp controls makes any difference. The PCB is in good condition, and with a careful look I see no obvious solder bridges, past repairs, scorching or disconnections.

Of course the symptoms lead me to figure the issue is after the preamp stage, and that it is mains-related hum (post-rectifier?) but I`m really not sure how best to further troubleshoot this.

Please does anyone have any other suggestions (or any encouragement!). I have several basic multimeters to hand, could make an audio probe, but have no oscilloscope. I'll really be very very grateful for help... I love this amp, my first one specially for acoustic guitar, but the hum is really disappointing. I can`t easily afford to get this professionally fixed, and also know both how thrilled I`ll be to fix it with my own hands, and how much it would make it `mine` ;-)

Thank you,
Rob