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Messages - Hawk

#211
Roly and G1 thanks great info and I soaked it up (no pun intended). I'll resurrect my limiter and put my analog voltmeter to good use. Thanks also for the cap discharge info! :dbtu:
What heat would you use when soldering/desoldering on pcb's?
#212
On the schematic the master volume feeds to seperate/somewhat identical circuits feeding two TDA7293's.

1. If this is considered a Stereo set-up what makes it sound like Stereo? Wouldn't connecting a single TDA to two speakers give us the same sound? Why two circuits?

2. The schematic is confusing! It shows Pain L, Pain R (what do those stand for? Pan Left, Pan Right?), and then two variable resistors as if there are two master volume controls which there are not. What am I missing here?

Thanks!
#213
Thanks Roly for taking the time. Instead of a limiting lamp I have inserted a household wattmeter which shows current draw (I like to know what current the amp is drawing. Or I've also inserted an ammeter between the fuse leads to make sure this wattmeter is accurate and indeed it is). Maybe it's time to trust the limiter, and I have used one in the past, and it's just with all that glowing I find it hard to tell if it's on the cusp of drawing too much current and the fuse will blow! Guess that comes with experience. I can't tell you how much I appreciate this site and people like yourself as this is such an isolating business and it's hard to find someone locally who you can "apprentice with" as there aren't that many of us around. thanks!

Later today I will inject the signal at the master volume, great idea!--I also picked up a an RCA VoltOhmyst but not sure if that will give me a more accurate reading than the modern DMM. From what I've read it won't due to loading. What would you suggest? I value you your years of experience!
#214
No worries. I'm new so will definitely use the one thread for the same amp. I will take your advice and create some guitar loops to inject into the amp and signal trace. Also, I'll check the LED's to make sure they measure correctly (looks like I'll have to take the board out and measure from underneath--too bad they make these things so hard to service!)

I noticed that D11 is O ohms in both directions and is charred. Not sure if that has something to do  with it but that can't be good. Later today I will replace the diode and see what, if any, difference I notice in sound. From theory  I'd say that this diode will accept both the neg and positive of the ac wave and will always be "on" as it has no resistance in either direction and therefore no forward voltage drop. Does this create AC on DC line?  that can't be good, Hmmm.....
#215
The notes on clean channel decay normally, but not on the distortion channel. Instead they sound for awhile then suddenly stop as if being sucked out of thin air. Something is damping the natural decay of the notes...any suggestions? Transistors? Caps?

Also, using my scope and a 1 KHZ signal injected, and the Distortion Signal selected, the signal is very distorted on the Scope. Of course to me that makes sense as this is distortion, but what kind of distorted signal should I be seeing on the Scope? There is nothing as readable as a sine wave with some distortion on the wave, or signs of saturation or cutoff, just a wierd distorted signal which looks the same at all volumes, just larger and smaller values of the originally crazy signal...does this have a lot to do with the first three lines of my query?  thanks for reading this!
#216
I've been reading Jack Darr's book, Electric Guitar Amplifier Handbook. I love  it!

1. He refers to hooking up an ammeter to the collector of the output transistor to measure current while slowly bringing up your variac and keeping an eye on the current (I'm pretending the amp is blowing fuses and trying to discover the problem). I chose to use the MG250DFX as my practice mule, so I can't physically unhook an output transistor collector and hook in my metere. Instead, I'm dealing with the TDA7294 DMOS audio amp. Do I break the wire connection to Pin 14 and hook in the ammeter there??? (This amp works so I'm just getting used to being hands on and am not trying to fix it, at least not this part--some quick dying of notes on distortion channel which I'll deal with later).

2. On the clean channel, I injected a 1 KHz, 50 mv signal into the amp. I hooked up a Weber VST Dummy Load on 8 ohm impedance. I hooked up an rms voltmeter across it. I turned up the master volume to full and slowly brought up the gain until the top of the wave started to flatten then pulled it back a little. At that point I found approx. 3.8 volts across the dummy load of 8 ohms. Using the formula, Esquared/8 I come up with 3.8x3.8/8=1.08 Watts. What does this reveal? It's obviously not the full output power of the amp.

Thanks!
#217
Thanks DrGonz78! Appreciate the response. I follow what you are saying.
#218
How does the DG212CJ function in the post secondary transformer set up? I've tested and found the 5V fed from the voltage regulator and also found +/-15V on the DG212CJ. I found 15V on pin 8 of the 4558DD Dual Op amps in the schematic above. Since I haven't looked under the board is it safe to say that pin 8 of one of the op amps is feeding the voltage to the DG212CJ?

I'm having a little trouble reading the schematic as I don't see where they would take off the 15 volts to feed the pre-amps section. Where does that usually happen?  Thanks, Hawk (the new guy!)
#219
Thanks Everyone, that was a huge help! Enjoy your day!
#220
So, basically, it's safe to assume that I can run the amp without any load at all, therefore disconnect the speakers.
#221
Thanks for all the input everyone!. I want to open up the amp and test voltages etc.to help with my knowledge of transistor amps. I want to attach a scope to the ouput as well as a voltmeter. Basically, I thought it would be safer to remove the speakers and replace with dummy loads while I take measurement in and out of the amp.  Hopefully that makes sense.
#222
Hello Everyone,

New to this forum. New to some aspects of guitar amps,or perhaps many!. Bought a Marshall MG250 and wanted to hook it up to my Weber Dummy Load Box, but I soon realized that there are two speaker outputs on back of amp, left and right, min 8ohms, 2 X 50w rms into 8 ohms. My first guess is that I need a second dummy load. I understand how parallel/series speaker hook ups work but not quite sure if I can hook up the speakers together and run a line into a quarter inch speaker cable into the dummy load. The speakers are 8 ohms each. Would they be hooked up internally in series, 16 ohms? If they were parallel they would be  4 ohms, yet the caution on the back is 8 min ohms. The two outputs have thrown me off! Can I somehow hook this amp into one dummy load? Thanks very much.