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Messages - DrGonz78

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Amplifier Discussion / Re: Match Output Transistors?
« on: August 15, 2019, 03:51:49 PM »
Recently, I did a purchase order from Peavey and was a bit surprised at the cost of the MJ15024 & MJ15025 transistors, $10 each. I ended up buying them from Mouser for about $5-6 each.

Amplifier Discussion / Re: Match Output Transistors?
« on: August 14, 2019, 04:09:24 PM »
When I have one failed output transistor failed in a set I typically will replace the entire set. Not only that I will replace the drivers and pre-drivers that associate with those output transistors. You want to get it just working again but you want it to keep working for many years to come. There was one case where I had replace 3 output transistors on one side of a PA system and left the working originals in place. That amp still works to this day and that was over 5 years ago. In that case I was feeling mighty frugal and got really lucky. I still recommend against trying to get lucky. 

Amplifier Discussion / Re: AMPEG B2R Problems-Help Please!
« on: August 13, 2019, 03:39:16 PM »
Amps with actual electronic problems generally don't "magically come back to normal".

I sure do miss Roly. Very good quote to get stuck in your head. I have many statements like this one in my memory bank from so many great techs I have met here and other places online.

So if you get no luck calling Acoustic there appears to be an option to buy the service manual/schematics on this site here for $20.

Schematics and Layouts / Re: Univox Stage 65b
« on: March 29, 2019, 03:41:18 PM »
Check out this link and all the information to see if any of it relates to your amp.

Amplifier Discussion / Re: Vintage 1967 Astro Amp
« on: March 19, 2019, 04:03:01 PM »
For some reason the link to the pictures is broken and not working.

Amplifier Discussion / Re: Help with Gallien-Krueger 200GT guitar amp
« on: March 03, 2019, 03:19:52 PM »
Typically a solid state amp does not have an output transformer but this amp has one. Are you using a load on the output? The norm with solid state amps is to not have a load connected while servicing the amp and once it is stable you connect one. So should smackoj have a dummy load or speaker connected to the amp?

Schematics and Layouts / Re: Crate GFX50
« on: February 02, 2019, 03:00:22 PM »
The great thing about Loud Tech. today is that they put a web page together for various schematics. Possibly they are not indexed to be searchable by google so they never turn up on a search. Here is what they have there...

Amplifier Discussion / Re: Grounding techniques
« on: December 31, 2018, 02:57:30 PM »
What amp is it? Does this mean you have a 3 prong power plug where the ground is broken off or something? Or it could be you have an old amp with a two prong, which involves some conversion. Or it could mean you have a modern amp with a two prong cable that is polarized and the manufacturer just did not utilize the ground plug option. So as Joe asked what is the concern here?

The Newcomer's Forum / Re: Carlsbro GLX100 footswitch query
« on: December 29, 2018, 03:14:56 PM »
Here is what I have for schematics. Not sure this is the amp that you have but it is something related at least.

DrGonz78 seems to have more information [that I have a -/+18v power supply] & sees no instability problem.

Look and read this little tidbit... Note the statement on this page "If you have low impedance speakers use +/-18V supply." This was mentioned as way of saying if you running lower than 8 ohms then... do it this way.

It is saying that if you run the chip at max power supply -/+25v supply that it will only be stable with an 8 ohm load. Now typically an amp designed with a 2050 chip the power supply will be run around -/+22vdc. So the power transformer will be a 32vac supply but with a split power supply that will be -/+16vac .

16 * 1.41 = 22.56vdc >>> We multiply the voltage by 1.41 as to figure out the added voltage when the AC is rectified to DC. So most amps I have come across running tda2050 chip will have very similar set up as what I have mentioned.

You can actually confirm your voltage on your amp with the chip disconnected. I would be guessing that the power transformer is 24vac w/ center tap and it results to a split power supply of -/+12vac. 12 * 1.41 = 16.92vdc. Check out your voltages on your amp to be sure.  :dbtu:

Note that when I say that a 2030 chip puts outs only 14 watts that is at 0.5% THD. It will dissipate up to 20 watts but at the expense of increased harmonic distortion. This is with a -/+18vdc power supply.

"Thanks to its high power capability the TDA2050
is able to provide up to 35 W true RMS power into
a 4 ohm load at THD = 0%, V S = ±18 V, f = 1 kHz

and up to 32 W into an 8 ohm load at THD = 10%,
V S = ±22 V, f = 1 kHz."

That is from the data sheet and it even implies that running it at 4 ohm load that you should use -/+18vdc supply. Makes sense to me more each time I think about it. Now confirm your power supply voltages to make sure. Which if a 2030 chip suggests -/+18vdc supply then I would think its very close to that indeed.

With the TDA2030A chip running 36v power supply or rather a -/+18v supply it will be max rated at 20 watts as per data sheet. So now you will have the TDA2050 in place that can handle 32 watts at 8 ohms or the claimed 50 watts with 4 ohm load. In those examples though it is with a power supply capable of delivering 50v or -/+25v from the power transformer. Your amp will still have the -/+18v power supply and will have a new chip that is highly overrated to your amp's power supply. Honestly looking at the TDA2030 data sheet I think it's really only putting out 14 watts RMS with a 4 ohm load. I would be surprised if the 4 ohm speaker caused any trouble for TDA2050 chip.

I would just replace the chip as it is most probably bad.

You can measure those two bigger filter caps with your DVM to see if they are holding a charge. Chances are the caps discharge rather quickly and probably only millivolts will be stored. Still just put your meter across each one and check the voltage so you know. You don't have to discharge any of the other smaller caps.

Well if set to max range of 200mv the meter would not be able to read +/- 1. What you are measuring is simply an out of range symbol for the meter. Do you have 200mv, 2v, 20v, 200v, 700v as the range selections? Try putting the meter on 200v setting and then what does it say?

Edit: Also the burning smell is probably that diode leg cooking so yes don't keep the amp on very long at all. Just take measurement to confirm DC on the output.


I will measure DC voltage at the speaker connection using a VOM [is this similar to DC-Offset measurement in link above as described above?],& with amp on ---correct?

I no longer know the polarity of the red & blue speaker wires coming from the amp to the speaker. 
Can I damage the VOM by connecting the positive probe of the VOM to the negative speaker wire, or vice versa?

You probably already figured this out but Yes with the amp on.

Also if you don't remember the polarity of the speaker wires you can use your DVM to figure it out. When doing resistance checks always turn off the amp and even unplug it for added safety. Measure blue wire with Red probe and place the black probe to the chassis, and do the same with the blue wire. Blue is probably negative and red is probably positive. The negative terminal will be grounded to the chassis and will read near 0 ohms resistance. The Positive will terminal will have a bit of resistance relative to ground. This method should work for this amp.

When you measure to check for DC voltage at the speaker leads it will not damage your DVM having Red probe on negative or vice versa. Black lead to negative and Red lead to positive terminal. As example you might see 20V that way and when you reverse the meter leads it will then display -20V. So it just changes the reference point of ground.

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