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Messages - DrGonz78

#31
Amplifier Discussion / Re: Vintage 1967 Astro Amp
March 19, 2019, 04:03:01 PM
For some reason the link to the pictures is broken and not working.
#32
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?
#33
Schematics and Layouts / Re: Crate GFX50
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...
https://supportloudtech.netx.net/loud-public/#/asset/1671
#34
Amplifier Discussion / Re: Grounding techniques
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?
#35
Here is what I have for schematics. Not sure this is the amp that you have but it is something related at least.
#36
Quote from: Roark on September 04, 2018, 05:15:41 PMDrGonz78 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.
http://www.youritronics.com/audio-amplifier-with-tda2050/

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.
#37
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.
#38
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.
#39
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.
#40
Quote from: Roark on August 21, 2018, 05:21:23 PM

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.
#41
Typically we say don't buy it on eBay since there are so many fakes there... However, I have personally bought from this seller before and feel okay recommending him to others. You can search mouser.com or digikey.com for the LM1875 IC chip but you won't find brand new manufactured TDA2030, TDA2040 or TDA2050. So that is why we go to eBay to find them and buyer beware of fakes. I still feel safe buying this particular chip from the seller I listed though.

https://www.ebay.com/itm/1-x-TDA2050-32W-Hi-Fi-Amplifier-IC-USA-SELLER-FAST-FREE-USA-SHIPPING/301246868204?hash=item4623b66aec:g:uV4AAOxy69JTGjHO
#42
Yeah look at the picture here and I circled the output IC chip. It is bolted down to what we refer to as a heat sink, which helps the component expel heat through the metal. Look on that chip that is bolted down and try to read what it says. The diode is in question too if it is burnt as well as the trace of copper on the board that it connects to.

Edit and ADD... The IC power opamp should look identical or similar to this...
#43
The hum you hear is DC voltage on the speaker and that is bad for the speaker, so disconnect the speaker when powering up the amp. At this point I would guess the IC chip is probably a TDA2030 or something similar. The other thread you started on the MEF website had G1 asking you to measure DC voltage at the speaker connection. Do this but remove the speaker first since it probably has DC volts and it's not worth leaving connected at this point. So start by identifying the IC chip in there connected to that heat sink. Look at the top body of the component for a part ID.

P.S. Not sure but that D6 Diode looks like the top leg got smoked a bit?? Please confirm...
#44
Not sure if they will support you on this specific product?? But I did call Fishman when I was fixing loudbox mini and they were very receptive to my call. In fact, they offered repair rate that seemed reasonable and they helped me out with schematics. Now that might be the result that the model I was trying to fix was far past being a warranty repair. Not sure about your product but it is worth the time to call... They have general 1-800 type number and international type too.

https://www.fishman.com/contact/