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Messages - DrGonz78

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16
Amplifier Discussion / Re: Acoustic 220 quieter volume than expected
« on: November 07, 2019, 03:23:10 PM »
I am actually not familiar with this model and have not worked on one before. Basically we just need to break the amp up into sections, Preamp & Power amp. If the preamp is putting out a good healthy signal then the problem is pointing to the power amp and vice versa. Conquer and divide is the term for such an exercise and we want to know where in the circuit to focus our attention.

Edit: Typical issues are sometimes good to look at but can easily get us looking the wrong direction for the fault in the amp. Best to troubleshoot in a logical way first. Once we determine if the amp has a preamp problem or power amp problem then the next steps to to troubleshooting will follow. For example, we might have a power supply issue where signal is not passed through either the preamp or power amp. So honestly testing for power supply voltages might actually be the very first step in the process. Since testing preamp and power amp is simple we sometimes just jump right in to check that first.

17
Amplifier Discussion / Re: Info on Sunn Stinger 100 amp
« on: November 06, 2019, 02:53:46 PM »
I think that EL34World website download is the Japan version. Either way just email Stratopastor and he'll help you out, he's a nice guy.

18
Amplifier Discussion / Re: Acoustic 220 quieter volume than expected
« on: November 01, 2019, 03:49:22 PM »
First thing to check is the preamp output and poweramp in mono jacks. The main instruments signal path passes right through those jacks. Often those jacks get dirty and the signal starts to fade as heads through to the output section of the amp. Easiest way to test the jacks is to just simply connect the two jacks together using an instrument cable. Does plugging this patch cord from preamp out to poweramp in make any difference the volume?

Later you can connect the preamp out (Acoustic 220) to the poweramp of another working amp to test the preamp section. Then take the preamp out of another amp (Guitar signal, iphone preamp works too) and input it into the power amp input of the Acoustic 220 amp. That way you can find out if there is a problem with the preamp or poweramp.

https://elektrotanya.com/acoustic_220_guitar_amplifier_sm.pdf/download.html

19
The Newcomer's Forum / Re: Peavey Audition Plus
« on: October 16, 2019, 03:43:19 PM »
Look inside the amp chassis and confirm if there is a 2030 type opamp power IC. Not sure how closely related your amp is to the schematic posted but we'll find out as we go. Start by confirming the 2030 and then we're off to the races.

20
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.

21
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. 

22
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.

23
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.
https://www.musicparts.com/products.asp?Company=Acoustic

24
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.
https://www.ssguitar.com/index.php?topic=4398.0

25
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.

26
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?

27
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...
https://supportloudtech.netx.net/loud-public/#/asset/1671

28
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?

29
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.

30
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.
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.

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