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 1 
 on: April 23, 2018, 06:33:44 PM 
Started by aruption - Last post by aruption
Well I got the new AD130s in the mail today and installed both of them in the Pianet.  It's hard to tell, but it seems that the output is not quite as distorted at max volume.  However, the same buzz/hum is still present across the board, and the volume pot still behaves the same way it always does (and affects the buzz/hum level as well).

Here are the voltage readings I am getting on all the transistors now (left to right E B C):
#2 AC107: 1mV, 95mV, 195mV
#3 AC125: 1mV, 98mV, 249mV
#4 OC308: 1mV, 157mV, 335mV
#5 AD130: 2.4mV, 456mV, 5.8mV
#6 AD130: 9.9V, 10.3V, 11.5V

Also I measured the voltage on (what I believe is) the Zener diode: anode 1mV, cathode 94mV.

How do those measurements look?

 2 
 on: April 22, 2018, 10:24:19 PM 
Started by Clyde - Last post by Clyde
Roland seems to wants to deal with 'authorized service' shops and not so much with DIY repair.  At least with schematics and internal components/parts.  So... I ended up with this Alps 7 pos switch from Mouser.  Works perfectly.

 https://www.mouser.com/ProductDetail/688-SRBV170501

There were also 2 ea 1K ohm 0603 SMD resistors that were open.  These were 2 of 8 located on the amp selector baby board that the switch was mounted on. Replaced same and all is well. 

Initial problem was only two of the amp select choices was working.  The above replacements corrected the issue.  But in poking around the rest of the amp I could not help notice the Alps pots are not held together by much.  Closer inspection led me to believe that this amp had been dropped on its top or stacked with something on top that placed pressure on the control knobs and pushed the back out of some of the pots (selector switch included).  I'll be replacing these Alps with something more robust. 


 3 
 on: April 22, 2018, 08:06:50 AM 
Started by El Scorcho - Last post by phatt
You will find that the PCB printing sequence does not line up with the schematic used.
Although they are *Almost the same* there are a some differences.

*Elscorcho* has likely just assumed the PCB numbering is the same as Schematic but if you swap the numbering it makes sense. :tu:

D1&D2 are not at the input they are right before the power amp on the input to VR5 (master volume), maybe Diodes were removed at factory and a cap across the master volume to block excessive treble. Makes sense as the diodes would surely restrict the max volume level. So the cap is likely a factory rethink after pcb's were made. (not uncommon). xP

@ Elschorcho;
As JMFahey has noted it's close but not quite the same. Your best bet is to redraw the circuit. Yes it's tedious but you will learn a lot in the process. 8|

You can use the existing schematic as a guide but don't cheat triple check every connection. (wise to do it over several days)
These are single sided semi translucent PCB's and if you hold them up in front of a strong light you can easy see the tracks while looking at the component side of the board. This allows you to work out where the tracks go. I've done this hundreds of times myself to back track odd circuits that have no schematic.
At the end of it you will have an accurate schematic and a better understanding of how the amplifier works.
You will also help others who may have you model amplifier.

BTW, when you quote components please write down *EXACTLY* what is written on the component. i.e. if it says 273 write that,, don't try to convert it to what you think it means.
A capacitor might read 2u2 or 2.2uF which means the same thing.
just write down what you read as most teck guys will know what it means.
hope it helps,, Phil.

 4 
 on: April 21, 2018, 09:03:55 PM 
Started by El Scorcho - Last post by Jazz P Bass
I believe that TR2 is the incorrect transistor.

The schematic calls for a BC212, which is a PNP transistor.

Your notation states that a BC182 is installed in that position.
BC182 is an NPN transistor.

Also, I do not know what is going on there at the LTP (TR4 & TR5) but those two transistors should be the same type & hopefully from the same batch so that there characteristics match.

 5 
 on: April 21, 2018, 02:57:22 AM 
Started by El Scorcho - Last post by El Scorcho
Here's what I've got so far....

 6 
 on: April 20, 2018, 03:48:04 PM 
Started by Billythekid - Last post by Billythekid
Thanks for getn back to me im with you.as for the generator i have a function generator that allows signal to go out the headphone output jack. Ive also built a signal tracer test probe from online video schematic. So it gos func.generator or the file you provided out of a sound source mp3 amp small bluetooth player.  Where do i go from there?? Signal tracing?? I have multimeter as well.

 7 
 on: April 20, 2018, 05:10:38 AM 
Started by HamSandwich - Last post by phatt
I'm no expert but I had no issues with isolated power (i.e. plug pak/wall wart)

These are the 2 transformers I mentioned which powered many power amp circuit ideas. I can't see any issues *As long as the audio circuit common is connected to a metal enclosure*
AFAIK, IF? the Metal frame of a transformer is bolted to the case and there is no earth then you might have hum issues but that is likely unsafe anyway. xP

As can be seem in pic, these transformers have no earth pins and the case is plastic (Double insulated)
By simply connecting the secondary output neg of TR1 to the pos of TR2 you have +30/0/-30VDC. You now have a split rail supply :tu:
Zero volts becomes circuit common which you ground to the case.


As for Smode supply,, it's a hit and miss game.
Some might work but a lot will induce a high frequency whine on the audio signal and filtering that out is no easy task.
Unless you can find a Sw-mode that is dedicated for audio use I'd be wary. 8|

Those LM1875 chips are found in a lot of small budget amps and the transformers in those are often under rated yet they still punch out a good 10 watts.

A power amp can only output the power that is available from the transformer so the transformer current is the limiting factor.

Let's say a given amp circuit needs 4 Amps to deliver 50 Watts, now if the supply can only deliver 2 amps then the wattage will be more like 25 watts. As the current limit is reached the supply rails droop.
In the case of my two transformers shown that 37VDC drops down to 28 Volts because they can only deliver 400mA each.
But still some of my tests where loud enough to keep up with a drummer.
The transformers only ever ran warm.
I had no hum issues except when I forgot to ground something in the audio path.
Phil.

 8 
 on: April 19, 2018, 04:23:48 PM 
Started by Stephenl - Last post by J M Fahey
They are very different, biasing will drive you crazy.
I would build 2 independent gain stages, each biased properly, join inputs and use a couple mixing resistors to mix outputs, say 2 x 100k or 220k.
Or use a 500k linear "blend"  pot and you can pan from one to the other or get both same level when set to the middle.

 9 
 on: April 19, 2018, 04:17:20 PM 
Started by Billythekid - Last post by J M Fahey
Smartphones are wonderful but not they cann do *everything*  and *at the same time*:

Simplest test instrument is generator, I would play this MP3 file https://www.mediacollege.com/audio/tone/files/440Hz_44100Hz_16bit_30sec.mp3 from *any* other player you have available, say an old "pendrive MP3 player" to an old phone to netbook/computer/tablet , so you free the phone for higher purposes.
Set player to continuous/loop/Repeat 1 because it lasts 30 seconds, so it self restarts all the time.

As of the scope, not sure *how* will you get signal inside it, does it  have an external mic/line in?

If it only can get signals "in the air" , through its voice microphone, it´s not much more than a toy.

If you have a Laptop or PC available with such inputs, then it´s a different situation.

 10 
 on: April 19, 2018, 09:22:54 AM 
Started by Stephenl - Last post by Stephenl
I was thinking of trying this, but I don't have a lot of experience using FETs. I'd like to try the conventional arrangement of common Anode and Cathode resistors...but I'm guessing the LND150 will need something different for biasing.

Thanks in advance for any education on this.

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