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Where to start?

Started by Gremlin, December 18, 2014, 12:16:20 PM

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Gremlin

I'm new to the forum and a big hello to you all.

A short background on myself. I have  very basic knowledge of electronics

(I know what most of the parts in an amp are called and can determine

most values) Spent some time doing rework/repairs on circut boards for a

computer firm replacing bad chips, resistors, rewirering for updates and

fixing broken etches. So at least remove and replace are not a problem

for me. Where my problems start is that the boards I worked on came from

the techs with little orange stickers that said what needed to be

replaced or fixed. So I have zero diagnostic experience and my eyes and a

good multimeter for diagnostic tools.

With all that said: I have an Eleca EAA 60 amp that was said to be brand

new but never worked. The customer was given a replacement and it was

given to me by the guitar shop I deal with. What I have done so far is to

plug in the amp and a guitar and turned on the amp. The power light comes

on and I get a small speaker pop but no sound. So I have removed the amp

head and did some visual inspection. While poking around in there I

noticed the one etched board connected to the A1695 power transistor was

loose and actually came off with little movement.The board is just a

direct etch from wire to transistor. Can I just resolder the wires direct

or is the board there to keep the wires away from the heat generated?

I'm almost positive this is unrelated to the original problem. So my main

question would be where to start? I had thought about either plugging a

cd headphone outlet into one of the inputs and working my way down the

preamp board and seeing if I see signal or just running continuity from

the input lead down the board and seeing if there is a break somewhere.

At any rate where would you start?
Thanks in advance.
The harder I try the behinder I get

DrGonz78

Welcome to the site! At first glance it almost looked like you had snipped off those transistor leads... So you basically wiggled that small PCB connected to the transistor and it came off just like that? That needs to be fixed first. The heat transferred from the transistor is occurring at the heat sink, so I would think connecting the wires directly to the transistor would be fine. Maybe wipe off(99% isopropyl & a q-tip) a bit of the thermal paste near the leads of the transistor and solder the wires directly. This should work for some temporary testing, but make sure to document where each wire connects.
"A person who never made a mistake never tried anything new." -Albert Einstein

Gremlin

Thanks for the welcome DrGonz78.  It does look like I snipped the PCB off. Yes I was doing a visual inspection and putting some slight pressure on the different components to make sure the solder joints were tight and the PCB just snapped right off. Looks like they used a little too much heat when they did the solder there and didn't seat the legs all the way in to the thicker part of the legs. The plugin on the board is a one way plug and the etching is a straight shot to each lead so doing them one at a time should keep them at the right connecton.

My appoligy for the double spacing in my first post, I copied and posted from notepad and having to leave for work I didn't have time to go back and fix it.
The harder I try the behinder I get

Gremlin

Got the transistor wired back in. I think I read/heard somewhere I shouldn't have continuity between either speaker lead to chassis ground. Is this correct??
The harder I try the behinder I get

g1

  For starters, the speaker should not be connected until you verify there is no DC at the amp output.
  With the speaker disconnected, check the amps output leads.  The - lead is often grounded so don't worry if you have continuity to chassis on the - wire.
  The + lead should not give continuity to ground.
Before connecting the speaker, when you turn the amp on measure for DC volts between the leads that go to the speaker.  There should not be any substantial DC there, maybe up to 50 millivolts.

Gremlin

Thanks for clearing that up for me g1. No continuity on the positive lead to the speaker. Voltage between leads to the speaker about 38 millivolts. Rail voltage on amp board +/- 40.3 volts. Kind of running blind here, have no schematics. Talked to Eleca and they said they might be able to dig one out for me but may take a while as the amp is discontinued. Meanwhile what might the typical voltage going to the preamp boards?
The harder I try the behinder I get

g1

  If the preamp has IC's, they will typically run on a split supply of approx. +/-15V.
It will often be derived from the main supplies, but the voltage dropping may be done either at the main supply or on the preamp board.
Seeing as we are completely blind about what this amp even is (not a common brand or model), some pictures would definitely help.
  As you did not have fuses blowing, and there is no DC on the output, you are probably ok now to connect the speaker and check for sound.
Does the amp have an effects loop (send and return jacks) ?

J M Fahey

A couple things can be deduced from pictures:

1) the amp does have ICs , most certainly Op Amps, visible at least on one board.
So it will most probably have +/-15V supplies
You can check that at least the IC seen is getting -15V on pin 4 ;  +15V on pin 8  .
Which is which? :  read the IC code, google its datasheet, it will show the pinout.

2) from what you say, that power amp looks fine:
a) it has <50mV DC at the speaker out
b) is not blowing fuses
c) it receives proper +/-40V
d) speaker slightly pops on turn on.

so I wouldn't waste time on it

3) what worries me a lot, doubly so in your case, is that the amp is a jigsaw puzzle of tons of submodules, all interconnected by myriads of push in, probably non soldered but just pressed/crimped connectors.

Each and one of them , if bad, means a "traffic stop" for signal.

How will you find it?  :-\
There's a reason they gave you that amp for free: it's a nightmare even for a regular Tech.

The standard recommendation: "inject signal and follow it with a scope" won't help you very much.
No forgetting it's an "orphan" amplifier, no schematic, etc.

4) There's (basically) 3 options here:
a)  get an (experienced in SS and MI amps) Tech
b) junk or donate it (what the shop did)
3) if after testing that power amp works (which I think it does)  you may junk the complex dual preamp (with plug in modules to boot  ::) ) and EQ and effects modules , wire an input jack straight to the Power Amp, and use it as a 100W power booster, driving 1 or 2 12" speakers to boost any small but tasty amp you have into a firebreathing dragon.

It will give muscle to any small tube or typical 15W practice amp.

You may also drive that power amp straight from any modern floor multi effects unit or any simulator, such as POD, V Amp, etc.

Gremlin

Thanks for the info guys, no there is not any effects loop g1. Yes there are at least 4 IC's on the pre-amp boards. Can't really see if there are any on the chorus main board or not yet.
I understand we are flying blind here, with an amp that is most likley considered a throw away amp, but I am working on it to get a better understanding of how all these things relate and work together. Being it is an accoustic amp I figure it might be a good setup for my 3String Cigar Box Guitar.
The shop I got it at is closing it's doors the first of the year and walked off with this amp plus a Fender Frontman 25R that had a couple scratchy pots that I replaced and is working fine and the head from a  Peavey 240t Deuce that had been dropped off a truck and had the front damaged breaking three pots and two of the tubes. Got the front panel on it straightened out and the pots replaced just waiting on the tubes.
At any rate I think we are narrowing down the problem on this one. I am getting no power to the preamp boards so will have to figure out where the voltage is broken down.
The harder I try the behinder I get

Gremlin

#9
I have attached a pic of where I think the problem is. I believe the voltage regulator on the left in the photo is bad. By following the etches on the board and checking voltage I am getting a -15.4 voltage out of the one on the right, the one on the left I am getting various +mv readings. Am I on the right track?

I pulled this out it is a L7815cv

The harder I try the behinder I get

Roly

Quote from: GremlinI am getting a -15.4 voltage out of the one on the right, the one on the left I am getting various +mv readings. Am I on the right track?

Oh yes, most likely.  Well deduced.   :dbtu:

78nn
78 - +ve 3-pin voltage regulator
15 - volts (1 amp, and thermally limited)

These are the regulators for the (nominal) +/-15V supply rails for the preamp.  There are two reasons why the +ve one may be near zero, it could have failed open, or something in the preamp could have failed short to ground.  On the odds (and the observation that it's getting hot), the latter is more likely.

One trick to tell which is to (somehow) disconnect the output lead of the regulator and see if the voltage on the lead jumps back up to about +15 volts.  If it does, the regulator is okay and just protecting itself against a short somewhere in the preamp (which you now have to locate).

If the voltage is anywhere else, too low or too high, then the regulator is stuffed and needs to be replaced (however it is still possible that there is still another, primary, fault that caused its failure).

These supplies will go to a connector then cables to take them to the preamp board, and perhaps to the Fx boards.  If the preamp connector is a plug and socket it may be easiest just to pop this off and see if it restores the +15V at the regulator, if so your short is on your preamp board {and the prime candidate is the op-amp electrically closest to the input socket.  Are any op-amps getting hot?}


Gleenings;
Eleca EAA 60 amp

Quote from: http://music-electronics-forum.com/t26455/#post227380The op-amps are 4558s, power to the op-amps are +-14.68 VDC. ... Supply rails are +-34.68V.

4558 = LM833, TL072, TL082 etc etc, just about any dual op-amp.
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

Yes, that's suspect.

Regulators usually get 20 t0 30V raw voltage in, give 15V out , polarity positive or negative .

So if one of them offers correct -15V and the other does not offer matching +15V , yes, that shuts off all Op Amps and probably a couple extra things , rendering the amp mute.

Possibilities:

1) it might not be receiving its 20 to 30V raw voltage, check that.

Beware that pinout on the + and the - regulator is not thye same, download datasheet and draw pinout on a paper, then tape it to a wall nearby.

2) the regulator itself might be bad

3) there might be a short in the +15V line which pulls voltage down and kills the regulator.

As Enzo once said, shorted Op Amps generally short + to -, meaning they equally compromise both sides, but in a few cases they might short to ground.

With amp off and suspect regulator removed, measure continuity/resistance from the suspect +15V rail to ground.

Use various scales, at least the 2k one, the 200r one and the diode test one, both ways.

Good luck :)

Gremlin

Thanks guys,  It is getting 22.4 volts at the input if my memory serves me correctly. I have pulled the regulator out of the board and will throw a couple rc battery packs in series for 24 volt input and check it to see if it's  good or toast. Then will go from there.
The harder I try the behinder I get

Roly

Quote from: Gremlinrc battery packs

Please, with tests such as this, put a car tail lamp globe or similar between the battery and everything else.  If the regulator happens to have an input short your battery pack can deliver enough current naked to fry the chip between your fingers.  Some sort of current limiting, any sort of current limiting is really vital.  {I still have the scar on my thumb after 50 years}.
If you say theory and practice don't agree you haven't applied enough theory.

Gremlin

Thanks for the reminder Roly. I actually ran my + lead through a 30amp discharge board I made to condition my battery packs. Regulator is bad. It is my understanding that these regulators are pretty trouble free so now will run some test (thanks for the direction FM) to see if it was just a bad regulator or if there is a problem somewhere in the preamp section that caused it to go bad.
The harder I try the behinder I get