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Messages - J M Fahey

#16
The Newcomer's Forum / Re: Hello from the new guy
June 03, 2018, 05:36:08 PM
Lots of people improve their amps by swapping the "factory" one for one of the "famous/expensive" ones, say they pull the "black painted" "house label" speaker from, say, a Peavey or Fender or Crate amp (all USA made of course) and put an expensive (and doubtlessly good sounding) Celestion V30 or G12H or Creamback there.
They have no use for the old black nameless speaker so they junk it, or offer it on Craigslist for $20 (and even so they don´t sell) or a garage sale for $5/$10 tops.
Guess what? if the amp was USA made (any one through the 90´s and Peavey up to 2000 or so) then it´s 90/95% possible that speaker was made by Eminence, and wholly comparable to any Legend series.
In fact I think Legends are basically a selection of their most popular OEM offerings, think the generic speaker inside a Fender or Peavey, just labelled and put inside a printed box (instead of an OEM brown or white generic box) so it can be sold to the general public.
They will work very well in your amp, and for a very low cost.
#17
In a nutshell, since you revived this old topic:
1)
QuoteThe amp stock uses an TDA2030 for power. On a tip at another forum, I subbed in an LM1875 which is a pin for pin match.  Per the spec sheet, looks like it's capable of a bit more power (nearly double?) using the existing power supply.
No, exact same power, exact same sound.

2) 
Quotewill require a new speaker to handle the additional power which will be the second mod.
No additional power so no need for a new speaker.

3) want bto try different clipping? fine, go ahead.
#18
And they are not "flattened" , they are that shape from the beginning.
They are basically a thin ceramic disk with metallized faces.
#19
Quote
I'm looking for advice on splicing the output cables from fairly matching rated amps with the same speakers [celestion 12s]
NO
If you splice *output*  cabled from one amp to another you are joining *outputs* and that blows amps as Phatt said.

QuoteI have had speaker issues with a 2x12 Stage 160 that is Not a ripped cone. when plugged in, they have a very audible hum that's harsh and responds to input frequency. I pulled the amp down on stage on accident one night.
Nice to know but irrelevant.
Quote
I now since use the amp to preamp my cybertwin
IF you mean you connect Stage *preamp* to drive Cyber Twin *power amps*  with same signal, and each CT power amp drives its own speakers, yes, that can be done.
Quotewith the same speakers. the CT is rated 130 watts, and the Stage is rated 160
Nice to know.

Read my lips: NO.
Each power amp (one in Stage, 2 in CT) drives its own speaker(s) and nothing else.
#20
Those output transistors, specifically Tr10 are VERY dead, VERY shorted, not sure about your measurement technique.
TR10 must show like a dead short CE ... yet you declare it "good".

Please measure it out of circuit (pull it out), both CE and EC , on Diode scale, NOT resistance scale, buzzer or any other, just the one which has a small diode printed on it, and report what the screen shows, just the raw number, not interpretation, no "it reads fine", not "it shows no resistance" or whatever, just the digits on the screen. Thanks.
#21
Not exactly done to death, in fact I don´t know of a single case .
#22
And in any case nowadays chokes are not used any kore becausetb they are the very expensive solution.
If you are annoyed by ripple just increase capacitance.
#23
"Read"  this Audiobook (just play it in the background while you do something else which does not require concentration) : Make room make room!!! by Harry Harrison.
https://www.youtube.com/watch?v=W_Tgj1Byz50
Written in the 60´s, it predicts a super populated World with all resources worn out for good, no more farm animals, no more fish, only crops are Soy and Lentils.
50 Million people just in New York City.  :P
No oil, no energy, people runs fixed bicycles connected to generators and batteries to cover minimum power needs.

Curious, I checked how much power can a standard Human produce (not an Athlete): some 60/70W for a couple hours or 50W steady all day long.

Not sure about Solar Power but doubt it will be much more, *per person* , if it has to power *everybody* .... say 400 or 500 Million people.

So forget tubes, air conditioning, fridges, clothes ironing , elevators, power tools, and anything power hungry.

We *will*  be able to keep our basic Electronic "toys": tablets, phones, maybe a well designed Laptop, radios, maybe a low power TV set so life won´t be that boring.
Oh, and low power lighting such as LED lamps.

Although most will have to work 12/14 hour shifts since we won´t have machine power to help us any more.
Back to the farm and horse/Ox pulled ploughs?
Back to manure fertilizer? (forget current synthetic fertilizer made from Oil).
Why not?
Amish and Mennonites (we have quite a few in Argentina and neighbouring Countries) never left the old ways and all I know seem to be *happy*  people.
#24
Since you have 2 amps and 2 speakers, connect one speaker to each amp.
If you have a stereo program (Stereo MP3 from Phone or Player) connect them to a stereo input jack and use the proper stereo cable; if for mono program (say Guitar, Bass, keyboard, etc.) connect both inputs together to a single input jack.
#25
Phatt hit the nail in the head  :tu: and I´ll continue:
1) that is a ¨**terrible** circuit, VERY early Germanium Technology.
Very advanced ... in 1960 or so, obsolete by the next year  :o

2) that amplifier design is **terrible** : worse than Class A , besides having 25% efficiency (Class A is 50%) it passes full idle current through the speakers, a HIGH idle current I might add, it shift voice coils out of the gap, go figure.
Terrible design.
You will solve nothing by "repairing" it, just *replace*  it.

What is supply voltage?
I guess some 14 to 16V raw, before the capacitance multiplier.
Let it feed the preamp only,  which is a light load, and:

a) remove the cheesy original amplifier.
Simplest is to plain remove from the board (use a good solder sucker to clean pads first) the parts enclosed in red.
b) beef up the 1000uF main filter cap to a 4700uFx25V one.
You will have some 14/16V there which you will use to feed a:

c) TDA2003 amplifier.
Buy a ready built one from EBay for less than $5, feed it available 14/16V, connect it to speakers but first wire them in parallel, for a 2.5 ohm load.
TDA2003 is designed to drive down to 1.6 ohm, so it will be happy with that.

Now you will have som 8 to 10*clean" RMS watts, enough to play at home in a living room or similar with very good sound.
To play in a Church/Club, etc. you connect line out to an external amplifier, like you already did.

The original amp was capable of 1W (if that much)  and intended for quiet practice at home, at table radio levels, not more.

d) you can replace the cheesy series volume pot with a 10k to 25k one, Audio/Log preferred, but even a Linear one will perform better than the current series one.
You just connect it from coupling capacitor to ground, and wiper to TDA2003 amp board, which I guess already has an input capacitor.
If in doubt, upload board schematic an pictures when you buy it.

You will be (pleasantly) surprised :) 
#26
Quote from: Stevo1987 on May 28, 2018, 05:29:04 PM
Yes, I know that.  :) I just can't see it because of the heat sink (?) or grounding arrangement on top of it.
Ok, do this:
1) check that you have proper voltage at the power supply, we expect something around +18V DC

2) you have 2 *possible* Pin 1 , on opposite diagonals.
*One* of them will have same voltage as from supply ... that is Pin 1.
Mark it somehow, a felt marker dot, a drop of nail enamel, and you´re done.
Then count other pins relative to this one, following the datasheet.

3) measure DC voltage at ouput pin 12, we expect 1/2 the supply voltage, so some +9V DC

4) measure bootstrap volotage on pin 4 , we expect 3/4 of supply voltage or around +12/13V.

5) now to preamp: we expect some 6 or 7 V on Q1 collector.

IF DC voltages are reasonable, then active components are working, lack of audio most probably comes from connection problems, dirty/open pots and jacks, etc.

Do NOT replace parts at random, no matter whether they are "cheap" or even free, you are damaging the PCB, the *only*  component you can not buy over the counter or even at the (long defunct) Factory.
***THE PCB IS THE AMPLIFIER, PARTS ARE JUST ACCESSORIES***
#27
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.
#28
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.
#29
Thanks fuzzythebear  :dbtu: :dbtu:
Schematics are ALWAYS useful.
Somebody else may need them today or tomorrow  :cheesy:
#30
The Newcomer's Forum / Re: Marshall 5010 Needs Help
April 12, 2018, 02:11:46 PM
Started comparing your PCB and the published schematic and just fron the very beginning I see parts designations will NOT match, so you will have to apply what you see on PCB to the paper version.
Actual *circuit* (who connects to who else and how) must match 99%  :o
Just as an example and the first I checked, you go on: paper shows a single jack connected to one of Op Amp inputs through a 0.1uF cap "&C2" ; your amp has 2 input jacks which are mixed by mixing resistors #R1 and #R2 , 68k each .
I am calling paper circuit parts "&"  and actual PCB ones "#" because plain labels R1, R2, C1, C2, etc. differ on both and can be very misleading.

Also checked that yours has Darlington power transistors (MJ2500/3000), the early versions of standard MJ2501/3001 and not MosFets, which further confirms it´s the basic same circuit.