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Messages - GIMAGUITARS

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1
Hello All.  This is what has been done now:
1.Jumped the input jack leads.
2. Checked for bad traces/ track = all tracks and traces good
3. lifted one leg of all resistors and tested= all within 5% tolerance

CONTINUING PROBLEMS:
-low gain input jack not working
-hi gain input- plug breaks first "switch" for sleeve, tip can touch second switch and there's  some noise (even RF radio) but, cannot break second switch or else amp squeals.   

other than that everything else seems to be in order.  I believe the problems are now isolated to the input section.


2
I'll type this in a "Less offensive case".  I prefer cap letters as it's easier on my eyes.  That being said... No, I have not contacted Randall.  Any suggestions?

3
HELLO EVERYONE! I'M LOOKING FOR A PEDAL LAYOUT OF THE RED CHANNEL FROM THE RANDALL RG100ES THAT ISN'T THE "PREAMP FROM HELL" MUNKY VERSION.  I'M OKAY WITH RUNNING IT AT THE STOCK 24 VOLTS WITHOUT MODDING IT OR DROPPING VOLTAGE.  IF SOMEONE HAS THE LAYOUT FORMATTED WITH ALL CONNECTIONS NECESSARY FOR THE BUILD WITH OR WITHOUT TRUEBYPASS AND A DC JACK IT WOULD BE APPRECIATED.  SO, CHECK YOU FILES IF YOU HAVE IT AND POST IT.  THANKS ALL. 

4
Thanks Roly for the words of encouragement!  This amp is dying to relive its days of glory and is right on the brink of it.  There was a bit of a snafu with both low and high gain inputs when I acquired this amp.  Further down the traces there are a few small dark spots on the track.  I will jump past the poor spot and post results when completed.  It's kind of funny,  I just finished building the Noisy Cricket 1/2 watt amp from junk parts out of old electronics and it works flawlessly but This Marshall can't catch a break. 

5
I pulled all 4 of the T0-92 transistors and one of the BC184 transistors was bad. ordered the part.  Will update when part arrives and is installed.  Transformer and diode bridge was also retested and both are in perfect working condition.  Power section is good.



UPDATE:  new transistors installed, amp has now has volume and is quite loud.

Now the only problems are:  when 1/4 is plugged into high gain, it can pass through the first "bump" and touch the 2nd bump and have sound.  If the plug passes through the second bump, there is a high pitched whine/ squeal.

2nd problem- low gain input doesnt seem to work at all

NOTE:  both inputs have been cleaned with contact cleaner thoroughly.

6
ROLY,  Yes I'm aware that the electrolytic caps are polarised and were installed as such according schematic and the blatant +/ - signs printed on the component side of the board.  Also, I haven't been "blindly" replacing parts as many of these parts were missing from this amp when I acquired it.  It puzzles me that someone would rob parts from (at the time a perfect working) Marshall amp to repair a Crate. In regards to replacing the electrolytic caps the originals were no good and I wouldn't trust them being 25/ 26 years old. 

TEST: Lift one leg on R6 and R11.
RESULTS: measurments at MAIN SUPPLY RAILS (C17 & C18 2200uf/ 50v) still reading around 13.2v.

Now that it has been established that the output stage is good, what is the next step?

7
LOUDTHUD...  I removed the IC (socket and IC chip are properly soldered and seated) and retested the same components that ROLY suggested and the results were:

MAIN SUPPLY VOLTAGE:
C17= 13.4v
C18= 13.2v

SUPPLIES TO PREAMP:
C2= 0.0v
C6= 0.0v

ACROSS OUTPUT:
MEASURED 0.03V

As you can see all the measurements with the exception of the SUPPLIES TO PREAMP, are the same.
All measurements have been double checked using a UEI DL49 digital voltmeter,( Borrowed from a friend) which proved the original measurements on my DMM were just as accurate.

In regards to using my laptop to test the circuit, I'd prefer to use a cd player which can also be borrowed.  Will I have to butcher a 1/8 cable and splice in to the circuit?  If so where would you suggest placing the leads from the headphone 1/8 wire?

8
The Newcomer's Forum / Re: Gorilla GG-20 output problem
« on: January 12, 2013, 12:07:21 PM »
NOTHING SCREAMS LIKE A GORILLA.  For those of you who have never owned or seen a Gorilla amp, this is the slogan on the sticker they placed on the back of the speaker. http://s1325.beta.photobucket.com/user/GIMAGUITARS/library/

9
The results are in!
MAIN SUPPLY VOLTAGE:
C17= 13.4v
C18= 13.2v

SUPPLIES TO PREAMP:
C2= 2.73v
C6= 6.32v

ACROSS OUTPUT:
MEASURED 0.03V

QUESTION 1: The low volume applies to ext. cab. as well as internal speaker.

QUESTION 2: The pitch of the hum is a very low B- B flat.

QUESTION/TEST 3: I do not own an MP3 / CD/ Cassette player. My music is primarily enjoyed by cds in the car or online.  Can you suggest an alternate testing method?

10
Correction. My multimeter has a DC volt range up to 500v with settings at 2v, 20v, 200v, and 500v.  I blatantly forgot.  How do you suggest performing the measurements you ask for?

11
Unfortunately I do not own a DVM.  However, power rails are 15v as stated in the schematic.  What would the DC voltages at the speaker output signify or point towards?   

12
Hello all!  Thank you in advance to all who read, reply and assist in reviving this amp back to life.
 Let's jump right into this project.... 

I recently picked up a Marshall Lead 12 that has a volume/ output issue. 

ABOUT THE MARSHALL LEAD 12:
This amp currently sells on ebay in head or combo form from $75-$150 so please do not suggest that I just go buy one as I know it is on the verge of working. The ultimate plan is to slave the lead 12 into a power amp as it sounds (to my ears) close to a JCM800 for a solid state amp and even better with a distortion pedal (tube screamer) as most would suggest.  There is very little info online about the lead 12, combo-head conversions, and modding solid state amps.  I do have pictures of the chassis and a schematic for the 3005 and the lead 12 reverb ( I'd like to build the driver and recovery circuit and add reverb but the amp has to be working first).  This particular model was made in 1986 and its got mojo.
I'll try to be as detailed as possible to better aid those aiding me.
-The amp is 12 watt R.M.S / 8 ohm
-120v   40/60Hz  35VA
- The layout of the amp from left to right on the front is:
high input--low input--gain--volume--treble--middle--bass--line out-- power switch

A FEW NOTES ON COMMON SENSE WITH THIS AMP:
-This is an 8 ohm head and was run through an 8 ohm cab, so there's no impedence issues
-all solder joints are good/ have been re-tinned
     

SYMPTOMS:
-very low volume when plugged into external cab
-When volume knob is turned all the way up there is a semi-high pitched hum
-no noticible change when gain knob is turned

WHATS BEEN DONE/ REPLACED:
High gain (mono) input plastic like original
Low gain (stereo) input plastic like original
zener diode 9.1v 0.5a- like original
Power transistors - mj2501 and mj3001
genuine marshall volume pot.(1m) and gain pot (22k)
2 external speaker input jacks (2 x mono plastic) wired through chassis instead of hole through bottom
IC Socket added / Replaced original MC1458 with same model IC
New power cord has been rerouted through back of chassis
All electrolytic capacitors (power- 2x 2200uf/50v , c13/c15/c16 22uf/ 25v , c14 100uf/ 25v)
Pots and jacks have been sprayed with deox-it and left to dry for a few days at room temp.


THEORIES FOR MALFUNCTION/ NOTES:
-Possibly transistors which are untested (1x bc212  , 3x bc184).  BC212 is a PNP and BC184 is an NPN.  both are low power audio amplifiers.
-Also untested are 5 film capacitors (box shaped) 2 x .022 m 400, 2 x .22 k 100, 1 x .047 m 250.  Actually all the caps are untested as I don't have a capacitor meter. 
-all resistors have been tested (in circuit) and are operating within in their 5-10% range with one exception.  R4, which is a 47k ohm resistor and is reading at 18.5 k ohms after new zener diode was installed.
-there is one fuse in this amp and it hasn't blown once.
-bridge rectifier (W005) has been tested in circuit and works.
-everything else is original.
-Lighted power switch does light up


I think the problem is with the transistor(s) as the amp receives and generates power but the audio signal is not being amplified or passing through to be amplified. I have heard and read that bad potentiometers are sometimes the culprit, could this amp be the victim of malfunctioning pots? 

Thanks for checking this out and helping if you can! 

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