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Author Topic: Marshall G 30RCD Practice Amp REPAIR  (Read 10875 times)

EnglishChannel

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Marshall G 30RCD Practice Amp REPAIR
« on: November 21, 2013, 11:23:45 AM »
Hello.

This is my first post.
I have a Marshall G30RCD practice combo that has been sitting around for awhile. I got it for the kids years ago. Dusted off it looks brand new, very clean inside and out. When turned on it plays, on both clean and overdrive, but no ability to change volume, tone or gain. So, in opening it up I discovered that 5 of the control pots were broken off at the base.


The pots broken are CLEAN CHANNEL - Volume, Bass, Treble. OVERDRIVE CHANNEL - Gain, Bass.
The rest of the pots (OVERDRIVE CHANNEL - Contour, Treble, volume, & MASTER - reverb) look fine.

So, the broken pots will get replaced. I expect that will fix the problem.


My question is, should I replace ALL of them?
I suspect the amp was either dropped or pushed over which resulted in cracking the 1st 5 pots.
Is age a problem with these cheap little pots?


Any advice is appreciated.
« Last Edit: November 21, 2013, 11:40:26 AM by EnglishChannel »

g1

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Re: Marshall G 30RCD Practice Amp REPAIR
« Reply #1 on: November 21, 2013, 11:45:42 AM »
 With inexpensive practice amps, it is not uncommon for the circuit boards to be secured only by the pots.  Often what happens is the amp falls on it's face while there is a cord plugged into the input jack.
 This often causes the board to bend and breaks the pots.
Inspect the input jack to be sure none of the solder joints have cracked.
The best way to check if some pots are good is with DMM measuring ohms.
Also you can look with a magnifier to see if the carbon track is cracked (in the same area where the bad pots are cracked).

EnglishChannel

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Re: Marshall G 30RCD Practice Amp REPAIR
« Reply #2 on: November 21, 2013, 12:01:25 PM »
Thanks for the quick response.

I take it then that unless damage is evident, the pots that survived will not start cracking off.
No reason to fix what isn't broke.

I cannot seem to find a source for the schematic.
One of these is marked B200K (omega), I assume a 200K Ohm pot.

The others are marked A1M (omega), which I assume means Alpha 1 Meg ohm Audio Taper pot.

There is a B200K pot as the TREBLE control of the overdrive circuit. Can I assume the broken B200k is for the treble control on the clean circuit? Don't know why, but I would've though volume instead.

Or can you tell me of a good source (in USA) for the schematic and these little pots?

Thanks again.
« Last Edit: November 21, 2013, 12:27:59 PM by EnglishChannel »

Enzo

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Re: Marshall G 30RCD Practice Amp REPAIR
« Reply #3 on: November 21, 2013, 01:50:07 PM »
The B in B200k means linear, so a 200k linear pot.   If you can't find that, a 220k or something will do.

The A in A1M means audio taper, yes.


I believe these are common 16mm size pots.   With knurled split-shaft.

I am not a fan of replacing things when not necessary.   But considering the trauma this unit faced, if there are only a couple left unbroken, I think I WOULD replace them all.

Your schematic is below.

Roly

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Re: Marshall G 30RCD Practice Amp REPAIR
« Reply #4 on: November 22, 2013, 10:58:55 AM »
Agree with @Enzo, normally I'd only replace what's broken, but this amp has obviously suffered a major trauma at some stage.  If it were a repair for a client I'd do the lot, pots being only a couple of bucks, but for myself I might economise and accept that I may have to dive back inside somewhere down the track and "finish" the job.  So it's really a matter of your personal taste (since you seem to be fairly at home inside the gubbins).

But do clean the input sockets and closely inspect their solder joints for damage while you are inside.  The move to PCB mounted sockets has been a backward step; people trip over leads and wrench these joints all the time, very common fault.

Just be careful ordering replacement pots, they come in a number of variants and these are PCB mounting, and the shaft type must also match.  The nearest value I could get locally to 200k would be 250k, but that would also be acceptable - the exact values are not critical.
If you say theory and practice don't agree you haven't applied enough theory.

EnglishChannel

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Re: Marshall G 30RCD Practice Amp REPAIR
« Reply #5 on: December 09, 2013, 12:50:35 PM »
To all,

I was able to find a parts list for this amp (no schematic) but that is enough to help me place the right pots in their proper places.

Thanks to all for your responses. For now, I am waiting on parts for this one.

Too many other pressing projects to work on.

Thanks again.
Cheers!

tubeAMP

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Re: Marshall G 30RCD Practice Amp REPAIR
« Reply #6 on: December 09, 2013, 01:05:40 PM »
might should consider loosening the face plate mounting screws in case the amp winds up on its face again.  something has to give either mounting screws or pots :dbtu:

Enzo

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Re: Marshall G 30RCD Practice Amp REPAIR
« Reply #7 on: December 09, 2013, 05:46:27 PM »
Um... I did post the schematic a couple posts up.

adam2a3

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Re: Marshall G 30RCD Practice Amp REPAIR
« Reply #8 on: February 20, 2015, 11:57:32 AM »
Um... I did post the schematic a couple posts up.

Hi everyone, I do not start a new topic as I would like to mention an idea of mine about a mod of this particular amp (G30R CD) here.
I am thinking about adding a real power amp to it, and I have 2 ideas:
1. To lift the C40 cap going to pin1 (input of power amp chip) and sent it to the grid of an ECC83 phase splitter tube driving a pair of EL34's, the standard Marshall power amp section, JTM 45 and the like.
2. Do not modify anything inside, just remove the speaker cables and sent them to the 8 Ohms side of a small push pull output transformer, and the 10K+10K directly to the grids of a nice big tube pair. The center (ex HV tap) can get the appropriate bias voltage.
So only 2 output tubes will be added, chosen from a very long list like 6V6 6L6 EL84/34/509 6146 GU50 KT... going even up to an 811!
I would give a try to a 600 Ohms/10K+10K interstage transformer as well, but maybe the TDA chip is too powerful for this.
Any comments?

Roly

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Re: Marshall G 30RCD Practice Amp REPAIR
« Reply #9 on: February 20, 2015, 03:49:40 PM »
Well you need to do a few sums.

1. What is the signal level at "Power Amp" in the circuit above?  (what voltage gives 30 watts into 4 ohms, divided by the voltage gain of the TDA2050?)  Will this be enough to drive the PI you decide to use?  Since the preamp supply is +/-15V this must also be the maximum peak signal voltage available at "Power Amp", and this won't be enough for most power valves if you use a Split-Load or "concertina" PI; you will need a Long Tail Pair or other PI with voltage gain to obtain full grid swing on the OP valves.


2. Same sort of question.  If you use an 8 ohm to 10kp-p you are going to get a considerable step up of signal voltage, rather more than you need I suspect.  I think you will find that a 600 ohm to 10k will be more of the order you need.  Transformer power handling isn't an issue since the power valve grids are effectively an open circuit, we are only dealing with voltage, not current (except under overdrive conditions, but the current and therefore power is still minimal).


You are going to need a capable HT/B+ power supply.  This is my answer to the high voltage transformer problem;

http://www.ozvalveamps.org/ava100/ava100psu.htm

HTH
If you say theory and practice don't agree you haven't applied enough theory.

tome

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Re: Marshall G 30RCD Practice Amp REPAIR
« Reply #10 on: December 16, 2016, 09:50:15 AM »
 hello
  I   have  a marshall G 30Rcd and  the traces  have  been burned off  the board , I am not  able to  read  the schematic , does someone have  a picture of  the traces, so  i  can  see  what  goes where,
thanks

g1

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Re: Marshall G 30RCD Practice Amp REPAIR
« Reply #11 on: December 16, 2016, 12:22:59 PM »
  Traces do not burn up for no reason.  If you repair them without finding the cause, they will just burn again, stressing more parts and possibly doing more damage.
  If you are unable to read schematics, you should probably take it to a repair shop.

yonibenitez

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Re: Marshall G 30RCD Practice Amp REPAIR
« Reply #12 on: March 01, 2020, 10:33:16 AM »
Thanks for the schematics. Really useful.