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Randall RG200D Preamp section. Help?

Started by Mattxorz, October 05, 2011, 12:37:00 AM

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Mattxorz

I'm trying to rip the preamp out of an old Randall RG200D combo amp, and I've got a few questions. The amp quit working a while ago, and I'm pretty sure the power transformer is shot. I don't know too much about amp anatomy, but I do know a little about electronics. That said, here are my questions. I know I'm quite novice, no need to flame- help is why I'm here.

1.) My ultimate goal is to remove the preamp section, and use it in conjunction with the poweramp of one of my other amps. To do this, I need to figure out all of the connections between preamp and the rest of the amp, and sever the ones I don't need. I've done almost all of that, but there are a few connections that I am unfamiliar with.
   -The first is a 3 wire connection between preamp and poweramp pcb's (3 pin clip on the poweramp end that separates into 1 red, 1 blue, and 1 black wire going to different places near eachother on the preamp end). This is separate from the preamp's line out that goes into the power amp- I know that is elsewhere. On the power amp pcb, the connection is labeled "CN2," and on the preamp side it's labeled "P101"(red), "P102"(black), and "P103"(blue).
   -I also am unsure which connections coming from the power transformer are which. There is a two pin (2 wire) clip that I'm assuming is kind of the "main power" right in the corner of the control pcb of the preamp, connected via the same clip on the transformer (labeled "CN1"). There is also one red wire going from transformer to preamp labeled "QC15" and "+15V" on the transformer pcb, and "P104" on the preamp pcb. (It's close to "P101-103" from earlier). There is also a black wire going from what seems to be the bottom of one of the huge capacitors (the size of a D battery) in the transformer section to somewhere underneath the preamp pcb. I haven't unfastened the preamp and taken it out of the chassis yet to see exactly where it connects.

If someone could let me know what these connects could be, I'd be very appreciative. Also, If you have any ideas on how to power this preamp once I've gotten it away from the poweramp and transformer, I could use that advice. I'm hoping it's as simple as figuring out how much juice it needs, and finding an AC adapter to patch in. Thanks in advance!

joecool85

Matt, sounds like your project is coming along nicely.  Please post some photos of the PCBs you are working with, it's very difficult to help with just text descriptors.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

phatt

Quote from: joecool85 on October 05, 2011, 08:00:22 AM
Matt, sounds like your project is coming along nicely.  Please post some photos of the PCBs you are working with, it's very difficult to help with just text descriptors.

Plus one :tu:

@ Matt,, Have you considered asking about fixing the poweramp/psu?  That might be a lot less hassle 0:)

Might just be a blown diode in the psu which may end only costing a couple of dollars to fix.

Phil.

Mattxorz

-The first image is overall layout of the pcbs.
-Second is the red wire going from transformer "QC15" and "+15V" on the transformer pcb
-Third is that same red wire (on the right) connecting to the preamp pcb "P104" Also in that picture are the 3 wires going from preamp ("P101"(red), "P102"(black), and "P103"(blue))  to poweramp  ("CN2").
-Fourth picture is those same wires from a different view, showing how they connect to the poweramp pcb.
-Fifth is that black wire going from the big cap to underneath the pre control pcb
-And sixth is that double red lead that I believe to be the preamps power, clipped on both the transformer end, and the preamp control end.

As for fixing the power supply unit, I wouldn't know where to start. If you think you could help me with that, I'd much rather fix it than be ripping this whole thing up. If it helps, the pdf I attached looks like the schematic for the power supply. (I say looks like because I found it on another site labeled as "RG200 power supply," but not specifically for the RG200D. Not sure if that makes a difference; it at least looks pretty close. Thanks so much. Quick response times!

phatt

Hi Matt,

Good effort. :tu:

OK Go here :    
http://music-electronics-forum.com/t16529/

Note that *Mr Fahey* (a member here also) will likely see this and when he has time He will pass comment. (He is better qualified than most)

My guess is the Power amp section has some design issues and my next guess is the same problem is happening to your Amp also.

In the end you maybe better served by inserting a whole new power amp but meantime read the link for clues as to what maybe wrong.

I have no doubt that if the transformer is not broken then the Amp is probably fixable.

Phil.

Mattxorz

That's the thing. I'm pretty sure the transformer is the problem. The power switch and indicator light are wired across the transformer. They don't light. This amp was given to me, and I really don't want to invest too much money in it (seeing as how they go used for like $150 anyway). If I can fix it for the cost of some tiny parts, that's fine. If not, it's not worth my money. I'm a college student (no, not in electronics), and this is just kind of a free-time hobby.

joecool85

Quote from: Mattxorz on October 06, 2011, 04:32:03 PM
That's the thing. I'm pretty sure the transformer is the problem. The power switch and indicator light are wired across the transformer. They don't light. This amp was given to me, and I really don't want to invest too much money in it (seeing as how they go used for like $150 anyway). If I can fix it for the cost of some tiny parts, that's fine. If not, it's not worth my money. I'm a college student (no, not in electronics), and this is just kind of a free-time hobby.

Could still be something else.  Do you have a voltmeter?  If you do, I would suggest checking for voltage directly off the transformer, before it even hits the diodes.  If not voltage is present, disconnect the transformer from the rest of the circuit and then check for voltage (be careful not to zap yourself or cross any wires).  Transformers can crap out on you, but it's a lot less common than other (much cheaper) parts.

I started this hobby while in college as well, as a New Media major - not exactly an EE, and also with very little money.  It can be done, and it's a lot of fun.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Mattxorz

Yea, I've got a multimeter. What should the direct transformer voltage read? I'll test it tomorrow when I get back to town.

phatt

OK,, have time so I'll try 8)

Have you checked the FUSE?
Looks like you have one of those IEC Mains cons which has the fuse hidden inside the housing.
Some of those units are second rate quality and the metal tabs on the fuse holder can break off.
If all is well then move on.


Ok watches, rings and jewelery OFF, shoes ON feet.
Next find where the AC mains EARTH wire connects to the metal chassis.

If No Chassis Earth? then it's likely unsafe.

I personally check the continuity of the power chord EARTH to chassis is actually present before I even open it. Hint..

Establish the *Secondary wires* that come from the Tr Should be at least 3.
(not the primary wires as they are Mains AC, that stuff can kill you instantly)
The voltage from Tr is AC of course and the bridge rectifiers (Diodes) turn it into DC Voltage and those big blue Caps filter the DC.

Set DMM to read AC volts. (use highest setting first)

Now find those two large electrolitic Capacitors on the board,,, find where they both join,,, that should be circuit common or circuit ground. (often it is also the Chassis Ground)

Now black probe on the common ,,, read the other 2 wires that exit from the Secondary of the transformer.
If the Tr is working then you should get a reading,,,might read somewhere between 20 to 30 VAC.

The picture is a bit obstructed but you may find 3 wires exit the Tr and connect via one of those white con blocks.

Clearly one Con goes to the poweramp board and my guess is;
Switch the DMM to read DC Volts,,,,, you should read a DC volatge that is 1.4 times the AC Voltage.

If you Have AC present but No DC then the rectifier section is blown.
----

If you get nothing,, Don't give up yet...
Next, Power down,, UNPLUG EVERYTHING!!!!

Go back to mains side.

Now set the DMM to Continuity or lowest Ohms setting. (Some have a beeper for this test)

Continuity with Ohms,  should be close to Zero Ohms.

Check ALL 3 wires from plug pins all the way to the fuse.

You can easy read from the cons at the back of the Chassis input socket.
Ground should be obvious,,Green or yellow or both,, (Depends where you live).

The likely culprit should be the Hot wire which normally passes through the internal fuse.
A bit tricky to explain but you should get continuity all the way to the switch on the front.

For gods sakes don't try and read Main Voltage as a lot of DMM units are not MAINS rated and should not be used. 

All this needs to be checked before you assume the Tr is blown.

Phil.

Mattxorz

The fuse was the very first thing I thought about when the amp first crapped out on me. I checked all over. I don't see anything in this amp resembling a fuse. I'm still definitely novice, but I would hope I could identify a fuse. I had just shrugged it off. If that really is the problem, I'm going to have to kick my own ass. Any clues where it could be?

Mattxorz

OMFG. You're right. The fuse is built into the mains in. Imagine that. Let me now commence the kicking of my own ass.

phatt

Quote from: Mattxorz on October 10, 2011, 04:24:23 AM
OMFG. You're right. The fuse is built into the mains in. Imagine that. Let me now commence the kicking of my own ass.

;D
I once re-built a whole engine gear box and for a week it rattled/clunked everytime I slowed down/changed gear.
Idiot me only finger tightened the flywheel to the engine and in haste to get back on the road,,, I forget to torque the bolts up. :-[ :-[ :-[

So I know how you feel. 8|

The concept of the fuse holder is quite smart as you are forced to remove the mains cord before you can access the fuse which makes it close to idiot proof.
Great but the quality of build on some of those units is questionable.
Keep us posted.
Phil.

joecool85

Glad you found it, but be careful bypassing the fuse with anything (like foil etc).  It very well may be drawing much more current than it should and that's why it blew the fuse.  When you get rid of the fuse, the amp could blow itself instead of the fuse.  You got lucky so far and it hasn't, just keep it in mind.  Better to be safe and wait till you can get a fuse than possibly hurt yourself or your amp.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com