Randall RH100 issue

[Blacksack]

Hey guys.  I don't know if this question is getting the right exposure in the Newcomers section so I figured I'd try it over here.  This is basically copied and pasted from my first post.

This is my first post and as you may have guessed, I came here in search of an answer to my problem.  I have a Randall RH100 head powering a cab with 2 Carvin Brittish series 12s.  I have used it very little but a couple days ago when I went to power it on, it was making no sound.  I swapped guitars and still nothing.  I swapped probably 4 or 5 cords and still nothing.  Made sure the guitars were turned up, plugged them into another amp, etc.  Something is wrong with the Randall and I can't quite track it down.  I have noticed with the guitar turned up all the way and the level and gains turned up all the way, I can hear a tiny amount of sound (whatever I'm strumming) come out of the speakers.  Also, it's either clear or metal, so I know that part of the amp is working.  I'd just say the total output of the amp is probably 1% of it's potential?  I've taken the amp apart, searched and scoured for burnt spots on the cirvuit board, bad resistors, loose this or that, I just cant find anything visually and though I am not brand new to electronics, I'd have no idea where to start as far as formal troubleshooting.  :grr   Anyone had this happen? 

P.S.  As a side note, the one thing I noticed is that the tiny amount of sound I can here is not effected one way or the other by the moving of the master volume control...

Thanks.
-Andy

[J M Fahey]

I think we should make this answer a stickie, because it´s asked everyday.
Obviously it´s a *very* common problem.
Start by plugging a known good guitar cable from loop send to loop return. Might be labelled as preamp out/power amp in.
Any improvements?
Good luck. :tu:
PS: also check the headphone out: do you have good headphones volume, but still no speaker volume?

[Blacksack]

Right on, thanks for responding.  I'll try it tonight.  You're saying to just take a good cable and plug in between those two jacks, leaving the cab plugged into the main out and the guitar plugged into the main in, right?  What should I expect to happen?

[J M Fahey]

You should expect the sound to come back 
Fact is, there are leaf switches (a thin strip of metal *just* touching another one) inside jacks, which disconnect your preamp so the sound comes from the effect instead.
Sort of a "free service to the community"
Problem is, after some years those contact surfaces rust/corrode/get dirty/worn or lose springiness, you lose contact , you lose sound.
Plugging a guitar cord you are forcing a connection.
The headphone jack also has a similar one, to automatically disconnect your speaker , and can also have a similar problem.

[Blacksack]

Well, I snuck home @ lunch time hoping for the best but found that plugging, unplugging and fiddling around with the send, return and headphone jacks all yielded no results. Same thing, a couple decibles of sound, they change according to what channel I select and the sound goes away if I turn down the gain and sub-volume knobs. Still the master volume switch is unresponsive and looks like a trick to replace. I do have tools, a DVOM and other resources if there is somewhere I need to check for power or a switch or knob I need to jumper or short to eliminate. At this point I may have folded but the last person who ran a repair business in Fairbanks moved away. I'm learning a lot though so this is probably the best route for me to take.

[J M Fahey]

Start by googling (and posting it or at least the link) the RH100 schematic, so we all talk about the same.
The basic procedure is to download some test signal , a 1KHz or 440Hz MP3 works great, then you play it in your MP3 player (or PC or burn it to a CD) in the repeat mode, so it goes on forever, and send that signal to your amp input.
You´ll need a stereo mini plug (headphone) to mono 1/4" guitar plug.
You play the continuous tone, and follow signal along the path (the "map" is the schematic)
Somewhere along the path your signal disappears .
Then you start searching around that area.
A practical example:
5 minutes ago I finished repairing an amplifier I had made in 2001 or 2002.
Worked flawlessly for 10 years, until it "died" a week ago.
I found nothing gross about it, all voltages fine, but it had a sound as loud as a mosquito ... and equally buzzy.
I traced signal along it, until I found that the Treble pot received signal on both ends, but the center leg was dead.
I replaced it and everything was as-new.
The problem?: the center leg is a rotating contact (which you move with the knob) lightly touching a semi-circular strip of resistive material.
Said contact was worn or lost springiness or had some dirt or lint.
Best solutiom is just to replace it.
After 10 years service it deserves some rest!!!

[Blacksack]

This is as close to a schematic as I can find.  I have written Randall and requested a schematic, I don't know if that'll work out for me or not.  If I download the MP3, make the cord and supply it to the amp, what kind of voltage will I be looking for to know that it's there or not?  Also, on a side note, do you have any idea how to remove the separate pots on this amp without cutting the entire circuit board loose?  I have the knob pulled off and the small nut and washer off but I don't see how the pot is actually supposed to release from the board.

[J M Fahey]

Well, *if* that is what they sell as a Randall (and I think they do), it´s a piece of cr*p.
They´ve managed to kill the most characteristic part of Randall sound, its grinding buzzy ballsy distortion. How´s that?
That´s what happens when investors who have no idea of Guitar sound (nor care) buy a Company, just for the label they glue to their product. :grr  :trouble
Oh well.
Anyway you only have the power amp there, let´s wait for the Preamp.
As an example, look at this schematic of the "real" Randall, the RG100.

You´ll see that they inject a 6mV signal at the input, and it gets amplified, step by step,  along the path.
Voltages shown in an oval are DC voltages; those in a rectangle are AC (signal) ones, the ones I told you to measure.
They show 40mV on Q1´s drain leg (same appears on top of Green Channel Volume control, then 2.1V (or 2100 mV) on Q2´s drain leg, same as in Q3´s source leg and on top of Green Master control .... and so on.
Someting similar happens on the Red channel.
*If* some AC voltage is not there as suggested, or is *much* lower than expected, then *there* you have a problem.
That´s the general purpose troubleshooting procedure for any amp with your symptoms, no matter the brand .

[Blacksack]

Thanks, JM.  I've got a ton of work to do so I guess I'll get on finding that MP3 file or something similar and riggin up a cord.  I have doubts that the schematic I found is the schematic for my particular amp, but hopefully both of the schematics can lead me in the general direction.  Randall wrote me back and said "Uhhhhh, take it to a repair center  -n-stuff..."  So no schematic from them.  I guess we'll see how it goes!  I wonder what the difference between the RG and the RH is.

[dhkv69]

Hey guys, im kinda new to this place and i figured this would be a good way to start me off...

My Head seems to have some feedback problems and im not sure what it is, ive ran it through three differant cabs but i currently dont have one, and its had the same feedback problem through all of them. It seems when i turn my level up to about a quarter or so, if my masters up to about half or so it starts to squeal and it doesnt matter how close i am. The only way to solve it is to turn down the level on my gain and then it dissappears, its not to much of a problem to me but i would just like to know if theres something i can do about it.

[J M Fahey]

Does it stop squealing if you turn your guitar volume to 0 ?
And what about the tone control when set to "bass" ?
Did you try another guitar cable?

[Blacksack]

Well, I have given it a shot.  I tried sending a tone through the amp, I've tried diagnosing it with a short circuit tester, I have gone over the circuit board with a comb, I just cannot figure it out.  I can't seem to follow the pathway of the signal of the tone.  On the opposite side of the the housing from the internal circuit board is a large, heavy thing wrapped in windings.  What is this, could it be bad and is there a fuse anywhere I need to check?  Is there a way to test the continuity or the resistance in this thing?  Just seems like my sound is there, just not amplified.   xP

[joecool85]

Blacksack, please answer the questions J M Fahey posted above, then we will go from there.

[Blacksack]

Well, it's been a while since I posted, I've been trying out the solutions that JM gave me but to no avail.  I am having a hard time following the power flow (tone signal) through the amp circuit board.  Anyhoo, at some point, another forum member (DHVK69?) jumped in with a different problem, seems there may be some unanswered questions in his case.  If there are any questions either JM or anyone else needs answered or any I may have missed, I'll answer what I can. 

[joecool85]

I was referring to these questions:

Does it stop squealing if you turn your guitar volume to 0 ?
And what about the tone control when set to "bass" ?
Did you try another guitar cable?