Randall RH100 issue

[Blacksack]

Phatt, thank you for your thoughts.  Joe and JM, judging by your responses, it appears as though i have pulled a major boner.  If you look at the first photo (IMG 3853) in reply #20 on page 2, you'll see how I have things plugged in.  When I purchased the setup, the cab was plugged into the "Line Out - Class 2 wiring" jack.  I had used it like that until recently, it was never plugged into the 4 ohm jack, which is located on the opposite side of the amp housing.  How it is in that picture is how I have been running it.  I honestly didn't not know any better to change it.  Have I inadvertently fried something or ruined something?  I really had no idea, dont lose patience with me just yet!  LOL!  I hope that this is not the solution to my entire problem.  Also, if I am supposed to be running off of the 4 ohm jack, have I destroyed the Line Out jack?  I thought that we had discussed bypassing the "phones" and the "4 ohm parrallel" as they are two jacks never used?

[phatt]

BlackSack,, You have 3 outputs on that Amp.

1/ speaker out (consisting of 2 sockets)
2/ Line out (line level means low signal Like what comes out of a pedal)
3/ You also have a Headphone Socket.

Only one will drive a speaker,, Winky.

Should now be obvious that speaker goes to speaker out.

It's beyond my telepathic ability to say why your speaker was plugged into the line out. maybe someone hot wired it?
Mayebe the previous owner knew lees than you? LOL

Either way like I said,,take the Hot wire go to speaker out,,, bypass everything else and it should work.
(Assuming nothing has been damaged before hand)

There is NO 4 Ohm output as such.
the 2 Speaker sockets simply allow you to plug an extra speaker into the Amp.
Though one socket may disconnect the internal speaker. Not sure?

Most common value for speakers is 8 Ohms and joining two together will change the load (seen by the power Amp) to 4 Ohms.

This is about as low as you would want to go otherwise the amp will overheat and at full pelt will likely destroy the Power Amp stage.

If you care to note it actually states on the back panel (Spk Out)
Minimum load total = 4 Ohms.

I would simply plug an 8 Ohm speaker into the speaker out as intended ,,, Then if it works ?? Quit while you are ahead and count your blessings that it has not damaged anything.


As already explained the H/P Socket disconnects the speaker out when in use ,,,so that internal switch may have broken.
That maybe the only problem so bypass it.

Phil.

[leeuwa]

OK.
Do you have Effects/Loop send/return jacks?
If so
1) plug a known good cable from send to return , and your guitar at the input. Do you get normal sound?
2) plug a cable from the send jack into another, working amp. Sound?
3) plug the line/loop/pre out from another amp into your loop return. Sound?
Test these first, I have others to ask you, but let´s go step by step.

And, yes, there was a mixup between posts. Oh well.

My Randall RH100 is not working. I did the tests suggested by M Fahey, and I know now that the power-amp part is allright.
Test 1: no sound.
Test 2: no sound.
Test 3: YES, there was sound.

The guy who gave me this amp told me that the amp acts like a tube-amp. Before it gave no sound at all anymore, the amp made sound after a couple of seconds. What can be the problem? Potmeter with bad contact? Some bad condensator?
I replaced the input-jack already, because it looked worn. But this was not the problem, amp still not working.

Adrie

[J M Fahey]

One detail: were you injecting audio at the preamp input, say a friend playing guitar or some CD/MP3 , while you tried 1 and 2?

Just checking.

If no sound still, after setting the controls to sensible values, you'll have to trace audio inside, to check where you lose signal.

Please post schematic .

[leeuwa]

One detail: were you injecting audio at the preamp input, say a friend playing guitar or some CD/MP3 , while you tried 1 and 2?

Just checking.

If no sound still, after setting the controls to sensible values, you'll have to trace audio inside, to check where you lose signal.

Please post schematic .

While testing I used a guitar. I got original schematics from people at Randall itself.

All the parts look alright, nothing 'burnt', no burn-smell. What's the best (simple) way to trace the audio in pre-amp part of the amp?

Adrie

[J M Fahey]

1) to simplify things get some MP3 player and play some music or a 1kHz tone http://www.mediacollege.com/audio/tone/download/ (best)  into your preamp input (set the player to repeat_1/loop so it goes on forever), kludge an "Audio Tracer cable" out of an old guitar cable and plug it into a working amp input.
You leave the best plug on the cable, clip the other end, separate hot and ground.
Solder hot to a red crocodile connector; a 10 inch single insulated wire to the twisted ground sleeve and the other end to a black crocodrile.
Tape/heatshrink all connections for safety.
Now you clip the black to ground or chassis, and since you have that 10" ground wire you can freely move the red crocodile and touch different key points and listen through the auxiliary amp (any will do, even a 10W beginner's combo) whether you have audio or not at that point.
Start with the input jack itself to check you are actually sending that music or tone to the input jack.

If that schematic matches your preamp, measure/check in this order:
* input jack
* IC1a pin 3 , a.k.a. IC1a-3
* IC1a-1
* IC4a-3
* IC4a-1
and so on
I think by now you get the idea: we are testing *all*  preamp Op Amps at input and output and checking that signal reaches and leaves them.
Follow the signal path from input to preamp output, remember that signal splits into different channels (clean/dirty) so follow each branch, they will eventually reunite at the end.
Same with Reverb and loop.
In some places you will lose signal even in a good preamp, as in having a volume control set to 0 or same with tone controls, also you might have switched to "the other" channel, so move pots, switch channels and reverb on/off, etc.
When something does not react as expected, there you have a suspect, we grab the multimeter and start searching for abnormal values.

As you see, shotgunning (replacing parts at random hoping to hit the bad one) is MADNESS, and you add more problems than you had at the beginning.

EDIT: be CAREFUL with the crocodile tip, do not short the pin you are testing to the one by it.
Why a crocodile?
Because often you will need to leave it clipped while you test something else, they are too large for Op Amp legs but they are usually connected to something else, a resistor or capacitor, and you leave it clipped there.

The best probe is the "hook" self grabbing type, but besides being more expensive (that's not the point)  usually you have to mail order it:

[leeuwa]

Thanks for all the detailed information. I get the picture now how to trace the signal through the pre-amp. I ordered some hook-probes on Ebay, and are going to connect them to an 'old' guitarcable.
I'm going to try fixing the amp the next couple of weeks I think. I let you know the results here.
Thanks for all your help, I appreciate it very much.  :dbtu:

Adrie