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
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?
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?
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.
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.
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!!!
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.
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.
(http://spaceryerson.com/pics/img%20RG80-A-SCH234.jpg)
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 .
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.
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.
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?
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
Blacksack, please answer the questions J M Fahey posted above, then we will go from there.
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.
I was referring to these questions:
Quote from: J M Fahey on December 16, 2011, 07:50:27 AM
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?
I know, but I think those recent questions asked by JM were directed at the other guy (DHKV69) who jumped in on the discussion. My amp is not sqealing, so I cannot get it to go away, there is no difference when I adjust the tone control and as outlined in my first post, I have tried nothing short of a billion guitar cables. I have rattled and swapped the cables going to the cab from the head unit as well. Just a tiny bit of sound. The sound switches from clean to distortion when the appropirate channels are selected on the footswitch and/or the head unit but when turned all the way up, it's barely audible. Also, the only switch that seems to make no difference whatsoever is the master volume control. I tried to remove it for continuity/resistance testing but the contacts seem to be soldered directly to the circuit board.
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.
Thank you JM, I will get on this series of tests when I get home. Thankfully, our main guitar guy has left his amp in the band room so I have a test rig to fiddle with. I know you've had me plug into the pre-out and the headphone jack to make sure that it's not switching the sound and then that switch sticks, and it didn't make a difference but I'll try these out and let you know. Thanks a lot for taking the time to help me on this.
-Andy
Test #1 yielded no results. Test #2 yielded results. I plugged my guitar into the input on the Randall and plugged a line from the send jack on the Randall into the guitar input on my other amp and sound was there. First time I have heard amplified guitar in a long time! Channels switched back and forth well between clean, distortion, etc. Test # 3 yielded no results, dead as a doornail. There were two jacks that I tried on the good amp, one labelled "Line Out" and one labelled "Send", neither of them made any sound on the Randall when plugged into the return jack.
Well, your preamp is fine.
Your power amp not.
Let´s check connectors first
I follow the schematic you posted.
1) when you turn it on/off, do you hear *something* on the speaker?
Plop, hiss, pop, buzz, whatever.
2) if not, test the speaker connecting it to some other amp or to a battery.
Well, I cranked it up and tried to see if I could hear anything when I turned it on and off. Nothing. I then plugged the cab cord into the out jack on a Spider III and the cab is making noise like it should, so I guess it's now definitely narrowed down to somewhere in the amplified power? I'm going to try and post a couple of pictures of what I have. I don't think they'll be of any technical use but just to give a little better idea of where I'm at. I have the amp tore down so I can access the circuit board/electronics and I can flip it to get at the heatsink and whatever that suspect looking contraption with the built in thermal fuse is next to it on the top...
Yes, your pictures were useful, specially the first one, showing th guts and the back panel.
So your power amplifier is mute.
2 possibilities:
1) it works but the sound can´t come out:
the schematic shows that the sound, before going to the output jacks, goes through "something" enclosed in a rectangle, calles "phones".
It does not show it in detail. It usually involves switching the speakers off when plugging headphones, it often fails (weak/worn contacts).
You also have a switch on the "4 ohm" jack.
To bypass them, grab a spare jack, solder two 12" wires to it, say red and black, solder black to ground (you can use th ground of any of the output jacks) and red straight to the power amp output rail (top of R27 or where R24/25 meet).
Plug your cabinet into that floating jack. Any sound?
2) the amp *does* have an electronic "mute" circuit, usually activated when too hot.
If it fails, it may work all the time, so your good amp mutes anyway all the time.
Check the jacks first as in "1".
Good luck.
Whew. Ok. I whipped up the vagrant jack and after fiddling and fiddling with it, got it to work. I was initially getting only loud feedback, then I plugged into my trace elliot to test the pre-amp again and figured I'd give it one more shot and it started working, loud as hell. So this would be indicative that the switch in either the "phones" jack or the 4 OHM out may be bad? I have a couple pictures of tonights fiasco I'll try and get up here. In the overall shot, I labelled the 4 ohm out jack in the lower left of the photo, so you can get a good idea of how the wiring is routed from there to the board and up to the "phones" jack. There is a close up of the "Phones" jack and it's corresponding, 3 way connection and then there is a close up of the circuit board where I finally found big, fat R27 resistor, wrapped a wire around it and had sound. Does this mean the heat activated mute safety is not the problem?
:dbtu: :dbtu: :dbtu:
Starting by the end, It looks like the thermal mute is not the problem.
At least one of the jack switches is bad (worn/weak)
Since they are all the same vintage, the still working ones must be close to useful life end anyway, so replace (carefully, pampering the board) all 5: loop send/return , headphones , speaker jacks.
Google some supplier which has an online catalog with good drawings or pictures (or check jack factory catalogs , such as switchcraft or Cliff) to be able to order with precision.
Or just buy 5 "mono-switching jacks" (4 legs) and 5 "stereo-switching jacks" (6 legs) and cover all bases, once you have them you replace then one by one.
*If* too complex, there is a "ghetto repair" possible just by bypassing the switches with a piece of wire, but *if* you plug headphones they won´t mute the speakers and soon.
But as somebody said once: "did Jimi Hendrix need a f*ck*ng headphone jack?"
Good luck.
Man, that is just so hard to fathom, this thing is like brand new. It must just have been a bad switch from the factory, this probably has less than 4 hours of ON time since unpacking from it's factory box. :( K, so are the stereo jacks the ones with the switches that mute to the cab jack or do I need to get all 10 switches (stereo and mono) and if so, which are which? This process will require me to desolder the old switches off the circuit board and solder new ones onto it, correct? I haven't done that before. I saw yesterday in Radio Shack when I bought my floater jack that they had kits for small (scale) projects like that, with desoldering "bulbs" that look kinda like an inline primer bulb on a boat gas tank. I'm a heavy equipment mechanic, so I think in lamens terms when my electronic experience compares to this stuff. Too bad Randall didn't have some sort of bulk parts to replace. A new "Phones" assembly would be nice, just plug it in and bolt it to the faceplate. Course, it'd probably also be $5,000...
Blacksack, I don't think Juan realized how new this amp is. I'd say just replace the broken jack and be on your way.
So JoeCool, you think the best bet would be to contact Randall about a replacement "Phones" jack?
Quote from: Blacksack on February 02, 2012, 06:02:12 PM
So JoeCool, you think the best bet would be to contact Randall about a replacement "Phones" jack?
Yes. If it's as new as you say it is they might even send you a replacement jack free of charge.
Man, I'm having a hard time trying to get any info from Randall. Online parts seem to be knobs and tubes (apparently, not for my amp). Do you think parts from this listing on eBay would fit my amp?
http://www.ebay.com/itm/Randall-RH-100-Amplifier-parts-/330681656902?pt=LH_DefaultDomain_0&hash=item4cfe29a646
I am not sure of the diffrence, looks a little older possibly. Is that a G1, where mine's a G2? I wouldn't want to get it and find it has slightly different electronics. Damn, this is just a never ending hassle. I've probably spent the $200 dollars worth of bandwidth typing on here when I could have just bought a new one! >:(
Quote from: Blacksack on February 03, 2012, 02:43:00 PM
Man, I'm having a hard time trying to get any info from Randall. Online parts seem to be knobs and tubes (apparently, not for my amp). Do you think parts from this listing on eBay would fit my amp?
http://www.ebay.com/itm/Randall-RH-100-Amplifier-parts-/330681656902?pt=LH_DefaultDomain_0&hash=item4cfe29a646
I am not sure of the diffrence, looks a little older possibly. Is that a G1, where mine's a G2? I wouldn't want to get it and find it has slightly different electronics. Damn, this is just a never ending hassle. I've probably spent the $200 dollars worth of bandwidth typing on here when I could have just bought a new one! >:(
No worries, you just need a basic jack. You should be able to get one from mouser.com, partsexpress.com, jameco.com or even smallbearelec.com depending on what you need.
If you can post a good pic of the jack maybe we can help you find one.
Even though the amp is new I would replace ALL the jacks:
I've been buying returns from musicians friend fixing them and reselling them:
About half of the returns are bad jacks corroded contacts.
A lot of these "new amps" have spent a long trip in a container from China!
Only do this if your soldering skills are good. It's very easy to damage the traces on these amps So if you doubt your desoldering techniques don't try to fix waht aint broke :D
Well, I am not very confident in my desoldering skills (or soldering, for that matter) on circuit boards. I got back down and took a few pictures of the headphone jack. One with it plugged in still, one with it unplugged and one of the underside so a person can get a good idea of how maybe contacts (legs?) it has. I wonder if Randall (or the guitar manufacturer) builds these with the specific connectors or if this might be a common component available from a retailer with the litte connector and everything ready to go already. I looked, but to no avail.
You will not find the little board as shown.
Maybe you can find the exact same jack, *somebody* must have it, but sometimes catalogs don't have good pictures to help you choose.
Not even Randall shows the headphone out schematic, just a little square where the little board is plugged. :duh
I would just bypass the whole thing (losing the headphone option, which sounds *horrible* anyway) and solder the speaker out jacks straight to the proper points in the PCB.
The little "phones" rectangle shows 4 pads which *should* be labelled so in the PCB:
PA+ / Ext+ <-- join them with a small piece of wire and
PA-/Ext- <-- do the same
Now you have bypassed (and killed) the headphone out.
Cover the jack with a piece of tape, so nobody ever uses it.
Now to the "4 ohm out" , do a similar thing:
just in case, post pictires of it, both above and below the PCB.
We are bypassing and killing two troublemaking connections, which anyway are never used.
If you do Juan's method, I would take it one step further and remove the "phones" plug and board all together and simply wire nut the appropriate wires together.
I apologize for letting this sink into the oblivion for yet another couple weeks. I have some photos to upload before I started wire nutting, soldering or anything else. I am not exactly sure what points I need to connect together to bypass the phones jack and the 4 ohm. The first picture is a close up of the 4 ohm board and the wire connections on it. There is a ground switch on the left and the two jacks for the 4 ohm parallel. The second photo is a close up of where the two big wires meet at points labelled SP+1 and SP+2 on either side of a large coil-butron labelled L1. The third photo is a larger, overall shot of the whole unit, the phones jack belonging in the upper left hand corner and the 4 ohm board in the lower left hand, respectively.
Quote from: Blacksack on March 05, 2012, 07:20:24 PM
I apologize for letting this sink into the oblivion for yet another couple weeks. I have some photos to upload before I started wire nutting, soldering or anything else. I am not exactly sure what points I need to connect together to bypass the phones jack and the 4 ohm. The first picture is a close up of the 4 ohm board and the wire connections on it. There is a ground switch on the left and the two jacks for the 4 ohm parallel. The second photo is a close up of where the two big wires meet at points labelled SP+1 and SP+2 on either side of a large coil-butron labelled L1. The third photo is a larger, overall shot of the whole unit, the phones jack belonging in the upper left hand corner and the 4 ohm board in the lower left hand, respectively.
Can you post a picture of the bottom of the board that shows where the phones and output jacks are?
**edit**
By "bottom" I mean the underside with all the soldered joints.
Me neither.
Draw a schematic of what´s on the jack board.
Label parts as they are in the PCB, wire colors, connection points, etc.
Yes, I can do that. I think there are five seperate PCBs, the tiny one with the phones jack (pictured in an earlier post), the larger one with the 4 ohm paralell jacks, an Identical one on the other side with the speaker outputs and the send/return jacks, a larger one in the center where the two large black round ...apparatuses... are and finally the largest PCB with all of the control knobs. I'll take a photo of the underside of the 4 ohm PCB and try to label what is what. Taking photos of the other three would require some intense dissasembly, I'll do it if it will help. Lemme know, and thanks again!
So, my guess would be that the amp is switching grounds here, as I think the black and the blue from the 4 ohm are both grounds and the black and the blue on the "phones" jack look to both be grounds. Here are some photos, two more of the "phones" jack PCB (top and bottom) and two of the 4 ohm PCB (top and bottom). If they are just swapping grounds on me, could I just run the correct wire directly to ground on both jacks to bypass the jack switch? Ugghhh...
So, does this sound like the right path? If so, which one needs to be grounded, or do they need to be tied into each other and then grounded? Or am I totally off base...? :loco
Quote from: Blacksack on March 14, 2012, 11:44:50 AM
So, does this sound like the right path? If so, which one needs to be grounded, or do they need to be tied into each other and then grounded? Or am I totally off base...? :loco
I think you are on the right path. To check, try using a wire to "jumper" the appropriate pins and see if it does what you want it to. If it does, solder the wire in.
I tried jumping the phones connector. I wiggled the cables. I plugged and unplugged connections on the circuit board. I rigged this connector to that ground, on and on and on. I can't get the damn pile of crap to work! So then, I plug my cab into the 4ohm output instead of the "line out" jack. Everything works and sounds normal. What is the 4ohm jack used for normally and can I just use it as my dedicated "line out"? And does this make anything more evident as far as the problem?
Quote from: Blacksack on April 03, 2012, 10:54:07 PM
I tried jumping the phones connector. I wiggled the cables. I plugged and unplugged connections on the circuit board. I rigged this connector to that ground, on and on and on. I can't get the damn pile of crap to work! So then, I plug my cab into the 4ohm output instead of the "line out" jack. Everything works and sounds normal. What is the 4ohm jack used for normally and can I just use it as my dedicated "line out"? And does this make anything more evident as far as the problem?
"Line out" is for connecting to a PA or recording device, your speaker cab should always be connected to "speaker" etc. In this case "4ohm" is where you would connect your cab. Out of curiosity, have you been trying to drive the speaker cab with "line out" this whole time?
QuoteI can't get the damn pile of crap to work! So then, I plug my cab into the 4ohm output instead of the "line out" jack. Everything works and sounds normal.
You must be joking :o :o :o :o :o
Just plowed through all this,,, phew
Best bet is remove both H/phone socket and line out,,, then take the speaker hot directly to the *Speaker out socket* (I''m assuming the speaker common is already soldered to that socket)
Oh do remember that speaker common is ground lifted so it's not chassis or circuit common.
If you do ground it the amp will run hotter.
Phil.
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?
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.
Quote from: J M Fahey on January 27, 2012, 04:18:03 AM
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
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 .
Quote from: J M Fahey on April 20, 2015, 10:35:42 PM
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
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:
(http://img.dxcdn.com/productimages/sku_134129_4.jpg)
(http://www.robotroom.com/Circuit-Board-Tips/Wire-Loop-Test-Point-Hook.jpg)
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