Hello, newbie here
I've been playing guit-fiddle for more than 3 decades. Got me a Randall RG100 150w head from the 80's (not sure what year). Been a vintage Marshall guy my whole life - love my Marshalls which I'll never sell! But, a couple years ago I got rid of a guitar at a music store and wound up with this RG100 head as part of a trade.
It worked fine for the first 6 months, but it was quite noisy at idle with a loud "waterfall" with "a little hum to it" sound - even on the clean channel and no pedals or anything even plugged into it. Kinda weird. By contrast, my Marshalls are very quiet at idle going through the same Marshall cabinets. I've tried the Randall into other cabs, like Fender, and Ampeg - same result.
Anyways, the clean channel quit working completely. The green light on the front lights up - but no sound, even with volume turned all the way up. Another peculiar thing is that the distorted channel has lost major tone. It no longer has any power and sounds very "tinnny." This gets worse after about 15 minutes of constant playing, until there's barely any sound at all coming out. Both of these symptoms occured at the same time. It has been this way for 2 years - but hasn't gotten any worse.
I'm speculating that the idle noise may be some kind of grounding issue. The power loss may be the capacitors. I really don't know.
I've taken it to several amp repair shops, and the techs at two different shops said that they weren't sure what was wrong with it, and didn't want to take it on. They said the amp wasn't worth it. One of the techs said that it had an RG80 board inside of it. But, another tech said it was normal for Randall to put RG80 boards inside RG100 heads in the 1980's. I've seen the RG80 board inside of it.
The techs took it apart and couldn't visually see anything wrong. They told me that this amp is just junk and not worth fixing, and somebody may have done something weird to it before they traded it in at the music store (like swapping the boards of 2 incompatible amps or something along those lines). Now I'm stuck with it. The guitar I traded in was worth about $200. The value of these RG100 heads are about $200-$250 when nothing's wrong. Mine is now worth $0 since it barely works.
I've considered sending it off to an electronics repair shop rather than going the guitar amp repair shop route. But, that may not work out. I really would hate to just throw it in the garbage or give it to the Goodwill. But, I don't know how to fix it myself either.
I'm very technical minded. I've been an auto mechanic for 15 years, and I have pretty good knowledge of electronics and electrical theory and troubleshooting basic DC circuits. (I know amps are AC) I have a DVOM but no lab scope. But, I'm no amp repair tech - that's for sure!
Does anybody have any ideas? I'd love to fix it for as cheap as possible (or have it fixed as cheaply as possible). I don't have that much money these days, kinda broke with the high gas prices and crap. So, a "quick" and cheap fix is the best for me. I'm not a milliionare!
If I knew what exactly would fix it for sure and the cost is under $100, I'd probably go for it.
Advice and ideas much appreciated.
Thank you in advance,
Kernal Flagg
Well, the RG80 and RG100 are technically almost the same circuit. The amps have slight differences - like different mains transformers – but technically the circuit and the boards are just the same for both.
You can find a schematic for a '81 model from schematic heaven but there might have been some revisions since then so I wouldn't trust it blindly. The schematic covers models RG80-122SC, RG80PH, RG80ES, RG100ES and RG100H so you can see that they used the same circuits/boards in a very wide line of products. Most manufacturers do this even today since it is very cost-effective.
These are highly rated amps in circles that appreciate brutal distortion tones (e.g. death metal and such genres). Definitely not junk - far from it, actually.
I really don't have any ideas except the usual things: Measure the rail voltages and see if there's something wrong with them. If not proceed to trace the signal from the input towards the output and see where it breaks/goes bad. Sounds like a component failing – could be a contact problem as well.
Thanks Teemuk.
I have downloaded the book you have on here and I will read through it. I watched one of the techs make some checks on my Randall. He grounded out the caps on the bench (they sparked almost like an arcwelder). He said that was good - shows good capacitance. The voltage on both sides of the two caps was 48V +/- 0.1V. He said that was good - no problem there. I'm skeptical on this though.
I'm with ya on checking the whole circuit. He must have overlooked something.
I'm not sure of the scematics. Mine has "RG100 HB" stamped on the back of the amp. Is that the same as "RG 100 H" ??
If I recall, the last time (a couple years ago) I tried to fix this amp, I ran across this problem. I wan't able to get schematics for an RG 100 HB - couldn't find them anywhere. I contacted Randall and they said that there was an "HB" model made that was different than the "H" model. I looked at the schematics of RG80 and RG100 and they are different (quite a bit different if I recall). But, I may be wrong.
Dear kernalflagg. I think I can help you with your amp, but to make sure we´re talking the same, post here the schematic *you* have, so I´ll suggest you some things to check, to see how close it is to your actual amp, and then suggest some voltage and resistance measurements. To begin with you´ll need a "series lamp" fixture, and your digital multimeter. Bye.
oK, thanks JM
The schematic is saved in a pdf. I don't know how to post that on here.
No problem, mail it to: juanmanuelfahey@gmail.com I do have a schematic and a couple of board pictures, but I want to be sure that we are talking about the same, so when I ask you to measure , say, voltage on Q104´s base or something like that, we agree.
I split my PDF into a few functional blocks:
Quote from: J M Fahey on June 07, 2008, 08:11:07 AM
I split my PDF into a few functional blocks:
J M Fahey,
What is the trimmer pot for?
Bias.
Don't mess with it needlessly.
If in doubt, set it to its minimum value, it's the failsafe position.
Looking at the second pic (red channel), you can see where the clipping diodes are switched with the "pull" treble knob.
When you do this there is a huge volume drop in that channel (by design).
I'm trying to wrap my head around a fix for this issue.......keeping the clipping diodes on but compensating for the huge volume loss.
Are you repairing or building one?
Neither.
I own several and they all have this "feature"......and I'd like to figure out a mod that would retain the extra diode clipping in the red channel, but compensate for the extreme volume loss.
That way I could keep the extra gain/compression at a giggable level.
That brutal volume loss comes from the fact that it really does not *increase* sustain, because it does not increase gain when you pull that knob, but it switches those diodes in.
The original Zeners clip at 4.7 V peak; the diodes at 650mV; so you increase clipping but your signal drops to .65/4.7= 0.14 the original level.
You need 7x gain added after that to compensate.
The only way is to pull R28 (10K) and add there a mini-board with something providing that gain.
It might be a FET or bipolar transistor, or a TL071 or similar.
Increasing gain somewhere else would not do, because it would also affect the Green channel.
The amp design is so simple (good) that it does not have many options for tweaking.
Try to get a link to somebody talking about the mod you mentioned earlier, maybe reading it I might have some other idea.
Did you ever get any resolve to this issue? If not, let me know. I just may have the anser for you.
BMW FREQ
Dear BMW.
Randalls are popular here (SSGuitar ;)), so even if we never again hear from the original poster , I'm *sure* your posting that solution here will certainly help many, so please do.
Thanks in advance.
JM Fahey:
On your third picture, there is a small blue elctrolytic cap from circuit board to chassis. It lands right near the PS filter caps. What is that cap? What part of the board is it attached to? I ask because I don't have that on my amp.
Regards,
JHow
Don't know.
I didn't take the pictures myself but were downloaded, now I don't remember who or where they posted them originally.
Just an educated guess: many amps try to avoid ground loops by separating PCB ground from actual chassis metal, yet it has to be grounded somehow to let it act as an electrostatic shield.
The solution?
They are joined *only* through a low value resistor (typically 10 ohms) which is parallelled by a capacitor, often 10uF; sometimes they are also parallelled by a couple diodes, one pointing each way, .
Look at modern Marshalls.
Obviously Randall pioneered *many* things we take for granted today, they were always groundbreaking.
Hey everyone, it's me, I'm the OP to this thread. Sorry, I haven't been back here in a while. I have had a lot of pressing issues I've had to deal with the last couple years, and I have had zero time whatsoever for my guitar. (Has it really been that long? Wow – time sure does fly!)
Anyways ... I have started to work on this amp again to and I have some new information about it that I'd like to share and possibly get some help in fixing it.
I typed up a pretty long (but clear and understandable) update to this thread, and I would like to include some schematics and jpgs.
HOWEVER, I get nothing but errors when I try to upload the attachments.
My questions:
What's the deal with attachments here? Am I supposed to have a min # of posts to do attachments, or how does this work?
If someone could fill me in on this, I'd appreciate it thanks.
Not sure about the # of posts, let joecool answer that, but as far as attachments, I've never been able to "glue" them here directly from my own computer, I always uploaded them to some free server such as imageshack, then clicked here "add an image" and included the "direct address" imageshacked provided between "img (insert image address here) /img" tags.
Preview the message before definitively posting it to check it.
Works for images (gig/jpg/maybe png).
For pdf, you'll need another general purpose server (not image-only) and just post the direct link here.
RE: My Randall RG100 HB head
(I will try to make this as clear as possible, with minimal confusion, so try to bare with me)
After doing some research on this amp, I have determined that it is a reissue of the RG100 ES. It was produced in the mid-late '90's during Randall's "Washburn era." It has an RG80 main board, which is identical to the older SS RG80 model. There isn't much info on the RG100 HB because they lack the tone quality of the original RG100, and so they're not all that popular anymore. Nevertheless, I would still like to try to fix this amp if at all possible.
Pic of the amp from the front:
http://img717.imageshack.us/i/frontqpe.jpg/ (http://img717.imageshack.us/i/frontqpe.jpg/)
After hours of examining all the electronic components on the main board of my amp and tediously comparing them one-by-one to all the different schematics, I have determined that without a doubt, the schematic I'm attaching to this post is the correct one for my amp, and it is an exact match.
SCHEMATICS:
Because the resolution is poor, I had to break the whole diagram up into 4 parts. I overlapped them a bit, and I included a "whole" one so you can see the overall layout of each individual part.
http://img59.imageshack.us/i/rg80whole.jpg/ (http://img59.imageshack.us/i/rg80whole.jpg/)
http://img97.imageshack.us/i/rg80lefttop.jpg/ (http://img97.imageshack.us/i/rg80lefttop.jpg/)
http://img696.imageshack.us/i/rg80leftbottom.jpg/ (http://img696.imageshack.us/i/rg80leftbottom.jpg/)
http://img220.imageshack.us/i/rg80righttop.jpg/ (http://img220.imageshack.us/i/rg80righttop.jpg/)
This is the main one I am looking at:
http://img7.imageshack.us/i/rg80rightbottom.jpg/ (http://img7.imageshack.us/i/rg80rightbottom.jpg/)
ALSO – see the gif named "SSPower" in J M Fahey's post from reply #6 on page 1 of this thread. I will post the link to it here for convenience:
http://www.ssguitar.com/index.php?action=dlattach;topic=816.0;attach=408;image
After taking another look at the guts, I found a burned resistor – which is something new that I did not notice before (R4 on the schematic) and 4 missing emitter (ballast) resistors: R2, R3, R12, and R13. (Some knucklehead that owned it before me must have cut them out with wire cutters). I have had this amp apart before, but I had never looked close enough at it to see the missing resistors.
NOTE: I am AMAZED about the fact that this amp has been to THREE different amp repair shops, and NONE of the techs even noticed that FOUR BALLAST RESISTORS WERE MISSING!!! ... LOL
Gut shots:
Note: In one of the gut pics, I have circled the places where the missing resistors go in RED, and R4 is circled in YELLOW. I circled R1 because I would like to verify what value it is supposed to be, because it was soldered on the backside of the PCB, and I'm not sure if it has been replaced in the past.
With markings:
http://img576.imageshack.us/i/randallrg80circuitwithm.jpg/ (http://img576.imageshack.us/i/randallrg80circuitwithm.jpg/)
Plain:
http://img26.imageshack.us/i/copyofrandallrg80circui.jpg/ (http://img26.imageshack.us/i/copyofrandallrg80circui.jpg/)
I have run into a few snags:
The problem with the schematic, although it is the correct one, is that it does not give the resistor values, and since I have no resistors in place to go by, I don't know what the correct value (or type and construction) of the resistors are supposed to be, or where to get order replacements from.
After looking at all the other available schematics from that era of Randall amps, I have figured out that they all pretty much use the same or very similar "piggyback" power output circuits, which is where these resistors belong.
In fact, on my main board, R4 and R14 are identical 100ohm/0.5w resistors. I verified this by looking up the color bands on the resistors themselves. So, R4 is taken care of.
My main issue lies with the 4 missing ballast resistors I mentioned above.
My questions:
1) Since I know that R4 and R14 are the same on the SSPower schematic (which has the resistor values in the diagram), ... and I know that they are identical and physically match the values on the SSPower diagram (by their physical color bands) ... can I conclude that R2, 3, 12 and 13 on MY main board should be the same as the values written on the SSPower schematics?
For example:
The schematic "RG80 Right Bottom" for my board lists my missing ballast resistors as R2, 3, 12, and 13, which are numbered as: R55, 57, 56, and 60 on the SSPower schematic.
Note: R2 on the RG80 Right Bottom schematic = R55 on the SSPower schematic, R3 on the RG80 Right Bottom schematic = R57 on the SSPower schematic, etc... and so forth.
Also note: There are other schematics that also use the same power circuit design, and they all have very similar - if not exactly the same - resistor values.
Then the correct values would be as follows (according to the SSPPower schematic):
R2 (R55) and R12 (R56) = 0.6ohm/7w
R3 (R57) and R13 (R60) = 0.27ohm/7w
Also . . . R4 (R53) and R14 (R54) = 100ohm/0.5w
2) Were all the ballast resistors used for these amps pretty much the same type and construction – just different ratings? What type were they if anyone knows? What company were they made by?
3) Where would I be able to find the correct resistors? (I have looked into electronic component dealers that deal in "vintage/obsolete/hard to find" components, but do I need to go that far?
Or can I use some I found made by "Ohmite" that were pretty close value-wise? But I'm not sure they are the right type. And I found a few others that were close as well, but again, not sure if they would work either...???
If anyone can shed light on this, I'd surely appreciate it. Thank you.
In my RG80/112SC these resistors are just rectangular, wire-wound .6 / .27 Ohm 10% 7-Watt.
I think mouser will have 7 or 10 watt replacements with similar dimensions: Maybe have a look at this PN at mouser280-CR10-0.27-RC ?
http://www.mouser.com/ProductDetail/Xicon/280-CR10-027-RC/?qs=gI4ZKBuUnui5tgBooyyNvg%3d%3d
Hi kernalflagg.
Today I'm too tired to cross-check the schematics, (Hospital health check early tomorrow) but rest assured that they are 95% the same, with small variations (the 2-4/8-16 ohms switch) and little else.
Parts are very common and inexpensive, maybe size or color varies, no big deal.
Tomorrow night hopefully I'll try to assign old (known) values to modern parts.
Stay tuned.
Quote from: JHow on April 18, 2011, 01:21:49 PM
In my RG80/112SC these resistors are just rectangular, wire-wound .6 / .27 Ohm 10% 7-Watt.
I think mouser will have 7 or 10 watt replacements with similar dimensions: Maybe have a look at this PN at mouser280-CR10-0.27-RC ?
http://www.mouser.com/ProductDetail/Xicon/280-CR10-027-RC/?qs=gI4ZKBuUnui5tgBooyyNvg%3d%3d
Cool thanks man! :tu:
On the spec sheet, they are listed as "wire wound cement filled ceramic."
Questions:
In
your amp, does all 4 look like the sam type and construction as those in the pic from the link you posted?
I doubt they would be vertical mount, but just to make sure, they are axial right ?
The only thing that bothered me was that I couldn't find any 7W ones listed on the spec sheet. I wonder if it makes a difference to go with 10W or not. I know that they had a lower heat dissapation temp, but I'm not sure how that would affect it, if it will at all.
I found some made by Ohmite (PN# TUW7) that listed both the 0.6ohm/7w and 0.27ohm/7w but I'm not sure if any of these are right type and construction. They don't give an actual photo of any of them, just a drawing.
Also for the TUW7, they give a "range" of values (0.1 to 680) for ohms, instead of specific values. I wonder if that means you can order any value that is in the range? (I guess I could contact them and ask)
I'm also wondering if I need to be concerned that thye are rated at 500V?
Here's the link:
http://www.ohmite.com/cgi-bin/showpage.cgi?product=tuw_tum_series
Quote from: J M Fahey on April 18, 2011, 10:49:35 PM
Hi kernalflagg.
Today I'm too tired to cross-check the schematics, (Hospital health check early tomorrow) but rest assured that they are 95% the same, with small variations (the 2-4/8-16 ohms switch) and little else.
Parts are very common and inexpensive, maybe size or color varies, no big deal.
Tomorrow night hopefully I'll try to assign old (known) values to modern parts.
Stay tuned.
Cool. BTW, I have to do some major yardwork at my uncle's house for the rest of the week, which is a half day's drive from here. So I may not be back for several days.
Quote from: kernalflagg on April 18, 2011, 11:23:04 PMIn your amp, does all 4 look like the sam type and construction as those in the pic from the link you posted?
Yes. They are rectangular, axial. The orginal ones are brown, The mouser xicon ones are white, but that isn't a concern.
Quote from: kernalflagg on April 18, 2011, 11:23:04 PMI doubt they would be vertical mount, but just to make sure, they are axial right ?
Yes, you want the axial ones, they lie flat on the board.
Quote from: kernalflagg on April 18, 2011, 11:23:04 PMThe only thing that bothered me was that I couldn't find any 7W ones listed on the spec sheet. I wonder if it makes a difference to go with 10W or not. I know that they had a lower heat dissapation temp, but I'm not sure how that would affect it, if it will at all.
As to the wattage, 10w rating will be okay versus 7W original. As to heat dissapation temp, I didn't see that spec, but 10 watt rated device should dissipate more power than 7W, so should not cause a problem.
Quote from: kernalflagg on April 18, 2011, 11:23:04 PMI found some made by Ohmite (PN# TUW7) that listed both the 0.6ohm/7w and 0.27ohm/7w but I'm not sure if any of these are right type and construction. They don't give an actual photo of any of them, just a drawing.
These should also work. Your big concerns are resistance, power rating, will it fit and can you find a supplier that has it. You have your board and hole spacing so you can measure before you buy and tell if it's too long or wide or not.
Quote from: kernalflagg on April 18, 2011, 11:23:04 PMAlso for the TUW7, they give a "range" of values (0.1 to 680) for ohms, instead of specific values. I wonder if that means you can order any value that is in the range?
Yes, the description you linked to is giving the range of values. You have to check if you can source the specific standard value (.27 or .6) you need from your vendor. I looked at mouser and digikey but I didn't see the TUW series in values you need and in-stock. They had the 15w, but that is pretty chunky size: 12.5 mm W x 49 mm L x 11.5 mm H
http://www.mouser.com/Passive-Components/Resistors/Wirewound-Resistors/Wirewound-Resistors-Through-Hole/_/N-7fx9i?P=1z0x6u4&Keyword=ohmite+TUW&FS=True
Quote from: kernalflagg on April 18, 2011, 11:23:04 PMI'm also wondering if I need to be concerned that thye are rated at 500V?
How much voltage does the circuit have on that part? The whole power supply is only about 82 volts in mine. Those parts are probably around 41 volts, n'est-ce pas?
I managed to get a couple of close-up pics of some ballast resistors from an RG100es.
I have a few questions:
1) Do you know if the ones in your amp were made by Colber?
2) Do they look like these (http://img818.imageshack.us/f/rg100esballastresistors.jpg/)?
Quote from: kernalflagg on April 25, 2011, 02:22:01 AM
I managed to get a couple of close-up pics of some ballast resistors from an RG100es.
I have a few questions:
1) Do you know if the ones in your amp were made by Colber?
2) Do they look like these (http://img818.imageshack.us/f/rg100esballastresistors.jpg/)?
The ones in mine look exactly like the brown one on the top of your picture. They do not say "Colber" on them.
:-\
Because my amp is an RG100HB, and not an RG100es, I still don't know if these are the correct ballast resistors for my amp. The correct schematic for my amp does not have any resistor values on it. (:tu:) I am presuming that they are the same, but it is possible that these are not the right ballast resistors - at least resistance and wattage wise.
RE: Mouser and Ohmite
I will have to contact Mouser and Ohmite to see if I can get the exact resistors I need, (that is - if I can determine for sure that the 0.6/7w and 0.27/7w are the correct ones for my amp) and if I can order just a few of each type instead of like having to order a lot of like 100 or something.
Don't worry too much about version.
Main differences lie in the preamp; poweramps ara basically the same, specially the ballast resistors used.
Thay .27/.6 ratio between them was typical of Randall to even current sharing at full power while lightening the load on drivers.
Acoustic also used something similar, Peavey a distant cousin.
Don't worry about trademark or colour , only about value, ratings or size.
Mouser will happily supply you with just one if you like, I suspect Ohmite will have a minimum order of a few thousand dollars.
*** Update ***
Hello everyone, I have done some research on getting ballast resitors. I called Ohmite and talked with their customer service dept, who referred me to a couple of their distributors. Although they were able to locate the 0.27 and 0.6 ohm 7 watt power resistors, they are indeed only available in lots of 500-700 each (and they cost like $0.50 each). So, forget that idea!
However, I did come up with a couple of viable options that may work, and I am hoping to get some feedback:
1) The I can get 0.27 and 0.62 10 watt power resistors through Mouser. Is the extra 0.02 ohm resistance going to matter?
2) Mouser also has 0.56 ohm 10 watt power resistors. Will these work in place of the 0.6 ohm ones?
***best option, IMO
3) Mouser also has 1.2 ohm 10 watt power resistors. So...
Use the 0.27 ohm resistors, and use two 1.2 ohm resistors soldered in parallel for each 0.6 ohm resistor. According to this formula, it should work:
Required R = R1*R2/R1+R2, so if each R is 1.2 ohms, then R = 1.44/2.4 = 0.6 ohms
The only issue with this may have to do with the amount of space available on the board and it's chassis location, etc, But I think I can make it work with a few minor mods.
What do you think?
I'd go with the 0.27 and 0.62 ohm 10w resistors from mouser.com and be done with. Should work perfect.
Quote from: joecool85 on May 04, 2011, 09:12:20 AM
I'd go with the 0.27 and 0.62 ohm 10w resistors from mouser.com and be done with. Should work perfect.
You don't think the extra 0.02 ohms (+3.33% resistance) will make a difference? I think this would increase the rail voltage (+/- 48V) to ~49.6V. (48*1.033 = ~49.6 ... Is this correct?)
Quote from: kernalflagg on May 05, 2011, 02:04:38 AM
Quote from: joecool85 on May 04, 2011, 09:12:20 AM
I'd go with the 0.27 and 0.62 ohm 10w resistors from mouser.com and be done with. Should work perfect.
You don't think the extra 0.02 ohms (+3.33% resistance) will make a difference? I think this would increase the rail voltage (+/- 48V) to ~49.6V. (48*1.033 = ~49.6 ... Is this correct?)
That shouldn't matter what so ever. That's a very small change.
I'm with Joe :tu:
In my limited understanding R3 and R13 would be around 100 ohm, 1/2watt.
and R2 and R12 are the high current 5Watt Wire Wound main emitter resistors where anything from .33 ohm to .5 ohm would work.
This looks like a straight forward DC power stage with bootstraped output. I doubt a bit either way will effect it,, that said read your DC on the output when up and running to make sure it's close to Zero.
Phil.
Quote from: phatt on May 05, 2011, 09:37:30 AM
I'm with Joe :tu:
In my limited understanding R3 and R13 would be around 100 ohm, 1/2watt.
and R2 and R12 are the high current 5Watt Wire Wound main emitter resistors where anything from .33 ohm to .5 ohm would work.
This looks like a straight forward DC power stage with bootstraped output. I doubt a bit either way will effect it,, that said read your DC on the output when up and running to make sure it's close to Zero.
Phil.
From looking at the schematic, R4 and R14 are the 100 ohm, 1/2 watt resistors. R2 and R12 are 0.6 ohm/7watt power resistors, and R3 and R13 are 0.27/7watt power resistors. However, the actual schematic that matches MY amp has no resistor values marked on it. For the values, I am going off of ANOTHER scematic with the same type of power output circuit. However, other parts, such as the input stage are totally different as what is actually on my amp.
I can get the 0.27 ohm resistors no problem. But I cannot find 0.6 ohm ones without having to order umpteen thousand from Ohmite.
Getting back to the Mouser resistors:
For the 0.6 ohm resistors, I think 0.62 ohm is better because it will DECREASE the amps of current flow; thus reducing the chances of frying something. On the other hand, they do have 0.56 ohm resistors, but those will allow more amps to flow through the circuit, and will increase the chance of frying something.
I realize that it may not affect it much either way
under most operating conditions...
However ... what about at HIGHER volume levels?
Am I correct for assuming this? Or is it still not going to matter too much?
Hi again,
I understand your concern but really I doubt there would be any difference between .47/.56/.62
Let's put it this way if it did then something else is very wrong with that circuit and it would be considered unstable and bad design and would have likely died an early death.
Lets assume the amp WAS quite stable and may have run for many years without issue.
just throw in some .47 ohm 5 watt WW or close and test the thing.
Phil.
I'd still go with the .62 ohm myself, but Phil is right, most anything close should work just fine.
Whoops?
I should read before I open my mouth. LOL.
Those 4 output transistors are connected in a rather different configuration. (headscratch)
Normally they run them in parralel but this is not the case here. xP
If you are worried then you could just piggyback two 5watt units of double the ohms.
i.e. 2x 0R47 will reap 0R23,,close enough to .22 ohms, wink.
I just checked the output devices in my comparison chart book and found 2N6254 are compatible with 2N3055. (a common device)
Anyway having just re read the first posting again to refresh my memeory I'm struggling to understand that you said the amp worked but had issues then you say it stopped.
I can't see how the amp could have possibly worked with no emmiter resistors? :loco
Although the VAS stage may have been able to push a little wiggle out to the speaker even with those output devices removed from circuit but that would be very low output.
The fact that you have a burnt resistor on the board says the amp may have had a major problem and was never solved.
Anyway I'll just add,
The reality for the novice when it comes to discrete power Amp circuits is that 90% of them are high current DC coupled opamps and as such are hard to fix as when one device blows it often takes out several other devices.
(The DC current is very large so if it's got DC issues then it's going to blow something eventually)
Without the right gear (and a fairly good handle on what to do they can bend your brain but speaking from *my novice* expiditions into building many poweramp circuits that went up in smoke I did learn what to basically look for.
With a light bulb limiter wired in the mains or a couple of 47 ohm 5 watt resistors in place of DC rail fuses, Then With your meter set to read DCV, read the DCV on the 3 major points that will indicate the *DC conditions* of the Amp.
The basic aim of these designs is basically keeping the bases of Q8 and Q9*** as close to Zero DCV as can be done,, while Also keeping the output terminal very close to Zero VDC.
( *** Q8/9 on JM Faheys pics)
Those are the 3 points to test,, some designs will tolerate even 500mV offset at base of Q2 and at the output but if it reads over 1 volt then there might be a problem lurking.
The AC wiggle (your guitar) can then ride on this DC and wiggle up and down between the 2 voltage rails. (that's how Amplification works)
BUT if those DC paremeters are not set up correctly then all hell breaks loose and no doubt something will get mighty hot from DC Current draw.
That is a very simplified explanation so if you want to understand more about it then google like mad. 0:)
You might find plenty to read at ESP site (Rod Elliot) He does write his webpages with the novice in mind and he certainly helped me out with some hard stuff.
http://sound.westhost.com/amp-basics.htm
Phil.
Quote from: phatt on May 06, 2011, 10:22:08 AM
...
I can't see how the amp could have possibly worked with no emmiter resistors? :loco
...
The fact that you have a burnt resistor on the board says the amp may have had a major problem and was never solved.
I had the amp at 3 different amp repair shops, and NONE of the techs spotted the missing emitter resistors! ;D
The burned one is a new development since I initially briefly looked at it a few years ago. I did not notice the missing ballast resistors then either,
because I am indeed a novice amp tech. That is why I took it to the 3 different amp techs to begin with becuase I knew I did not know what I was doing really. None of them noticed the resistors missing and one of them charged me $80 AND COULDN'T FIND ANYTHING WRONG! ;D :trouble
That is why I have took it on myself, even though fixing amplifiers is not something I normally deal with. Nevertheless, I do have experience troublshooting electronic and electrical circuits ... on automobiles.
But YES the amp STILL works with 4 missing emitter resistors and a burnt R4. :loco
It even sounds half way decent, but has that horrible buzz, which was not there so much when I first got it!
OK, I ran into a few snags again. The available length area for each power resistor on the main board is ~25 to 30mm. This was plenty for the original 7 watt resistors, which were ~ 22 mm in length. The Mouser 10 watt resistors, which are the ONLY ones available, are 49mm in length. Width and height are not an issue.
How could I make this work? Can I ...
1) Fold the leads of each resistor underneath the resistor and solder them in place? Or...
2) Mount the resistors at an angle (like 45 degrees) with one end off the PCB board? Or...
3) Use 5 watt power resistors from Mouser that are only 22mm in length, even though the originals were 7 watts each?
My concern with options 1 and 2 are:
What if the leads heat up, and the resistors "fall" over and short out against the board?
My concern with option 3 is:
Using a lower power rating resistor may fry the amp.
Any ideas/suggestions? (What about using vertical mount resistors instead?)
None of the options you sugested sound like a good one.
1 & 2: Mounting components floating off of the board just isn't a good idea. I wasn't thinking of it falling over but that might be possible. I was thinking of it breaking off.
3: I'm pretty sure that 5 watts will be too little, this is probably the worst option.
Vertical mounting might work, but your going to have to figure out how to mount it, I'm not sure the existing contact points will work without extra wire. You'll also have make sure they will fit like this inside the case without bumping into anything.
One question, will the resistors be lying down, or hanging off the side/upside down when the amp is in the normal position? If they are on top then if you can find away to support them being off the board then #1 should definitely work. I was thinking of something like standoffs glued to the resistors, they will likely need to be ceramic as I'm unsure plastics will take the heat and the metals will be conductive and there appears to be traces where they might need to go. This might also work if they aren't on top but I'd be reluctant to try it unless it was my only option. (edit: you'll need glue that can take the heat, not really sure what the best type would be)
Maybe these 7Watts from mouser, wirewound, 22.2 mm length?:
66-W22R270JRLF
66-W22R470JRLF
Quote from: substratus on May 12, 2011, 01:15:13 AM
None of the options you sugested sound like a good one.
1 & 2: Mounting components floating off of the board just isn't a good idea. I wasn't thinking of it falling over but that might be possible. I was thinking of it breaking off.
3: I'm pretty sure that 5 watts will be too little, this is probably the worst option.
Vertical mounting might work, but your going to have to figure out how to mount it, I'm not sure the existing contact points will work without extra wire. You'll also have make sure they will fit like this inside the case without bumping into anything.
One question, will the resistors be lying down, or hanging off the side/upside down when the amp is in the normal position? If they are on top then if you can find away to support them being off the board then #1 should definitely work. I was thinking of something like standoffs glued to the resistors, they will likely need to be ceramic as I'm unsure plastics will take the heat and the metals will be conductive and there appears to be traces where they might need to go. This might also work if they aren't on top but I'd be reluctant to try it unless it was my only option. (edit: you'll need glue that can take the heat, not really sure what the best type would be)
You know what ... YOU just brought up a very good point:
These will be hanging upside down the way that the amp normally sits! This means that I don't really have to worry so much about them falling over onto each other or onto the board. I will defeinately take another look at this.
Quote from: JHow on May 12, 2011, 10:03:44 AM
Maybe these 7Watts from mouser, wirewound, 22.2 mm length?:
66-W22R270JRLF
66-W22R470JRLF
Despite the fact they are vitreous enamel, and not ceramic like the originals, I think they should still work. I believe the enamel will make the amp run quieter but their heat dissipation characteristics are not as good as ceramic. That shouldn't be a problem as far as I can see.
Resistance-wise:
The 0.27 ohm ones are correct, but the 0.47 ohm ones may be a little far from the originals that were 0.60 ohm. since these are in the power output stage of the power amp circuit, I'm not sure if the smaller resistance will have a detrimental effect when the amp is cranked up to higher volume levels...?
According to Ohm's law:
less resistance = more amperage, which may possibly = expensive compressed smoke
That Randall amp needs those .27/.62 ohm resistors as shown in the schematic.
It was an old trick (also used by others such as Acoustic) to even the current sharing.
You have a length problem here.
Those 49mm long ones are way too much.
In 22 mm/5W:
1) you can use a 5W .27 ohm without a problem because it dissipates less than one half what the .62 ohm does.
Why did they use a 5W then? because buying a smaller one costs practically the same and simplifies bulk ordering (as from Ohmite)
2) substitute each .62 ohm with two .33 ohm ones in series.
Mount them this way:
In your hand, put two of them vertical.
Each of them will have an upper lead and a lower one.
Twist upper leads together, solder and clip them.
Now you have a nice vertical .66 ohm, 10W resistor.
Open it slightly into an inverted "V" shape and insert free legs into the PCB.
Done.
Quote from: J M Fahey on May 13, 2011, 05:30:24 AM
2) substitute each .62 ohm with two .33 ohm ones in series.
Mount them this way:
In your hand, put two of them vertical.
Each of them will have an upper lead and a lower one.
Twist upper leads together, solder and clip them.
Now you have a nice vertical .66 ohm, 10W resistor.
Open it slightly into an inverted "V" shape and insert free legs into the PCB.
Done.
Juan, wouldn't that give him a .66ohm 5w resistor? I think he would need two 1.32ohm resistors in parallel to give him a .66ohm 10w resistor.
No, power is evenly shared, whether they are in series or parallel, so far as both have the same resistance.
If you make them dissipate 5W each, the pair will dissipate 10W.
I suggested .33 ohm resistors because they are very popular and easily available; don't know if you can find 1.32 ohm 5W ones.
In fact, a "normalized" value would be 1.20 ohms, if available.
I think they jump from 1 to 1.5 ohms or something like that.
I guess you're right Juan. I suppose I was thinking of how many volts would cross the little fellas, not watts. Big difference.
Quote from: J M Fahey on May 13, 2011, 05:30:24 AM
That Randall amp needs those .27/.62 ohm resistors as shown in the schematic.
It was an old trick (also used by others such as Acoustic) to even the current sharing.
You have a length problem here.
Those 49mm long ones are way too much.
In 22 mm/5W:
1) you can use a 5W .27 ohm without a problem because it dissipates less than one half what the .62 ohm does.
Why did they use a 5W then? because buying a smaller one costs practically the same and simplifies bulk ordering (as from Ohmite)
2) substitute each .62 ohm with two .33 ohm ones in series.
Mount them this way:
In your hand, put two of them vertical.
Each of them will have an upper lead and a lower one.
Twist upper leads together, solder and clip them.
Now you have a nice vertical .66 ohm, 10W resistor.
Open it slightly into an inverted "V" shape and insert free legs into the PCB.
Done.
Sounds like a plan. But for the 0.60 ohm 7 watt power resistors, what about just getting vertical mount power resistors that are in stock and available in either 0.47 or 0.68 ohm and both are 7 watts like the schematic calls for? Are these too far from the specs in either direction?
such as these:
http://www.mouser.com/catalog/specsheets/XC-600039.pdf
*Electrically* they are the same; *mechanically* not, because these have 5mm leg separation; you said yours are 22mm, *big* difference.
You can't mount them in the available PCB.
My idea of mounting two smaller ones in an inverted "V" shape, is that you can separate the legs any distance you want.
I see they follow a normalized series of values, fine.
For .27 ohm use .22 ohm; for .62 ohm use .68 ohm, if available, or 2 .33 ohm in series as suggested.
If you want to be even more certain, search and post here a similar resistor page showing horizontal 7W ones in 22 mm , if available, or in 5W , still in 22mm.