Randall RG100 Head ... power loss

[JHow]

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

[J M Fahey]

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.

[kernalflagg]

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.

[J M Fahey]

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.

[kernalflagg]

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/

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://img97.imageshack.us/i/rg80lefttop.jpg/
http://img696.imageshack.us/i/rg80leftbottom.jpg/
http://img220.imageshack.us/i/rg80righttop.jpg/

This is the main one I am looking at:
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/

Plain:
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.

[JHow]

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

[J M Fahey]

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.

[kernalflagg]

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

[kernalflagg]

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.

[JHow]

In 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.

I 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.

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.

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.

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.

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.

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?

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

I'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?

[kernalflagg]

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?

[JHow]

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?

The ones in mine look exactly like the brown one on the top of your picture.  They do not say "Colber" on them.

[kernalflagg]

 
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.

[J M Fahey]

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.

[kernalflagg]

*** 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?