Silverface Fender Twin has nasty bass distortion

[bob p]

That looks like brown-black-orange to me.

That's what I was thinking. The difference between yellow and orange is 10x. Well, that's good news! The problem may be solved.

Trouble reading resistors is why I recommended clamping an ohmmeter across the resistor to see what was originally there. One thing that I've learned over the years is that it's hard for me to trust my eyes, my memory, schematics and especially layout diagrams. I've encountered errors attributable to all of them. Just as a matter of course, when I go through an amp I print out a schematic and write the pre-service parts measurements and voltages on it. Then I print out another one for the post-repair assessment. That way I always have before/after documentation, so I don't have to rely on memory.

Personally, I find that reading resistor values is getting harder and harder. And I'm not just referring to similar colors like yellow and orange, which can be hard to tell apart on an old CC resistor that's been subjected to heat and the resulting discoloration. Today the resistors are shrinking. Today it looks like the manufacturers are shrinking the size of the through-hole parts down to the scale of the SMD parts, to the point that 500mW resistors are impossible to read without a magnifying loupe. For example, these are all current production 500 mW metal film resistors. Some are a lot harder to read than others:



http://music-electronics-forum.com/t21277/

As a result of the shrinking resistor sizes, I find that I have too much trouble reading those flea-sized 5-band resistors, so I just sort them with an ohm meter when I get them mixed up.

[überfuzz]

You should issue a reward, five red balloons to the one that pointed out the issue. That you then obviously didn't care to investigate. Well well, we'll see.

[g1]

Ok, so the blue layout drawing has a typo for the dropping resistor values in the doghouse. gtrplayr1976 has verified he has the proper 2.2K and 10K (not 100K) values. As Enzo pointed out, a 100K there would not work. With only 5mA through it, a 100K would drop 500volts.
So it would appear that the original poster (whymeworry) had dropping resistors put in by someone using that incorrect layout drawing. In the photo he posted it is clear they are not stock type parts.
If nothing else, good to know that this bad drawing exists. People working on amps that do not have electronics knowledge will tend to use layout drawings rather than schematics that they do not understand. So the issue could come up again.

[bob p]

But notice also that the node of the 100k dropper says it goes to the trem controls?

I hate these old fender schematics that are drawn to cram everything onto one page. I find them hard to read. And it's not uncommon to have errors in them. I've recently been banging my head against the Super Twin Reverb/Studio Bass schematics that are just as bad if not worse than this one.

I'm not sure that I understand where the 100k dropper says that it's going to the trem controls. What I see as I trace the rail to the left from the TR2 choke are 100k plate load resistors. Following the rail upward from the choke, I see a 2k2 Pi resistor followed by a 10k secondary Pi resistor, and then a bunch of 100k plate load resistors. By droppers, are you referring to the Pi filter resistors in the first and second stages, or to the plate load resistors? Just want to be sure that I have the lingo right.

[bob p]

If nothing else, good to know that this bad drawing exists. People working on amps that do not have electronics knowledge will tend to use layout drawings rather than schematics that they do not understand. So the issue could come up again.

There's no doubt that the mistakes on the layout diagram are going to be repeated in the future, especially if the layout diagram gets used by the paint-by-numberss crowd. In a situation like this, it might be a good idea to point out the discrepancy to the person who's hosting the bad schematic/layout PDF online. It would be nice if they could add a third page to the PDF that mentions the discrepancy. Another option would be to just correct the schematic and send the updated schematic their way.

I've cleaned up some McIntosh and Carver schematics and passed them along to the speciality sites that host those sorts of things.

[Tom Phillips]

I had never seen a white line on blue background layout drawing like the one posted above until it showed up here. In addition, I have never seen an older style Fender layout drawing that included the circuitry under the doghouse. Has anyone else seen such a Fender drawing?

Stuff like the 100k resistor mistake is why a successful tech / builder needs to understand basic electronics. Otherwise one is doomed to repeat - repeat - repeat mistakes and use shotgun parts replacement as their main repair methodology.

The color codes on new small off brand resistors are certainly a real PITA. Some of them look like they are using up leftover pastel paint colors. (Is that light black, dark brown or burnt orange?) For nice easy to recognize vivid colors it's hard to beat the old Allen Bradly CC resistors. They could also be used to create sculptures as published on the covers of some electronics magazines of yesteryear.

[Enzo]

And to add, a lot of modern resistors have an extra value stripe, so a 10k changes from brown-black-orange to brown-black-black-red. The traditional gold and silver tolerance stripes are now joined by reds and others. And some have an extra stripe for stability rating.

I agree with Tom, I have never seen a reverse-blueprint Fender layout drawing.

[g1]

I'm not sure that I understand where the 100k dropper says that it's going to the trem controls. What I see as I trace the rail to the left from the TR2 choke are 100k plate load resistors. Following the rail upward from the choke, I see a 2k2 Pi resistor followed by a 10k secondary Pi resistor, and then a bunch of 100k plate load resistors. By droppers, are you referring to the Pi filter resistors in the first and second stages, or to the plate load resistors? Just want to be sure that I have the lingo right.

Bob: page 2 of this pdf: http://music-electronics-forum.com/a...win_rev-2-.pdf The blue layout drawing shows the 2 resistors at the filter caps in the lower left of the layout drawing. To the right of the 100K it says "10 vibrato controls"

But notice also that the node of the 100k dropper says it goes to the trem controls?

Another typo, wire #10 then reappears below ch.1 middle pot, going where it should, to V2 plate resistors.

I had never seen a white line on blue background layout drawing like the one posted above until it showed up here. In addition, I have never seen an older style Fender layout drawing that included the circuitry under the doghouse. Has anyone else seen such a Fender drawing?

I've never seen such a drawing either, I thought it was an original blueprint type, do you think it's bogus? Here's another with all the same drawing, revision, and ECO numbers, yet the values are corrected. TwinMVS-CHASSIS-F.pdf
I assumed it was edited as it looks like a custom pdf (still has the "10 vibrato controls" typo), sorry to further confound this convoluted issue, I think I've had enough of it .

[Chuck H]

The color codes on new small off brand resistors are certainly a real PITA. Some of them look like they are using up leftover pastel paint colors. (Is that light black, dark brown or burnt orange?)

Almost lost my beer laughing. It definitely happens. Trying to interpret resistor color codes when slightly off colors are used as markings. Funny and frustrating at the same time.

[52 Bill]

I've never seen the blueprint drawings before this, but I don't really look for that stuff very often. I'd question the authenticity of them as there are so many mistakes and inconsistancies with real Fender product. All of the drawings that I've seen from that time period were hand drawn and not computer generated.

[Tom Phillips]

…I've never seen such a drawing either, I thought it was an original blueprint type, do you think it's bogus? Here's another with all the same drawing, revision, and ECO numbers, yet the values are corrected. [ATTACH]22702[/ATTACH]…

I do think that the drawing you reference is bogus. At least I don’t believe that it is an original Fender drawing. The properties description tab of the PDF lists “Application: AutoCAD 2010 (18.0s (LMS Tech))” However, it is interesting to have a copy. If someone were to make clean high resolution copies of all the old amps AND they would get the facts right it would be very useful. I could be that someone had access to some unpublished CBS era documentation that was used as a starting point. I.e. scanned in the original then added the CAD info.

[bob p]

The color codes on new small off brand resistors are certainly a real PITA. Some of them look like they are using up leftover pastel paint colors. (Is that light black, dark brown or burnt orange?) For nice easy to recognize vivid colors it's hard to beat the old Allen Bradly CC resistors. They could also be used to create sculptures as published on the covers of some electronics magazines of yesteryear.

As it turns out, those brand new flea-sized resistors are name brand Vishay products and those large sized resistors on the right are generic Chinese "Multicomp" branded resistors. The trend for the first tier manufacturers is to go to 5-band color coding like Enzo mentioned, and to miniaturize MF resistors. In searching for metal films, if you want 4-band resistors in a large size, today's market limits you to the second tier manufacturers like Multicomp that don't even publish noise specs in their datasheets. The itty bitty little resistors are actually the highest quality that's available from the best manufacturers.

I still prefer the look of the old Allen Bradley CC resistors. I just don't like the crackles and pops that come with them.

[bob p]

Bob: page 2 of this pdf: http://music-electronics-forum.com/a...win_rev-2-.pdf The blue layout drawing shows the 2 resistors at the filter caps in the lower left of the layout drawing. To the right of the 100K it says "10 vibrato controls"

Another typo, wire #10 then reappears below ch.1 middle pot, going where it should, to V2 plate resistors.

Well, that explains why I didn't see what you were talking about. I was looking at the schematic and not at the blueprint.

I'm thinking that somebody tried to update the layout to improve it's readability and made mistakes. Whenever I update a schematic, I add my name and a list of changes to the info box so that there's a provenance to help track-down who's responsible for misteaks.

[g1]

I do think that the drawing you reference is bogus. At least I don’t believe that it is an original Fender drawing. The properties description tab of the PDF lists “Application: AutoCAD 2010 (18.0s (LMS Tech))” However, it is interesting to have a copy. If someone were to make clean high resolution copies of all the old amps AND they would get the facts right it would be very useful. I could be that someone had access to some unpublished CBS era documentation that was used as a starting point. I.e. scanned in the original then added the CAD info.

Tom: the CAD version (http://music-electronics-forum.com/a...-chassis-f.pdf) has been edited with correct values for the dropper resistors. The "blue" version (http://ampwares.com/schematics/100W_...F_Twin_Rev.pdf) has no CAD notes, but yes, it still could be bogus. Because it's on ampwares, it gets a lot of exposure so this issue could crop up again.
Mostly just trivia now, but for the heck of it I sent an email to ampwares to see if they cared to comment on the authenticity of the blue layout drawing. Not really expecting to hear back though.

[Enzo]

Or call Fender and ask if anyone is aware of such drawings on old gear.