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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 10:19 am
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BMW2002Ti wrote:
Yes, the 100mfd/100VDC cap will work fine for the 80mfd/75VDC and 70mfd/100VDC caps.

WATCH THE POLARITY! On all caps. I usually changed them out one-by-one to avoid disaster.

Also, check to be sure all resistors in the power rail (between the caps) are correct value (resistance & wattage). I'd go metal oxide or wire wound here --- for stability and long life.

After all is done, do a thorough check with a DVM (in resistance DC mode) to be sure all the caps are hooked up correctly. And you have no shorts --- before firing up.

A Variac or the old "light-bulb" trick should be used on first fire-up. It could save you a world of pain.

http://www.radiomuseum.org/forum/curren ... ester.html


It concerns me that the OP is buying capacitors to replace the ones currently in the amp without really understanding their function and placement in the circuit. I have seen no effort on the part of the OP to trace out the power supply and figure out what SHOULD be in there (both caps and resistors) and how they are connected. Instead, he is just being told that "yes, you can replace this cap with that cap".

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 10:35 am
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Nope

Don't get me wrong guys, I'm not going to do it myself.

Bill, I can't trace it because its hieroglyphics for me...I feel it would be stupid to try and read a schematic only understanding half of the information. I ordered the book, and Im waiting for it to be delivered.

Im buying the caps in the hopes that the tech will say to himself "well this is a slam dunk...Amp+schematic+proper capacitors....All I have to do is install..."

...this to me is infinitely better than "well, I don't have the parts, but...maybe the guitarist doesn't know anything about tone....what caps do I have laying around...that are close enough" (ala what happened in tech repair round one, and what was missed with tech round two...)

We have some good techs up here In Toronto, but...there's like 3 of them. So they all have about 100 amplifiers to repair in their shops...I doubt they have the time or interest to pour over my amp, check every cap and make everything perfect.

...But I do :) And I can give them the right parts?

Thanks
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 10:41 am
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Bluesky, Yeah you are prolly correct. Dilemna is, if the OP is dead set on replacing the caps --- whether to give information or let it go...


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 11:53 am
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ParksideDrive wrote:
...But I do :) And I can give them the right parts?

Thanks
Dave


That's the whole point. Without going over the schematic and tracing things out, you don't have any idea if the parts you are providing are correct or not. They could be just as wrong as what is in there now. The only way to tell is with a detailed comparison between the schematic and the amp itself.

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 11:56 am
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BMW2002Ti wrote:
Bluesky, Yeah you are prolly correct. Dilemna is, if the OP is dead set on replacing the caps --- whether to give information or let it go...


I think I have said all that I need to say, so I am bowing out of this thread. Not upset with you, just concerned for the OP and what might happen with this amp if he continues on this path.

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 12:04 pm
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Thought you guys might like to see/hear a video clip.

http://www.youtube.com/watch?v=bm3chUok ... ture=g-upl

Not sure what to make of your post Bill...On the one hand you're saying "dont do any work on the amp, you don't know what you're doing" on the other hand you're saying "you need to read the schematic" which is another thing I don't know how to do...


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 2:07 pm
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Sounds like you've got a lot of distortion riding on the notes. Off-hand, this could be:

1.) Bad speaker(s).

2.) Or that crazy cap/resistor setup under the doghouse. This maybe putting too much or too little voltage onto the input/gain/EFX stages and leading to noise & way too early distortion.

3.) Or a number of other singular or combination of things. Best left for discussion AFTER #1 & 2 are ruled-out.

Note: under the big 220mfd/285 or 300VDC caps lies two 100k-ohm resistors, These are critical for dispersing the voltage evenly across the two 220mfd caps that are in series. The reason Fender did this was to get the equivlence of 110mfd capacitance, rated at 550-600 VDC. I know of no single axial cap, except some humongous motor-run ones & cap cans, with this kinda ratings. You will never get good filtering, if these resistors are out-of-spec. They are in the first filtering stage. So, the voltage to the 6L6GC and all the 12AX7 and 12AT7 are downstream & effected by its integrity.

What Bluesky was prolly talking about is re-drawing the Fender supplied schematic --- to show only the PS section (caps/resistors/choke delivering the B+ to the anode plates of all tubes). Then, comparing this to the layout of your amp. It seems that not only are some caps not OEM, but some resistor values seem questionable, too. Until, all is back to OEM values, it would be impossible for you or a tech to measure correct voltage values.


Your amp's schematic:

http://www.webphix.com/schematic%20heav ... _schem.pdf


Re-draw the power supply section to look more like this example:

Image

Or the lower right portion of this schematic, including the two 70mfd/350VDC caps and voltage reading points: B, C, and D.

http://www.webphix.com/schematic%20heav ... _schem.pdf


Last edited by BMW2002Ti on Tue Oct 23, 2012 2:24 pm, edited 2 times in total.

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 2:21 pm
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BMW2002Ti wrote:
What Bluesky was prolly talking about is re-drawing the Fender supplied schematic --- to just show the PS section (caps/resistors/choke delivering the B+ to the anode plates of all tubes). Then, comparing this to the layout of your amp. It seems that not only are some caps not OEM, but some resistor values seem questionable, too. Until, all is back to OEM values, it would be impossible for you or a tech to measure correct voltage values.


Exactly!

Statements like this scare me:

ParksideDrive wrote:
2 x 500v 47uf side by side- I think these may be the biggest offender. The schematic makes no mention of 47uf, and he has 2 of them in there, more than doubling the capacitance. I would like to replace with 2 x 500v 20uf, but it seems F and T doesn't make those (they make a 22uf) but sprague does..... http://thetubestore.com/ca-at-20uf-500v.html. Does the 2 micro farads make any difference


It is not known what function those capacitors are performing in the circuit without tracing things out and comparing the schematic and amp. Without that knowledge, replacing them with different value caps MIGHT make the amp sound better or it could make the amp sound worse or even damage the amp. It is not wise to randomly replace parts without knowing what those parts do.

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 7:56 pm
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Image

OK I drew it out as per your advice. My drawing is based as If one was to look for the most direct route to the large power capacitors. I have noted the resistors that are on the way.

If I was to attempt to draw every connection on the line, it would essentially be a recreation of the schematic. In terms of a straight line, the path that takes me to the 2x20uf/500v caps would T-intersect with a line to V5 on the way, and another point would T Intersect with a path to the 4x 6l6's. I find that confusing, since I believe those 20uf/500v caps are right beside each other in the doghouse?

In certain places on the schematic it will say -58v, +410v...what does that mean?

In order to follow the path in the amp and compare, I would need to really get in there. With them being on the other side, I would need to remove the caps to follow the terminations...or I may be able to trace the wire colors...

For the resistors: Image

http://brunningsoftware.co.uk/ResVal.htm

It would appear the larger one, at bottom right is 4+7+00= 4700 ohm 4.7 k resistor with a 10% tolerance.
On the left 1+0+00=1000ohm=1k resistor with a 10% tolerance
Its hard to see but, up top that looks like a 2.2k which may actually match the schematic (not sure until I get the values of those caps, hidden by foam)

Anyways, thats my drawing of the schematic as per Bmw, and Bluesky's advice.

Thanks for the advice
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 9:48 pm
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OK. Now you're getting somewhere.

I'm not going to check your drawing for accuracy, but I think a few things should be obvious:

1. The two 47 mfd caps in your amp don't appear anywhere in your schematic. You need to trace them out in your amp to see what they are connected to.
2. The 2.2K resistor is the dropping resistor for the bias supply. It drops the -62 VDC to -58 VDC.
3. The two 100K resistors associated with the 220 mfd caps in the schematic don't appear in your amp, unless they are hidden under the board or somewhere else.
4. The 4.7K and 1K resistors are associated with the two 20 mfd caps. They are part of the circuit that supplies plate voltage to V1, V2, V3, and V4. It could be those two caps that have been replaced by the two 47 mfd caps in your amp.

There may be more discrepancies if you look closer, but those are the most obvious.

The -58 VDC is the bias supply feed to the 10K bias balance pot. The 410 VDC is the plate supply voltage for all the preamp tubes. If you follow it around, you will see that it goes through a series of resistors at each tube that drop it down to the required voltage to the preamp tube plates.

That's about all I see tonight. It's late and time for bed. You did good. :wink:

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 9:56 pm
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Parkside Drive, I'd re-do the schemo to clearly show the postive and negative (ground) rails of the circuit. And separate the BIAS POWER SUPPLY from the main B+ power supply line. I whipped out my pencil and quickly did it. Now, double check the components, as I really whipped this boy out fast. I will recheck it, too.

Good luck!


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 10:10 pm
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BMW's drawing clearly shows what I was talking about when I mentioned tracing the 410 VDC supply out to the preamp tubes. Bias circuit is clearer too.

I sure hope Fender fired the guy that drew the original schematic. What a mess. :lol:

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 10:40 pm
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Edited schemo...

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 11:01 pm
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BMW2002Ti wrote:
Edited schemo...

Image


This looks like what's under the doghouse of my '78 UL Twin Reverb (one-owner, never been fooked with).

Arjay

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Wed Oct 24, 2012 7:56 am
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I think BMW's schemo clearly shows that two of the 20 mfd caps were replaced by 47 mfd caps in the OP's amp. I don't think that is necessarily a bad thing.

Now the OP needs to locate the two 100K ohm resistors associated with the two 220 mfd caps and see what is under the foam.

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