1974 Twin in for Repair Blowing Fuses

[Platefire]

Got this in today. Customer had already pulled all tubes and it was still blowing fuses. I pulled the chassis and did an initial inspection. It SS rectification--nothing burnt there. Everything looked OK on the board and wiring as far as not seeing any burn marks or anything suspicious. When I tried to pull the dog house cover and removed the four screws it wouldn't come off. Not even when slightly prying it with a screwdriver. From past experience the dog house cover always come off easy after removing the four screws. I am thinking something may have welded itself to the cover but there are no visible burn/char marks from the outside.

Is there anything I'm missing that would be holding the doghouse cover on?? Don't want to get rough with it until I know that's the only alternative!  Thanks, Platefire

[Willabe]

Not that I can think of.

The cover might be stuck because the caps were leaking bad? Cap shorted and spot welded the cover to the chassis? Mmm, pretty extreme.   

I'd try taping it with a hammer but use a wood block in between the cover and the hammer.

That should free it up.   

[Platefire]

OK thanks. I couldn't imagine anything else that would hold it on there like that. It stuck pretty good. I'm thinking that may be the source of all the fuse blowing. Platefire

[Willabe]

Yeah shorted B+ filter cap(s).

They can explode and spew a nasty mess. 

[sluckey]

There may be some foam rubber between the caps and the doghouse. Sometimes that foam rubber kinda melts and acts like an adhesive. Just keep prying around the doghouse with a screwdriver. Get a bigger screwdriver or putty knife if needed. It will come off.

[eleventeen]

You could try heating it up with a hairdryer. usually heat softens adhesives. Heat > pry, it should come off.

[Willabe]

There may be some foam rubber between the caps and the doghouse. Sometimes that foam rubber kinda melts and acts like an adhesive.

Yep, seen that happen, putty knife is better than screw driver in this case.

You could try heating it up with a hairdryer. usually heat softens adhesives. Heat > pry, it should come off.

Good idea too. 

[Platefire]

OK, Good Thanks! I will try the putty knife and hair dryer heating.

Meantime, I got my 50 year High School Grad reunion tomorrow---1965 to 2015--count it up! The frozen filter cap standoff will just have to wait  Platefire

[Willabe]

I'm sure your taking your best and only gal to the dance, have a great time!!!!!!!   

 


 

[Platefire]

Yelp, the wife of my youth and high school sweetheart Connie. Next year 2016 will be our 50th anniversery.

BTW---the addition in the picture is Molly, one of our 12 grandchildren. Platefire

[alerich]

Yelp, the wife of my youth and high school sweetheart Connie. Next year 2016 will be our 50th anniversary.

I have no input on the ailing Twin but this is wonderful.

[Platefire]

Thanks! Well back to the twin, pryed the doghouse cover off and it was the foam between caps and cover that was sticking. Original paper caps and if you look close you can see some gunk on one of the leads that may have come out of one of the caps. The fuse holder is broke, so I got to replace that before I do any test. What would be the easiest way to test the PT out??? Test up to standby or disconnect PT from SS Rectifier? Platefire

[DummyLoad]

> What would be the easiest way to test the PT out

pull the rectifier; temp short with test lead across the busted fuse holder; use a lamp limiter.

i love twins. thinking about picking this one up on payday...

-pete

[mresistor]

Time to replace those caps too. Wouldn't a shorted filter cap blow a fuse? Methinks yes. Why not replace the old leaking and possibly shorted filter caps and the fuse holder, then test it and see if it will go to standby. If not done so already, it might be prudent to replace all the electrolytic capacitors, like the 25uf/25v cathode bypass caps. I'd use 25uf/50v. And the 50/70 in the bias circuit.  BTW - Those two electrolytics before the stdby switch read 100mfd but all the schemos for silverface twins show them as 70uf. (AC568, AA769 and AA270) If the all the electrolytics are original they are over 40 years old!
An ounce of prevention is worth a pound of cure.

DL  I think you missed that this one is SS rectification.

[eleventeen]

Yeah, you pretty much have to replace the ecaps, all of them. You want to check the internode resistors as well...for a grand total of $1.50 in parts for 2-3-4 qty 2 or 3 watt resistors, I just feel better replacing those with metal films, mount them a little bit off the parts board to help dissipate heat. I also like to replace the 6 qty 100K plate resistors with MF's but this is much less necessary on a 70's SF versus a 60's blackface. I also like to replace the dual 220K "balancing" resistors across the series connected 70uf caps in the power supply. Just on the theory that these parts get big stress over their lives.


The grand total of parts you *maybe ought to replace* versus those you *must* replace is under $5. Seems silly to overthink it.

There are 5 or more ways to test the PT, but none of them truly test the HV output under load unless you simulate the load somehow. You could do this on the unfiltered rectifier output with the caps removed; 100 mils load at 450 volts would occur with a 4500 ohm resistor but that resistor needs to be 45 or more watts, a biggy. I would venture to guess that the Fender Twin PT failure rate is in the area of 1/2 of 1%, ergo, IMO most techs would replace all those caps on the assumption that the PT is just fine.  Unless it stinks or has goop dripping out of it.


 

[PRR]

> Wouldn't a shorted filter cap blow a fuse?

First, maybe second filter caps, yes, usually.

Subsequent filter caps feed through resistors so large that the worst-case load hardly warms the whole-amplifier fuse. These typically take out the dropping resistors, fuse is fine.

While I understand keeping a good woman, good caps do go bad and by the looks, I would replace those altogether. It isn't such a major trauma as replacing your woman.

[Platefire]

This 74 Twin is very clean inside the chassis. It looks like the original circuit is UN-molested. I'm not sure if it has ever been cracked open before---don't look like it! The cab is pretty raggedy but intact except for the missing back-plate. It's got the original speakers Fender Special Design except one of the speakers has a slight hole in it. The customer deals in vintage effects pedals and said he got it on a trade for some pedals. I think it has been setting up for a long time. No need to try to convince me it needs new caps, those old caps are wasted and I wouldn't think of reusing them. I do want to test the PT before I order new caps/& stuff just to make sure it's alive and not shorted out. So I think the easiest thing to do is disconnect the two leads to the SS diode rectifier circuit and turn in on--if it don't blow a fuse, it is probably OK but I think I would want to read the ACV on those two leads just to be sure. The schematic says 340VAC. I had trouble finding the schematic but I think I've narrowed it down to the SF 100 attached. The Schematic seems to agree with what I've got under the hood--with the pull knob master volume. The customer is wanting to get it going and I need to give him an idea of the total scope of work---so I need to be sure about the PT. Platefire

BTW--I'm holden on to that woman, the paper caps has to go

[eleventeen]

Just be sure you are not exceeding many meters 600 volt rating by testing the full HVAC output. Half at a time is fine, CT to one leg, CT to the other leg.

[Platefire]

Thanks for the warning! I've destroyed enough meters, don't want to loose another one. Platefire

[mresistor]

Well, so much for usng the Fender Amp Field guide, it doesn't even list the SF100 and SF135 in the Silverface section, or I would have looked at those schematics. That SF100 is the one with the 100uf primary filter caps. 

[Platefire]

Installed new fuse holder/3 amp SB fuse, disconnected PT secondaries to rectifier circuit, turned power on---didn't blow any fuse Checked the voltage on the secondaries at 345VAC each, the schematic says 340VAC--so I think I can safely say the PT is good!

Trying to remember how I did the last dog house cap/resistor change. What I can recall if I remember correctly, I think I pulled the little board out. positioned the components on the board, soldered what connections I could with it off and then soldered the power supply leads to the board after it was in place. If you don't do this very often, hard to see best method. Some experience sharing would come in handy now  . Thanks in advance, Platefire

[sluckey]

Take a good pic of the board so you will know how to put the caps back in. They may not all have the positive end on the same side of the board. Making a drawing may come in handy also.

Replace that bias cap also. It's probably on the diode board.

[Platefire]

Thanks, hadn't really considered replacing the bias caps but I guess it's a good idea. Just pricing Sprague cap, Man! They are so high now. What are the next best cheaper alternative? platefire

[sluckey]

Plenty of good affordable choices. Just follow the capacitor link at the bottom of this page.

[eleventeen]

Shhhhhhhh....it's a secret. 20 u or 40 u / 450 caps radial caps (PC mount) are under $2 each.


For the two that are stacked in series....I split a 4" length of PVC pipe the long way with a hacksaw, and using one of the halves as a jig, pre-assemble the two caps and their 220K balancing resistors, with an insulating spacer in between the caps. Coming out of that ass'y is a + lead and a - lead. Then I silicone the two caps to the parts board and place a weight on the PVC half-pipe until the silcone dries. When it's dried, the caps are in perfect alignment and it works fine. And you don't end up with a huge glob of silicone.


No kidding. Those caps in Sprague axial are about 6 * $13++ each.

[6G6]

Clever idea!

[Platefire]

That sounds like a good idea to try if it was my amp but don't think I want to try something radical to the customers amp. Think I may go with the FT's unless the customer wants to spring for the Spargue's.

I've got another amp from the same customer that is a Mesa Boogie Mark 111 head. Guess I'll start another thread on it. Platefire

[DummyLoad]

> Think I may go with the FT's unless the customer wants to spring for the Spargue's.

excellent choice. 

--pete

[Platefire]

Preparing to order Filter Caps for this one. Gave the customer a price on Sprague and F & T and he chose the F & T.
F & T has 22/500 but not 20/500, so I guess that's close enough. What are you guys doing on the bias caps that are 80/75 in these vintage amps. I think the closest thing I found was 100/100. I know higher voltage is no problem, just not sure about the extra 20uf. Platefire

[sluckey]

100/100 is fine. The extra gives even better filtering.

[Platefire]

Thanks for confirming, Doug's got my order. Platefire

[Platefire]

Got the new filter caps in and the Twin is now up and running. Sure glad that slucky suggested getting new bias caps too because when I first fired it up it was running way under Biased at -38 V but when I put the new bias caps in it went up to schematic spec -52. The amp is running way weak but I think it has the original tubes. He just wanted me to get it going and no restoration, so until further instructed, guess I'm done. Thanks, Platefire