1971 Fender Twin Reverb- Blows fuse

[silverfox]

I'm supposed to take a look at a Fender Twin Reverb, (customer said it was a Silver Face Reverb but I don't think I've heard of that before). Turned power on and it blew the fuse. Aside from a likely recap, are there any common failures particular to this model that cause that? I do know it could be power tubes, rectifier, or caps. Just looking for some average experiences prior to metering it.
Thanks for the assistance,
silverfox.

[silverfox]

Just found out it is a Silver Face Delux Reverb, 1971. I'm thinking to check the death cap first, diodes if it's solid state. Then all the voltages. I'll bring it up with the Variac.
silverfox.

[silverfox]

So far I replaced a 2.7k 10 watt resistor after the first filter cap. Previously I got 500 volts to one side of the resistor only. After I replaced the resistor the amp came back to life but now has a serious hum. There was also no signal passing through as indicated by pluggin in an input cord and touching the end. Pulled all the preamp tubes but that didn't make a difference. Still had the hum- Sounds like 60 cycle. The voltages on the caps all look good. Not too sure if I'm going to take this project on. Did no harm and haven't charged anything. At least got it to hum as previously there was nothing.
Suggestions?
silverfox.

[2deaf]

2.7K 10W resistor?  500V?  This sounds like a 135 from later in the decade.

[silverfox]

I haven't been able to find the exact schematic, but did notice as I put it back together last night there is on the chassis in back a tag that stated 135 watts. Not sure if that was the total power consumption or the rated output as I didn't check to see which specification. However, it only has 4 6l6GC output tubes which would be questionable to obtain 135 watts output power from.
Conversely, the PT and OT are rather large for a 100 watt amp. Is it ultra linear? I never got that far in tracing the OT connections. This was, is only preliminary. I currently have a really busy schedule and don't think I have the time to invest. More a favor at this point. The interior wiring is done very nicely compared to most of the Fender factory assembly jobs I've seen on line. Nice and neat, not a nest of snakes.

[sluckey]

I haven't been able to find the exact schematic

Schematic in in Hoffman's schematic library.

[mresistor]

The 135 was ultra-linear with screen taps on the ot which should be easy to tell.  '71 Twin shouldn't be a 135 though. You could look at the date codes on the trannys to tell the appx year of mfg. of the amp. Could also look the chassis serial number up.  Twin 135's didn't come out until '77, but the one you have could be one. It would prolly have a master vol on it too.  We could help with identity with codes or pics..


135 schematic  https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_135_schem.pdf


Usually these older amps need the electrolytic filter caps replaced as well as all the other electrolytics in the pre-amp and bias supply.  The interior wiring -- are the wires sheathed in pastel colors? or traditional colors. Neat wiring would indicate it is a late 60's early 70's and therefore a 100W or less amp..  does it have a balance pot and non adjustable bias?

[sluckey]

That 2.7K 10W resistor already identified it.

[mresistor]

so it's a 135UL twin then.. 

[silverfox]

mresistor- That is the schematic I ended up using as it was the closest I could find based on the ss rectifier, noise suppression caps on the diodes,  and the 2.7k resistor. Thanks for the confirm. I'll have to check and see what the output rating of the 6L6GC are. I didn't notice the UL taps as I used the 470 ohm resistor as a voltage check point and didn't realize there were OT taps connected to them.
Now I'm making some more progress on information: https://www.premierguitar.com/articles/22353-ask-amp-man-restoring-a-1979-fender-twin-reverb
Thanks for the FB.
It has the pastel colored wiring. I recall seeing the hum balance pot, I think it said that. The bias pot, can't say for sure. Very large transformers for a 100 watt.

sluckey- The customer first told me it was a 1971. Everything I looked at from that era had a tube rectifier so I was very uncertain. I'll see if I can find a layout.

silverfox.

[mresistor]

I've not been able to find a layout for the later 135UL twins..   would like to have one though.. 

[jjasilli]

When it comes to what people say about an amp -- it's hearsay.  Trust if you want, but verify.  E.g., check pot codes for dates.  "Blown fuse" is a documented fault condition.  Use repair flowcharts & checklists.  Post if you need help finding them. 


Meanwhile: always suspect tubes 1st.  Personally I would replace entire PS batch of components from rectifier > filter caps > dropping resistors > plate resistors.  These parts tend to age together gracefully until one fails.  Replacing only a dead component can trigger a daisy chain of successive component failures.  So it's good practice to replace them all at once.

[silverfox]

As a quick attempt to eliminate a potential tube problem, the last step was to pull all the tubes and power it back up. Still hum. Based on that I concluded likely a cap problem.
silverfox.
I believe this is the layout: http://thevintagesound.com/ffg/schem/twin_reverb_aa270_layout.gif
silverfox.

[2deaf]

I believe this is the layout:

Not even close.  Although if you look at the 135, they used the same old board with several modifications.

[sluckey]

...sluckey- The customer first told me it was a 1971. Everything I looked at from that era had a tube rectifier so I was very uncertain. I'll see if I can find a layout.

...I believe this is the layout: http://thevintagesound.com/ffg/schem/twin_reverb_aa270_layout.gif

None of the Twin Reverb models ever had a tube rectifier. That aa270 layout is very different from your 135W UL Rhodes era amp. Major differences in the power supply. I've never seen a layout for the 135W UL. It's possible you may find one on the Fender website.

[silverfox]

I told the customer I would see if I can find a knowledgeable tech. As a last resort I would work on it and start with a recap at about $140. If I do, I'll post some pics of the chassis.
Suspended for now. Thank you for all the assistance. For those of you that are unfamiliar with my tube amp repair practice it is more of a hobby and I haven't charged anything at this point so I'm free to walk away having done it as a favor.

silverfox.