Fender Hot Rod Repair -now I'm blowing fuses

[TerryD]

I replaced the input jacks and the two 5 watt resisters that overheat the board on my friend's  along with the diode next to the overheating resisters. I put it back together and now the fuses are blowing and I can't figure out why.  I look and then I look again.  I'm not very knowledgeable Help!!

[mresistor]

Hello.  Which Hot Rod is it? Deluxe or DeVille? Do you have a schematic or can you find a schematic and post the link?


Can you post some pictures of the inside of the chassis please, this will help us to help you.

[Willabe]

If you scroll down the page in Doug's forum, you'll find the link for Doug's tube amp library.

It's probably in there.

https://el34world.com/charts/Schematics1.php

[mresistor]

If you scroll down the page in Doug's forum, you'll find the link for Doug's tube amp library.
It's probably in there.
https://el34world.com/charts/Schematics1.php

https://el34world.com/charts/Schematics/files/Fender/Fender_hotrod_deluxe.pdf


I think the OP better check his wiring of the new jacks.

[shooter]

these 5W?
just a quick look though, might help to indicate WHICH 5W and diode since the diode there is a Zener
by "replacing" a burnt up part without knowing what caused it to burn up will just result in what you're seeing, the original problem, since resistors seldom fail cause they just feel like it

[TerryD]

The 5 watt resisters in all of these are  too close to the board by the machine.  I was just replacing them because they burn up the area around them and funk up their own solder.  Get them up off the board and your ok.  I don't even know what to do with the schematic.  Thanks


It's the Hot Rod Deluxe

[mresistor]

OK Terry   do you want to help? Then post the pictures, especially the input jack area that you worked on.


You don't know what to do with a schematic? Seriously?   


The input jacks are important and if you can't read a schematic then maybe take this amp to someone that can.

[tubeswell]

The 5 watt resisters in all of these are  too close to the board by the machine. ....  Get them up off the board and your ok.  ...

Errr not necessarily. True, the heat from the resistors WILL eventually burn the board, but it will also burn other parts nearby, including diodes, and cause other problems with relays and channel switching etc, BUT that's not the only common issue with these amps - one or more (or all) of the filter caps are probably bad as well. Merely getting the resistors away from the board won't fix other problems. If the mains fuse blows, its because there is too much current being drawn through the PT, which indicates a short circuit in the power supply part of the circuit (or in the PT itself). You have to do more testing with a current limiter and/or a VARIAC and figure out what's causing the short, and fix that. Or take it to a tech

[mresistor]

Was the amp blowing fuses prior to you working on it?

[shooter]

 
I quit reading after 5W overheat


the "tip" is a valid one for new troubleshooters, they identify the crispy part but neglect the "why it's crispy" til they replace it and, well, crispy stuff still happening 

[mresistor]

I quit reading after 5W overheat
the "tip" is a valid one for new troubleshooters, they identify the crispy part but neglect the "why it's crispy" til they replace it and, well, crispy stuff still happening 

He said  "now the fuses are blowing" inferring that they weren't before.  Of course not enough information. 

[AlNewman]

I think if you started blowing fuses AFTER the repair....  It's time to take it to a tech.  Hopefully it's just a simple fix at this point.

[TerryD]

I had a tech friend of mine look at my work and said it was fine. But he is not up on tube amps.  I used to do this more regularly.  I used the old Fender picture schematics This board helped me with lots of amps.  Anyways yes after the repair.  I'm replacing the IC caps which are 25 years old.  I'm hoping against hope.  What's the next thing after those??  huh

[stratomaster]

Build a lightbulb limiter is the next step. This will enable you to probe around and find the short without endangering the amp or blowing through fuses.

[AlNewman]

I had a tech friend of mine look at my work and said it was fine. But he is not up on tube amps.  I used to do this more regularly.  I used the old Fender picture schematics This board helped me with lots of amps.  Anyways yes after the repair.  I'm replacing the IC caps which are 25 years old.  I'm hoping against hope.  What's the next thing after those??  huh

That's fed by the low voltage supply?  I guess try the caps 1st, and as Stratomaster mentioned build a light bulb limiter to test before firing up.  Good luck.

[mresistor]

Well folks I realized that I've seen insides of these amps before and I should have known that the input jacks are pc mounted limiting mistakes in replacement by a lot.


Is there any chance that some those IC filter caps were spooging white electrolyte out of there vent hole?


Be careful with the ribbon cables if you're pulling the board out to desolder the filter caps.

[mresistor]

Terry you might watch this video about repair of a HRD amp. Notice the heat damage to the board at around the 37.5 minute time stamp.


How to build a light bulb limiter Amp_Scrapbook.pdf



Here's a video on how to build one

i=A-jRdmosDpGkmnJD


and mine is very basic and has served me very well many many times. 

[TerryD]

As always. Thank you.


For this one time thing couldn't I grab a lamp and put it in series on one side of an heavy duty extension cord?  Just pull the plug instead of a switch.

[AlNewman]

The photo mresistor shared doesn't have a switch.  But at the minimum, you should have a male grounded plug, a lamp socket to screw your bulb into, and a female grounded receptacle to plug your amp into.

[mresistor]

Terry this is all you need  you can't see it in my photo of mine but both cords are three conductor with Hot, Neutral and Ground.

[TerryD]

Perfect perfect

[plexi50]

I have replaced those 470 ohm 5 watt resistors many times. They still get hot enough to vibrate loose being in a combo cab. So i came up with an idea the other day when replacing some again. Once the leads of the resistors are bent over and soldered they are in place. But being raised off the board does not give them solid support. Once the solder starts to get to the melting point and the resistors come loose again from their traces. Take a toothpick and your pliers and flatten the toothpick just enough to slide under the (2) 470 r resistors. Put a spot of glue on the tooth pick before sliding it underneath the resistors. This will keep them in place solid and wont be able to move from vibration when playing the amp for extended time periods.

[TerryD]

I like where your going with this though I can't "exactly" see it in my minds eye.  Wouldn't the toothpick wood kind of burn?  I have my resister like 3/4 inch above the board


Please forgive my ignorance.  I've used a limiter like the simple one mresister has shown me above when I simply upgraded old filter caps in old Fenders, long ago. 
But, What am I looking for the limiter light bulb to do that indicates I have a problem with too much current draw?

[Willabe]

I like where your going with this though I can't "exactly" see it in my minds eye.  Wouldn't the toothpick wood kind of burn? I have my resister like 3/4 inch above the board.

He's just saying to be able to have air touching all around the resistor so it helps cool it. If you think wood will burn, use something that is fire proof.

I think a tooth pick won't give enough space off the board. I think it should be at least a good 1/4" or a little more.

I've used a limiter like the simple one mresister has shown me above when I simply upgraded old filter caps in old Fenders, long ago. But, What am I looking for the limiter light bulb to do that indicates I have a problem with too much current draw?

When you 1st turn on the amp the limiters bulb will glow brightly for a few seconds as the B+ filter caps draw current to charge up. As soon as their fully charged, the bulb weill dim to almost no light output. Amps healthy.

If the bulb doesn't dim back down or only dims back down a little, there's something wrong, amps drawing too much current. In indicates a short or partial short.

The lower the bulbs wattage, the more it will protect the amp and the brighter it will glow at turn on and with a short. You need to try it with a healthy amp 1st to see how bright it will glow at turn on.

With a 2 x 6V6 amp = 60w bulb, 2 x 6L6/EL34 amp 100w bulb, even a 4 x 6L6/ELe4 amp. With a single 6V6 amp I use a little 40w bulb.

[Merlin]

The amp basically has three power supplies: B+, heaters, and the bias/low voltage stuff. It should be easy enough to discover which of the three is blowing the fuse, just by disconnecting the three transformer secondaries and then reconnecting them one by one (until the fuse blows). Remove the power tubes while doing this, since they won't have bias.

A possibly relevant video:
https://youtu.be/51eXP65oJaw?t=196

[TerryD]

All of this stuff is so helpful.  Thank you.

[TerryD]

OK I put in the new filter caps.  I waited about 1-2 minutes for limiter light to dim but it did not.  That's long enough right?  All I have is a multimeter. Any other usual suspect to check/replace?

[mresistor]

Follow Merlin's advice    I think you could use the LBL doing this

[TerryD]

Not sure how to follow Merlin's advice.  I may be out of my depth.


Any last ditch efforts for a novice/layman to maybe get lucky?

[Willabe]

When you 1st turn on the amp the limiters bulb will glow brightly for a few seconds as the B+ filter caps draw current to charge up. As soon as their fully charged, the bulb will dim to almost no light output. Amps healthy.

If the bulb doesn't dim back down or only dims back down a little, there's something wrong, amps drawing too much current. In indicates a short or partial short.

The bulb should only glow bright for a few seconds.

Somethings wrong. 

[AlNewman]

Start by pulling the power tubes and see if the bulb dims or not.  If the light dims, then you can check voltages at the points indicated on the schematic.
If your soldering work was done properly, and you didn't, (for instance), install a diode or capacitor backwards, it's possible one of the ribbon cables were broken when you took the board out of the amp to replace parts.

If the light doesn't go dim, follow Merlin's suggestion, (with the power tubes still out!), to determine which section is causing the fault. 

[TerryD]

so I'll check the ribbons one by one.  I'll just look for a beep touching each end.  Any tips on basic way to follow Merlin's advice.  Thanks.

[stratomaster]

His advice is to literally just disconnect the secondaries and reconnect them one at a time until the fuse blows.  There's no more basic advice than that.

[TerryD]

I'll show you basic.  What are the secondaries and how do I disconnect them?

[BrianS]

Oh my.

[AlNewman]

so I'll check the ribbons one by one.  I'll just look for a beep touching each end.  Any tips on basic way to follow Merlin's advice.  Thanks.

You didn't explain what happened when you pulled the power tubes and tested with your light bulb limiter.

I still think that you should take it to a reputable tech.

[Willabe]

I still think that you should take it to a reputable tech.

[TerryD]

Thank you thank you everyone!!!   


As I was checking the ribbons I noticed a little little ball of solder that fell between two of one of their posts.  One little flick and wallah.


Please one more stupid question as I don't see the exact layout of the input jack slots on the board as I have.  I'm using switchcraft jacks.  I've got input #1 positive and ground.  Some say input #2 goes to the positive and to the bypass on the board.  The bypass has two holes ..do you solder it to both holes?


The older point to point boards were much easier than this crap.  Thanks again for your patience.  Thank you Lord!