Fender 75 eats fuses like candy on a movie night.

Discussion in 'Amp Tech Center' started by Dreadneck, Apr 8, 2020.

  1. Dreadneck

    Dreadneck TDPRI Member

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    So I recently aquired a Fender 75 tube amp.
    At the beginning it worked fine, some noise though but i thought that it was some of the caps or heaters that was acting up.

    So I put it on the bench and all hell breaks loose!
    The filter caps has been changed but the old electrolytics inside was still there.

    I changed those and the coupling caps aswell.
    I re-soldered the heaters which was needed.
    The resistors on the output tube sockets was original, so i changed those aswell.

    The only mod i have done is getting rid of the hum balance pot and placed two 100 ohm resistors on the pilot light.
    The hum balance seemed shot anyways.

    But... Now the amp blows fuses.
    The fuses that was is the amp when i got it was waaay to high.. Like 10A fuses.
    So I changed those to the correct value. (I did this before i put it on the bench)

    There is one 3A slow fuse directly after the on/off switch and two 500mA slow right before the rectifier. (the two 500mA fuses is not on the schematic, but i live in Sweden so I suspect it may have been added when the amp was imported. I've looked at others that has these fuses aswell.)

    I used a variac, turning the voltages up slowly and monitored the voltages inside the amp.

    When the variac is at 168VAC i reach the correct voltage meashurements inside of the amp. However we have 220VAC in the wall here. I have made shure that the correct primary lead is hooked up on the PT.

    Something is drawing way to much current and I can not for the life of me find out what.....


    I've got no tubes in the amp. I've tried with known good tubes aswell but to the same result.

    I have taken meashurements on the OT and those seem fine...



    Does anyone have an idea of what I am missing??
    Any help at all would be very much appreciated!

    I attached the schematic.
     

    Attached Files:

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  2. Peegoo

    Peegoo Friend of Leo's

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    Is it blowing the fuse with no tubes in the amp?

    If not, plug only the rectifier tube in and try that. If the fuse pops, you know the issue is early in the circuit. Continue one tube at a time until the fuse blows (turn off the amp when adding the next tube, then switch it on).

    You can also do this kind of tube test with a 'light bulb current limiter' you can easily make yourself.
     
  3. Dreadneck

    Dreadneck TDPRI Member

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    Yes it blows the two 500mA fuses without any tubes at all.
    At one point I was able to add all of the pre amp tubes without the fuses blowing, but when I tried with one of the output tubes the fuse shot imidiately. Even the 3A main fuse.

    It is a solid state rectifier. I have checked the diodes and they seem fine.. I have been suspecting the rectifier but I can not find anything wrong with it..

    Yes I actually built one of those but the bulb died a while ago and i have yet to find another.. They are really hard to come by.
     
  4. dan40

    dan40 Tele-Afflicted

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    What happens when you remove the two 100 ohm resistors forming the artificial centertap? The PT appears to have a green/yellow CT but it's connection point is a little strange to me. It's possible the 100 ohm resistors are causing the problem.
     
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  5. Teleguy61

    Teleguy61 Friend of Leo's

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    I agree.
    Remove the two 100 ohm resistors, and see if that changes things, first step.
     
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  6. sjoko

    sjoko TDPRI Member

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    show us some pictures of the inner side of the chassis


    Does the center tap wire ( red/yellow ) still connects to the central point of the in series connected filter caps ?

    ( otherwise it doubles the rectified voltage :eek: )


    You can leave the 100 Ohm resistors in ,....that should be oke
     
    Last edited: Apr 9, 2020
  7. Dacious

    Dacious Poster Extraordinaire

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    With no power tubes in the circuit voltages will be very high. Ignore that for the moment

    If it's blowing fuses with the rectifier in it might still be gone even if it passes the diode test.

    Otherwise, your power transformer is possibly toast.
     
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  8. sjoko

    sjoko TDPRI Member

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    I agree !
     
  9. Peegoo

    Peegoo Friend of Leo's

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    Yeah, if the resistors don't change anything, the PT = Post Toastie.
     
  10. uriah1

    uriah1 Telefied Gold Supporter

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  11. KC9KEP

    KC9KEP TDPRI Member

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    Do you have a dim bulb test socket? I.e. a conventional table lamp bulb in series with the ac mains? This will allow you to test without sacrificing fuses. If the lamp comes on full brilliance when you power up your amp, you’ve got a short. Keep disconnecting transformer secondary wires until you can power up the amp without the table lamp bulb lighting at full brilliance. If you’ve disconnected every secondary wire and the lamp still comes on, all that’s left is the power former .. good luck.
     
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  12. Dreadneck

    Dreadneck TDPRI Member

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    First of all, thank you all for your response!

    I will try this. Do you think I should put the old hum balance back aswell?



    Yes it does. It does however split out to other areas and goes through some resistors as well. Something I can't see in the schematic.



    Sadly no.. I made one some time ago but the bulb has died since and I have yet to find another....

    Here are some pictures:

    20200409_184944.jpg 20200409_184954.jpg 20200409_185024.jpg 20200409_185038.jpg 20200409_185054.jpg 20200409_185123.jpg
     
  13. Dreadneck

    Dreadneck TDPRI Member

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    Woo! Progress!!

    So now I got the amp up and running without blowing fuses with all tubes installed!

    It was a connection between a resistor and a wire that was bad on the strip behind the PT. I soldered them together in a good connection and BAM! Amp was working!

    As you were saying the voltages got normal with the tubes in place.

    So the PT seems to be in good shape!

    What happens now is that when I turn the volume up the amp starts to feedback/howl like crazy!
    And there is humming with the volume down.
    Something is not right around the inputs and tone stack.


    I took photos of the input section:

    20200409_213745.jpg 20200409_213801.jpg 20200409_213815.jpg 20200409_213826.jpg

    My theories is that eighter the inputs and tonestack is wired wrong/bad or it is the subber caps that is missing from the pre amp tube sockets that is causing this horrible squeal.
    I have heard before that the Fender 75 is prone to instabillity and oscillation.
     
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  14. Peegoo

    Peegoo Friend of Leo's

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    Awright! Good work.

    The squealing may be something as simple as lead dress. Sometimes an amp will oscillate due to proximity of wire leads with one another and components in the chassis.
     
  15. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Hi Dreadneck,

    I was going to suggest looking there, thats where the major problem on mine was hiding, it also had a bad joint, and the one resistor was burnt through, but whilst trying to replace it, and refresh all the solder joints, the whole strip just crumbled through long time heat exhaustion caused by the close proximity of the hot resistors. So I replaced the whole strip.


    Good luck finding that. Try tapping and pocking with a chop-stick to find sensitive areas. Your lead dress does look to be more wild than it should be, so as Peegoo points out, that could be the problem.

    I set up a page for owners of this amp here on the forum fender-75w-lead-amp-with-reverb-1980-82-owners-club.
    There are a few of us here, and it's good to have a central thread for swapping experience.
    One really important thing to know about this amp, is that it not only, only works properly with the foot-switch connected, but that it must also have the bias set with the foot-switch plugged in, because it changes dramatically when it is disconected.
     
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  16. Dreadneck

    Dreadneck TDPRI Member

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    Thank you!

    That makes sense and eighter way a good place to start. I think I'll redo the entire tonestack and input section. There is a lot of sensitivity there when I poke around.



    Same on mine! The strip is broken in half. I ordered a new one so I will replace it as soon as it arrives. A the moment i have just replaced the resistor and connected it directly to the wire.


    Yes I did a lot of poking there and it is very sensitive around the input jacks especially.
    Well I am not proud over the three resistors on the input to be honest. Not my finest work... It was made in hurry and lack of wire.
    However I have been spending a lot of time replacing wires and completely rewired other parts, because I can tell someone has been in here going crazy with the soldering iron and has done a terrible job.

    One thing that surprised me was that the wire from the input to V1 is not shielded.
    To my experience shielded wires is what you expect to see here.
    Now I do not have any shielded wire at the moment but I think of ordering some and put there.
    My Dual Showman and my Studiomaster has shielded wire and an JCM900 I had once also had shielded wire.
    I can not hurt right?
     
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  17. Dreadneck

    Dreadneck TDPRI Member

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    Oh! And thank you for posting the Fender 75 owners thread!
    I will make a post there and say hi! =)
     
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  18. Dreadneck

    Dreadneck TDPRI Member

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    Ok so about time for an update.

    I have rebuilt the strip that was broken. I actually removed all the transformer wires for alternate current from a strip next to the power tubes and sealed them of with shrink tubing.
    The strip they were attached to, I mounted the circuitry from the broken strip behind the PT.
    I also replaced the resistors and wires with new ones while I was at it.

    I replaced the wire going under the turret board from the input to pin 2 on the first pre amp tube with a shielded cable.

    So right now the amp works...
    However it is very unstable and I do not trust it.
    It makes noise like wind and scratching that is affected by the master volume.
    The guitar sounds beautiful through the amp though, but the noise is disturbing.

    Last but certainly not least.
    If I turn it on when plugged in to the wall or with the variac turned up, the 500mA fuses blow instantly. If I turn it on and slowly turn up the voltages on the variac it does not blow the fuses.
    Does anyone know what voltages it should be on the red secondary wires before the SS rectifier? I get around 370-375 VAC on those wires and the fuses are rated for 250 V so that is an obvious issue. But I do not know what voltages the red wires is supposed to have.

    Now with the variac I run the amp on 220 VAC but we have 230 VAC in the wall here in Sweden.
    I do use a variac mostly with my tube amps because I do not trust the wall voltages and I think that when many of these amps were imported, they were modified for 220 VAC which we had at that time.


    Well that is how far I've come in the process. I really like this amp so I will not give up on it just yet.
    But with the amount of wires in this amp, good lead dress and layout is a big challenge.
     
  19. dan40

    dan40 Tele-Afflicted

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    The higher voltage through the fuse will not cause a problem. It's the current that blows fuses. Provided there are no wiring issues, shorts or bad capacitors, the fuse is blowing from the large inrush of current when the amp is first powered on and the e-caps are charging. By bringing it up on a variac slowly, you are slowing down the large rush of current that the caps are drawing so this is likely the reason that the fuse does not blow. The 500ma fuse should not blow under normal power up conditions though. There may be a problem further down the b+ line that is causing a larger than normal current draw.

    Edit; I am not familiar with the circuit...does it have a standby switch? Does the fuse blow when taking the amp from standby to playing mode or does it blow when using the power switch?
     
  20. Dreadneck

    Dreadneck TDPRI Member

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    Oh yes that makes sense.
    I will follow B+ and see what I can find. Although it is hard to pin down where the problem is. I tried other tubes, I have replaced all caps inside the amp (the ones in the dog house has been replaced by someone else but it looks alright visually) and I've rewired lots...

    The amp has a standby switch on the schematic, however I have yet to see one I Sweden that still has that. It seems the standby and ground switch has been removed by those who imported the amp. It is the same with my Dual Showman Reverb.
    Do you think it would benefit the amp to install a standby switch?
     
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