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Old Oahu amp issue . wont power on

Discussion in 'Amp Tech Center' started by mkster, Oct 12, 2017.

  1. mkster

    mkster Tele-Afflicted

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  2. Preacher

    Preacher Friend of Leo's

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  3. BobbyZ

    BobbyZ Doctor of Teleocity

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    If I'm thinking right a 3A is what a Twin Reverb uses and a Princeton Reverb is 1 amp.
    The schmatic for it will (should) have the rating. Otherwise looking at amp's schmatics with similar tubes will get you in the ballpark or a tube manual and some math.
    However it blew a fuse that big and it needs to see a tech ! Something is very wrong and my first guess on an old amp is electrolytics but it could be a tube or something else. Hopefully the power transformer is ok because that often becomes the "fuse" in a case like this.
     
  4. mkster

    mkster Tele-Afflicted

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    I have checked the fuse with what i had both ends are still connected , so it my not be my problem after all . I was planing on buying some 2 amp slow blow , but it seems like the problem is elsewhere .

    It worked fine no issue , i disconnected it a few weeks then nothing . The switch dose nothing either on stand by or On , could the rectifier do that , i will test the switch i have all the tubes and spare transformer .
     
  5. dan1952

    dan1952 Friend of Leo's

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    Fuses don't go bad, and buying some slo-blo fuses doesn't fix anything. It amazes me that some folks don't realize that a fuse blows when something is wrong in the circuit. Fuses are PROTECTION DEVICES to keep from doing further damage. THE PROBLEM IS NOT THE FUSE!
     
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  6. mkster

    mkster Tele-Afflicted

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    I agree , its not blown either its the wrong value so its useless , hence the need to know the values for a right one .

    But still the amp is not lighting up any ideas ?

    Thanks in advance

    Marc
     
  7. BobbyZ

    BobbyZ Doctor of Teleocity

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    Normally the pilot light runs off the heater filiment winding and that circuit is separate from the high voltage that goes through the rectifier. So with a bad rectifier the pilot still works. A look at the schematic would really help me here.

    Sounds like the fuse isn't blown but there's no power coming out of the PT right?
    If so I'd be inside the amp with a voltmeter hoping to find a break in the primary wiring before the PT. Like a bad switch or solder joint on the fuse.
    If you have ac from the plug in to the PT and nothing on the secondary side you have new paper weight.

    Needless to say being inside an amp, expecially when it's plugged in isn't for every body !

    Oh the outlet has power right ?
    Simple things like that can get you too. Just the other night while comparing two amps I shut one off to try the other, it's on, a little hum told me that but nothing from the guitar at all.
    Yeah the cord was still plugged into the one I switched off. . . . .
     
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  8. mkster

    mkster Tele-Afflicted

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    [​IMG] [​IMG] This is the schematic i have drawn , and one similar made under supro name.
     
  9. BobbyZ

    BobbyZ Doctor of Teleocity

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    Yeah check the power going into the PT, as you can see on the schmatic there ain't much between the wall plug and the PT to go wrong.
    Then check the secondary side. This PT has three secondary windings.
    6 volt filiments. I'd start there, without that circuit working the pilot and tube filiments won't light up.
    Exept the 5Y3 because that filiment runs off a 5 volt winding of its own.
    And of course the high voltage winding.

    Best case from what I'm hearing is the power transformer just isn't getting juice, that's a cheap fix. After that the filiment circuit has a bad joint, but I didn't see anything about the 5y3 still lighting up?
    Obviously worst case is the PT and hopefully that ain't it. Checking voltages will norrow it down real quick.
     
  10. Wally

    Wally Telefied Ad Free Member

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    This amp likely has had a problem for sometime....hence the over-fusing. Or...perhaps....some previous owner only had that 20amp fuse and used it when a power surge took out the correct fuse. At any rate, that large fuse has probably led to the failure of a more expensive part....transformer??? this amp needs a tech....if the transformers are good, then it definitely needs to be recapped. Hopefully, an electrolytic failed without damaging a transformer.

    Note to self: Never turn on an unknown amp without pulling the fuse to assure oneself that it is the correct value. The first thing I do when assessing an amp for purchase or repair is to pull the fuse. I once found a perfectly rolled cylinder of aluminum foil in a rare Vox AC-100 while the owner was bragging that the amp had never failed to work. 3 of the 4 EL-34 power tubes were shorted.
    I keep those 10, 15, and 20 amp fuses for my old Chevy pickup.
     
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  11. mkster

    mkster Tele-Afflicted

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    Its been a while , the amp is still on the fritz ... I am now looking into it . I don't have a proper fuse for it so will not power it on , but the power transformer seems , i have check for continuity the power transformer , primary , heater , rectifier coil ... Could it be the 5y3 ?
     
  12. timfred

    timfred Tele-Meister

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    You could pull all the tubes, including the rectifier, and the pilot lamp should still light if the lamp, fuse, power switch, power transformer, and power cord are all ok.
     
  13. trobbins

    trobbins TDPRI Member

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    Marc, it seems like you have a multimeter and have confirmed that the AC mains plug pins show a certain resistance when the fuse is put back in, and the power switch is on ? What resistance did you measure ?

    Further to timfred's test (where the rectifier valve is removed), if the pilot light doesn't come on when the mains switch is on, then I suggest putting the 12AX7's back in and checking if their filaments glow (just in case the pilot light is now faulty). And of course if nothing is working, then check your mains AC socket can power something else.

    If you do get signs of the 6V heater voltage working then I suggest not putting the valve rectifier back in until you have some tech help to measure that the power transformer HT secondary winding has the appropriate voltages (as the secondary winding can easily fail open-circuit if either the rectifier valve arcs, or the first 20uF e-cap fails short, or the output stage fails in a manner that pulls down the B+.
     
  14. Wally

    Wally Telefied Ad Free Member

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    Tech time.....continuity of the windings is one thing, but that does not assure that the transformer is good. Have you checked to see if there is continuity between any of the windings and the frame of the transformer. Such continuity would indicate a short to ground and a bad PT. Even if these metering tests do not show a problem, a transformer under load may then exhibit problems that do not show up in such continuity tests. Without a proper 2A slo blo fuse, one is on a short road. You need some fuses. You also need a current limiting light bulb rig for testing and diagnosis.
    There could one of a fair number of causes of whatever the problem/s is/are. This amp will need to be recapped at any rate. IF the power transformer is bad, that will need replacement first. It has been three years since you opened this thread. Have you been study tube technology and amplifier repair in the interim?
     
  15. mkster

    mkster Tele-Afflicted

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    Hi Wally , i have been looking into tubes en servicing for 20 years , i got a good base but i am scared to blow stuff up . I have a replacement PT in hand but i would rather like to keep it to build a clone. There are plenty of caps to change , i am starting on those. I need a new power chord 3 prong i need to look into it a bit i know i might need to remove a cap.(is it the .02 on the fuse)

    I remember starting the tube madness when Fender reissued the 59 bassman , i bought the aspen book i think the 3rd edition.

    I got some of the cap already , i have to got get some electrolytic one . I may go get them later .

    I have the fuse 2amp sloblow ... I will check the amp without the rectifier ,i think i should see about 350 volt between pin 4&6 and 6.25 on 2&9 if everything is ok. correct me if i am wrong.
     
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  16. Wally

    Wally Telefied Ad Free Member

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    If you see the expected voltages on the high tension and the heater filament..both for the 6.vac supply and the 5 volt rectifier supply with no rectifier in place, then my advice would be NOT to go any further until the electrolytics have been replaced. I see a 20mfd and two 10mfds In that power rail...all at 450vac. There is one electrolytic bypass cap on the input gain stage.
     
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  17. mkster

    mkster Tele-Afflicted

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    I am calling to see if i can get one locally , a 10-10-20 all-in-one would be amazing otherwise the wiring will be something to see , lol . I think i will recap first anyway .
     
  18. Wally

    Wally Telefied Ad Free Member

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    Not having seen a decent shot of the open chassis, I can’t speak to the difficulty of using single axial e-caps versus a multisecti9n. Here is one that might work out....but we don’t know the diameter of the original can??.

    https://www.tubesandmore.com/products/capacitor-ce-mfg-475v-20201010-f-electrolytic

    Be aware that you do NOT want to buy an old electrolytic. Whether they have been in circuit or not, electrolytic caps degrade....just as do dry cell batteries. This is probably why there are date codes on e-caps. (;^)
     
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  19. mkster

    mkster Tele-Afflicted

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    I just came back , i have pretty much all i needed . Unfortunately its all NOS. I even got my self some saddles .
     

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  20. Wally

    Wally Telefied Ad Free Member

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    As I noted, NOS with electrolytic caps is NOT advisable. I am going to think that that multisection can is old production....decades old, probably. If you can take a better picture of the codes beneath the values...right side up...we would have a better idea, but I never use NOS electrolytics. Here is 9ne that I let slip through the cracks....that code reads 677 9703. 667 is the manufacturer’s EIA code, 9703 indicates the third week of 1997. That is 23 years old. I am not going to use it....I lost whatever Impaid for it...$26.96 wholesale back then.

    EF5A66AF-A174-4861-93E5-945D50EBBD24.jpeg
     
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