fender Blues Jr III problem...help me, if you please??.

Discussion in 'Amp Tech Center' started by Wally, Nov 8, 2019.

  1. Wally

    Wally Telefied Ad Free Member

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    Okay, I have a problem here that I cannot cure. A Blues Jr III came to me w the V5 EL84 blown. I checked things out and replaced a bad screen grid resistor. Fired it up....big time 60hz hum. All bias voltages are correct. Plate voltages are correct. I got to looking for AC voltage....whoa!...540VAC on the plate of V4 power tube. There are diodes on the OT primary legs, so I replaced the diode on that leg for V4....no cure. I wondered about the coupling cap on that side of the output. Voltages were weird, so I replaced that cap. All of the Dc voltages are now looking correct there, but there is still 540 VAC on the plate of V4. That voltage is not on the primary of the OT when the OT is separated from the power tubes. All filter caps are filtering the AC, it seems, because I do not find that AC there. Does anyone have any insight as to the source of this high AC voltage?

    I have not reviewed all of the BillM work on these amps. Here is a link to the schematic.

    https://schematicheaven.net/fenderamps/blues_jr.pdf

    any help is appreciated.....this one has me stumped.
     
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  2. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Have you 'scoped the AC to determin its frequency? I'm thinking maybe check there is no HF oscillation...
     
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  3. Whatizitman

    Whatizitman Friend of Leo's

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    Geez. You're asking us?

    Anyone work on it before you did? Replaced the PT, mixed the primaries up?

    Please be gentle with my ignorance. Don't crush my dreams. :oops:

    OMG: I meant secondaries. You can ignore me now.
     
    Last edited: Nov 8, 2019
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  4. Wally

    Wally Telefied Ad Free Member

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    Bendyha, it is 60 hz.
     
  5. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    I suppose you have tried other obvious things ....like V4 tube replacement...
    ...checked the two 47 Ohm center-tapping filament supply resistors....and the voltages there.
    ..checked for no socket shorting "trace" ..

    You mention "Voltages were weird" (at the grids?)..how d'ya mean ?...ac riding the bias?
     
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  6. Wally

    Wally Telefied Ad Free Member

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    Bendyha, when I first started on this, I was focused on V5 since it was blown. I found the bad screen grid resistor, and that straightened that socket/tube out. But.....then...V4 was giving me a different plate voltage reading. Bias voltages on the two sockets were identical...a bit over -11 vac. The hum made me look and find this high VAC on V4’s plate. Changing the diode on that primary did not change anything, but when I changed the coupling cap on that side, the plate voltages...DC voltages...camp into balance, and the bias voltage on both tubes now comes in around -10.5vdc. The source of this AC has me baffled....it is learning time for me. I will look into the filament balancing...but I don’t see how this could cause the AC on the plates of the power tubes. the amp does not appear to have been worked on before. I will look at the possibility of shorts in the socket.
    thanks for the suggestions.
     
  7. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Learning time for us all..
    Hm..back to my first thought of oscillations..The screen grid resistors..they are very small at only 100 Ohms, and although I don't know the amps layout, I imagine they are on the circuitboard rather than hugging up against the tubes socket.
    Try 500K direct on the socket perhaps....might not be the answer, but worth a try.
     
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  8. Wally

    Wally Telefied Ad Free Member

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    I shared the info below with Bendyha and though I would bring it here for everyone to ponder. I am baffled. Today I am going to check the OT to see if the primary and secondary windings have not shorted to each other.

    Here is what I have researched to date...in addition to the basics that are in the thread.
    I checked the sockets using probes from pin to pin. This is something I have never done before...so if this is all normal, please forgive my concern. I started trying to check for continuity/shorts from pin to pin with the EL84s removed from the sockets but my meter would show me a voltage before giving me the resistance reading....Fluke meter....so I switched to DC voltage metering. pins 2,3,4, and 5 Probe to either pin 7 or pin 9 will all read whatever residual voltage is in the filters...whether that voltage is 3.6, 20, or 120..VDC. like I said, I have never taken such readings before and don’t understand why there is continuity across those pins. Pins 2 shows continuity with pins 3,4, and 5 of 234Kohms. Pin 2 and 7 read a varying continuity from 220K to 1meg. Pin 3 to the heater filament reads 24ohms. The heater filament reads 4.5meg to the plate. These results are identical in both power tube sockets.
    When power is applied to the circuit but without the power tubes installed, there is no AC voltage on the plates. With only V5 installed, V5 has 75VAC on the plate with proper Dc voltage...336vdc. In this scenario, the empty V4 socket has 350VAC on the plate and 336vdc there. With both power tubes installed, both tubes have 325vdc on the plate, V5 shows 3.3vac on the plate, and V4 has 538VAC on the plate. Note:yes, that high voltage AC has moved from V4 to V5 with the installation of both tubes. Also in this scenario, there is a hum. When testing the voltage on V5,P7, this hum ceases. When testing V4,P7 the hum increases. V4 is noticeably hotter than V4....and I am not keeping the amp under power any longer than necessary to take these measurement. That said, V4 is the tube that was blown when the amp came to me.
    I tested the PT and OT winding resistances, and all seems correct when not under a load. The brown winding with the +/-15vdc is acting correctly. The heater filament voltage is good across all tubes.
     
  9. King Fan

    King Fan Friend of Leo's

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    I won’t be any help, Wally... IIRC some BJs in the early 2000s had bad filter caps, but you tested the filtering. Are those ceramic sockets? mounted on a PCB? Bad socket, bad PCB, bad ribbon connector, etc? Sorry, just spittin' in the wind, but looking at the scheme, and not being wise in the ways of the Force, could it still be an OT problem?

    [​IMG]
     
    Last edited: Nov 14, 2019 at 10:46 AM
  10. Snfoilhat

    Snfoilhat Tele-Afflicted

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    If the bias supply is extraordinarily ripply, like from a bad bias supply, can those ripples showing up on the control grids somehow get reflected onto the OT primary and look like AC voltage? 60Hz is the frequency of the heaters and of a half-wave rectifier, right?
     
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  11. D'tar

    D'tar Friend of Leo's

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    pcb carbon tracks contaminating neighboring circuits/traces.?

    socket carbon contamination?

    There is really only one place to get that high 538VAC right? PT pre rectifier.
     
  12. D'tar

    D'tar Friend of Leo's

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    Wally... Crazy thought here. Try a new battery in your meter. Only reason is having a negative AC voltage reading is unusual no?
     
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  13. corliss1

    corliss1 Friend of Leo's

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    I would also tend to agree that high voltage AC has to be from the PT secondary before any rectification. Mostly I'm commenting to follow along since this is interesting, and I haven't run across that in any of the hot rod series stuff I've played with, although I've seen plenty of open/burnt/high value screen resistors on blues/pro juniors.
     
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  14. Wally

    Wally Telefied Ad Free Member

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    D’tar, I have taken measurements with more than one meter, but I will check the battery...batteries. Thanks.


    All PT secondary voltages are correct. Voltages following the rectifier are correct. Voltages out of the filter section are correct. No AC to be found at those points...although I am sure I will quickly revisit those points when I take look at the OT primary/secondary aspect per Bendyha’s suggestion.

    I appreciate the responses.
     
  15. corliss1

    corliss1 Friend of Leo's

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    What if you just disconnect the OT? Even easier with those spade plugs.
     
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  16. LightningPhil

    LightningPhil TDPRI Member

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    Does the hum vanish if you pull any of the preamp or phase inverter tubes out?
     
  17. D'tar

    D'tar Friend of Leo's

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    What happens if your full wave rectifier is half failed? Half wave 60hz rectification instead of full wave 120hz? While dumping ac into the circuit? <$1 worth of diodes perhaps?
     
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  18. LightningPhil

    LightningPhil TDPRI Member

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    If it fails short - the most likely way for diodes to fail - then it’ll stress the mains tx and blow the fuse. If it fails open, then you’ll probably over heat the tx and perhaps push it into saturation - as it would effectively have dc going through it.
     
  19. Wally

    Wally Telefied Ad Free Member

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    Disconnected, there is no AC on either end of the OT primary.


    No.


    I’ll recheck, but it seems the answer to Corliss’s question above covers this.....there is no AC with the primaries lifted from the connection to the plates....unless my confusion over this problem has me not remembering. I checked the voltage off of the rectifier, iirc....and there is no AC at the filters.
     
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  20. corliss1

    corliss1 Friend of Leo's

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    So I think I've decided this can't be happening. There's just no way to get that much AC at that point in the circuit.

    What if you take your meter, set to AC volts, and measure a 9V battery? Do you get a weird reading with that?

    I'm not saying your hum isn't an issue, but I think your meter could be reading *something* else on top of the DC at that point.
     
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