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Thread: Bias problems---

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    Bias problems---

    working on an orange TH-30 . It has 2 of the el84s red plating , the other 2 do not. Its not the tubes .

    Looking at the scem , ive checked all resistors from V5 all the way to pin 2 of the el84s , ground is good , no bad solder joints , and while i had the pcb up i replaced the .1 caps , c45 / 46 coming out of V5 .

    with no el84s installed im getting 280vdc at pin 2 of V 6 /7

    at V8 / 9 , i get about 0 to 4mv..the meter just kind of jumps around--it wont settle down on a number .

    Any thoughts ??

    TP13 is 290vdc at "C" so that looks ok according to the scem.
    Attached Files Attached Files

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    Supporting Member The Dude's Avatar
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    What resistance do you measure across C45 and C46?

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    Supporting Member Randall's Avatar
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    Which two are red plating and which two are not? V6 and V8 share a cathode resistor and bypass cap, as do V7 and V9. That 4 output/2 output switch and it's pal 3w 100K R84 are worth a look also, since it is how two of the tubes get their cathode grounds.

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    It's weird, because it WAS working fine.....

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    v6 / v7 sockets are tubes going red.

    i just found something..R84 tests shorted in circuit..but out of circuit tests 100k . both ends of r84 are going to ground no matter where the half pwr is set . R 84 has to be shorted ?? or am i missing something ...

    the half pwr switch tests ok

    R83 / R85 OK

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    Supporting Member The Dude's Avatar
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    If the resistor checks good out of circuit it's good. The half power switch switches out 2 tubes by adding the 100k into the cathode path of those tubes. The switch shorts across the resistor for 4 tube operation. If your problem was there, V6 & V8 would be red plating instead of V6 & V7. I believe the problem is that 280vdc at pin 2 of V6 & V7. You need to find out where that's coming from. That's why I asked the resistance across C45 & C46. Even if it's not the caps themselves (since you changed them), there may be another current path there- a solder bridge, debris, socket issue, etc.

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    ok--sorry i missed that post-i will measure those here in about 30 minutes----in circuit right?

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    Supporting Member The Dude's Avatar
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    Yes, in circuit. Basically, we want to know if the plate voltage from V5A is somehow leaking over to the grids of V6 & V7, whether through the cap or some other means. Mostly interested in the resistance reading across C45, but comparing to C46 gives us more data.

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    220k and 240k

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    Lifetime Member Enzo's Avatar
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    TP10, TP11 are the 120 ohm cathode resistors for the power tubes. Either of them shorted to ground?

    Your caps from the PI to grids may not measure shorted, but can still leak DC when facing hundreds of volts.

    Lift C45 from the circuit and see if the voltage goes away.

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    Quote Originally Posted by Enzo View Post
    TP10, TP11 are the 120 ohm cathode resistors for the power tubes. Either of them shorted to ground?

    Your caps from the PI to grids may not measure shorted, but can still leak DC when facing hundreds of volts.

    Lift C45 from the circuit and see if the voltage goes away.
    tp10/11 are not going to ground.

    ill have to pull the board again to pull c45

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    don't forget the joker g1's Avatar
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    Have you checked resistance from pin2 of V6 or V7 to ground? Should be approx. 150K.

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    Quote Originally Posted by g1 View Post
    Have you checked resistance from pin2 of V6 or V7 to ground? Should be approx. 150K.
    V6/7--the meter slowwwwwly creeps up to around 120- 125k ? this is over 10 or 15 minutes until the meter times out and shuts off

    V8/9 instantly measure 150k.

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    Quote Originally Posted by Enzo View Post
    TP10, TP11 are the 120 ohm cathode resistors for the power tubes. Either of them shorted to ground?

    Your caps from the PI to grids may not measure shorted, but can still leak DC when facing hundreds of volts.

    Lift C45 from the circuit and see if the voltage goes away.
    c45 removed--still have 230 vdc there at pin2

    so its maybe something to do with the v5 area... ??

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    Last edited by Valvehead; 03-30-2018 at 01:23 AM.

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    Lifetime Member Enzo's Avatar
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    Start looking for conductivity or foreign material on the board in that area. Also look for a tip of a component lead nearby touching that trace. If so, trim it back. Follow the trace from pin 2 to anywhere it goes, looking for anything along the sides of that trace somehow touching it.

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    Last edited by Enzo; 03-30-2018 at 01:58 AM.
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    Senior Member SoulFetish's Avatar
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    Lift one end of r76 and r78. Is the voltage still there?
    (Assuming you already pull those tubes)

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    Quote Originally Posted by Valvehead View Post
    V6/7--the meter slowwwwwly creeps up to around 120- 125k ? this is over 10 or 15 minutes until the meter times out and shuts off

    V8/9 instantly measure 150k.
    Does R73 measure 150K ? Does one end measure zero ohms to ground?

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    Quote Originally Posted by Enzo View Post
    Start looking for conductivity or foreign material on the board in that area. Also look for a tip of a component lead nearby touching that trace. If so, trim it back. Follow the trace from pin 2 to anywhere it goes, looking for anything along the sides of that trace somehow touching it.
    All looks good.

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    Quote Originally Posted by SoulFetish View Post
    Lift one end of r76 and r78. Is the voltage still there?
    (Assuming you already pull those tubes)
    no---no volts at pin 2

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    Quote Originally Posted by g1 View Post
    Does R73 measure 150K ? Does one end measure zero ohms to ground?
    yes and yes

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    So with C45 removed, you still had 230V at the grids of V6 & 7 ?
    But with R78 & R79 lifted it disappeared?

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    Quote Originally Posted by g1 View Post
    So with C45 removed, you still had 230V at the grids of V6 & 7 ?
    But with R78 & R79 lifted it disappeared?
    Yes sir

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    Sounds like maybe board leakage, not sure from where.
    You still have the EL84's out?
    Earlier you said 280V at the grids, now it is 230V?
    Point C is 290V? How about pin1 and 6 of PI tube?

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    Senior Member SoulFetish's Avatar
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    After you report back with the voltages g1 was asking about, I want you to try something..
    Pull the Phase Inverter tube, solder r76 and r78 back in circuit, and power back on. What's the voltage at the grids now?

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    Quote Originally Posted by g1 View Post
    Sounds like maybe board leakage, not sure from where.
    You still have the EL84's out?
    Earlier you said 280V at the grids, now it is 230V?
    Point C is 290V? How about pin1 and 6 of PI tube?
    ok--i was measuring wrong--sorry--


    el84s out , PI in , all resistors / caps in
    v6 220v
    v7 220v
    v8 0v
    v9 ov

    point C 355vdc

    PI tube pin 1 353v
    pin 6 354v

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    Last edited by Valvehead; 04-01-2018 at 10:52 PM.

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    Quote Originally Posted by SoulFetish View Post
    After you report back with the voltages g1 was asking about, I want you to try something..
    Pull the Phase Inverter tube, solder r76 and r78 back in circuit, and power back on. What's the voltage at the grids now?
    PI pulled , el84s pulled

    v6 258v
    v7 258v
    v8 0v
    v9 0v

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    Senior Member SoulFetish's Avatar
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    Okay. Just in case you measured something wrong, replace the coupling cap feeding those two grids. Its cheap and thats the logical source of the problem. Report back

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    I replaced the .1 caps before

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    Senior Member SoulFetish's Avatar
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    Okay.
    When you power up are those voltages present on the grids immediately or do they climb to those voltages after some time?

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    Quote Originally Posted by SoulFetish View Post
    Okay.
    When you power up are those voltages present on the grids immediately or do they climb to those voltages after some time?

    Ok... i just got back from vacation last night. Ready for the madness ? I powered it up to test just now .... and now I have 15 vdc at the grids of V6-7 and 0vdc at v8-9................yep !

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    Are there any signs that something might have been spilled in the amp? I wonder if there was moisture somewhere that has dried out- maybe under a socket or something? I might hit the board with some compressed air and/or hair dryer. That, and a thorough inspection.

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    Quote Originally Posted by Valvehead View Post
    Ok... i just got back from vacation last night. Ready for the madness ? I powered it up to test just now .... and now I have 15 vdc at the grids of V6-7 and 0vdc at v8-9................yep !
    Well I’d say that’s progress! Only thing is you came back from vacation a little early. Go away for another two days or so, then come back and those voltages should be right we need them to be.

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    Absolutely stumped here....

    I did a hail mary and installed some new caps and resistors tonight ..i dont know what else to do at this point ? I cleaned the sockets and pcb with alcohol and contact cleaner..i dont see anything wrong anywhere. No cracked joints or lifted traces...no solder blobs or hairs .

    So everything from the out of the plates of V5 is brand new ....also replaced R3 / c3 / c4 . All grounds check good . Ive worked through the circuit checking continuity .


    same result = V6/7 super high volts ............v8/9 nothing .

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    Quote Originally Posted by SoulFetish View Post
    Well I’d say that’s progress! Only thing is you came back from vacation a little early. Go away for another two days or so, then come back and those voltages should be right we need them to be.
    I think Mercury is in retrograde until Sunday. Still needs two more days.

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    "Absolutely stumped here...."

    It appears that the PCB itself is bad.

    Have you tried contacting Orange for a replacement?

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    Quote Originally Posted by Jazz P Bass View Post
    "Absolutely stumped here...."

    It appears that the PCB itself is bad.

    Have you tried contacting Orange for a replacement?
    no, but i can try that

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