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1-9 of 9 messages
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Need Help on ThunderBolt HF Amp
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by N5RVZ on January 18, 2005
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I have restored this amp and the only problem is the HV pops a fuse every time the HV Plate switch is thrown.
I have replace the following.
(1) Original 866's with solid state
(2) HV Cap's with new ones
(3) New HV wiring underneath, from HV caps to 4-400's
(4) Replaced existing 4-400's with other sets
(5) Got HV Plate transformer rewound
I am at a loss what is still the problem! When I disconnect the HV at the caps from the rest of circuit. I have AC HV at the end of HV section transformer Coil L101. Doesn't blow the fuses at this point. Something is shorted after this point!
Can anybody help?
Thanks in advance.
Keith - N5RVZ
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RE: Need Help on ThunderBolt HF Amp
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by AF4K on January 19, 2005
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Keith - you should eliminate one thing at a time.
STEP #1 -
Try disconnecting the rectifier from the HV
secondary of pyour plate transformer. Does it
blow the fuse now?
If so, then your transformer is bad or your wiring is wrong. Check all wiring for shorts.
Use an ohm meter to mneasure the resistance of your transformer windings. Use the LOWEST ohms range.
They should be something ABOVE zero ohms, even if they are only an ohm or two.
STEP #2 -
If it all looks OK up to that point, you should
try connecting JUST the rectifiers (no caps or load.)
Does it blow the fuse now? If so, then you might
have some shorted out rectifiers. This could have occurred because a) They were bad to begin with. OR
b) Because the inrush current to the capacitor(s)
is too much for them. You nee dto limit the inrush current, because the original circuit had tube
rectifiers that allow a bit of E/R current limitation
through the 866As.
Let us know how you are progressing after the above two steps. Consider adding some resistors in series
with your rectifiers. I have never done this but
I hear that some guys add like about 10 ohms at 20
watts to ease things up for the circuit. Remember,
with today's higher line voltage AND the higher plate voltage you get from the sand state rectifiers, you have probably increased the plate voltage by 10 to 20% over what it was in the original application!
KEITH WROTE:
HV pops a fuse every time the HV Plate switch is thrown.
I have replace the following.
(1) Original 866's with solid state
(2) HV Cap's with new ones
(3) New HV wiring underneath, from HV caps to 4-400's
(4) Replaced existing 4-400's with other sets
(5) Got HV Plate transformer rewound
I am at a loss what is still the problem! When I disconnect the HV at the caps from the rest of circuit. I have AC HV at the end of HV section transformer Coil L101. Doesn't blow the fuses at this point. Something is shorted after this point!
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RE: Need Help on ThunderBolt HF Amp
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by W7DJM on January 19, 2005
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Some good advice above, but left out one thing.
Do you still have the filaments transformer (winding) hooked up at the rectifiers?
If so, that winding could be "leaking" to ground.
Also, double, quadruple (??) check the size and type of fuses you are using (Radio Sh...?) and make SURE they are correct. Make sure the fuses are not low voltage type.
This could be the choke. temporarily disconnect. There is no reason you cannot put the choke in the GROUND (center tap) side.
Is the center tap permanently grounded, or switched, and if switched, does this affect the problem?
I just looked at one schematic at bama, shows SW105, must be a "lid" safety switch. Is that open?
I don't understand what you mean that you have "AC at the end of L101?
If the caps are disconnected, this should be pulsed DC, and will probably read on an AC meter, I'd be for "rigging" a temporary filter cap right there and see what you have.
Try hooking up the rectifiers one at a time. ARE YOU SURE they are not in backwards? (It isn't clear to me whether you soldered up diode strings or used plug-in replacements.) You want the band on the diode toward the DC output, the bands of the diodes hooked together.
From AC winding------->|---------to DC(choke)
About all you can do, is disconnect everything, and go step by step.
Hook up the rectifiers ONLY
Hook up the L
Hook up ONLY the caps.
What are you using for caps, are you sure they are good?
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RE: Need Help on ThunderBolt HF Amp
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by WB2WIK on January 19, 2005
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You can't arbitrarily replace old parts with new parts on the Thunderbolt. I don't know why you needed to rewind the HV transformer, but:
-Solid state rectifiers can create a lot more DC potential than the original 866's, and that alone is often a bad thing because everything that follows the rectifiers in the T-bolt is only rated 2500V maximum. If you achieve higher DC levels than this, all sorts of "interesting" things might happen.
-Also, this was a choke-input power supply in the T-bolt. When you went to solid-state rectifiers, did you short out the choke, or leave it there? If you shorted it out, or if it shorted itself out, now you'll have a whole LOT more DC potential than the amp was designed for, and I'm not surprised it blows fuses.
-You say you replaced the "high voltage caps," but what do you mean? The original T-bolt used just one, single high voltage filter cap in the B+ supply. It is installed horizontally beneath the chassis and was only rated 2000V working, 2500V peak. It was oil-filled and from the days when PCBs were used in the oil. Since these amps were from the late 1950s, by now the caps have all leaked. What exactly did you do, here?
WB2WIK/6
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RE: Need Help on ThunderBolt HF Amp
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by WA2CWA on January 19, 2005
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Steve said "original T-bolt used just one, single high voltage filter cap in the B+ supply"
Later version Thunderbolts used a bank of filter caps (6 I believe, schematic not handy) rather than one filter cap plus some other changes.
Early version, later version, same book- two different schematics
Pete, wa2cwa
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RE: Need Help on ThunderBolt HF Amp
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by WB2WIK on January 20, 2005
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Pete, I didn't know that. The T-bolt I had for several years had just the single cap, and it did leak! I replaced it with a non-PCB oil-filled of similar size but better ratings, since the "new" cap was about 30 years newer.
In the "later" version, does the power supply still use the big choke?
73
Steve WB2WIK/6
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RE: Need Help on ThunderBolt HF Amp
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by WA1RNE on January 20, 2005
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Given what you have told us, I would say the most likely suspect is the interlock switch, SW105, located dead center in the rear of the chassis.
Sounds like its doing its job.....
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RE: Need Help on ThunderBolt HF Amp
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by WA2CWA on January 20, 2005
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Steve:
Schematic dated 10-8-63; 6, 80 mfd caps in series (total 13 mfd), each shunted with a 120K resistor; all hung on the output side of the large choke L101(5-25); 866's replaced by 3B28's. Didn't compare both schematics line for line for other changes.
pete, wa2cwa
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RE: Need Help on ThunderBolt HF Amp
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by W4XKE on March 11, 2005
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There might not be anything wrong with it. The first thing that I would suspect is a high in-rush of current to charge the filter caps. You might be able to cure the problem with thermistors or with a soft-start delay relay that initially connects a resistance in series with the primary, then shorts out the resistance to direct feed it. The larger the filter caps, the more initial current they are going to draw. Let us know what you find out when you get it going. Respectfully, Johnny W4XKE
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