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Post subject: Smokin Caps AB165
Posted: Mon Feb 21, 2011 7:18 pm
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Hi Folks,
I have a SF Bassman. 1967 per the serial number of A36024.
When I got it I plugged it in and it made a gut wrenching
noise.
Opened it up and the power supply caps, the 4- 20ufX500 showed
leaking and the the 2 resistors were burned and cracked.
Also noticed that someone had put in a 250ufX450v cap and a 1K
resistor there on the rectifier board. Schematic calls for a 50ufX50v cap and a 470 ohm 1W resistor. I changed those two out.
Next replaced the 4-20ufX500v and the burnt resistors, then turned it on...slowly the same friggin noise came back and two of the 20uf caps begin to smoke and blew out oil..
I don't know how you would test a power tranny but I suspect this one is shot!
Arjay, am I correct?
I have looked at those on Antique Electronics and CE Dist. they have a few offerings. The Hammond says it's for the 022814 which is in the Bassman.
Would that be a good choice?
Hopefully the output tranny hasn't been damaged...it does have output.
Thanks for any help from you folks.
Tube Dude


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Post subject: Re: Smokin Caps AB165
Posted: Mon Feb 21, 2011 8:32 pm
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Hi tube dude,

I would not suspect the power transformer, if it was shot you certainly wouldn't smoke the power supply caps. Sounds like something bad in the supply itself or in the output section, output tubes, output transformer, etc. You should try your tests/power up, with all tubes removed first. :idea:

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Post subject: Re: Smokin Caps AB165
Posted: Mon Feb 21, 2011 8:54 pm
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shimmilou wrote:
Hi tube dude,

I would not suspect the power transformer, if it was shot you certainly wouldn't smoke the power supply caps. Sounds like something bad in the supply itself or in the output section, output tubes, output transformer, etc. You should try your tests/power up, with all tubes removed first. :idea:


+1

I'd be suspect of one or more diodes in the rectifier stack -- excessive AC amperage into the filter caps is never a good thing (they're supposed to only see minor ripple current).

Hammond iron is good stuff though, if it turns out to indeed be the P/T.

HTH

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Smokin Caps AB165
Posted: Tue Feb 22, 2011 6:15 am
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More then likely the caps are original and need to be changed, a bad transformer would not cause a cap to leak.And old cap would cause a cap to leak and make funny sounds and smell.


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Post subject: Re: Smokin Caps AB165
Posted: Tue Feb 22, 2011 7:46 am
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Try this... Get a bunch of fast-blow 1 or 2 amp fuses. Pull the legs off the rectifier bridge. Pull legs off heater lines. Pull ALL tubes. Have amp connected to speaker (of any kind with correct impedance -or- a hefty load resistor). When you hit the switch (power switch, only---amp on standby), does the fuse pop? If power switch pops the fuse, then, it a good possibility your power tranny is shorted. If no popped fuse...

Try again with heater lines connected. See if the fuse pops or not. If your tranny pops fuses... Check the heater lines for shorts or broken solder points. If all look ok with heater line (including line to lamp)...

Pull all wires off the PT (including ground points) and test the continuity of the primary and secondary lines. Power off, just testing static DC resistance. Should have some resistance across leads to CT for secondary (like 20-80 ohms). And same for end to end on primary. Esp test each end to iron lammation for shorts. If you get near zero ohm from any lead to lammy, prolly shorted tranny.

If tranny doesn't pop fuses... I'd get a new set of UF4007 diodes and BE sure to wire them correctly into the circuit, for that main rectifier section. Disconnect the first set of filtering caps (70mfd/350VDC caps) and see if you get around 425-450 VDC (it's going to fluctuate because of lack of filtering).

Also, REPLACE the diode in the bias supply circuit. A UF4004 works fine. WATCH polarity of that new lytic, in the bias supply (I like 100mfd/100VDC here). If you get good VDC off the diode bridge and good negative 45 VDC off your bias supply...

I SUSPECT that when the last tech replaced those 70mfd/350VDC rated caps he didn't check the 220K-ohm, 1 watt resistors across the series'ed pair. They should be brand new or very close to each other in resistance, or the voltage load will not be distributed evenly across the two 70mfd caps. Check their resistance. In fact, I just replace them. 1 or 2 watt rating is fine. Bad distributed load = one cap takes most of the current demand and fails early.

In fact LOOK carefully under the hood holding those main filter caps. People often flip flop polarity of the lytics... which leads to instant meltdown. I usually try to replace each cap one-by-one, to avoid polarity issues). Check for shorts to ground. Bad soldering. I like to replace all resistors under the cap hood. Heat resistant 2 watts are nice.

Pull one leg of the choke (TR2) and check for shorts to its frame from each lead.

Triple check all your connections and POLARITY. No tubes. Use a Variac and slowly start ramping up the voltage. Measure the point off where the first cap section is (before standby). Should look good. And rise without incident as you crank the Variac. If it looks good, you can hit the standby switch and see if the other voltage points seem to be ok. Their voltage should be RELATIVELY raising wrt to the B+ at the first read point (B+>C+>D+, etc). If any look too high or low. Stop the Variac. Turn off. Drain caps. And RELOOK at all the soldering points, ESP THE GROUND POINTS.

If all looks ok. Measure the anode voltages on the 6L6GC and the gain stage/EFX/PI tubes, WITHOUT the tube inserted. They'll measure kinda high, because of the lack of load from the tubes. But, should be within range (+20%).

If ok... Insert a pair of known good, but expendable 6L6GC's. You should get that faint hum when you hit the standby switch. Check the bias. Adjust if necessary. Seem ok?

Insert phase inverter, !2AT7. How's the amp sound now? Maybe a bit more hum or other noises, but not howling at you.

Follow with all other tubes. One by one. Hopefully things will be ok.

Oh yeah... replace fast blow fuse with standard 2amp Slo-Blow. I'd remove the "death cap" and spring for three-wired power cord with green grounded right to chassis, as it enters the amp circuit area.

One last thing, since your under the hood getting all greasy. Replace the grounding straps from each 6L6GC with a PRECISE (2%), one-ohm, 1/2 watt resistor. You them be able to measure the dropping voltage across this resistor and calculate the idle bias current. (With one-ohm resistor, mV measured = 1 mA of current).

Good luck with all that!

http://www.ampwares.com/schematics/bassman_ab165.pdf


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Post subject: Re: Smokin Caps AB165
Posted: Tue Feb 22, 2011 9:13 am
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BMW2002Ti,

Dude, what are you smoking? I want some! :lol:

You have a couple of good points, checking cap polarity, pulling tubes for tests, but the rest???

A hand full of fuses for troubleshooting is ridiculous. Sounds like an old Army technique, like using barbed wire for electrical wire. If you really want to do it the Army way, put some permatrons in there, they won't blow, but the bad components will. :lol:

If the power transformer is shorted or grounded, the existing fuse would already blow, and if the transformer is open, then the caps wouldn't smoke. It is highly unlikely that the power transformer is the problem in this case.

No point in just replacing diodes (or any components) for the heck of it, check the diodes to see if shorted or open, that's all that is necessary. If they are not shorted or open, it would be foolish to replace them.

If you start pulling wires and components needlessly and randomly, you can actually cause more damage than you started with. Not a good way to troubleshoot.

Troubleshooting works much better than troubleguessing. :wink:

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Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: Smokin Caps AB165
Posted: Tue Feb 22, 2011 9:31 am
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Shimmy, I've had bad PT's that don't pop fuses, but blow caps. Fuse is really only on primary side. A short in secondary side can pop diodes and caps. I only recommend pulling wires to test the PT. And when does it matter if you do pull wires? Only if you do not connect them back, correctly?

It sounds like caps under the hood issue. But, you can't always be sure. Popping a fast blow 1 or 2 amp fuse is not going to cause catastrophe, like leaving a slow blow 2amp in and trying to measure voltages around the unit. Obviously, his fuse didn't blow BUT his caps do.... :?

You have some good points. But, let's not degrade the Army. And barbed wire. You do it your way. Fine. No need to inply that I'm outta my mind or smoking $@!& doing it my way.

Best to have Variac, in any case.


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Post subject: Re: Smokin Caps AB165
Posted: Tue Feb 22, 2011 9:50 am
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Nothing against the Army, I love those guys. It's just that many times, guys in remote locations didn't have the necessary tools and equipment, so they had to improvise with whatever was available, and they did it quite well. My Army buddies are the ones that turned me on to the permatrons. :lol:

Yes, a variac is indispensable, great tool. 8)

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Post subject: Re: Smokin Caps AB165
Posted: Tue Feb 22, 2011 10:13 am
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Shimmy, Yeah my fault. Was in hurry to drive kids to school. Looking back at OP, I'd first look under the cap hood. Make sure the polarity of ALL caps is correct. It only takes one cap, put in wrong direction, to blow the whole works. Be sure grounding wire is connected to correct point.

If this all looks ok, the only other way I know of to check the PT is with a DVM or scope. And you'll still need to pull the secondary and heater wires from the amp. I hope this isn't the issue.


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Post subject: Re: Smokin Caps AB165
Posted: Tue Feb 22, 2011 7:20 pm
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shimmilou wrote:
Nothing against the Army, I love those guys. It's just that many times, guys in remote locations didn't have the necessary tools and equipment, so they had to improvise with whatever was available, and they did it quite well. My Army buddies are the ones that turned me on to the permatrons. :lol:

Yes, a variac is indispensable, great tool. 8)



Gee, I remember an HF transmitter that had a bad parasitic suppressor. Since we were an isolated station, and Brooklyn Supply didn't have any anyhow, We enlisted the Chief Engineman who happened to have his fly tying tools with him. Unwound the coils one by one and wrote down the final number, grabbed a new resistor out of stock and rewound the coils. Bear in mind, the coil wire is hair thin on those things. Soldered in the new rebuilt component, fired up the transmitter, and voila!! I don't think Brooklyn Supply ever did send us that new parasitic suppressor. :mrgreen:

Art


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