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Unread 05-22-2011, 04:27 PM   #1 (permalink)
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jvm 410h blowing capacitors

so the head arrives and i pop it open,want to fix it quick im all jacked!
upon removing this beast i find a blown resistor r83 (82k) and cap near it c53 (10u 160v) is bulging.
also fs6 is blown

so im guessing power tube problem and hurry up and replace tubes and above mentioned parts.

fire it up and POW! c53 blows the stuffing out of its top.
so now i look a little closer!
and maybe d16 d17 d20 d21 look like they have been resolderd or got real hot

im no expert in amp repair but know enough to drain caps and stay alive and what not!

this is later model head without the problematic fuses

bought this head used knowing that it had problems( i like a challenge)

what does anyone think is going on here?

havnt had time to do any other diagnosis yet
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Unread 05-22-2011, 04:43 PM   #2 (permalink)
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Re: jvm 410h blowing capacitors

That would be the heater elevation circuit. If R83 blows, then C53 will most certainly blow because the loading provided by R83 goes away, which places the full HT voltage across C53.

When you replaced C53, did you also replace R83 with one of the same value and power rating?
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Unread 05-22-2011, 08:37 PM   #3 (permalink)
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Re: jvm 410h blowing capacitors

yes i did replace r83 with same value.
original resistor was 1% all i had was a 2% but same resistance value (82k).
could that be the problem?
does it really need to be that tight? 1%

also it was a 1/4 watt 82k that was in the amp does that look correct to you?
or should that be 1/2 watt or even larger?

could it be that i didnt get a good solder connection to the board on r83?
would it cause the cap to blow? or would voltage dead end at cold joint?

amp did have two bad power tubes and two out of 5 ecc83 where also
bad.

i ran it with only two power tubes but did have all pre tubes in place
when it blew c53 but did not hurt r83

when i first opened it up i noticed someone had replaced c53 before
and r83 was toasty looking so i replaced both.

thanks for your input ,im learning much here!
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Unread 05-22-2011, 10:01 PM   #4 (permalink)
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Re: jvm 410h blowing capacitors

Quote:
Originally Posted by bilbobagins View Post
yes i did replace r83 with same value.
original resistor was 1% all i had was a 2% but same resistance value (82k).
could that be the problem?
does it really need to be that tight? 1%
No it doesn't have to be that tight.

Quote:
Originally Posted by bilbobagins View Post
also it was a 1/4 watt 82k that was in the amp does that look correct to you?
or should that be 1/2 watt or even larger?
Although that resistor only dissipates about 100mW, I would honestly go with a 1-2 watter there but that's just me.

Quote:
Originally Posted by bilbobagins View Post
could it be that i didnt get a good solder connection to the board on r83?
would it cause the cap to blow? or would voltage dead end at cold joint?
Absolutely...if the resistor didn't have good contact. That resistor is what pulls the voltage at that node down to the 80-90V elevation voltage. Under that voltage the cap is fine. But once that resistor either blows or loses contact, the voltage on the cap gets pulled right up to the B+ rail voltage via R81 and...kaboom!

Quote:
Originally Posted by bilbobagins View Post
amp did have two bad power tubes and two out of 5 ecc83 where also
bad.
Since that resistor opened up, which allowed the elevation voltage to be pulled right up to the B+ rail voltage, I would replace every valve in the amp.

Now...before you put the valves and the replacement cap in there, I would replace R83 (or resolder the joints), then check the voltage relative to ground at the junction of R83 and R81 (basically where the + terminal of C53 will be connecting to) and make sure it is no higher than 90V. If it's higher than that then you still have an issue.

Did you check the integrity of the PCB tracings that connect to that resistor? If the trace that takes the grounded side of R83 to ground is burned open, this could very well be your issue.
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Unread 05-22-2011, 11:19 PM   #5 (permalink)
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Re: jvm 410h blowing capacitors

fantastic man thanks for the hints!

i started to lean towards the idea that i had a bad joint on the ground side of r83 also.

gotta inspect a little closer but i bet we are on track now
maybe tommorow night i can get a little free fun time to check it out.

let you know when i know more
thanks again
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