U-225 R

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Andy Le Blanc
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Joined: Sat Dec 22, 2007 1:16 am
Location: central Maine

U-225 R

Post by Andy Le Blanc »

I have a univox on the bench. The client opted to convert
the rig to el34 from 6873. Easier to source and more affordable
than NOS. I have had to rebuild the bias supply. I went with an ampeg solution.
Resistor network off the HV taps on the PT.

The problem developing, cascading across the PCB as I rework
the circuit is the failure of carbon comp resistors.

I was thinking I should put the whole rig in a low temperature
oven to desiccate the pcb and comp resistors.

Really do not want to repopulate the pcb with resistors.
Don't think I've ever seen one this bad.

Ideas?
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R.G.
Posts: 1223
Joined: Tue Dec 02, 2014 9:01 pm

Re: U-225 R

Post by R.G. »

It's a question of objectives.

If it's giving you that kind of failures, with resistors failing as the work progresses, you can't ever trust any of the resistors. The whole assembly is likely to continue to slowly fail as it's used. If your objective is to make it work long enough to get it out the door, work fast and get it stable enough for the customer to come pick it up. The chances are that it will continue to have failing parts after it leaves. If the customer is a real customer, not a friend, they will likely blame you for any failures "soon" after the trip to the shop.

If your objective is to have it leave the shop in a more reliable condition, dry out the whole thing and replace the resistors.

Since resistors are less fungible than caps, I'm assuming you've already done all you can with caps.
Stevem
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Re: U-225 R

Post by Stevem »

I think you mean EL34 from 6973 ?
If so I can't see how that amps power transformer is going to have the needed reserve 1 amp of heater current to do that swap!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Andy Le Blanc
Posts: 2582
Joined: Sat Dec 22, 2007 1:16 am
Location: central Maine

Re: U-225 R

Post by Andy Le Blanc »

1 12at7
2 12ax7

@ .9A

2 6bq5/el84

@. 1.52A

Tot. 2.42 A

PT 6.3 rating 3A
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Andy Le Blanc
Posts: 2582
Joined: Sat Dec 22, 2007 1:16 am
Location: central Maine

Re: U-225 R

Post by Andy Le Blanc »

I know its close. I'd much rather have more as well.
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Stevem
Posts: 4551
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

Re: U-225 R

Post by Stevem »

You may have to think about dropping off some drive signal to the84s as the original outputs needed a good bunch more if I recall right!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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