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Post by wildcat444 on Oct 11, 2020 9:51:09 GMT -5
[And there's the problem. Everyone who used to make "just one" part (or two) available for you to walk in and buy has gone extinct. ...And yes, the board is our custom design. Thank you 2600, I am restoring my own radio including parts on the BA Board. The challenge sourcing parts that you tell was the reason for my original question. What I would like to know is if the LM340T15 regulator that you use on your custom BA board would also a suitable replacement part for the stock BA board since the LM340T15 is available at Mouser where I am ordering? Mike's Radio Repair uses the NTE152, the original part was a 2n5295, but neither are available at Mouser and I am overwhelmed by all the parameters to make a proper selection for Q1 on my own, despite having some experience in electronics. Thank you sir!
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mattbee
Mudduck
Tram & Browning Enthusiast :)
Posts: 38
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Post by mattbee on Oct 12, 2020 6:31:42 GMT -5
Is there any particular reason why you choose to replace the regulator? Replacing ALL the higher watt resistors, and electrolytic capacitors on the BA board would resolve 99.9% of the possible failing problems
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Post by 2600 on Oct 13, 2020 0:22:48 GMT -5
The specs for Q1 are not that critical. It only serves as an "emitter follower" circuit. The voltage at the emitter lead will always be about 2/3 of a Volt less than the zener voltage applied to the base lead. This is also called an "open loop" voltage regulator circuit, but it works well enough.
Original part is rated for 40 Volts, 4 Amps and 36 Watts. The gain is not important for this circuit. The heat sink is too small for more than a Watt or so. Decent substitutes would be TIP3055, MJE3055T, TIP41A, B or C. It's a brute-force circuit, so small differences in the specs have no impact on how it works.
Just why are you replacing it? I have seen only the smallest handful of them fail out of literally many hundred specimens over the decades.
Probably the single most-reliable part on the BA board. I do have one suggestion. Never use a fuse rated more than 1/8 of an Amp rating on that board. And use a capacitor rated for 450 Volts for C625. A glance at the schemo shows that the fuse will take the resistors out of line, removing the ground circuit from the big audio tube. But C625 is on the wrong side of the fuse. If the tube shorts, and puts hundreds of Volts onto C625 the original 150-Volt rating will cause it to break down and short. Naturally if it does this it bypasses that fuse and your modulation transformer could be the next casualty. Using a 450-Volt part insures that if the fuse blows, the audio tube will shut down no matter what.
Oh, and we use the 7815 Voltage regulator because it just works better, and has a built-in current limit. Customers who want to 'borrow' 14-Volt power to run a frequency counter need more current than the stock heat sink allows, so we make that bigger as well. The cost difference is small enough to be worth it.
73
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Post by wildcat444 on Oct 13, 2020 12:14:40 GMT -5
The specs for Q1 are not that critical...Original part is rated for 40 Volts, 4 Amps and 36 Watts. The gain is not important...The heat sink is too small...Decent substitutes would be TIP3055, MJE3055T, TIP41A, B or C...Just why are you replacing it?...Never use a fuse rated more than 1/8 of an Amp rating on that board...use a capacitor rated for 450 Volts for C625. 2600, this is fantastic information... thank you so much!! You and Mattbee asked why I am replacing Q1. I have no way of knowing how much heat stress these semiconductors have endured through the years, so in addition to replacing caps and resistors, I figured I would also replace the zener and regulator since they are cheap at $0.26 and $0.80 respectively. The top of the Mylar cap is no longer yellow, it's brown so I'm replacing that as well. In addition, I'm looking at Mouser for the Molex connectors too since they are browning. Might as well replace both male and female ends while I'm at it for well under a buck each. The advice for C625 is a good one. I am replacing all caps with 450v where possible. The tip on the BA fuse is also a very good one which I will follow! Right now, I'm going through finding all high wattage resistors and electrolytics at Mouser, and trying to figure out any other parts I need to order so I can get as much of this in one order as possible. I already have to put in another order through CE Dist., so I'm trying to keep my number of orders in reason. /Derrick
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Post by pattyboy on Apr 22, 2021 9:08:59 GMT -5
I went ahead and replaced every item on the "BA" board in my D201A with the same values that came out. I am lucky, I have a electronics surplus store about 10 miles away. I used an NTE152 regulator as that is what came out. I also used larger wattage, 1% and 5% resistors as well. Now I'm in process of relocating the board up front by the speaker. That should relieve some of the heat issues.
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Post by pattyboy on May 14, 2021 10:28:34 GMT -5
I'm in the middle of a BA board rebuild as well. However, I decided to relocate the board to get it away from the tube heat. I plan to mount it upside down up front by the speaker. I'll make a jumper harness and plug it in like that. The capacitor voltage value is great info. I'll change it out today. Thanks
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