When i first opened for visual, no supply fuse. Toasted resistor divider which goes to lower 4 A6 pnp devices. Did a quick continuity check looking for CE shorts on A6s. Found 3 with BC
Shorts. As show in pic I proceeded to pull all 4 outputs. 3 have passed. The single survivor was limping along with ~ 2 mA
Of Collector leakage. He will be retired as well.
Cause or effect? Don’t quite know yet.
Pic of backside. Rated E for everyone.
Turned my attention to amp board. I first noticed the roached resistors. Both appear to be 1k ohm . There are absolutely no markings on pcb.
A total of 18 transistors10 x MPSA42’s
5 x MPSA92’s
2 x MPS6571’s
1 x 2N4250
There're a few other pieces of silicon consisting of 6 diodes.
3 electrolytics and some film and tant capacitors.
5k bias trim pot. No trim for offset other than front end cascode diff pair.
In this version a pair of moto 42/92’s is used as pre driver. Not as beefy as i would of expected.
This unit looks untouched so i suspect I'm looking at original components. Assuming this unit was used hard and has seen Venus type temperatures ….very impressed that it made it to year 2023.
The MPS6571 cascode devices and the 2n4250 concern me. I plan on testing each device. I suspect these have some high beta properties which may be difficult to match with modern replacements(IF NEEDED).
I’ll need to inspect the supporting circuits to see what else is damaged. I did notice the 47uF/50V is now open.
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as it seems that you know your drek, I have no other remark than - please continue with porn
https://www.zenmod.in.rs/treshold-treasure-while-papa-was-there-400a/
https://www.zenmod.in.rs/treshold-treasure-while-papa-was-there-400a/
please continue with porn. Thanks for the clearer schematic and the CLASS A BIAS/Smores party instructions.
More nudes to come.
I assume you've seen this version? Would one leave the pre drive as the A42/A92? I was considering KSC3503 KSA1381 but I suspect their gain may be too low.
I've mapped out a new schematic for this version. Couldn't seem to find one. It's a mash up of a couple of the schematic versions of 400A floating around on DIYAUDIO.
Current Replacements for 2A4250. And or MPS6571?
More nudes to come.
I assume you've seen this version? Would one leave the pre drive as the A42/A92? I was considering KSC3503 KSA1381 but I suspect their gain may be too low.
I've mapped out a new schematic for this version. Couldn't seem to find one. It's a mash up of a couple of the schematic versions of 400A floating around on DIYAUDIO.
Current Replacements for 2A4250. And or MPS6571?
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output stage for this version. Other than the emitter resistor values and a few coupling caps , same. Love the collector nut attachments for the bipolars. Just don’t have enough of those nut to board attachments nowadays.
Sorry looking red dangling output connection
That was once attached to RCA jack.
Sorry looking red dangling output connection
That was once attached to RCA jack.
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Don't trust the DCA when measuring bigger transistors. They may seem to work measured with 5 mA, but at higher voltage and currents they may misbehave.
Check all outputs, and test all emitter resistors carefully. I have repaired two 4000, one -78 and one -79, totally different VAS and driver circuits.
Everything worked great except those 1k resistors, emitter resistors, cascodes and output transistors, but I did a recap anyway.
Figge
Check all outputs, and test all emitter resistors carefully. I have repaired two 4000, one -78 and one -79, totally different VAS and driver circuits.
Everything worked great except those 1k resistors, emitter resistors, cascodes and output transistors, but I did a recap anyway.
Figge
Updated output section for CFE1078 amp board. Rectifier R711XE in TO-3 package included. There is a TO-247 package with shared cathode that works as replacement if needed. The one in this unit is fine.
A DIY’er had a schematic for a version cfg379 of 400A model. Other than a few minor drawing errors it matches for the the cfe1078 board. There are some passive components that differed as well but very minor. It uses the same silicon
MPSA42/92 complimentary pair to stand off higher voltages. 2N4250, MPS6571. Along with 1N4007 and 1N914/1N4148.
(18 transistors total. 6 diodes)
You can label the transistors on the board it will help follow the location of the circuit.
A DIY’er had a schematic for a version cfg379 of 400A model. Other than a few minor drawing errors it matches for the the cfe1078 board. There are some passive components that differed as well but very minor. It uses the same silicon
MPSA42/92 complimentary pair to stand off higher voltages. 2N4250, MPS6571. Along with 1N4007 and 1N914/1N4148.
(18 transistors total. 6 diodes)
You can label the transistors on the board it will help follow the location of the circuit.
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Post rebuild board. All transistors on amp board are EBC. So if you do use alternates stay alert . I replaced all diodes 1N914/1n4148 and 1n4004/UF4005.
The diff pair MPS6571 were 920/420 hfe’s not well match over time. Other hfe’s varied from 10-170 . Pre drive stage had collector leakage. I replaced accordingly.
Bias 5k pot was replaced with Bourns pot . Several other resistors were replace as well
Next step is to rebuild output/heat sink plate. Emitter resistors will also be inspected
All three Ecaps replaced 2x 47uf and 1x 470uf
A few additional caps were replaced accordingly.
The diff pair MPS6571 were 920/420 hfe’s not well match over time. Other hfe’s varied from 10-170 . Pre drive stage had collector leakage. I replaced accordingly.
Bias 5k pot was replaced with Bourns pot . Several other resistors were replace as well
Next step is to rebuild output/heat sink plate. Emitter resistors will also be inspected
All three Ecaps replaced 2x 47uf and 1x 470uf
A few additional caps were replaced accordingly.
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Next up are the outputs. The quad upper right bipolars in pic below were shorted and now replaced with tranny’s , new micas and lube.
Rated E for everyone.
Going down the line the betas have drifted enough that i will be refreshing every last one. Beta ranges 20-180 for pnp’s and
40-130 npn’s. None of npn’s were shorted and i could of lived with new micas and lube. But i have opted not to live on the wild side.
Improved pwr disp , current and voltage ratings. Check.
More to come.
Rated E for everyone.
Going down the line the betas have drifted enough that i will be refreshing every last one. Beta ranges 20-180 for pnp’s and
40-130 npn’s. None of npn’s were shorted and i could of lived with new micas and lube. But i have opted not to live on the wild side.
Improved pwr disp , current and voltage ratings. Check.
More to come.
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As for right now, Rochester Electronics has more than an ample stock of both of these ( in the Fairchild flavor), at a per unit price of below 3, yes, THREE US CENTS per transistor!. I am going to double check this, as one part of the 9014DBU item page says it's per 1000, and the 9015DBU page says 0.Lately I've been using SS9014 and 15 in the DBU category.
Nice and cheap, but its not a really critical - mostly you just want high beta.
https://www.rocelec.com/part/01t4w00000PQLpyAAH-SS9014DBU
https://www.rocelec.com/part/01t4w00000PQLq3AAH-SS9015DBU
I am interested in this thread because.. I recently acquired a 400A, needing some TLC. I have yet to go over what schematic may or may not apply to it, but I am doing some research beforehand.
Please keep us posted on progress!
John
More info on my previous stocking post for the SS9014DBU / SS9015DBU components @ Rochester Electronics
Good News - Bad News - Good News!
I spoke with Wendy in sales, and she did verify that you can purchase these transistors in a quantity down to 1 piece if needed...
BUT.....
There's a minimum order total that must be reached ( for total of any and all items), and that value is... $250 USD
BUT......
I am also probably in need of replacing all of the output BJT's, as every post I have seen where someone has removed them
and checked gain, some were nowhere near the original matched values that were tested and installed in production.
SO.....
Rochester does have an ample stock of reasonably priced (currently $3 USD ea cheaper than Digikey) Onsemi MJ15022 & MJ15023 outputs, which, if purchased for this and a few other amp projects I will need them for, that will also be suited to using these for replacements, will put me well over that
needed price to execute my order. The MJ15022/23 combo is rated at 200V , which is higher than the originals and still has the
250 Watt max rating, so these should be well suited at a lower cost than the 250 Volt rated 24/25's ( which Rochester does not have in stock as of this post).
Good News - Bad News - Good News!
I spoke with Wendy in sales, and she did verify that you can purchase these transistors in a quantity down to 1 piece if needed...
BUT.....
There's a minimum order total that must be reached ( for total of any and all items), and that value is... $250 USD
BUT......
I am also probably in need of replacing all of the output BJT's, as every post I have seen where someone has removed them
and checked gain, some were nowhere near the original matched values that were tested and installed in production.
SO.....
Rochester does have an ample stock of reasonably priced (currently $3 USD ea cheaper than Digikey) Onsemi MJ15022 & MJ15023 outputs, which, if purchased for this and a few other amp projects I will need them for, that will also be suited to using these for replacements, will put me well over that
needed price to execute my order. The MJ15022/23 combo is rated at 200V , which is higher than the originals and still has the
250 Watt max rating, so these should be well suited at a lower cost than the 250 Volt rated 24/25's ( which Rochester does not have in stock as of this post).
Good luck with the Rochester purchase, last time I contacted them, they had the $250 min order value as well as a minimum line value for each item.
I can confirm the MJ15022/23 are perfect after just helping with a Stasis 2 overhaul, yes buy more than needed to get to get matching sets, as per Nelson's advice. Jon Soderberg advised output matching to 15% at worst case. We were able to get 5% matched.
I can confirm the MJ15022/23 are perfect after just helping with a Stasis 2 overhaul, yes buy more than needed to get to get matching sets, as per Nelson's advice. Jon Soderberg advised output matching to 15% at worst case. We were able to get 5% matched.