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Old 9th April 2005, 02:59 AM   #1
sulky is offline sulky  United States
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Default Threshold 400a Parts Substitution & Mods.

I am in the process of repairing a Threshold 400a Amp that blew a channel. I have been able to source replacements for most parts but am having a bit of difficulty with crossing over a couple parts to currently manufactured pieces. When examining the schematic:

http://www.passlabs.com/np/400a-4000.tif

I have found replacements for the A6 & A8 with MJ15025/4 units, the MPSA42/92 and most everything else are readily available.

However, I am having difficulty with the two transistors marked as FT317 & FT417 located near the center of the schematic. On the actual physical board these are RCA-1A15 & RCA-1A16 transistors. Can anyone suggest a good substitution for these that is currently produced, any onsemi cross-over part? I'm pretty sure someone has dealt with this substitution with the other pass designs...

Also, I see that the schematic portion for the "4000" indicates a .02/100v cap across the output transistors, located on the upper & lower right of the schematic. The 400a does not have this cap. So, I am wondering if this cap is there to add stability to the output circuit? Also,would the addition of this cap to the 400a circuit be a good idea?

One other curious observation is that the schematic calls for 0.68 resistors on the output transistors whereas the actual board has 0.33 resistors instead. I'm debating whether or not to switch over to the 0.68 or stay with the 0.33 since I may need to replace a few anyway.

Thanks for any help or advise.
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Old 9th April 2005, 08:16 AM   #2
Netlist is offline Netlist  Belgium
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Hi Sulky

Here's some data for the RCA transistors:

/Hugo
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Old 9th April 2005, 03:38 PM   #3
sulky is offline sulky  United States
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Netlist / Hugo:

Thanks for the data sheet, that should make things easier...

After checking around I am thinking that the onsemi D44H11 / D45H11 would be good substitutions. They are a different case style type than the original but I think they may be a good selection.

http://www.onsemi.com/pub/Collateral/D44H-D.PDF

Anyone see anything I might be missing or have any other good potential substitutions? Feel free to make sugggestions...

Later
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Old 9th April 2005, 04:26 PM   #4
gl is offline gl  United States
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sulky,

I built this circuit (the 4000 version) 25 years ago and substituted the onsemi MJE15030 and MJE15031. As I recall the FT317 and FT417 were Fairchild parts and were unavailable even then.

Regards,
GL
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Old 9th April 2005, 05:08 PM   #5
mvollb is offline mvollb  Germany
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I am in the process of repairing a Threshold 400a Amp that blew a channel

Hi,

I did the same a couple of weeks ago - i am just refering the schematics of the newer (bipolar) models and what types of transistors are used there. I take the japanese twin used in the stasis type amps. Take care about "beta matching" - check stasis 1 schematic at the usual place.

I replaced the input diff-pair with a MAT02, sounds different, not able to say better or not, but the price for one MAT02 compared to one MPS6571 is lower...in germany. I will use the saving for my new aleph project...

cheers
Michael
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Old 10th April 2005, 04:27 AM   #6
sulky is offline sulky  United States
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GL :

Thanks, I have had suggestions of the MJE15030 and MJE15031 from others as well. I had been considering the onsemi D44H11 / D45H11 but will be looking into your suggested components to see which would be the better choice.

Michael :

Thanks, I did a quick check of those MAT02 and they seem to be a nice dual-matched transistor. Did they make your amp sound warmer or more bright? Off hand I am not seeing any 6571's on the physical board. This 400a is an early unit and does appear to have a few small differences from potentially later units. Amazing how much components can change over the years... Don't get me started on the other amp projects. As I work on this one I keep eyeing all the other great designs that I need to toy with next. Good luck with your aleph.


--------------------

Interesting note, I was just checking over the physical components on the pre board. I quickly realized that the transistors at the bias are not A42/92 units per the schematic. On this board they used a single, six pin dual differential matched 2N2920 transistor, instead of two seperate matched A42's. The numbers and coding on the dual differential matched transistor for the A92's is not clear so I'm still deciphering that one.

http://www.semicoa.com/pdf/transistor/2n2920.pdf

Since this is one of the eairlier 400a amps they may have decided this dual-trans was to costly and changed to seperates on others later on during production. An interesting point none the less. This would definately help to easily keep close tolerances during production. I'm not sure if I should go with the same 2N2920 or replace with A42? A42 is cheaper but 2N2920 is nicely matched. I'll have to see if there is a better upgrade dual differential matched transistor available for these components. Really a nice clean design in this amp.

Later...
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Old 7th June 2006, 09:30 AM   #7
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Default Threshold adjustment procedure


I got some papers with my newly purchased classic 400A, and here are the "goodies" scanned. One page at a time..

Rolv-Karsten
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Old 7th June 2006, 09:31 AM   #8
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Default page 2

Page 2 of the adjustments
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Old 7th June 2006, 09:32 AM   #9
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Default psge 3

Page 2 of the adjustments
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Old 7th June 2006, 09:34 AM   #10
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Default page 4

Page 4 of the adjustments.
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