Well the odd part numbers have about 20% hfe of the rest. It's just as I suspected.
What's the recommended replacement? MJ15003?
What's the recommended replacement? MJ15003?
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All that's left to do is replace the -12dB LED which goes out after a short warmup. I measure 1.7 across the other lit LEDs, but 26V across the one that's out, so open LED it looks to be.
Not so much that the cascodes need matching than that they need a decent gain figure.
Those replacements were pretty pitiful at 20%
Those replacements were pretty pitiful at 20%
I suppose the low hfe devices were dragging down their neighbors. 3 of 4 devices had good gain. Just one device in each bank was hampering the ability to sink current into the load.
Nelson, I am the original owner of one of your first Threshold amps, the 400A. It has been working without a problem for all of this time. Now, in my later years, I am planning to build some of your later amplifiers. However, what would you recommend that I do to my "Old Faithful" 400As to bring them up to date, or to improve them?
Thank you, Mr. Pass. The 400A I mentioned above, had been kept in storage for most of the last 20 years. How can I determine if electrolytic capacitors need to be replaced?
Your amp is the best part of 45 to 50 years old now, so replacing all electro caps , both in the PSU and on the FE boards is very advisable.
Thanks for your reply, Gary. Do you have recommendations for quality caps? Should I go with electrolytic or a different type?
The large can types in the PSU must be electrolytic. You will need to measure the diameter and height to see what is available to suit the existing caps. The rule of thumb is that the DCV rating must be at least as high or higher than the existing caps and the capacity can always be increased to suit the can size available today. Based on three previous Stasis rebuilds I have been involved in, you should be able to find the PSU caps in same diameter and voltage ratings with increased capacitance of around 20% over the originals in a height that might be less than the originals. If you can advise what is the capacitance and voltage rating along with the can diameter and height of the existing PSU caps , I can advise further.
The electro caps on the FE board can simply be replaced with same voltage rating and capacitance. If there are any tantalum caps on the FE board - replace these with equivalent rated electro caps as well. Any film caps on the FE board should be OK.
I would also be replacing the emitter resistors with the correct resistance and power rating, especially if you have a mix now of different types in the OS.
The electro caps on the FE board can simply be replaced with same voltage rating and capacitance. If there are any tantalum caps on the FE board - replace these with equivalent rated electro caps as well. Any film caps on the FE board should be OK.
I would also be replacing the emitter resistors with the correct resistance and power rating, especially if you have a mix now of different types in the OS.
Also looking at the photo's you show of the OS, the output transistors have all been replaced by inferior NTE types and show 2 different NPN types ( SK3621 and SK 9237) and the PNP SK3846. This is not good.
When you look at the schematic Nelson has shown of the 400A, the pos side transistors are A8 NPN types and the neg side are A6 PNP types. For all the rebuilds I have been involved in, these older devices are replaced by the newer MJ15022G for the A8 (NPN) and the MJ15023G for the A6 (PNP). This is based on recommendations in the past by Nelson and Jon Soderberg who still does repairs and rebuilds of the Threshold Stasis amps ( he was the senior tech at Threshold back in those days. Best and cheapest source in the US for these is Mouser.
Also Jon advises that the OS transistors should be matched for Hfe at around 20% max tolerance. I have found that transistors out of the same manuf batch are usually within that tolerance.
If you are going to rebuild this amp, then I would be putting in good quality output transistors, along with new emitter resistors of the same type and replacing all electro caps. I have also recommended replacement of the PSU diode bridge(s) in the past and this is not a large cost compared to PSU caps and all the output transistors for both channels.
Believe me, if you do all of that you will have one hell of an amp to own. Of course the bias will have to be adjusted and output offset DCV will need to be checked. We can run through that when the time comes.
When you look at the schematic Nelson has shown of the 400A, the pos side transistors are A8 NPN types and the neg side are A6 PNP types. For all the rebuilds I have been involved in, these older devices are replaced by the newer MJ15022G for the A8 (NPN) and the MJ15023G for the A6 (PNP). This is based on recommendations in the past by Nelson and Jon Soderberg who still does repairs and rebuilds of the Threshold Stasis amps ( he was the senior tech at Threshold back in those days. Best and cheapest source in the US for these is Mouser.
Also Jon advises that the OS transistors should be matched for Hfe at around 20% max tolerance. I have found that transistors out of the same manuf batch are usually within that tolerance.
If you are going to rebuild this amp, then I would be putting in good quality output transistors, along with new emitter resistors of the same type and replacing all electro caps. I have also recommended replacement of the PSU diode bridge(s) in the past and this is not a large cost compared to PSU caps and all the output transistors for both channels.
Believe me, if you do all of that you will have one hell of an amp to own. Of course the bias will have to be adjusted and output offset DCV will need to be checked. We can run through that when the time comes.
Gary, thanks again for all of your suggestions. I spoke with Jon about the rebuild / upgrade. He suggested some of the same mods as you have. If I first gained some circuit-building experience by building an amp(s) as a F5M, Aleph JZM or ACA MinMax, would that be sufficient experience so that I could tackle most of the upgrade work on my Threshold 400A? If not, what would you suggest that would be good projects to learn from, including transistor, capacitor soldering, power supply building, etc.?
Paul
Paul
project
I agree with Zen Mod,the ACA is a simple start and then maybe an F5M.
You do want some experience to tackle the stasis rebuild.
While talking to Jon, did you ask him if he was interested in doing the 400A rebuild for you?
You do want some experience to tackle the stasis rebuild.
While talking to Jon, did you ask him if he was interested in doing the 400A rebuild for you?
You're speaking of the ACA MinMax? You're saying that is a much better amp than the F5M and Aleph JZM, or are you saying that it would be a much better amp to build because it requires much less skill to build?
Thank you Gary and Zen Mod. Regarding test equipment and other stuff needed to bias circuits, etc, what are your thoughts for equipment that wouldn't break the bank?