Re: Re: Threshold 400A Repair
Nelson Pass said:
I don't know how you are getting your hfe figures, but if it's
with a portable in-circuit meter you should probably forget it.
The 4250's beta is usually around 300-500, and you want outputs
in the area of 100 at 1 amp, but it's not a big deal. In production
they were selected for higher than the 50 you're measuring,
and I would be surprised if they're that low. The devices were
2n5878 and 2n5876, and similar 4 mhz to3-s will work pretty
well.
In the version you show versus the schematics I have posted
the difference is mostly the a92's and a42's used in parallel
instead of the heftier 5415's and 3440's. Yours is the earlier
version. I recall that yours does not have the cascode input
diff pair as shown on the later schematic. The cascoding itself
was not a big deal, but it did allow the use of MPS6571's which
have very high gain (~500), but can't take the voltage.
Nowadays, you might be able to find a high gain 50V device to
use instead of the a42's.
First of all, Thank You Mr. Pass for your response!
My gain checker is a Sencore TF46 Super Cricket Transistor/FET Tester. I have been doing out-of-circuit checks to get the gain figures I posted. I don't have a REAL curve tracer/hfe test rig, so I can't get actual beta readings under the operating conditions you mentioned.
I neglected to include the RCA1A15's and 1A16's (one each per input board) in my initial post, which tested OK with gains like the 2n3440 and 2n5415 pair.
I went through both input boards and replaced the MPS-L01/51's with matched MPS-L01/51's of similar gains.
The only difference between the Left and Right channel assemblies now is R48, 49, 50 and 51 (R48 in Parallel with R50, R49 in Parallel with R51). Left channel has 3-ohm, 1-watt Carbon Comp resistors, Right channel has 1-ohm, 1-watt Metal Oxides.
Which is best?
I was able to download a schematic and components list that Sulky had posted some time ago that pretty closely matches what I have (thanks Sulky!!), so I'm good on that front.
I only have a few questions left now:
Biasing -
1. I know you mentioned the 5-second touch rule for setting the proper bias, but is there a measurement I can make to set it a bit more accurately?
2. Should I set the pot to mid-range and go from there?
Power Supply -
1. What voltage/current ratings should I use if I replace the Bridge Rectifier with Fast Recovery Diodes?
2. I want to replace the 7800uF 75VDC Caps with 35,000uF 80VDC caps of the same physical size. If I add an inrush current limiting device in line with the mains fuse, could it cause problems?
Input Coupling Capacitor -
1. Current cap is 4.7 uF 25V Tantalum. Is there a better choice?
Thanks again for your help!
Terry Keith
Columbus, OHIO