BA-F1 restore

Here is the Left driver board - almost complete, I have 2 resistors to change and 2 capacitors to fit - which need to be ordered. All the Black Flags are gone, there are new trimmers and zener diodes, new electrolytics, some of these changed for 'films', some resistors have been replaced.
IMG_2299a.jpg

I will be dealing with the Right driver board next.
 
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I thought that the generally accepted standard was what you have done: Replace what you find on the board ("factory") unless you observe evidence of 'recent' replacement (after factory). This standard probably has an amendment to let you consider wisdom recorded here by AKers.

BTW: alt-159 = ƒ for Farad, but I don't see a shortcut for lower case Rho in this character set. Alt-234 = Ω.

Good choice, given the variables.
 
I have a question regarding C17 & C21 on the driver PCB - should they be 1000pF (Polystyrene) as per the schematic (and parts list) or 1500pF - a clumsy replacement (on the track side) by the previous owner of this BA-F1 ?

I am rounding up the very last additions to a parts order, so confirmation of this would be really helpful. ;)

Edit Update
I'm leaning seriously towards 1000pF on this one.

John if I had a BA-F1 I would crack it open and let you know what I have. I guess it probably wont make much difference going between 1 and 1.5 nF but still would be nice to get confirmation. I will let you know what I have when I acquire a BA-F1 myself, but I think going with 1nF as per manual is what I would do if I felt the 1.5nF was not factory installed.
 
but I think going with 1nF as per manual is what I would do if I felt the 1.5nF was not factory installed.

Thanks for that ;) I have gone with 1000pF, I notice exactly the same combination (1000pF + 390Ω) used in the AU-919 - that nailed it for me. :thumbsup:
 
Here is the Right driver board, more or less as found, - replacement transistors ringed in green, not ideal in my opinion - MJE340/350 pairs as replacements for 2SC2071/2SA939 pairs, and a mish-mash of resistors ringed in red. This is the channel that has non-original OP transistors as it has obviously suffered a major failure and has been repaired.

IMG_2301a-markup2.jpg

And a picture of the underside showing the added 1500pF (wrong value) capacitors, the original 1000pF capacitors had been removed.
IMG_2302.JPG
 
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This is one area where I don't dare play in the sandbox (capacitance/resistor networks) and the effect on phase shift. 1000pf would be my choice as well. Kale and a few others have scopes accurate enough to pull minute ring oscillation traits off a signal however my analog tectonics probably would not. even with its 200mhz bandwidth. Heck, probably so much stray capacitance off my old interconnect cables would be spending more time chasing my tail than the ring overshoot :)
-Lee
 
I have used KSA1381 and KSC3503 as subs for the 2SA939 and 2SC2071 as they appear to be a better match than the MJE 350 &340. The Ft are a little higher at 150& 150MHz vs 150 & 100 MHz, but I think that the MJEs are much slower.
 
I have used KSA1381 and KSC3503 as subs for the 2SA939 and 2SC2071 as they appear to be a better match than the MJE 350 &340. The Ft are a little higher at 150& 150MHz vs 150 & 100 MHz, but I think that the MJEs are much slower.

Thanks Steve, yes I am thinking about this, I have some others in mind, I just need to check their suitability.

I got the right driver board stripped of old components, pins cleaned, glue removed, and washed clean of dust. I discovered more burn marks under components including the 'famous' 390Ω resistors (ref: AU-X1), I will mount replacements up on ceramic beads, and revisit the other driver board to do the same.

I found some gold plated pins, and ring terminals, I also have TO-3P insulators, and have screw hole insulators - so I can mount the new OP transistors very soon.

I ran out of flux cleaner - so another order placed. :)
 
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John, I have 2 BA-F1's and matching CA-F1 here that I have yet to have time to rebuild. If you need me to look at anything in them for reference just let me know.

Thank you very much ;) - I will bear that in mind - very kind of you :thumbsup:
 
I have used KSA1381 and KSC3503 as subs for the 2SA939 and 2SC2071 as they appear to be a better match than the MJE 350 &340. The Ft are a little higher at 150& 150MHz vs 150 & 100 MHz, but I think that the MJEs are much slower.

I am going with your suggestion - an excellent match. :thumbsup:
 
Yes John, the KSA1381 and the KSC3503 are excellent replacements for the pre-drivers and the current source. I have successfully used these in this amp with no problems.
 
The PSU has gone from this....

IMG_2290a.jpg

To this, it was in a bit of a state - and I still have more cleaning to do as you can see.
IMG_2303a.jpg

The rebuilt driver board tests were completed successfully. I hope to further improve on bias stability by implementing Kale's innovative bias modification. All of this will be carefully checked again when I get the new OP transistors installed.
 
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And here is the cleaned up and re-installed PSU

IMG_2305a.jpg

I doubled the capacitance of the filter electrolytics nearest the switch shafts, and replaced the bridge rectifiers. Everything else remained as per the values that came out, although I did 'bump' the voltages of some of the capacitors.

As you can see there is much more 'space' around the components now, which should allow better ventilation. (compare with pictures in the post above).
 
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And only 2.6pF of Cob is excellent!

That is excellent ! As comparison the pre-drivers I swapped to in the 999, ksa1220/ksc2690 are 26/19 pf ! - they are rated at +1A and that's the problem - high Ic = capacitance. I am afraid to use the ksa1381/ksc3503 since they are only rated at 100mA with originals at 500/200.

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John - Your restorations are always so well considered and thoughtful, striking a balance of staying true the original design while only carefully and deliberately up-rating specs when called for :thumbsup:
 
Just an FYI, you might want to consider replacing the power switch. I have seen these go bad a couple times in these amps.
 
John - Your restorations are always so well considered and thoughtful, striking a balance of staying true the original design while only carefully and deliberately up-rating specs when called for :thumbsup:

Wow - that is a compliment and a half - thank you. :thumbsup:
 
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