Because passive component values tend to drift with age, replacing the capacitors and resistors in some HAMMOND AO-28 preamplifiers may be of some benefit.

We will assume that:

  •  you have a schematic diagram for your AO-28,
  •  you know how to read it, and
  •  you know how to solder.
So where can you find the needed components? The following piece by Scott Hawthorn makes recommendations for brands/types of capacitors and resistors you will need for the project, and where you can buy them!

 

Rebuilding a HAMMOND AO-28 Preamp
                                                                            by Scott Hawthorn

 

Recommended Suppliers:

  • Mauser Electronics: 1-800-346-6873
  • Antique Electronic Supply: 1-602-820-5411
Both companies will send you a catalogue.

Capacitors:
In answer to questions regarding caps, there are several different kinds of "orange drops." I used type 715P polypropylene film for .01 mF and above.

For the smaller value caps, I used dipped mica; Mauser El. 5982 series. I think they are rated at 500V. I used these for everything in the pF range, including the .0022 (.0022=2200 pF, .0047=4700 pF, and so on). You will need to be able to translate mF to pF in your mind when sorting out the discrepancies between the Parts List and schematic. I found a couple of errors in the list in my C3 manual, but your schematic should be correct.

For the smaller filter caps, I bought electrolytics from Mauser; their 140-HTRL Xicon series. For the power caps, I used their 140-XAL series.

Resistors:
I hope this starts a discussion! For the 1/2 watt, I chose all carbon composition (not carbon film) for less inductance. I was able to scrape these up between Mauser and AES.

I purchased the power supply resistors from Mauser:

  • 10W: 588 VL-10 series
  • 1W: NE 261 series (there might be a new substitution for these)
  • Pot for determining value for R40: Mauser 31VA605 (5 M)

 

PARTS LIST
  1. C1   470 pF
  2. C2   39 pF
  3. C3   100 pF
  4. C4   .33 mF
  5. C5   .01 mF
  6. C6   30 pF
  7. C7   24 pF
  8. C8   100 pF
  9. C9   33/25 electrolytic (from can)
  10. C10   .33 mF
  11. C11   .0047 (4.7K pF)
  12. C12   .0047 (4.7K pF)
  13. C13   1.0 100V (forgot where I got this one but it's 150v)
  14. C14   doesn't exist
  15. C15   51 pF
  16. C16   .033 mF [N.B.: A100 uses .02 here for bass roll-off]
  17. C17   680 pF (phono input, in expression control box)
  18. C18   220 pF
  19. C19   .0022 mF (2.2K pF)
  20. C20   220 pF
  21. C21   .001 mF (1K pF)
  22. C22   (trimmer)
  23. C23   .033 mF (A100=.02)
  24. C24   82 pF
  25. C25   .001 mF (1K pF)
  26. C26   33/25 electrolytic (in small can)
  27. C56   40/450 electrolytic (larger can)
  28. C57   40/400 electrolytic (larger can)
  29. C58   30/350 electrolytic (larger can)
  30. C59   10/350 electrolytic (small can)
  31. C60   47/350 electrolytic (small can)
BTW, This list does not include the percussion section, which might not need rebuilding...
  1. R1   3.9 M
  2. R2   1 M
  3. R3   .27 M
  4. R4   1.8 M
  5. R5   10 M
  6. R6   47K
  7. R7   2.2 M
  8. R8   .47 M (470K)
  9. R9   .82 M
  10. R10   3.9 M
  11. R11   doesn't exist
  12. R12   .27 M
  13. R13   4.7 M
  14. R14   10 M
  15. R15   2.7 M
  16. R16   47 K
  17. R17   1.2 K
  18. R18   10 M
  19. R19   .47 M
  20. R20   2.2 M
  21. R21   1 M
  22. R22   .82 K
  23. R23   22 K
  24. R24   22 K
  25. R25   100 ohms
  26. R26   anyone know where this is??
  27. R27   1.8 M
  28. R28   .27 M
  29. R29   .33 M
  30. R30   1.8 M
  31. R31   68 K
  32. R32   1.8 M
  33. R33   68 K
  34. R34   15 M (I had to make this:  8.2M + 6.8M in series)
  35. R35   3.3 M
  36. R36   .33 M
  37. R37   .27 M
  38. R38   390 K
  39. R40   T.B.D.
  40. R41   100 K
  41. R42   1.2 K
ALL OF ABOVE=1/2W

POWER SUPPLY RESISTORS:

  1. R91   900 ohms 10W
  2. R94   4700 ohms 1W
  3. R95   4700 ohms 1W
  4. R96   4700 ohms 1/2W
  5. R97   4500 ohms 10W
  6. R98   7500 ohms 10W
  7. R99   180 ohms 1W
  8. R100   2700 ohms 1W
  9. R101   2700 ohms 1W

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