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SIMPLIFIED - Wurlitzer Upgrading Model to 3700/1050/7500 amplifiers to more reliable/stable 3800 series amplifier.
Last updated
March 5, 2024
Upgrading Wurlitzer (American) amplifier from 556 (3700) to 556 Rev. 3 (late 3800) This was a factory recommended improvement (originally two steps), amp runs cooler and is more stable! The changes follow two procedures, depending on your starting point (check for missing/changed parts).
Click here for a scan of the original Factory document...
For the Wurlitzer 3700/1050/7500 model 556 amplifier and perhaps some early 3800s:
Upgrade your amplifier's two sub-PCBs from the original 556 (3700) to (3800-late) Revision 3 amp:
Parts needed for two amplifier sub-boards:
4 X 0R5 (0.5 Ohms) @ 5W, 4 X 3K6 @ 1/2W, 2 X 1K @ 1/2W, 2 X 3K @ 1/2W, two chassis mount fuse holders along with two 1.25A slo-blo fuses (if not present), 2 X 47/50V cap.
Instructions:
Note: I follow the new convention that for resistors the "R" can be used as the decimal. So 0R5 is Zero point Five Ohms (0.5 Ohms), and 3K6R is Three Thousand Six Hundred Ohms (3,600 Ohms).
1) R17 & R18 changed to 0R5 at 5 watts if the original is 0R47 2W tubular style resistor.
2) R23 & R24 changed from 470R to 3K6R.
3) R12 changed from 2KR to 1KR.
4) R8 changed from 2K7R to 3KR.
5) Add fuses to green wire to each board's output (edge connector pin 4) - 1.25A (1-1/4A) slo-blo.
6) Zener diodes Z2 & Z3 removed.
7) Remove R25 (1K8R) & R22 (1K8R).
8) R21 & R26 (4K7R) removed.
9) Verify that Positive lead of C1 is connected to base of Q1 (reversed from original placement).
10) Change C5 from to 47ufd/50V and that the positive lead is connected to ground/common.
11-14) these steps are not needed, the original transistors are used
15) Change the lable on this board from 140617 to 142315 Rev.3.
For the Wurlitzer 3800 amplifier insure the following steps were done.
Check the amp sub-PCBs model 556 Amp have been converted from "SI" (early 3800s) to 3800 Revision 3 amp - the S1 revision can be identified by checking if Z2, Z3, R21 & R26 are missing.
Parts that may be needed for two amplifier sub-boards:
2 X 1K @ 1/2W, 2 X 3K @ 1/2W, 2 X 47/50V cap.
Instructions:
1) Zener diodes Z2 & Z3 removed.
2) Remove R25 (1K8R) & R22 (1K8R).
3) R12 changed from 2KR to 1KR.
4) R8 changed from 2KR to 3KR.
(5-7) These steps are not needed, the original transistors are used)
8) Verify that Positive lead of C1 is connected to base of Q1 (reversed from original placement).
9) Change C5 from to 47ufd/50V and that the positive lead is connected to ground.
10) Label board 142315 Rev. 3.
Click here for the 556-R3 amplifier schematic.
That completes the changes! If you need clarification please send me a note! John :-#)#
(jrr-at-flippers-dot-com for those who do not have their browser set to open their email client) |
