Hi everyone,
I’m building the AA Electronics 18W TMB amplifier and have encountered a serious voltage drop at EZ81 pin 3:
Standby OFF: approximately 290V DC
Standby ON: approximately 12V DC
After checking the wiring against the supplied layout, I noticed two possible issues:
1. Standby and EL84 cathode connections
I'm totally pi**** off!
Have you ever bought a circuit board anywhere where neither the component labels (R1, R2, etc.) nor their values were printed on it?
Nothing! Absolutely nothing...
That's what I got after placing my order!
Check out the Black-Eye hot-rod board.
Dear Jochen,
The black eye board has been developed in another software (Eagle) a long time ago. The original version hasn’t been modified/ported to the new software, hence there are no numbers. You will see that all the other PCBs have them, as they are relatively new products. You can still use the layout as a reference.
I understand the frustration, but it’s the first I got this complaint on more than 200 boards sold.
I have built two of these before one with this transformer which I can't remember were it hook up and one with out. I assume I had a tap from my power transformer for this. I just don't see the separate transformer in the layout for the 102. I must be getting old and forgetful. I little help would be appreciated. Bill
The AA Dumble board has a Delon-type voltage doubler that feeds the 7812 voltage regulator from the 6.3 VAC secondary winding. Allows you to make 12 VDC from the filament supply so you don't have to install a separate 12 VAC transformer.
Is anyone else frustrated with the BETA version material list for fox board?
Subject: 18W TMB build solved – possible corrections for layout rev 1.0.0
I have solved it, my 18W TMB build is now working and sounds great. Thank you for designing this board.
While troubleshooting I found a few points where the wiring layout (rev 1.0.0, 05/05/23) does not match the schematic (rev 2.0.0, 28/09/24). I'm sharing them so other builders don't run into the same problems.
1. Standby switch and EL84 cathode wires swapped on the PCB
In the layout, the wire from the standby switch S1 goes to the pad above C21 (cathode node: C21/R27 130R), and the wire from EL84 pin 3 goes to the pad next to R26/C20. Following the layout, B+ was connected straight to the 130R cathode resistor when standby was switched on: the voltage dropped from about 290V to about 12V. According to the schematic it should be the other way round:
- S1 output -> pad next to R26 (C20+, together with the OT centre tap)
- EL84 pin 3 -> pad above C21 (C21+/R27)
After swapping the two wires, everything measured correctly.
2. EZ81: D1 and C26 drawn on pin 2
The layout shows D1 (UF4007) and one leg of C26 on pin 2 of the EZ81. Pin 2 is an internal connection; the anodes are pins 1 and 7. The schematic correctly shows pin 1.
3. Input jacks have no ground connection
In the layout, the sleeves of J1/J2 are only linked to each other, and J3's sleeve only goes to pin 1 of the MV pot (P7). There is no wire to ground, so the layout assumes the jacks are grounded through the chassis. With insulated (plastic Cliff-style) jacks, J1/J2, J3 and P7 pin 1 are left floating. In my build this caused loud hum and almost no signal on the Normal channel, and a very weak TMB channel. Adding a ground wire from the jack sleeves to a ground point solved it. A note or an extra ground wire in the layout would help.
A small extra tip: the layout shows the output transformer centre tap as a black wire. On the Hammond 1750PA the centre tap is red (blue and brown are the plates). I had red and brown swapped at first, which unbalanced the output stage.
Final measurements (no limiter, EL84s in, speaker connected):
- B+ (C20): 326V
- EL84 cathode (R27): 10.1V
- EL84 plates: 314V / 315V
- EL84 screens: 305V / 305V
I'm happy to send photos if that's useful. Is there a newer revision of the layout?
Best regards,
Ronald