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Ok, need some help here please. I got in over my head refurbishing my old Mark IIIs. I had bought them used and had them recapped in the early 1990's. I retired them after several years having moved on to solid state gear. I pulled them out of storage and one amp worked ok, the other blew some smoke after the capacitor can failed. I cleaned them up and started down the refurb path. Here is what I have done thus far:

1. Replaced the capacitors with the SDS board.
2. Replaced the power cord to a three prong cord.
3. Replaced the driver board with the Poseidon board.
4. Replaced any remaining capacitors/resistors.

After checking and double checking my work, both amps now give me a really loud constant hum. Here are the measured voltages:
Amp A:
V1-12AX7:
Pin 1: 56
Pin 2: 0
Pin 3: 1.71
Pin 4: 0.3
Pin 9: 0.3

V2-12AU7
Pin 1: 183
Pin 2: 0
Pin 3: 1.03
Pin 4: 0.4
Pin 5: 0.4
Pin 6: 189
Pin 7: -14
Pin 8: 1.01
Pin 9: 0.7

V3-6550
Pin 1: 2.44
Pin 2: 1.5
Pin 3: >1000 (this scares me!)
Pin 4: 335
Pin 5: -59
Pin 6: -59
Pin 7: 6.1
Pin 8: 2.47

V4-6550
Pin 1: 2.48
Pin 2: 9.6
Pin 3: 530
Pin 4: 553
Pin 5: -62
Pin 6: -62
Pin 7: 22
Pin 8: 0

V5-GZ34
Pin 1: 0.1
Pin 2: 536
Pin 3: 0
Pin 4: 444
Pin 5: 0
Pin 6: 445
Pin 7: 0
Pin 8: 534

A: 0
B: 0
C: 0
D: 4.7
E: 4.3
F: -63
G: -62
H: -71
I: 0
J: 505
K: 533

The second amp reads as follows:

Amp B
V1-12AX7:
Pin 1: 100
Pin 2: 0
Pin 3: 1.17
Pin 4: 0.4
Pin 9: 0.4

V2-12AU7
Pin 1: 125
Pin 2: 0
Pin 3: 4.86
Pin 4: 0.5
Pin 5: 0.5
Pin 6: 115
Pin 7: -19
Pin 8: 4.57
Pin 9: 0.5

V3-6550
Pin 1: 2.80
Pin 2: 0.6
Pin 3: 347
Pin 4: 348
Pin 5: -51
Pin 6: -51
Pin 7: 0.7
Pin 8: 2.80

V4-6550
Pin 1: 2.80
Pin 2: 0.9
Pin 3: 343
Pin 4: 348
Pin 5: -55
Pin 6: -55
Pin 7: 1.3
Pin 8: 2.80

V5-GZ34
Pin 1: 0
Pin 2: 370
Pin 3: 0
Pin 4: 0
Pin 5: 0
Pin 6: 0
Pin 7: 0
Pin 8: 372

A: 0
B: 0
C: 0
D: 2.1
E: 2.1
F: -55
G: -49
H: -63
I: 0
J: 233
K: 300

Any help would be appreciated. Thanks in advance.

Have you properly grounded the SDS board ?

The original star grounding scheme has been changed. On the original MKIII design, the power tubes (the bulk of the current) grounded right at the cap lug which was soldered to the chassis. Presumably your MKIIIs still ground to the chassis, but the power supply is on the other side of the chassis and connected by a screw connection. Change that chassis ground lug that you have the two 10 ohm resistors soldered to an isolated perf lug. Then connect a wire from there to the SDS cap board grounding lug (which is inconveniently placed beside the on/off switch). Or you could solder that return wire to the underside of the PCB at the '-' of C4, the B+ return path.

Shannon

Ok, thanks for the replies. A few more questions:

1. So I am inferring that screwing the SDS board to the chassis does not necessarily ground it? And even if it did, it is creating a ground loop hum?

2. What is an 'isolated perf lug'?

3. Do the posted voltages make sense for the improper grounding? I am thinking specifically for Amp B, the V5 or rectifier readings, specifically pins 4 and 6:

V5-GZ34
Pin 1: 0
Pin 2: 370
Pin 3: 0
Pin 4: 0
Pin 5: 0
Pin 6: 0
Pin 7: 0
Pin 8: 372

Thanks in advance.

rpochi wrote:Ok, thanks for the replies. A few more questions:

1. So I am inferring that screwing the SDS board to the chassis does not necessarily ground it? And even if it did, it is creating a ground loop hum?

2. What is an 'isolated perf lug'?

Yes, there's a large ground loop when you use the SDS board with the heaviest currents. The return path from the power tubes should be a single wire to the SDS board. Post a nice pic of your MKIII innards for a more precise diagnosis, but this is the problem.

rpochi wrote:3. Do the posted voltages make sense for the improper grounding? I am thinking specifically for Amp B, the V5 or rectifier readings, specifically pins 4 and 6:

V5-GZ34
Pin 1: 0
Pin 2: 370
Pin 3: 0
Pin 4: 0
Pin 5: 0
Pin 6: 0
Pin 7: 0
Pin 8: 372

Thanks in advance.

This DC measurement is missing the (large) AC voltage at ins 4 & 6. Just change your DMM setting.

Shannon

Ok, finally got back to work on the amps after the holidays and what not. Indeed, grounding the board was the culprit. Thanks for the help!