Gibson GA77 RET-L Lancer, help with zenor diode replacement

[davohilts]

Hello,
I'm rebuilding an old GA77 RET-L that has the Tel-Ray oil can echo and the schematic shows two 36V, 400MV zenor diodes. I assume these are being used in voltage regulation. When I look for zenor diodes I see many to choose from that are 36V. Can someone point me in the right direction in obtaining the proper replacement?? I was looking at NTE5085A as a possibility.
Thanks, Here is the schematic:
Dave

[g1]

The 400mV is a misprint, should be 400mW. The standard replacement for that would be 500mW. So some generic numbers for 36V 500mW zeners would be 1N5258B or BZX79-C36.

[davohilts]

Thanks g1,
I couldn't figure why the 400 MV designation, of course I never thought it was a misprint, I was just happy I could read it. A lot of the old gibson schematics have hard to read fine print.
Dave

[Enzo]

500mw = 1/2 watt

36v half watt zeners are cheap. 1N5258 is the generic, and there are others. Mouser has them for under 20 cents. And I checked Newark, because I thought they sold NTE, but entering NTE5037 just brought up 1N5258, and check it out, TWO cents.
1N5258B-TAP - VISHAY SEMICONDUCTOR - ZENER DIODE, 500mW, 36V, DO-35 | Newark element14

NTE5037 is the half watt 36v zener. NTE direct sells them for 90 cents each.

NTE5085 is the 1 watt version. NTE direct sells them for two for $2.29.

Compare that to 2 cents each for real ones.

[davohilts]

500mw = 1/2 watt

36v half watt zeners are cheap. 1N5258 is the generic, and there are others. Mouser has them for under 20 cents. And I checked Newark, because I thought they sold NTE, but entering NTE5037 just brought up 1N5258, and check it out, TWO cents.
1N5258B-TAP - VISHAY SEMICONDUCTOR - ZENER DIODE, 500mW, 36V, DO-35 | Newark element14

NTE5037 is the half watt 36v zener. NTE direct sells them for 90 cents each.

NTE5085 is the 1 watt version. NTE direct sells them for two for $2.29.

Compare that to 2 cents each for real ones.

wow! now that's what I call comparative shopping! 2 cents vs 90 cents!
Thanks Enzo!

[davohilts]

so I got my zeners and now I want to be sure that I install them correctly. From the schematic it looks like their purpose is to regulate voltage. The diodes have black bands on the glass bodies so would I connect the diodes with their bands pointing away from their connection point? Like connecting the anodes together and connecting one cathode to ground and the other to the 10K resistor and mode switch?

[52 Bill]

The diodes have black bands on the glass bodies so would I connect the diodes with their bands pointing away from their connection point?

Connect the two diodes together with both black banded ends together. Then connect the series string together across the input of the oil can circuit.

[davohilts]

Thanks 52 Bill!
Hopefully this is the final step in bringing this baby back to life!
Dave

[davohilts]

I have the amp up and running but have low output on both channels. The normal channel (2) has slightly more output than the effects channel (1). I have tried different power tubes, preamp tubes (6EU7's) and the phase inverter tube (6FQ7) without any effect. I have tested the cathode resistors on all the preamp tubes and they test within spec. I replaced all the filter caps and all electrolytics on the cathode resistors of the preamps and the .02 mfd (C18) on the phase inverter.
Does anyone have any advice on where to look or test. All the cathode voltages are close to spec, most voltages are slightly high due to the new power transformer I installed and the 120VAC coming out of the wall.
Thanks for any help.
Dave

[52 Bill]

If both channels are weak, then start at the power amp and work forward. What is the idle current of the output tubes?

[davohilts]

Thanks for the help again 52 Bill!
The power tubes are biased at 28mA at 455V