Help, no bias voltage

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sixpakldp
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Help, no bias voltage

Post by sixpakldp » Thu Sep 01, 2016 11:10 pm

I just did a complete rebuild of my metro 100 watt kit from the ground up. I pulled out my kit instructions and began following the test out procedure, when I get to measuring the bias resistors, I have never voltage. I do have 90-100 v at the 27k resistor, checked the diode directions and the bias capacitor directions which are all good. Any ideas what I can nail down the problem? My guess is it has to be an open circuit somewherenas I would assume the voltage on the 27 k resistor would not be there.

danman
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Re: Help, no bias voltage

Post by danman » Fri Sep 02, 2016 5:12 pm

We need to clarify which resistors you are referring to as "bias" resistors. Are you referring to the 1ohm current sensing resistors soldered between pin 8 and ground? Or...are you not reading a negative DC bias voltage at pin 5 on each power tube socket?

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Re: Help, no bias voltage

Post by sixpakldp » Fri Sep 02, 2016 6:44 pm

The 220k resistors. In George's test out procedure in the metro kit instructions one of the steps is to check for roughly-30 v DC at these resistors or at the 5.6k resistors on the power tubes. There is a check that if you do not see this voltage to check at the 27 k resistor for 90v, the diode direction and then bias capacitor direction. I have 90v and the diodes and caps are in the right direction. So need to know what might be the next step if those check out OK.

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Re: Help, no bias voltage

Post by sixpakldp » Sat Sep 03, 2016 1:44 pm

Here are some voltage readings with the power switch on and the standby switch off.

Readings are Pin 1 to Pin 9 for pre-amp tubes:

V1 -1.75, -0.27, 0, 3, 3, -1.75, -0.47, 0.01, 3
V2 -1.6, -0.5, 0, 3, 3, -2.3, -1.6, 0, 3
V3 -2.2, -0.3, 0.2, 3, 3, -2.3, -0.2, 0, 3

V4-V7 roughly same readings for all, listed Pin 1 to Pin 8:

0, 0, 3, 1.9, -1.5, 0, -.15

Tried a bunch of things last night, mainly ensuring the capacitors are all in the right direction. If I switch the Stand-By on, usually the HT Fuse blows. Really scratching my head on this one. This is the first time I've done a build, flipped the switch and it not work.

Also attaching some photos in case my eyes are just tired and someone can spot something wrong.

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upload gif from url

sixpakldp
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Re: Help, no bias voltage

Post by sixpakldp » Sat Sep 03, 2016 1:55 pm

Also, if I do flip-on the stand by switch I do get about 518 V at the HT fuse and I am reading good DC volts through the rectifier section, so I don't think it's the PT. I know it's got to be a stupid mistake (I hope).

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neikeel
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Re: Help, no bias voltage

Post by neikeel » Sat Sep 03, 2016 3:20 pm

Are you bias caps both properly grounded (I run the wire back to one of the output tube ground lugs)?
Neil

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Re: Help, no bias voltage

Post by danman » Sat Sep 03, 2016 4:18 pm

Neikeel brings up a good point. Be sure that the ground to the bias circuit is intact and solder joints on both ends are good. You can trace the bias voltage to see where you may be losing it. You should have positive AC voltage feeding the 27k and then on to the diode. After the diode, you will need to set your meter for DC voltage and keep tracing through each connection point in the circuit. It will show negative DC voltage after the caps. Check for voltage at the junction of the 47k and the jumper that feeds the bias voltage on to the 220k splitters. You should be able to find out where you are losing it this way but be sure that the ground wire is in place first.

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Re: Help, no bias voltage

Post by sixpakldp » Sat Sep 03, 2016 4:30 pm

No they are not! And my choke is where the ground should be. Thanks Neil and Danman. I bet that's it. Gonna get the iron out.

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Re: Help, no bias voltage

Post by sixpakldp » Sat Sep 03, 2016 4:51 pm

Grounded the bias caps and moved the choke, blowing the mains fuse after a few seconds of power on.

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Re: Help, no bias voltage

Post by sixpakldp » Sat Sep 03, 2016 7:41 pm

I'm thinking its a bad filter cap. Found a thread on the gear page regarding blown mains after only a few seconds of power up. Says it could be three things, diodes, filter caps, PT. Per the thread checked the diodes and they are good. Next I disconnected all of the secondary wires on the PT and no blown fuses. I'm thinking caps. What do you think and how to check?

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Re: Help, no bias voltage

Post by danman » Sat Sep 03, 2016 9:49 pm

Is it blowing fuses with or without the tubes installed? If the power tubes are installed, are they beginning to redplate when the fuse blows?

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Re: Help, no bias voltage

Post by sixpakldp » Sat Sep 03, 2016 10:28 pm

It was blowing without the power tubes. But with the secondary taps disconnected it does not blow the mains.

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Re: Help, no bias voltage

Post by sixpakldp » Sat Sep 03, 2016 10:33 pm

Also prior to having the choke hooked up to the wrong terminal and the ground from the bias caps missing, it did not blow the mains. Cor would only blow a fuse it you switched the standby on and it was the HT fuse previously. No power tubes, just pres.

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Re: Help, no bias voltage

Post by sixpakldp » Sat Sep 03, 2016 10:36 pm

Here the thread from the gear page I am using to trouble shoot. Page 4 has the method for troubleshooting the PT. I followed the check out procedure and the PT is good. Diodes also check, so if the thread is right has tone a filter cap.

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Re: Help, no bias voltage

Post by sixpakldp » Sat Sep 03, 2016 10:36 pm


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