Low Bias Voltage on Crate TV-120

[RWood]

This pesky Crate is just revealing one problem after another. At least it now stays on long enough, with no tubes, for me to do some diagnostics.

Bias voltage at the power tube grids is -14 VDC. Should be around -45. It is not adjustable. I am not all that familiar with fixed bias, or how to troubleshoot it.

On the schematic (below) I am getting 75VAC between the two PT secondaries for the bias circuit. After the diode, I get -34VDC to ground. And by the time it gets to the power tubes, it is -14.

Could one of you point me in the direction of which components I should test or replace? If I need to post additional information please let me know.

RWood

[Enzo]

Let's clear up the terms a little. The single secondary that serves the bias has three wires, green, yellow, and violet. They should be on posts J312,J29, J33 respectively. Is that the case here? You should see about 45VAC between the grounded green wire and the violet wire. After D17 it turns to DC, and should be about -62VDC there. If that is low, then so will be the voltages that trail off it. SO if your 62 is 30, I don;t doubt your 45 is 14,, or whatever.

Set your meter to AC volts, not DC and measure that -62v. There should be almost no AC there. If there is a lot of AC measured, then the filter caps are not working. C60 or C61 open or circuit board traces cracked or failed solder.


And just for science, follow the yellow wire from the transformer. Is that making the +20VDC after D16?

And since the tubes are out, check the B+. Is that up to about 500v and pretty clean?

[RWood]

Thanks for checking it out, Enzo.
Aside from the filament wires, all the PT secondaries are different colors than what is in the schematic, which might be part of the problems you'll see below. I started with the wires 'as received'.

The single secondary that serves the bias has three wires, green, yellow, and violet. They should be on posts J312,J29, J33 respectively. Is that the case here?

Here they are orange, orange and blue, but the posts J31, J29 and J33 on the board appear to be correct.

Here are my VAC readings between the three wires, identified by the posts they are currently attached to:

• J31 to J33 76.1V
• J33 to J29 0V
• J31 to J29 109.8V

Set your meter to AC volts, not DC and measure that -62v. There should be almost no AC there

And there is not. I assume this is with respect to ground. Set to AC, the meter shifts to a negative sign but the readout stays at .000

follow the yellow wire from the transformer. Is that making the +20VDC after D16?

No it is not. I get a reading of .03 VDC and it is fluctuating. I do show 24 VAC after the diode, though.

check the B+. Is that up to about 500v and pretty clean

When I take the amp off standby, with no tubes but with a speaker connected, I get a loud hum and I flip the standby back on for fear of blowing a fuse. Maybe once the bias issues are straightened out I will be able to move on and check that. I have replaced one screen resistor and checked the grid resistors and they are okay.

I appreciate your helping me with this!

RWood

[52 Bill]

Before you get too confused with the transformer wire colors, please notice that the schematics are for a TV-60 not 120.

Even though the boards may be the same, I wouldn't be surprised that the wire colors are different from one transformer to the other.

Maybe Enzo has the correct schematics.

[RWood]

Great point Bill and you are right - the transformer in the amp is labeled 94-650-40 while the one in the schematic is 94-613-30.

It's only the bias secondary colors that are different. The filaments are both yellow as I mentioned and the B+ are both red, which I failed to say earlier. Btw I do get 367VAC between the two reds, with them disconnected from the board. This looks good.

What confused me for a while is that there are additional wires coming from the secondary side of the PT. Two greens and a blue/white, going to three tabs which according to the board layout (attached) are dead ends. Maybe one of these leads was mistakenly swapped for a non-bias wire.

[Enzo]

Whoa, hold on there. No power tubes, but loud hum out the speaker when standby switch is on. Only way for loud hum to happen is if current flows through the transformer. That means either there is a shorted flyback diode, or very leaky one, or the output transformer primary has a short to frame or to secondary. Does this amp have flyback diodes?

Power off, standby off, measure resistance from the plate pins of the power tubes to chassis. You ought to get open or a charging indication. If you get a low resistance, then shorts are your beef. Check for shorts between windings and frame and also for shorted flyback diodes.

With this situation, bias is the last of your concerns.


I don;t have any TV series schematics in my paper files or my disks, and none on the LOUD dealer site. Oh wait, I do have a four part scan of a TV-60 from somewhere.


I am curious. Yellow wires for the heater? And a pair of unused green wires? What voltage is across the two green wires? And what voltage is across the yellow heater wires when tubes are gone?

generally I expect yelliw for 5v and green for 6v heaters. Red for HV winding we got.

[RWood]

Oh wait, I do have a four part scan of a TV-60 from somewhere.

That's the one I posted earlier in this thread.

I will check on the other things. I don't know where to look in the circuit for a flyback diode - maybe someone can look at the schem I posted and tell me.

Thanks,

RWood

[RWood]

measure resistance from the plate pins of the power tubes to chassis.

On one pair of tubes I get 2.2 ohms from plate to ground. On the other pair I get 51 ohms. This pair changes to 'open' when I unplug the blue OT wire from the chassis. I will flip the board and see if I can find the shorts.

Check for shorts between windings and frame and also for shorted flyback diodes.

Would D25,26,27,28 be the flyback diodes? I hope so; 1N4007s are values that I have at hand.

What voltage is across the two green wires? And what voltage is across the yellow heater wires when tubes are gone?

The green wires produce 181VC between them. The yellows put out 6.7VAC

Thanks for going through this with me. I've got a nice old Showman waiting for me if I can get through this pcb poc!

RWood

[RWood]

Hold the phone, D27 and 28 are toast. I will replace them and the other pair, for good measure.

[Enzo]

D25-28 ARE your flyback diodes. The amp will run fine without them, so you can continue to work on the amp if you like. Just replace them by the end of the repair work.

Those shorted diodes are enough to blow fuses, and you certainly couldn;t bias up the amp with them shorted.

There was 2.2 ohms to ground, and 51 from the other end? Wanna bet the primary winding measures about 49 ohms? Your 51 ohms is measuring that same 2.2 ohms at the other end, but through the 49 ohms of primary winding. So they add.

Well then I don;t know WHAT that transformer is intended for. Odd voltages all.

[RWood]

Enzo, replacing those diodes worked perfectly and got the B+ back to normal - thanks for that!

As for the bias voltage, even though it was lower than what the schem calls for I decided to give it a try, with bias checker in-circuit so I could monitor the current. If it got above 50mA or so (this amp uses four EL34s) I was ready to put it back in standby.

To my surprise the cathode current only went up to around 28mA and the amp sounds fine. With 470 plate voltage this appears to be right in range. Now I am wondering if the bias voltage on the TV-60 (two power tubes) schematic uses a different value than what the TV-120 requires, and that I was fine the whole time.

At any rate the amp works (aside from an odd volume pot that does not turn down to zero, but attenuates the highs below 3 on the dial) and will soon be going back to the owner. Thanks for your help with this, I really appreciate it!

RWood