Vox NT15 various problems

[rockman627]

Issues: 120vac in and using 1kHz sine wave
pentode and triode with bright switch- hum, no sine wave
pentode and triode with thick switch - hum, a little sine wave.

Measurements:
tag 4- 308vdc
tag 7- 325vdc
tag 8- 325vdc
tag 11- 308vdc
tag 12- 308vdc
V1,p9- 305vdc
V2,p9- 305vdc

q1-r1 328vdc
r1- tag4- r17 308vdc
r17- r16 169vdc
V3,p1 - 80 to 100vdc
V3,p6 - 80 to 100vdc
V4,p1 - 160 to 180vdc
V4,p6 - 109 - 125vdc

note r18 on the schematic is actually r17 on the board.
after r16 the voltages fluctuate about 20v.

Vox_NT15.pdf .

First thing I see is r1 is dropping too much voltage. It measures 1k ohms in circuit. Of course every voltage after that is low. Then after r16 voltage readings swing up and down 10-20 volts.


I would replace r1 but why is it dropping so much voltage if it is 1kohms?
r16 measures 7.8 k ohms in circuit.
I don't think filter caps are the problem since I get 328vdc on one side of r1.

Thursday I will take the board out and touch up the solder joints. If I have the proper resistor I will replace r1.
Any ideas on what is going on?

[Jon Snell]

R1 is a 1k resistor. It reads 1k. Why replace it?
Is it actually faulty?
Did you try playing music through it?
You state hum ... frequency may be a pointer.
Your measurements and from where to where? What is tag 7 for instance?
Touch up the solder joints ... for what reason I wonder.

Check your oscillator input levels. Refer to the service manual for actual values and beware, valves are known for having different anode voltages under multiple conditions so, all voltages can be +-20% and still be perfect.

[g1]

Voltage shown feeding R1 is an error. It must be more than 325V as the show 330V at power tube plates.
Excess drop across R1 when it reads correct resistance means excess current through it.
The big problem looks like the drop across R18(R17). Is it marked as 18K or is that another difference from schematic?

[rockman627]

The tags are on the right side of the schematic after the EL84s on the bottom half. So, tag 4 is after R1 and before R18. Since it is on the schematic, I think we should just refer to R18 and it'll be up to me to remember it is really R17 on the board.
Tag 7 looks like the plate for V2

[g1]

So what is the color code on R18 ?

[rockman627]

I go back to work Thursday.

[rockman627]

R18 looks like brown grey red red. Measures 18.1k ohms.
R16 looks like white violet red. Measures 9.1k ohms.
I was thinking of replacing R1 because although it measured correctly, there is way more voltage drop than the schematic indicates.
I wanted to touch up some solder joints because of the fluctuating voltages.
But now there is no signal at all just hum in triple mode.
I'm going to check voltages again.

[rockman627]

Q1-R1:325v
R1-tag4-R18:306v
R18-R16:166v
R16-R11:99v
Tag 5:326v
8:322v
12:306v
11:306v
7:323v
V1,p9:305v
V2,p9:305v
V3,p1:20v
V3,p6:35v
V4,p1:140v
V4,p6:74v

V1&V2p9 originally read 0v, I read this a few times, then I measured V3 &V4. I went back to V1&2 and then read 305 v.
It's almost quitting time so Happy Holidays to all. Thank you so much for your help and insights.

[g1]

Voltage drop across a resistor will be high if there is too much current through it. R1 has too much current through it because R18 has too much current through it. That is because R16 has too much current through it.
What is the voltage at each end of R13 ?

[rockman627]

R13 has 101vdc one side(same as R16-R11); other side is 36 vdc.

[Helmholtz]

.
I don't think filter caps are the problem since I get 328vdc on one side of r1.

Any leaky filter cap to the left of R1 can significantly lower supply node voltages with little effect on the voltage before R1.
I strongly suspect C14.

[g1]

C14 is a prime suspect as Helmholtz stated. Or V3 tube or one of it's cathode caps.
R13 shows a 9V drop across it on the schematic. That equates to around 1mA of current through it (9V/9100). You have around 7mA current through R13 (65V/9100).
The excess current is probably due to a leaky C14 or some problem in the V3 circuit.
You should be able to remove C14 and see if the voltage at the 36V end of R13 comes back up to where it should be.

[rockman627]

Just before Helmholtz posted, I saw a quick spark on the board. I waited for it to appear again, but it didn't. I didn't see any burn marks so I decided to remove the board and look for a burn mark underneath. I didn't see any but I did see lots of flux and sloppy soldering on the tube sockets and socket pins with very little solder. So I cleaned it up and touched up the solder. But when I put it back together R1 smoked in Pentode mode. I turned it off, then tried triode mode. This time R7 and R8 smoked. Now R1 measures 880 ohms , and R7 and R8 measures about 220 ohms. Should I first replace the smoked resistors before starting with C14?

[Helmholtz]

I recommend to measure power supply resistors disconnected from circuit.

[rockman627]

Thanks. I'll continue Thursday.