wd hd3 problem

[Princo]

Just finished building HD3, and doing some tests, all checks out except voltage on pin 7 (bias), left channel, it is only 2v instead of 13v, right channel is fine, swapped valves and same problem.
Checked board soldering and looks ok, what else could cause this.

thanks

[Princo]

Here are my test voltages:

L chan
a 206v
b 205v
c 197v

d 0.9v
e 106v
f 212v
g 2.2v???

R Chan

d 0.9v
e 112v
f 197v
g 14v

[Princo]

I just checked all again and found the voltage on pin 9 (control grid) is -45v, that should be zero, yes? no DC should pass.

What would cause this? failed cap C6?

[John Caswell]

It sounds very much as though the grid is not returned to ground. Check with DMM on both channels that the grid has a resistance to ground and compare the two.
Once I have had a look at the blurb I will add further comments

John

[Richard]

Hi, yes Pin9 should be 0V.

Looking at the older HD83 circuit but it should be similar,

All with respect to signal ground,

Voltage checks,
Pin1 102V
Pin 2 0V
Pin 3 0.9V
Pin 6 192V
Pin 8 191V
Pin7 13V
Pin 9 0V

Resistance checks,
Pin 3 to ground 1K
Pin 7 to ground 750R
Pin9 to ground 1M as John says.

[Princo]

Thanks, i checked again all voltages and resistances and swapped caps C6/C9.

L Chan R Chan
1 107v 110v
2 -0.3v -0.1v
3 0.9v 0.85v
4 Htr
5 Htr
6 205v 196v
7 2v?? 14v
8 205v 197v
9 -47v?? 0v

Resistance

1 1k 1k
7 750r 750r
9 1M 1M

All resistances are fine.

Could there be a problem with the valve base?

What could give -47v at pin 9

Jim

[Princo]

I also removed the left chan cap C6 completely and left open circuit, then:

pin 9 0v
pin 7 13.6v

Bang on!

??

[Richard]

“Could there be a problem with the valve base?”


Yes or the valve itself. You say you swapped valves, did you swap them L to R and did the fault follow or did the fault stay in the same channel?


If it is the amp and not the valve, there are few components in that area, so recheck/resolder Pin7 and Pin9 and the 750R and 1M connected to them on the faulty channel.


Also disconnect one end of C5/C8 470uF connected to Pin7 and ground in case it has gone short (it is an AC connection so shouldn’t affect your DC voltage checks) If voltages become correct replace the cap.

[Richard]

Ah, leaking C6, excellent, well done

[bob orbell]

Leaking or fitted revers polarity ? Bob