Question re TL 922 amplifier

[bob.martin]

Dear All,

My kenwood TL922 has developed a strange fault. When I switch it on (via an
Ameritron Soft Start module) I get a negative reading on the Ig meter
of -50mA and a positive reading of around +100mA on the Ip meter with no key
down.

Keying the amp has no effect and the relay does not energise. I have
checked the ALC and relay connectors and the coaxial cables

I've tried starting it directly bypassing the soft start box and this has
made no difference. Obviously I do not leave it on for more than a few
seconds as there is clearly a fault. I have had the covers off and can see
nothing untoward in terms of burnt components and there is no burning smell
and the fuses don't blow. Also both valves (3-500zs) light up.

Any suggestions as to what may be up before a trip over to Castle, has
anyone else seen similar

thanks

Jim G7NKS

[t w]

"bob.martin" <bma...@qxi.com> wrote in message
news:bIbNh.18965$7l1....@newsfe4-gui.ntli.net...

Get the diagram out & fix it yourself.Why do so many people just send stuff
off.A 922 is not rocket stuff.
Keith.
>
>
>

[Dave]

You have described the classic symptoms of the heater-grid short inside the
3-500 [at least one of them].

The heater is tungsten. Tungsten grows crystal 'whiskers' over time. It is well
documented physics. These whiskers are thin as silk thread [or thinner]. In many
instances they can be removed with a simple application of 110 VAC between the
heater and the grid. Some people will charge a 10 uF capacitor to several
hundred volts and discharge it into the grid/heater short circuit. This will
'cure' the 'whisker' issue [pop the fuze] in about 90% of the instances.

A second option exists. When you first power up the 922 don't panic over the
current being drawn and jump to turn it off. Wait for 10 to 15 seconds to see if
the filament heating and resultant thermal expansion will eliminate the short
for you. [This is much less expensive than buying a matched pair of 3-500Z's]. I
have been running my AL-80B with a transient [heating] grid/heater short for
over a year. Once the transient state has returned the circuit to normal
operation, the amplifier works 100% as designed. [BTW: I have a spare 3-500 that
I will use if the short eventually becomes a 100% hard failure].

/s/ DD, W1MCE

[bob.martin]

"Dave" <W1...@comcast.net> wrote in message
news:seidnY2B3YsaF5jb...@comcast.com...

Hi Dave

Wouldn't the 15 second run burn out the filament supply transformer though?

Jim G7NKS

[Dave]

bob.martin wrote:

> "Dave" <W1...@comcast.net> wrote in message
> news:seidnY2B3YsaF5jb...@comcast.com...
>
>>You have described the classic symptoms of the heater-grid short inside
>>the 3-500 [at least one of them].

SNIPPED

> Hi Dave
>
> Wouldn't the 15 second run burn out the filament supply transformer though?
>
> Jim G7NKS
>
>

I'm not aware of the 922 design details that contributes to the 'transformer' issue.

The normal filament current is about 14 amperes at 5 volts. The grid/filament
short generally draws less than 100 ma of grid current and less than 300 ma of
plate currnet. An abnormal current of less than 0.5 ampere is not a problem for
the major floating secondary winding of a filament transformer.

The design of the bias circuit can be a totally different circumstance. In
Ameritron amplifiers it is a self bias through a series of resistors and/or
zeners. My Al-80B uses about -15 VDC as cutoff bias. In the 922 ... I have no data.

Restated, remove the tubes and 'pop the short' if there is a concern in the 922.