| |
GM7HUD > UHF 23.09.07 23:21l 39 Lines 2068 Bytes #999 (0) @ WW
BID : F24803GM7HUD
Read: DG1VV GUEST
Subj: MM 432/100 Amplifier Bias?
Path: DB0FHN<DB0FOR<DB0SIF<DB0GV<DB0LJ<DB0RES<ON0AR<GB7YKS<GB7SYP<GB7ESX
Sent: 070923/1910z 77908@GB7ESX.#31.GBR.EU $:F24803GM7HUD [Witham, Esx]NNA V3.1
Hi all,
I'm trying to revive a Microwave Modules 432/100 Amplifier. This is a 100W
o/p solid state amp using 3x CM50-12a devices, 1 as a driver and 2 in
parallel for output. I have obtained a circuit diagram for a slightly later
revision which uses an MRF646 as a driver with 2x CM50-12a for the output.
Apart from that difference it seems to be very close to what is actually in
my example.
The amp has failed and there is evidence that someone has "been at it"
judging by the 20A fuse being replaced by solid wire and the numerous
melted bits where the soldering has been wielded. However, I'm slowly
making progress. The dead short across the rails was a shorted 22uF tant
decoupler on the driver. The RF switching is dead and I have no idea if the
VSWR protection and thermal shutdown works. But when keying the amp, bias
appears at the bases of all devices. I suspect that all 3 RF devices are
working to some degree. I know that RF devices can fail such that they no
longer work correctly at RF but exhibit DC characteristics that look OK.
What I don't know (it's not on the circuit) and I haven't found reference
to it on the net, is just what the bias currents should be set to. When
keyed, the amp draws some 3.5A which seems a rather high standing current.
So does anyone know what the current should be? What would be cool would be
for someone with such an amp to measure the standing current when the amp
is keyed but with no input applied. That would help me no end. I dont want
to do too much more with it till I'm sure the bias is OK. There's no gain
at present, but judging by the "friendly fire" soldering iron damage to the
input circuit, I wouldn't be surprised to find there's no drive getting to
the base of the 1st transistor. I'm hoping that it will be a faulty
component on the PCB I can change without having to remove the PCB, that
looks like a major undertaking.
So if you have a MM 432/100 amp, can you measure the current drawn when the
amp is keyed but with no input applied and let me know. Thanks in advance.
73 de Andy GM7HUD
Read previous mail | Read next mail
| |
