Hi All
Need some help
Thought my RM HLA300 low power output was due to one or more of the sd1446's going bad but now i'm not so sure.
Looked at the waveform with my oscilloscope on the base of each SD1446 transistor on transmit cw keydown.
Two of the transistors had a fairly pure sinewave at the base appx 0.4v peek to peek
Two of the transistors had a very distorted looking waveform almost like a square wave with a ripple on the top and half the amplitude of the other two transistors.
I think this is the source of the low power but thought it could still indicate two transistors are bad as the input waveform will be influenced by the feedback via the collector resistor.
So to be sure I swapped over two of the transistors expecting the bad waveform to follow the transistor but NO the bad waveform stayed at the transistor position.
I should explain the signal input to the amp is split into two signals by what looks like an active combiner in reverse.
Each signal then feeds the primary of a transformer. The bases of a pair of SD1446's are connected to either side of the transformer's secondary and bias is provided via a center tap in the secondary.
The other signal from the splitter feeds another pair of SD1446's via another transformer as above
The strange thing is the two bad transistor positions are not paired together they pair with each good transistor position.
So basically the secondary is providing a clean waveform to one transistor and a distorted signal to the other transistor in that pair.
There are only a couple of resistors and capacitors in each transformer - transistor circuit and these seem to check ok
Any suggestions as i'm running out of things to check?
Thanks
Stu
Need some help
Thought my RM HLA300 low power output was due to one or more of the sd1446's going bad but now i'm not so sure.
Looked at the waveform with my oscilloscope on the base of each SD1446 transistor on transmit cw keydown.
Two of the transistors had a fairly pure sinewave at the base appx 0.4v peek to peek
Two of the transistors had a very distorted looking waveform almost like a square wave with a ripple on the top and half the amplitude of the other two transistors.
I think this is the source of the low power but thought it could still indicate two transistors are bad as the input waveform will be influenced by the feedback via the collector resistor.
So to be sure I swapped over two of the transistors expecting the bad waveform to follow the transistor but NO the bad waveform stayed at the transistor position.
I should explain the signal input to the amp is split into two signals by what looks like an active combiner in reverse.
Each signal then feeds the primary of a transformer. The bases of a pair of SD1446's are connected to either side of the transformer's secondary and bias is provided via a center tap in the secondary.
The other signal from the splitter feeds another pair of SD1446's via another transformer as above
The strange thing is the two bad transistor positions are not paired together they pair with each good transistor position.
So basically the secondary is providing a clean waveform to one transistor and a distorted signal to the other transistor in that pair.
There are only a couple of resistors and capacitors in each transformer - transistor circuit and these seem to check ok
Any suggestions as i'm running out of things to check?
Thanks
Stu
