PALOMAR TX5500 Elite Line RF Amplifier.

[Rob-G]

Hi all, I have been given a Palomar TX5500 RF amplifier which has a fault. When 12 volts is applied it draws heaps of current. I have checked the 4 x MRF455 output transistors and the tester indicates that they are ok.
The previous owner said it was working fine between 10m-30m and he didn't drive it any higher than 12 watts. He is not interested in repairing it so it was donated to me.
Would anyone have ANY information on this specific amplifier such as a schematic, users manual, service manual, overlays of the circuit boards especially the small board behind the power selection switch as there appears to be a track that has broken and I am unable to see where it went.
Thank you - Rob-G

 

[nomadradio]

When 12 volts is applied it draws heaps of current.

Does this before you key it?

Bad sign, as a rule. What kind of check did you perform on the transistors?

73

 

[Rob-G]

Does this before you key it?

Bad sign, as a rule. What kind of check did you perform on the transistors?

73

Thanks for the comments.
Yes the heat sink starts to get hot the moment its connected to 12 volts even with the front power switch turned OFF. But it dose not blow the 35A power fuse.
I checked the 4 x output transistors Motorola MRF455 which show ok. The tester checks for, O/C Junction: S/C between C & D and B & E: O/C between C & E and Off Beta and High Leakage.
To my knowledge this amp has never had any more than 12 watts input which gives around 200 watts output.

 

[Rob-G]

 

[linearone]

If you didn't separate the collectors and bases from the circuit before using your transistor checker, you did not check them properly.
Sounds like shorted transistors..
Pull the bases and collectors off the board well, make sure no solder blobs stay around, use desoldering wick.

 

[Rob-G]

If you didn't separate the collectors and bases from the circuit before using your transistor checker, you did not check them properly.
Sounds like shorted transistors..
Pull the bases and collectors off the board well, make sure no solder blobs stay around, use desoldering wick.

I used a solder sucker and removed all four from the PCB and tested them out of the amp one at a time and the tester said they were ok. I may remove and test them again to make very sure.

 

[nomadradio]

A fault in a bias circuit and in the circuit that is meant to switch the bias OFF in receive mode could do this. Good RF transistors will draw current on standby if there is a source of base current.

Simply lifting the base terminal of each transistor would tell you if this is the cause. A DC meter onto the pcb foil where the base was unsoldered would show if the keying circuit isn't turning off the operating bias. My first test is to probe the now-loose base lead of each transistor. Typical transistor failure will show voltage leaking from the collector voltage supply. A good transistor will show zero Volts DC on the loose base lead.

Quick and simple, I'll guess maybe the pcb foils for the RF transistors' base terminals are showing voltage in receive mode. This would cause perfectly-good transistors to draw current in receive mode.

73

 

[Rob-G]

A fault in a bias circuit and in the circuit that is meant to switch the bias OFF in receive mode could do this. Good RF transistors will draw current on standby if there is a source of base current.

Simply lifting the base terminal of each transistor would tell you if this is the cause. A DC meter onto the pcb foil where the base was unsoldered would show if the keying circuit isn't turning off the operating bias. My first test is to probe the now-loose base lead of each transistor. Typical transistor failure will show voltage leaking from the collector voltage supply. A good transistor will show zero Volts DC on the loose base lead.

Quick and simple, I'll guess maybe the pcb foils for the RF transistors' base terminals are showing voltage in receive mode. This would cause perfectly-good transistors to draw current in receive mode.

73

Thank you - I think I have located where the damaged track went on the power selection board. I used a strong mag glass and there is a very fine mark where the track was, so I have repaired that. The next will be your procedure for testing the bias circuit. The output transistors in this amp are 4 x MRF455 Motorola - more to follow. 73.

 

[Rob-G]

Thank you - I think I have located where the damaged track went on the power selection board. I used a strong mag glass and there is a very fine mark where the track was, so I have repaired that. The next will be your procedure for testing the bias circuit. The output transistors in this amp are 4 x MRF455 Motorola - more to follow. 73.

I thought I would double check, and found that the amp only draws big current when its turned on at the front panel. I can connect it to 12 volts without it being switched on and it does not draw the big current. Does this indicate anything ?

 

[nomadradio]

Sounds to me as if the bias circuit is powered down by the power switch. The high current drain of the large RF power transistors usually calls for a direct connection from the hot side of the power to the collector circuits of the RF transistors. They get switched on and off either by base-bias current or the radio's drive carrier. This is more cost effective than using a power switch rated for the amplifier's full current draw.

73