Sunday 7 January 2024

Help needed with Index Laboratories' QRP+ radio

I need your help! 

But first some background. The Index Laboratories' QRP+ is a 1990's 5W all-band QRP transceiver from the USA. I bought mine at a club sale and it was supposed to be working fine.

But it wasn't! Never mind, we all love a
challenge, don't we?

It seems that the SBL-1 mixer is a common failure point so a replacement was ordered.

The rig comprises four boards, which are stacked and have interconnecting leads.

It is relatively easy to pull apart, and the mixer is on the top board.

After a big desoldering struggle I fitted the new SBL-1 (U5), which brought the receiver back to life. Unfortunately, the TX still didn't work. Back to the drawing board.

The next stage was to replace the final PA transistor, which is on the lowest board. This is a IRF510 (Q9) and I ordered a few off Ebay. I fitted a new one and in tests I found that I could only get about 0.1W out! (on CW) with the gate bias backed off fully anticlockwise. 

Power out should be about 5W.

Anyway, that's as far as I have got. I have attached a link to the circuit diagram and would appreciate any suggestions.


Well, some replacement IRF510s were sourced from the G-QRP club (thanks guys). Apparently, there are some "fake" IRF510s around that don't play well with RF.  After pulling the radio apart (again) and replacing the transistor, it was a little better - about half a Watt output. Anyway, to cut a long story short I have passed the radio to another G-QRP club member and I hope he has more luck than I did! 

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