UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

G0HZU_JMR · 19th Mar 2017, 6:42 pm

I did the pin 7 tests this afternoon and it showed 6.11V when cold although this wandered from 6.07 to 6.2V but once it was on for a while it was typically 6.25V with no signal. Note that I also set my PSU to give 12.67V at pin 11 of the LA1230 to match the homebase radio reading here.

I did try optimising the quad demod coil adjustment and this thing is very critical on the adjustment. It only has to be out by a tiny amount to wreck the audio. Note that I did this with a two tone test waveform to look at 2nd and 3rd order distortion on pin 6 (on an AF spectrum analyser) as I adjusted the coil.

I also put the tip of my little finger on the centre of the LA1230 chip and held it there and my thermal camera is correct. The centre of the chip runs quite hot. Much hotter than you would think! I assume that mine is OK because it seems to be working fine in FM receive. These things were never that great in terms of 'fidelity' (despite the name on the front panel

) but I think it is working as I remember back in the day. Not as harsh as a 134 radio but other radios will be nicer to listen to.

19th Mar 2017, 6:42 pm	#44
G0HZU_JMR Dekatron Join Date: Sep 2010 Location: Cheltenham, Gloucestershire, UK. Posts: 3,077	Re: Harvard H407 CB RX audio problem I did the pin 7 tests this afternoon and it showed 6.11V when cold although this wandered from 6.07 to 6.2V but once it was on for a while it was typically 6.25V with no signal. Note that I also set my PSU to give 12.67V at pin 11 of the LA1230 to match the homebase radio reading here. I did try optimising the quad demod coil adjustment and this thing is very critical on the adjustment. It only has to be out by a tiny amount to wreck the audio. Note that I did this with a two tone test waveform to look at 2nd and 3rd order distortion on pin 6 (on an AF spectrum analyser) as I adjusted the coil. I also put the tip of my little finger on the centre of the LA1230 chip and held it there and my thermal camera is correct. The centre of the chip runs quite hot. Much hotter than you would think! I assume that mine is OK because it seems to be working fine in FM receive. These things were never that great in terms of 'fidelity' (despite the name on the front panel ) but I think it is working as I remember back in the day. Not as harsh as a 134 radio but other radios will be nicer to listen to. __________________ Regards, Jeremy G0HZU Last edited by G0HZU_JMR; 19th Mar 2017 at 6:52 pm.