McIntosh MR71 FM Receiver Video #20 - Gremlins Be Gone

It's taken some time but I finally uncover the Gremlins that have been secretly hassling me while I work on FM receivers. The spectrum display from my SDR radio is again revealing things normally hidden from my view. With the Gremlins under control, I'm able to properly align T5 and T6 with a notable improvement.

[peteselectronicprojects]

Hi Jim, just for completeness sake most VTVMs, when set to measure DC, are designed for use with a 1 meg resistor at the probe tip. The DC scale calibration takes the volt drop across 1 meg  into account. If the radio manufactures specifies a VTVM set to DC for a particular measurement then their VTVM will have had the 1 Meg resistor at the probe tip the same as yours, they wrote the alignment instructions with that in mind. If you use a VTVM to measure AC then there is no resistor in the standard AC probe and because of that the frequency response of the VTVM is not very good for AC (not good enough for RF and radio repair). VTVM makers sold separate AC probes for that purpose. The SDR, with its 50 Ohm input will obviously have a highly detrimental effect on the radio without some sort of isolation. You are currently having success with the "loose coil" method I know but the way forward would be to use an active probe to transform the high impedance that the radio needs down to 50 Ohm for the SDR. Interestingly the probe attached to your "new" signal tracer is basically the "sort of" active probe needed in that it is a cathode follower, but is not really suitable as it stands. I know you like to learn by finding your own way but I found it hard watching you struggle so I had to comment on your last few uploads, I hope you don't mind. I spent many years finding my own way with this sort of thing and lost a lot of time that way so I know what its like. All 400 of us are willing you on to success, good luck - Pete
P.S. I'm not a repair professional, I worked in telecoms but not on the radio or repair side. I have learned most of what I know about electronics and radio as a hobbyist. Much more interesting than work! :)

[marcolokker]

hi, first sorry for my bad english.. The T6 coil aligment you neet a generator of 10.7 Mhz + FM modulation of 75khz . T6 is the FM demodulator. working is when tuning for max neg voltage you tune 1 of the t6 coils to 10.7Mhz -75kHz (to get max negative voltage) then you tune the othe coil to 10.7Mhz+75khz becourse the coil are in balance at 10.7 the output = 0 volt. but you can check this by connecting your volt meter to this 0 point en change your generator frequentie. you wil see if you move slidly lower then 10.7 you get a negative volt on the tp2 and if you go a litte above 10.7 you get a positive voltage on second look this can be a phase discriminator then coils are not set to those frequentions ( im a long time out of repairing radios) but the point stay you need a FM modulated 10.7 Mhz signal. to set t6. correctly.

[gdj]

Hi Jim
So you need a 10 M ohm per volt meter .
Just use your oscilloscope with a X 10 a probe .

[robertmalsbury]

Funny, I almost asked what you ohm
 per volt was? Use a 10 Mohm resitor