Marantz 2275 IF stage has problems

CJ · Aug 26, 2019

everything works on this receiver except the FM radio has no stereo. this is usually the discriminator, and sure enough , we have some weird readings on the last three transistors, wonder if this is not some type of limiting, or what the heck is going on, the transistors test fine, about hfe = 120 at 5 ma, thanks for any help on the low V-eb readings!

i am hoping the L201 can is not bad, ???

PRR · Aug 26, 2019

> no stereo. this is usually the discriminator

If the Mono is good, it "might" be the discriminator, but I'd suspect the Multiplex decoder first.

BTW, isn't that a Ratio Detector not a Discriminator?

CJ · Aug 27, 2019

i will check the multplexer chip, thanks!

i wonder if the 10.7 M Hz RF is biasing those transistors on but not showing up on the meter?

seems to be plenty of collector current thru the emitter resistor, but 0.4 v on the base shoule mean cutoff.

i do see rf on the scope when hooked to the collectors.

CJ · Aug 29, 2019

yes ratio detector, Foster Seely,

it appears that the 10.7 M Hz RF is causing the funny readings for V-be on the transistors down the line in the IF section, there is about 500 mv P-P on the bases of three of the transistors,

do not know if the RF is causing the meter to read funny or if it is the RF degenerating the base voltage, they are conducting as evidenced by Ohms Law on emitter and collector resistors,

abbey road d enfer · Aug 30, 2019

CJ said:
it appears that the 10.7 M Hz RF is causing the funny readings for V-be on the transistors down the line in the IF section, there is about 500 mv P-P on the bases of three of the transistors,

That is quite usual in FM IF strips, where the stages are operating at "limiting", which is in fact clipping.

do not know if the RF is causing the meter to read funny or if it is the RF degenerating the base voltage,

Neither; because the transistor operates in class C, the emitter current is pulsated and charges the emitter decoupling cap to a higher voltage than without signal. That is why the be voltage decreases as long as you go towards the output, the last stage being the one that is most clipped.

moamps · Aug 31, 2019

CJ said:
....except the FM radio has no stereo...

If the mono signal is ok, you should check does the stereo control circuitry work correctly. Check out DC signal on pin 8 of MPX IC, which should go to logic high when the RF signal is good (stereo is muted for the low level signal) and also check does the VCO in MPX IC oscillate at exactly 19kHz, pin 10 (trim it with R311).

CJ · Aug 31, 2019

thanks you guys! i forced pin 8 high by turning the muting balance control all the way left, the stereo light did not come on and killing rt/lft audio with the SG165 did not indicate that we had decoding, so we have a couple of chips on the way, uPC7587C2,

Marantz 2275 IF stage has problems

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