Marantz 2252B - FM Issue

[dtafil]

Hi All -

I'm working on a 2252B that one of the prior owners did some "servicing" on; enough on that,

Only remaining issue is not something I've bumped into before, and am looking for a little guidance if possible to debug.

Long story short is that both left and right channels play strong in stereo mode in AUX and PHONO, but on FM, stereo output drops about 90% down on the left channel, stays nice and strong on the right (the stereo lamp comes on for stereo broadcasts as expected). Pressing the mono button allows equal sound out of both left and right channels at the normal volume levels (meaning the left channel comes back on to the expected volume level, but of course, no stereo).

As I was planning on a full restoration/service, here's what I've done so far:

All pots/switches thoroughly flushed out 3X to rule that out. While I was working on the tone board, I also went ahead and recapped that... no issues, same left channel result (volume drop-off) for FM Stereo mode.

I then went ahead and recapped the tuner board. No change. I then pulled and tested all transistors on the tuner board; they all "passed" on my handheld tester, so for now, back in they went.

I did then go ahead and grab voltages from those 6 transistors with my Fluke DMM... a few of the voltages look off. Here's the readings as they sit now (first # is the actual voltage reading / then the service manual voltage):

Q201 ( 2SC1047)
B: 2.2V / 2.1V
C: 14.3V / 14V
E: 1.5V / 1.4 V

Q206 ( 2SC828 )
B: 0.74V / 0V
C: 0.01V / 4.7V
E: 0.0V / 0V

Q152 ( 2SC1317)
B: 3.3V / 3.9V
C: 8.0V / 7.8V
E: 2.6V / 3.4V

Q301 ( 2SC828 )
B: 0.06V / 0.04V
C: 3.3V / 0.7V
E: 0.03V / 0.0V

Q304 ( 2SC1344 )
B: 1.9V / 1.9V
C: 7.2V / 6.2V
E: 1.3V / 1.3V

Q303 ( 2SC1344 )
B: 1.9V / 1.9V
C: 6.7V / 6.2V
E: 1.3V / 1.3V

Any thoughts on how to proceed? Hoping to get his amp back to 100%. Thanks for any guidance! Dave

 

[catrafter]

Hi Dave!
First since the documention is always suspect, you might need to verify that the circuitry matches what's on the schematic.
Q152 is in the AM circuit so I doubt that is an issue unless it's a clue as to why Q301 is on.
Q202 is the IF amp, an HA1137W integrated circuit, which also contains the FM demodulator circuit.
Q206 is part of the FM muting circuit. I can't see why Q206 would be turned on hard unless there is an FM muting problem??? FM muting switch? R212?

Why is Q301 on? This is supposed to kill the FM MPX VCO to prevent interference when you have AM selected(J207???). I would think that with the VCO disabled, the HA1196 MPX decoder would output equal left and right mono signal.
BTW, the Mono switch mixes the left and right channels together just ahead of the balance control, which is after the FM circuitry.

Tom

 

[dtafil]

Hi Tom - Long time, hope all is well with you! I've been over in Pioneer and Fisher land for a while...

FM muting switch *appears* to be working fine, as depressing the switch (when in stereo mode) does mute the FM signal until the stereo light kicks on for a strong signal and sound is heard out of the right channel. However, the prior owner had gotten into this unit for an LED swap, and broke off a couple tabs for the meter shroud (he did not go through the faceplate side to release the tabs from the front). The reason why I mention this is (from the top of the unit), looking down at the top of the muting switch, the left front muting switch pin appears to be bent/pushed perhaps 45 degrees forward toward the faceplate, probably from trying to get the meter shroud to snap back into the faceplate from behind. The solder joints for the mono switch looked fine underneath, but I reflowed the 3 of them anyway, just in case something was amiss there. No change. Not sure how that switch internally works; are there separate left/right channel connections within the muting switch? If so, could I jumper 2 (of the 6 pins from the top) as a quick test? Just curious.

Re: R212 - I've got my Sencore SG-80, but the FM muting appears to be working fine for the right channel; not sure if it makes sense to dig into that adjustment yet (?)

Re: Q301... thanks for catching that, and the info. Need to understand a bit more what's going on there... it's definitely putting out equal L/R channel output in mono mode, just stereo mode that's way off.

 

[catrafter]

You should be able to find the problem by using off the air stations, so you shouldn't need your SGG-80.
I doubt it is the Mono switch, but I'm not positive.
I don't know if you have a muting problem, I only mentioned it because the voltages you posted about Q206.
A botched stereo lamp installation could have blown the lamp driver on the HA1196, but that probably wouldn't kill a channel's audio.
If Q302 is working OK, you should have audio on both pins 4 and 5 with a mono or stereo station tuned in.
Here is a datasheet for the HA1196 which might help you figure out what's wrong.
The test circuit diagram may tip you off as to where the problem lies.

Tom

 

[dtafil]

Hi Tom - Just checked pins 4 & 5 on Q302 with my scope - saw audio for pin 5 (right channel out); nothing for pin 4 (left channel out). Bad IC?

 

[catrafter]

Dave,
That would be my guess.

Tom

 

[dtafil]

I was thinking the same; ordered a couple of these IC's yesterday, will report back next week. Thanks Tom!

 

[dtafil]

And fixed. Swapped in a new HA1196 IC, and working perfectly. Thanks again Tom!

 

[tonebells]

Great work!

Tony