Marantz 4240 repair, help please

[markallen]

Late to the party, as usual.

Anytime is on time for me, Tom, thanks!

If you have one power amp with problems, you have 3 others to compare it to. With the volume down, you can take DC measurements and record them on a good channel, then do the same for the bad channel. Set your meter to read DC volts, measure with the red meter lead, black meter lead to chassis.

Take DC measurement where? I should probably know this, but am drawing a blank. Speaker outputs?

Anyway, if the power supply is smoking (w/o DBT), isn't the rest secondary until that is corrected? I mean, am I damaging something else by any extended power on in this condition? Do I go ahead and change out the failed diodes and see what happens? I'm thinking the question is why they failed . . .

As far as the power supply goes, H805 thru H808 can be replaced with standard rectifiers, 1N4004 thru 1N4007 should be fine.
Collector of H801 should be around +50VDC, Base at around +35.6, Emitter at +35VDC.
Collector of H803 at around +35VDC, Base at around +14VDC, emitter at +13VDC or so.
J805 should be at around 9VDC or so, I think it's just for the Dolby lamp, but I'm not sure.

Tom

Good information, what I'm looking for - when the PS is working properly, correct? Just want to make sure I'm not missing something, which wouldn't be the first time.

 

[catrafter]

Repair the power supply first, of course. When the smoking stops you can move on. Replace H805-H808 then take Dim Bulb measurements on H801 and H803 and let us know.
H801
E
C
B

H803
E
C
B
Once we get the power supply fixed, power up without the dim bulb.

Find one good power amp. That will be one that has the bias and offset adjustable and that will output clean sound to a speaker with a good signal on the input.

Make a chart like this. Note, each power amp board contains two power amps. The odd numbers are for one side, the even are for the other, so H702B is the same point in the other power amp as H701B.
H701
E
C
B
H703
E
C
B
H705
E
C
B
H707
E
C
B
H709
E
C
B
H711
E
C
B
H713
E
C
B
H001
E
C
B
H002
E
C
B
Turn the volume down or remove the input.
Write down the DC voltages for each point in the good channel.
Now you know what to expect while troubleshooting the bad channel(s).

Tom

 

[markallen]

Thanks, Tom, for your thorough response. There is good news to report: it appears the entire problem WAS in the power supply.

Went through the PS board, and replaced the diodes with 1N4007s. Checked the electrolytic caps, and found C801 was bad, and a couple of others were well outside allowances, so replaced them. Checked all three transistors off board, and all indicated as functioning properly.

Reassembled enough to ensure no shorts, plugged it into the DBT - the bulb lit momentarily, and basically faded out! YES! Checked amp audio and auto voltage regulator adjustments, all were off but easily brought to spec per service manual.

Sounds fine, everything appears to be functioning properly! Interesting to me that who ever replaced the main outputs on the L channel apparently didn't bother to adjust the amp per the service manual, as these specs were far more off than the untouched channel specs.

Thanks again for the help. Gave me the confidence to move ahead.

 

[markallen]

There may be one issue. This unit is equipped with an SQA-1 decoder. As I have no quad sources, I'm inputting a stereo source at AUX/CD-4 front. When I turn the selector to SQ Decoder, I'm getting sound from one channel. I'd think I should get sound from at least two. So I switched the input to AUX/CD-4 rear. Nothing.

Does this indicate a problem with the decoder? Strikes me that 3 of 4 channels on that device don't work - unless I'm missing something here. On all other Mode settings, the receiver sounds great, all channels work as I believe they should.

 

[catrafter]

The SQA decoder is only needed for CBS SQ encoded LPs. The built in vari-matrix decoder does a decent job with most sources.

Tom