Marantz 2240 Low FM Output

[zaskarx]

About a year ago I re-capped my 2240, it sounds great but about a week after I finished the recap the FM section output dropped to the point where it is only audible if the volume is turned up past 1 o'clock or so and even then only barely.

I've been living without FM since but would really like to fix it I've downloaded the service manuals and studied the schematics, I've verified that all boards related to FM have the proper DC voltage. I've also double checked the values, orientation, and solder work on all of the replaced caps.

I suspect a failing diode or transistor but instead of testing each one I thought I'd check in to see if anyone can help narrow down where the problem may be.

 

[catrafter]

I have a couple of questions that might help narrow down the problem:
Does the signal strength meter work properly?
Does the tuning meter center when you tune in a station?
Does the stereo lamp light on a strong stereo station?

Tom

 

[zaskarx]

Thanks for helping out Tom. The signal strength meter works and the tuning meter centers when a station is tuned in, however the stereo indicator does not light up.

On the MPX board there is an IC (H321) and from what I can tell pin 6 is responsible for lighting up the stereo lamp. When a strong station is tuned in I see 13V on pin 6 which is close to the 14V indicated in the schematic - but still no light.

 

[catrafter]

Pin 6 on H321 should go low to light the stereo lamp, see the partial datasheet below.

Is there 13VDC on J306 and on pin 1 of H321?
Is there 35VDC on J314 and around 22V on the end of R343 that doesn't connect to J314?
If so, I'm thinking that either the FET switch in the input path (H301) is turned off or defective. With a strong station tuned in what do you measure DC wise on the S,D and G of H301?

If you temporarily short the S and D of H301 together does that make it play at normal volume with the stereo lamp on?

Tom

 

[zaskarx]

Here are the results:

J306 - 0 VDC (guessing this is a problem?)
H321 Pin 1 - ~13 VDC

J314 - ~35 VDC
R343 - ~22 VDC

H301
S - 6.2 VDC
G - 6.13 VDC
D - 5.58 VDC

When S and D are shorted there is no change in volume and the stereo lamp does not come on.

 

[catrafter]

Hmm.
J306 should be zero when in FM mode. It's job is to put a positive voltage on the base of H302 in AM mode, which will turn H302 on and kill the VCO so it doesn't interfere with AM reception.
It could be a bad H321, HA1156W. Can you measure the DC voltages on all H321 pins in FM mode (mono button not pressed) with a strong station tuned in?
Do you have access to an o'scope and the skills required to use it?

Tom

 

[zaskarx]

Unfortunately I don't have a scope or the skill to use it, do you offer a class? I'd sign up! The guys down at the local makerspace may be able to help me out though if it comes to that.

As far as H321 goes here are the values in VDC, pin 8 is the only one that seems to differ from the schematic which shows 0 VDC:

1 - 11.89
2 - 3.02
3 - 4.96
4 - 9.16
5 - 9.15
6 - 13.42
7 - 0
8 - 2.29
9 - 2.29
10 - 1.51
11 - 2.30
12 - 2.30
13 - 2.30
14 - 3.07

 

[catrafter]

Pin 8 goes to zero to force mono mode and is controlled by several things I will post below, so I doubt that is your problem.

I just noticed that this receiver has Dolby FM. Is the level low with the Dolby button out and in?
Did you clean the Dolby switch?

If you can make it to Prescott, I'll bet rBuckner will give you a scope lesson, what say Randy?

Tom

This applies to the 2240, 2275, 2325 and probably the 2250B as well although some of the component designators are different.

FM Stereo MPX operation
IF signal strength is monitored by H308. If signal strength is above muting level, H308 is ON, H309 is OFF.
If there is excessive noise, then H311 is ON which is wire-ORed to H309 keeping H301 (FET) OFF.
If the discriminator is outputting DC due to the tuning being off station, either H314 or H315 is ON which are wire-ORed to H309 as well. This also turns H303 ON grounding Pin 8 of the decoder forcing MONO operation.
If the station is properly tuned, not noisey and the signal strength is high enough,
H308 is ON, SS OK
H309 is OFF, SS OK
H311 is OFF, Noise is OK
H314 is OFF Tuning centered
H315 is OFF Tuning centered
H317 is OFF, SS, Noise and center tuning OK then:
H301 is ON allowing audio and MPX signal to pass into the decoder.
H303 is OFF If SS sufficient for Stereo operation and MONO button is not pressed
H302 is OFF VCO runs (AM not selected)

 

[rBuckner]

...If you can make it to Prescott, I'll bet rBuckner will give you a scope lesson, what say Randy?
...

Sure thing! Bring the 2240 with ya too. Seriously PM me if you want to come up sometime. Fridays are usually the best but Saturdays can work.

 

[zaskarx]

Ok, you are on to something with the Dolby circuit. I had cleaned it (along with the rest of the switches and pots) a few days ago but it didn't seem to make a difference then.

After reading your last post I pushed the Dolby switch in and out a few times just for grins and the stereo lamp and full volume returned! Thinking that maybe I hadn't cleaned it thoroughly enough I cleaned the Dolby switch yet again with Deoxit, waited about 10 minutes, and unfortunately it is now back to the same problem.

I am guessing that the switch must be extremely dirty, as best I can tell the receiver had been a combination ashtray/cat bed when I bought it a few years ago so perhaps there is some lingering grime causing a poor connection. I'll try cleaning it again in the morning to see if that makes a difference and post the results.

A scope lesson would be great! Randy - I'll send a PM shortly.

 

[zaskarx]

Woke up this morning and tried FM, full volume, stereo light...for about 2 minutes then it went back to the same issue.

 

[catrafter]

Since the stereo light comes and goes with the problem, I'm thinking that it doesn't have anything to do with the Dolby switch, but is perhaps a loose connection or bad solder joint that is making and breaking as you flex things. See if tapping on the MPX (P300) board with an insulated tool, like a chopstick or something, leads you to an intermittent connection. Look very closely at the center conductor of the shielded cable connected to J301, it is fragile and easily broken.

If I were closer, I'd go to Prescott just to meet Randy and see his fantastic shop!

Tom

 

[rBuckner]

Tom - you know you are welcome! :thmbsp:

I also use the handle end of a screwdriver to tap on boards to check for meachanical intermittants.

 

[zaskarx]

Did a little tapping on the board with a screwdriver handle and sure enough seems to be something on the rear, left hand side of the MPX board. Going to look closely for cold solder joints, loose wires etc and will let you know what I find. Thanks again for the help guys, would never have gotten this far without it!

 

[rBuckner]

It could be a swollen electrolytic cap. When the swell that bad they can push the leads out until they're disconnected. If it's still close enough it could be doing what you're experiencing. One other thing to do then is to gently move the caps a bit individually with your non-conductive poking tool and see if it can be isolated.

Quite possibly it could be a cold/cracked solder joint too. Checking for this would require board removal. On some models this can be the worst board to get to! So of course that's why it's acting up.

 

[catrafter]

It's great that you have isolated the problem!

In another thread, I posted a procedure to access the MPX board on a 2275, the 2240 should be about the same.

Tom

 

[zaskarx]

Success! It ended up being a bad solder joint on cap 304, it is a real pain to get to and I must have mis-soldered it when I recapped the unit last year, rookie mistake. I re-flowed the solder and all is well.

You guys are a wealth of knowledge, the tap & poke method worked like a charm in pinpointing the exact source of the problem. Thanks again for sticking with me.

 

[rBuckner]

Niiiiice! :yes:

 

[catrafter]

Glad you got it!

Tom

 

[HMEmerson]

Success! It ended up being a bad solder joint on cap 304, it is a real pain to get to and I must have mis-soldered it when I recapped the unit last year, rookie mistake. I re-flowed the solder and all is well.

You guys are a wealth of knowledge, the tap & poke method worked like a charm in pinpointing the exact source of the problem. Thanks again for sticking with me.

I know this thread is 3 years old, but an old Marantz 2240 is still just that, right? I was recently given one that had not been operated in over 10 years. I had all the controls cleaned and it was really nice for a few days. The bridge rectifier blew out, and has been replaced.....probably not the end of the procession, but I digress....

Now I have been having the exact same situation with getting perfect FM stereo reception for about 2 minutes, then a little crackle, and poof: The stereo light is out, the FM volume is lower, etc. I video recorded the proceedings to show my repair guy, but more than that I'm going to show him this thread. By the way: I did the same 'experiment' from cold start with my CD player on. The stereo light went out, but the volume did not change at all, and the CD is still playing in stereo, absolutely. On the FM setting, it definitely goes into a mono mode. I set the FM muting level to minimum, and can only pick up a couple of stations with the FM mute button engaged. Also the signal strength meter responds, but not very much. I hope my repair guy's tap & poke leads to a similar finding.

Thank you all for being the fusspots that you are! I'm an acoustic guitar composer & teacher, and I'm much worse than you all

Best regards,
Howard Emerson