Marantz 1040 - Left Channel

[Kharrison]

Hello all,

I'm new here and am brand new to any kind of stereo repairs, so please forgive my ignorance. I recently purchased a Marantz 1040, seemingly in good working condition. After about a month of use there was a loud pop and now I get a strong buzzing from the left speaker anytime the amp is on. The buzz is constant regardless of speaker selector position, volume, source, etc.

Any thoughts on where to start?

Thanks in advance.

 

[Binkman]

Greetings and welcome to AK.

Is this your 1040?
edit deleted wrong link.

(loud pop sounds like a capacitor blew. would make said 'noise'. but lets be sure of model number.

 

[Kharrison]

No, it's actually this guy here: https://www.hifiengine.com/manual_library/marantz/1040.shtml

Definitely agree that it sounds like a cap. I downloaded the service manual and schematics from the link above, but wasn't sure where to start probing. Main amp, tone amp, power supply, etc.

Thanks!

 

[Lavane]

If the hum is in both channels I tend to check the power supply first. If only in one channel I start with cleaning all switches, and selectors. Then test for that hum in all inputs as well. Then check the amp board. Ive found many more problems on the amp boards in 70's Marantz amps over the preamp or power supply boards. A cap letting go could easily cause a hum. You may not see a bad cap. Ive replaced many electrolytic caps in them that looked ok, but had corroded leads and leaking fluid around the leads. The old glue used on some caps eat away and corrode them over time. If you see any of that old brown glue around any old cap you replace, clean it all off the board. It should peal off pretty easily.

 

[mbz]

Maybe excessive Vdc on the left channel, did not see and spkr protection/relay/not cap coupled.
Suggest measure dc voltage at left speaker terminals, should be less than 20mV. Could possibly be 1-20V

 

[willyrover]

Definitely agree that it sounds like a cap. I downloaded the service manual and schematics from the link above, but wasn't sure where to start probing. Main amp, tone amp, power supply, etc.

Thanks!

Divide and conquer.

You could uncouple the preamp from the amp by disconnecting the brown wire within the green circle from the amp board. (If hum is only on left channel, you could probably get away with disconnecting the white wire only.)

If hum continues, it's probably one of the capacitors on the amp board within the red square. Likely the purple, green, black, or gray one.

If the hum goes away, you'll have to check the preamp boards.

 

[Binkman]

Willy; nice pic.

 

[Kharrison]

Thanks all! Willy, the pic is a huge help. I just moved across the country so I'm still locating all of my tools, but I hope to start some troubleshooting this weekend based on the advice above. I'll post the results.

 

[Kharrison]

Hey, the good news is that I found my kit! I detached the white wire and the hum was still briefly present in the left channel, leading me to believe it's one of the caps on the Main Amp board. However, after roughly 2 seconds of being powered on in this state the fuse on F701 blew. I apologize for my ignorance, but what precautions should I have taken to prevent that?

Am I correct in assuming that my next steps are to replace the fuse and start replacing the purple, green and gray caps one-by-one and testing as I go?

 

[Oldsansui441]

Hey, the good news is that I found my kit! I detached the white wire and the hum was still briefly present in the left channel, leading me to believe it's one of the caps on the Main Amp board. However, after roughly 2 seconds of being powered on in this state the fuse on F701 blew. I apologize for my ignorance, but what precautions should I have taken to prevent that?

Hi, no you didn't do anything wrong, removing the pre-amp signal wire on the main amp board wouldn't have caused the fuse to blow, (l assume it was well out of the way and didn't short two components together etc). Don't replace fuse and power up, l would be checking the output transistors, one or more may be shorted or at least show some leakage, these can also present with a buzz when faulty, l have had it before.

Use the 6 way diode test to check output transistors, if you find any faulty check the preceding driver transistors, pre driver transistors, emitter resistors (the 4 large white ceramic wire wound resistors at botton of photo) etc.

As the other guys have suggested it would be a good idea to replace those main amp caps while you are in there. I repaired one of these a while back, they are a nice little amp and worth the time spent .

 

[Kharrison]

I replaced the grey, green and blue caps (and the fuse) last night and when I powered it up the loud buzzing was in both channels, where as it was only in the left before. My next step was going to be checking the transistors as mentioned above, but I'm wondering what I've done to make the problem worse. Any thoughts?

 

[woodj]

If the hum is in both channels I

He said left.

 

[Kharrison]

He said left.

Initially it was only in the left. After replacing 3 sets of caps on the Main Amp board it's now in both. If it had started in both I would have jumped to the power supply, but I'm wondering how my cap replacement has caused the issue to migrate to the right channel as well. Also, this isn't a minor hum, it's a very loud buzz regardless of volume position. It was actually loud enough that I'm worried I blew a tweeter on one of my speakers.

 

[Binkman]

Build a DBT (dim bulb tester) less than 10$ home despot and use a 100w bulb. Should give you an indication of ps issue. put unit in AUX no inputs hooked up and switches in off position & vol. off.

If dbt tests okay then rail voltages.. I'd pull the driver transistors to check and check any voltage regulator(s) they can go open and cause that buzz, too.

Don't do audio tests with good speakers. use cheap but measured rated ohms when you think all the bases are covered, including looking for cold solder joints.