Marantz 1250 phono stage issues

[Streamz]

I have a Marantz 1250 which I have had for many years. The phono2/mic has section has never worked but recently the phono1 section has begun to produce a loud hum over the audio and has now developed into no audio and just hum. When switching to another input the hum can be faintly heard across all inputs.

I thought it might be a bad capacitor so I have begun recapping the unit with Nichicon FG. I have replaced all of the caps apart from the 4 axials on the power supply (waiting for replacement) and the main cap which I wont replace.

The problem still remains. Even when running the unit with the P400 phono board out of the unit the hum is still there.

Any thoughts?

 

[mbz]

Not familiar with the 1250. Should check RCA connections be it solder to board or wires, looking for dry/bad solder joint. Also check the ground wire to the TT.
Will try and track down sm.

 

[Streamz]

the rear RCA jacks are not easily accessed on the 1250. I have tried pushing the RCA's to see if the hum changes but no luck. Could be a bad solder connection. I may have to pull this apart further.

Interesting point. With no RCA's connected at all the hum is slightly louder. When connecting a single RCA to one of the inputs the hum drops somewhat.

Turntable is grounded.

 

[mbz]

Suggest checking the J401 connector to the phono board, ie, unplug/clean maybe check for resistance of pins 8,9,10 to GND

 

[Streamz]

Will deoxit the connector. The P400 on this a single long connector. pins 8,9,10 on the P400?

 

[mbz]

Maybe easiest to measure resistance at either R401 or R402 and chassis, no need to clean connector if less than 1 ohm

 

[Streamz]

thanks. I'll check tonight.

 

[Streamz]

I have checked resistance between 7,8,9,10 and they all have negligible resistance to ground. Also checked continuity between the RCA jacks and the pins 7,8,9,10 and there is good contact. Checked the center connection of the Phono RCA jacks to the rest of the connectors and they check out. There may be a short between the Phono 2 center connectors and GND. When i put the multimeter probe into the RCA jack i had continuity with GND and the signal pin on the phono board. This didnt occur with Phono 1's RCA connectors.

 

[mbz]

Took a punt that it was a phono issue and that the low level hum was just some crosstalk
or leakage between inputs. Also louder in phono because of higher sensitivity. Hum can be
caused by many factors and somewhat tricky to track down.

Firstly, the short between Phono 2/MIC RCA and ground is normal behaviour, to short
the not used input to reduce noise.

A more systematic approach would be,
- Hum occurs on both channels? Yes would indicate a common area like power supply
- If hum only on one channel you could use an RCA cable from PRE-OUT to MAIN-IN but
cross the channels ie, LEFT PRE-OUT to RIGHT MAIN-IN and RIGHT pre the Left main in
- For completeness you should run the amp as a seperate, if you have another amp, ie,
run the 1250 as a PRE connected to the MAIN IN of another amp. If no other amp
available you could connect your IPOD/Ixxx to the MAIN-IN at ZERO volume and slowly
raise the volume.

Most probably issue with power supply and hum will be present in each of the above "tests"
The main filter caps are strong candidates, technically also the rectifiers however they
don't fail that often. Could also be a leaky/tired component in the regulated voltage supply
like H801,2.

Will try and work out next step, I assume you don't have an oscilloscope...

 

[Streamz]

I will try the pre/main connector options you suggest. I do have another amp to test with. Unfortunately I do not have an oscilloscope.

I have the 4 large axial caps for the power supply on their way to me. The current axial in there look original and though I cannot see signs of leakage the rubber ends are cracked

Will post the results of the pre/main connections

 

[mbz]

I have the 4 large axial caps for the power supply on their way to me. The current axial in there look original and though I cannot see signs of leakage the rubber ends are cracked

The main caps are candidates. They no longer hold their correct charge so there is more ripple, this ripple finds its way into the audio path via the voltage rails etc...

 

[Streamz]

after further testing these are the results.

Main out of Marantz to my Kenwood amp tape in. Same hum on both phono 1 and 2. No bleed through hum on the other inputs, just clean audio
Main out to Pre in on Marantz jumpered with RCA's. Same hum on both phono 1 and 2 and bleed through hum on other inputs
Hum is exactly the same on both channels in all conditions

Also of note with the volume all the way down and putting my ear to the amp itself I can hear a faint buzzing/hum from inside the amp itself.

Pointing to power supply caps?

 

[mbz]

Main out of Marantz to my Kenwood amp tape in. Same hum on both phono 1 and 2.

Results suggest to me that problem in phono stage or power supply that feeds phono stage since both channels affected. Will focus on hum issue
and hope the bleed through will "fix itself" once the hum has been resolved. Need a minute to check schematic, phono power supply...

 

[mbz]

The phono stage is fed with +/-55Vdc, this is also used to power the front end
of the power amp. Maybe it is a psu issue and the blled through is from the power
amp stage.

The +/-55V supply is regulated, not easy to track down. The main caps are C801,2 1000uf/70V.
You can inspect, maybe replace but doubtful this will be the issue. More likly a tired
transistor/zener/cap... in the regulated supply.

An oscilloscope would be helpful, time for another punt, need to identify if it's the pos
or neg voltage rail.

Measure AC voltage with black probe connected to chassis and red probe at J808, 809 or 810.

Measure AC voltage with RED PROBE connected to chassis and BLACK probe at J814,815,or 816.

Hopefully you will have a cheap MM like I do, it will think the ripple is AC voltage. Hopefully
one rail will be significantly large than the other.

 

[Streamz]

I have measured those voltages. Not sure how to measure AC voltage on my metre but J810 to chassis GND is 55.3v. J816 to GND is 68V

 

[mbz]

I have measured those voltages. Not sure how to measure AC voltage on my metre but J810 to chassis GND is 55.3v. J816 to GND is 68V

J810 55.3V is good
J816 68V firstly should be negative, assume typo, it should be -55Vdc.
-68Vdc suggests H802 or H804 has failed.

Double check voltage at pin J816 (or 814,5), expect -55Vdc
Diode check H802 and H804, incircuit should be ok, looking for a short.

 

[Streamz]

Yes you are correct it should read -68Vdc. What is the method for diode checking H802 and H804?

thanks for your help mbz.

 

[mbz]

Nice graphic from markthefixer. I should pay him royalties, I've used it often enough. Use diode test function availble on most
multimeters, two results expected, OL=open loop/open circuit, no current flow. 650mV, nominal diode drop, could be 550-850mV maybe?
RED/BLACK refer to probe orientation. The -68Vdc strongly suggests one of the transistors involved in voltage regulation has failed short.
This should cause an increase in ripple that's probably/maybe??? the cause of the hum...

 

[CohibaJoe]

Here is my Recap:
http://audiokarma.org/forums/index.php?threads/marantz-1250-recap-restore.620560/

 

[Streamz]

Diode test is a fail on both H802 and H804. tested some other transistors to be sure and they tested ok.

Will these be these give out on their own or would something else further up the chain have killed these transistors? Are replacements available?