Humming Problem in Technics SU-7200 Integrated Amp

[sjith]

Hi Guys,

I got a vintage technics SU-7200 stereo integrated amp last week (1976 model, aluminium front panel & wooden case). one channel was not working and it fixed by changing the fuse. controls were not smooth and fixed it by applaying video 90. cleaned the whole amp properly by CRC and resoldered all component solderings in the printed circuit board. now its working but giving excessive humm in both channels. this humming starts immediately after switching on the amp irrespective of input selection and volume bass treable control position.

please help me for solving the issue

thanks

 

[Audiodoc]

Possibly a grounding issue. Check if the star ground is in connection with the AC input ground. Also check if there are any ground loops.

 

[sjith]

@audiodoc
thanks
tre is a grounding point in the back panel of the amp. thts connected to the grounding point of the power outlet
note: the power chord of the amp is only two pin, no ground with the chord
can u detail about the "groud lops"

 

[wlg]

Use a three pin plug, connect the external grounding point to the grounding point on the plug (top point). If the humming still continues may be a capacitor (power) has gone bust.

 

[koushik]

Change the power caps if they are old or try a higher value caps. See if this works.

 

[omishra]

Infact all eletrolytic capacitors from power line to ground need to be replaced as they already lived their life and must have gone heavenly abode.

 

[ANILKUMAR]

Infact all eletrolytic capacitors from power line to ground need to be replaced as they already lived their life and must have gone heavenly abode.

Check the following
1. The Ground cable for each channel from the cartridge either disconnected or loosened
2. The ground cable with in the Turntable(TT) (the other end from the cartridge) is disconnected or loosened

3. Check if the ground section in the power supply section with in Turn table

4. If there is built in Pre-amp, check the ground cable

5. Ensure that the turntable is not placed above an amplifier or pre-amplifier or graphic eq.

6. Check if ground outlet cable from TT is properly connected to a Pre-Amp or Amplifier.

Anil

 

[sjith]

@wlg

according to the commment, the power filter capacitor replaced with new ones but no effect (its 3300 mfd/35 v) the new capacitors are small in physical size compared to the original ones. and I noted tre no bulging in top of the faulty capacitor. I heared if an electrolytic capacitor is faulty tre is a bulging ith top of the capacitor ? shall I go for a 4700 mfd/35 v instead of 3300 mfd ?

 

[sjith]

@ omishra

thanks

@ anilkumar

TT is not connected with amp 7200, its connected to a cd player

 

[pnreddy]

If the problem is still there after replacing the capacitors, check the input cables for faulty/broken shielding. You could remove all the input cables and switch on the amp. If the problem persists then it is within the system. Otherwise connect each cable and check for the culprit. Hope this helps.

@wlg

according to the commment, the power filter capacitor replaced with new ones but no effect (its 3300 mfd/35 v) the new capacitors are small in physical size compared to the original ones. and I noted tre no bulging in top of the faulty capacitor. I heared if an electrolytic capacitor is faulty tre is a bulging ith top of the capacitor ? shall I go for a 4700 mfd/35 v instead of 3300 mfd ?

 

[shankar bhandarkar]

Dear Friend,
I am a fan of PIONEER products,I am using SX 880 coupled with Boston Accostics VR 20,
this one set of SA 7800 and TX 7800 coupled with Bose speakers, The Issue is if ever I want these to be serviced in mumbai. I am unable to find a good technician who can attend me, Some of these equipments re arround 15-18 kg,travelling with this is not a joke atall, Paying the technician is no problem if I find one,with the Indian Mindset as is I am lost in mumbai... Can U advise please....?
S Bhandarkar

 

[aks07]

Dear Sir

The situation as of now is:
- Humm at both channel irrespective of the volume/
- PSU Capacitors replaced with no improvement
- No intermittent connections. Are you sure all the solder joints are good?

Prima facie it doesnt look like a ground loop issue to me. Please short circuit the input and power on the amp. If hum persists than the problem is further up. Power section is a prime culprit but you have already replaced the elcos. Check the diodes. Check other capacitors in power supply section. There would be small (anything between 1uf to 100uf) decoupling capacitors. Replace them all if you can.

What type of output devices this amp has? BJT or STK? If STK then dual channel type? Check the main rail that power this IC. There could be decoupling capacitors right where the main DC goes into the STK. See if they are ok.

Do you see any signs of previews repairs? If yes then it will be worthwhile to search for a schematic and cross check each components for value and specially polarity in case of elco cap.

As of now I think it is the power supply and its downwards components

 

[sjith]

Dear Sir

The situation as of now is:
- Humm at both channel irrespective of the volume/
- PSU Capacitors replaced with no improvement
- No intermittent connections. Are you sure all the solder joints are good?

Prima facie it doesnt look like a ground loop issue to me. Please short circuit the input and power on the amp. If hum persists than the problem is further up. Power section is a prime culprit but you have already replaced the elcos. Check the diodes. Check other capacitors in power supply section. There would be small (anything between 1uf to 100uf) decoupling capacitors. Replace them all if you can.

What type of output devices this amp has? BJT or STK? If STK then dual channel type? Check the main rail that power this IC. There could be decoupling capacitors right where the main DC goes into the STK. See if they are ok.

Do you see any signs of previews repairs? If yes then it will be worthwhile to search for a schematic and cross check each components for value and specially polarity in case of elco cap.

As of now I think it is the power supply and its downwards components

thanks sir
now its kept aside
collecting the things to replace
once everything is ready, start

 

[SIDEWINDER18X]

Hi Guys,

I got a vintage technics SU-7200 stereo integrated amp last week (1976 model, aluminium front panel & wooden case). one channel was not working and it fixed by changing the fuse. controls were not smooth and fixed it by applaying video 90. cleaned the whole amp properly by CRC and resoldered all component solderings in the printed circuit board. now its working but giving excessive humm in both channels. this humming starts immediately after switching on the amp irrespective of input selection and volume bass treable control position.
please help me for solving the issue
thanks

Every one has their own views & opinions. I respect that. But my point of concern is that... are you technically sound enough to tweak the amp.
If yes, then...... I would suggest you to diagnose the amp in sections to find out which stage is responsible for the hum.
Take the following steps & advance further if problem exists.
Disconnect the power amps input from the tone control pre-amp output.
If hum is gone pre-amp stage is responsible.
If hum is still there check (+ 0 - ) power lines. Check main ground ... is it connected to chassis ?. Check for corrosive joints & solder.
Can you measure off load quiescent current of the output stage individually. Check it should not be in any case more than 80 miili amps.
Primary Filter capacitors are very unlikely to get faulty in these amps. These are much reliable than Indian ones. Smaller one's can sometimes be the culprit.
Check those. Can you hear or feel any buzz,vibration or hum in the power transformer ?.
Keep informed on the status so that I can say anything further.
Vivek

 

[greenhorn]

if you have a smartphone, download a frequency analyzer and check if the hum is 100 hz or 50 hz. if it's 100 hz, you know where too fix it.

Also, you should have a bunch of decoupling capacitors. look for anything with 100uF or more with the -ve terminal grounded. I'd try replacing them too.

 

[sjith]

if you have a smartphone, download a frequency analyzer and check if the hum is 100 hz or 50 hz. if it's 100 hz, you know where too fix it.

Also, you should have a bunch of decoupling capacitors. look for anything with 100uF or more with the -ve terminal grounded. I'd try replacing them too.

yes I am using an android phone
do download the same apps and update the freq.

 

[sjith]

Every one has their own views & opinions. I respect that. But my point of concern is that... are you technically sound enough to tweak the amp.
If yes, then...... I would suggest you to diagnose the amp in sections to find out which stage is responsible for the hum.
Take the following steps & advance further if problem exists.
Disconnect the power amps input from the tone control pre-amp output.
If hum is gone pre-amp stage is responsible.
If hum is still there check (+ 0 - ) power lines. Check main ground ... is it connected to chassis ?. Check for corrosive joints & solder.
Can you measure off load quiescent current of the output stage individually. Check it should not be in any case more than 80 miili amps.
Primary Filter capacitors are very unlikely to get faulty in these amps. These are much reliable than Indian ones. Smaller one's can sometimes be the culprit.
Check those. Can you hear or feel any buzz,vibration or hum in the power transformer ?.
Keep informed on the status so that I can say anything further.
Vivek

thanks
I am not an expert. started with diy cassette players amps and speakers while schooling time. gradually stopped everything because I was away from home during studies and after job. career in core electrical field. now started again with an onkyo htib in 2011. noted all your findings and suggestions and will try my best (its a single board psu pre tone power amp everything in board)

 

[SIDEWINDER18X]

thanks
(its a single board psu pre tone power amp everything in board)

You can easily separate pre out from power amp on board. Open or disconnect the capacitor's one end from the power amp's input stage. That is where you need to understand schematics. If you do, it's a piece of cake. If you do not,:sad: then I advise you to take support from any tech person offline.
Vivek

 

[Doorz]

Hello, i just got this amplifier, and i had the same hum problem.
Checked the main caps c 301/302/303/211/212, all were ok.
The solution was to re-tighten the two screws which attach the heat-sink to the circuit board, as well as the two screws which attach the heat-sink to the chassis.
The hum has disappeared.
Before someone says "this thread is old", i know, but there may be someone in a few years who has the same problem, and this is the only thread i could find.