Neet help troubleshoot Pioneer 1280

Leokarma

Well-Known Member
I recently got a Pioneer 1280 and seller said that it doesn't work. I need your help to troubleshoot the problem if possible or else I'll give it to a professional for repair. So far I have done following:

1) Connected via dim bulb tester and the receiver turned ON like it should i.e it Turned on (all lights) and after 8~10 sec. the trip clicked. No fumes, no small, no problem.
2) I connected the receiver without dim bulb tested and the receiver turned ON and after 8~10 sec. the trip clicked exactly how it's supposed to. No smell, no fumes, no problem so far.

No further testing was done or speaker connected because I'm scared. That's where I need your help. What should be my next step or sequence of steps to test this receiver ? Can someone help ?
Can I connect the speaker and test ? Should I test DC voltage across speaker terminal ? Check for fuse etc ??
 
It would be good to download the factory service manual if you haven't already done so. Study the pages on amplifier adjustment, Section 10.3, and connect your voltmeter as instructed to perform the DC offset (DC balance in Pioneer-speak) and bias (idle current) checks.

It would be safest to do this with the DBT connected at first in case the adjustment potentiometers are intermittent and break contact. This can cause the amp to run away and destroy the outputs. If the adjustment can be performed on the DBT without incident, connect to line power and repeat the adjustment.

Once the DC offset and bias are set, it should be safe to connect speakers for a test drive.
 
Does your unit have the jumpers from Pre out to Amp in installed. If not, install a set of RCA cables and do a DC offset check before you try hooking up a speaker.
 
It would be good to download the factory service manual if you haven't already done so. Study the pages on amplifier adjustment, Section 10.3, and connect your voltmeter as instructed to perform the DC offset (DC balance in Pioneer-speak) and bias (idle current) checks.

It would be safest to do this with the DBT connected at first in case the adjustment potentiometers are intermittent and break contact. This can cause the amp to run away and destroy the outputs. If the adjustment can be performed on the DBT without incident, connect to line power and repeat the adjustment.

Once the DC offset and bias are set, it should be safe to connect speakers for a test drive.

Ok will do this tomorrow and report....
 
Does your unit have the jumpers from Pre out to Amp in installed. If not, install a set of RCA cables and do a DC offset check before you try hooking up a speaker.

Yes jumpers are installed. Will report the findings tomorrow
 
Thanks everyone. Here is what i did this morning....

DC Balance :

Both the channels set to near zero. The only problem is that the voltage on right channel is not 100% constant. The voltage on right channel flactuates between +/- 2.5 milli volt.

Idle current : set to between 10 to 25 Mill volt.

I have also tested voltage across speaker terminal and there is no voltage.

So now can I connect speaker and test ?
 
If you don't already have a set of disposable speakers, get some. First fire it up with a set of speakers that are of no value and let it run on those till you know all is good.
 
Offset and bias appear to be acceptable. If you have some sacrificial speakers as treeslayer suggests, try connecting them. If not, you can connect appropriate speakers but keep the power levels low and gradually adjust the volume up over time. It SHOULD be safe, but...
 
Here is the latest

~ I connected pair of speaker and there was no sound on both channels.
~ Tested Phono and on channel was working.
~ Connected an external pre amp via main Amp IN of 1280 and both the channels are working perfectly . So the power amp section is OK (good news)
~ Connected a power amp to the pre out of 1280 and only one channel works.

So based on above can we conclude that the pre amp section of 1280 is the problem or more testing is recommended ?
 
With the phono operating on one channel, do both channels operate if you depress the MONO selector?

The phono input feeds through ALL the audio frequency amplifier stages, so ALL of the flat, tone, and pre-amp stages are operating in at least one channel. Therefore, the AUX and TAPE inputs should also work on at least the one operating channel. If they do not, that is an indication that the signal is not getting from the input jacks to the preamp stages.

Since the power amp is functioning, it might be helpful to clean and de-oxidize the contacts of all the function switches and input source RCA jacks. The loudness, muting, tone, and all other switches might as well be cleaned while you're in there. This step alone may solve all the problems, or may not, but we cannot be certain of their condition unless they are cleaned and exercised, thus additional troubleshooting might be meaningless.
 
Please see some correction as below :-

1) If I connect pair of speaker to 1280 only Right channel works. Left is out. In mono mode both the channels works.
2) If I Connected an external power amp to the pre out of 1280 the result is same as above i.e. Right working and left is not. In mono both the channels works.
3) If I connected an external pre amp via main Amp IN of 1280 both the channels are working perfectly . So the power amp section is OK.

In 1 and 2 above the sound from right channel is distorted some time.
 
With the phono operating on one channel, do both channels operate if you depress the MONO selector?

The phono input feeds through ALL the audio frequency amplifier stages, so ALL of the flat, tone, and pre-amp stages are operating in at least one channel. Therefore, the AUX and TAPE inputs should also work on at least the one operating channel. If they do not, that is an indication that the signal is not getting from the input jacks to the preamp stages.

Since the power amp is functioning, it might be helpful to clean and de-oxidize the contacts of all the function switches and input source RCA jacks. The loudness, muting, tone, and all other switches might as well be cleaned while you're in there. This step alone may solve all the problems, or may not, but we cannot be certain of their condition unless they are cleaned and exercised, thus additional troubleshooting might be meaningless.

OK. Will DeOx and report....
 
Since the power amp is functioning, it might be helpful to clean and de-oxidize the contacts of all the function switches and input source RCA jacks. The loudness, muting, tone, and all other switches might as well be cleaned while you're in there. This step alone may solve all the problems, or may not, but we cannot be certain of their condition unless they are cleaned and exercised, thus additional troubleshooting might be meaningless.

:thmbsp::thmbsp:

A good bath of Deoxit and alls working like it's suppose to. There are some minor issues like changing of bulbs etc. which I'll take care. Thanks again everyone.
 
Nicely done. Now you can see the importance of clean switches and controls via firsthand experience. Don't be too discouraged if the unit starts exhibiting the symptoms again, requiring a second cleaning. It's fairly common. Use it for a while and determine how it is going. Since you've set the offset and bias, it should be ready for a good test drive. Pay attention to chassis heat as you start pushing more wattage, and be prepared to throttle back if it begins to get uncomfortably warm.
 
Nicely done. Now you can see the importance of clean switches and controls via firsthand experience. Don't be too discouraged if the unit starts exhibiting the symptoms again, requiring a second cleaning. It's fairly common. Use it for a while and determine how it is going. Since you've set the offset and bias, it should be ready for a good test drive. Pay attention to chassis heat as you start pushing more wattage, and be prepared to throttle back if it begins to get uncomfortably warm.

Great thanks...
 
Guys,

I screwed up.....

While cleaning the receiver I accidentlly dropped a small screwdrive right on the four metal fins on the power protection circui board (see attached picture with orange circle) and there was sparks in that area. You can see the burn marks below the 4 brown capacitors. Receiver was working perfectly fine below this incident. Although the sparks looked beautiful but now the receiver turns ON and within a sec turns OFF. Then after 4~5 secs. it'll turn ON again and then within a sec. turns OFF. This goes on.

Sorry guys but need your help again. Do you think I'll bee able to repair this ?
 

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That really sucks, it's possible you took out all 4 of those bridges. I would not power it up anymore, if you did not already fry the outputs you might if you keep powering up, and they are not available.
 
BUILD A DIM BULB Tester and DO NOT PLUG IN THE 1280 without it. Use a 100W bulb at minimum. This will limit further damage to it. Good possibility when the POWER DIODES shorted, they took out the OUTPUT Transistors. So those will have to be tested. I don't envy you this job. With that I'll bow out and let Watthour, Petehall, or Hyperion get on this bucking bronco and take a ride.

Unfortunately this gives you 1st hand experience with UN-INSULATED Tools. Wrap the shaft of your screwdrivers with a layer of electrical tape all the way down to the tip.

Larry
 
Sparks make my stomach drop! I had a relapse after reading your post!!!

Those are the "BIG DIODES" for the rectifier circuit which make DC power out of AC house power. The protection is reacting to the damaged power supply.

Unplug the Amp boards and check the outputs.
You can run the set without the outputs connected.
Check the Power supply voltages at the pins to assess the damage.
The PS might of survived as the protection is working.
Start looking for the diodes. Probably a Donor board.

Like Larry said insulate. Heat shrink is another alternative to tape.
 
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