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 Post subject: Pioneer SX-780 output meters not balanced
PostPosted: Mar Tue 03, 2020 2:51 pm 
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Joined: Mar Mon 02, 2020 12:59 am
Posts: 4
Newbie here, so apologies in advance if I use the wrong terminology. Feel free to correct. I won't be offended.

I have a 1979 Pioneer SX-780 that I purchased new. During the warranty one channel stopped working and I sent it to an authorized repair location for repair. They fixed it but ever since then the output meters have displayed different levels even though the sound level was the same right and left. It never bothered me because I always figured I would just replace the receiver when it died or I needed better. Well that never happened and I recently put the receiver back in use. The volume knob was scratchy so I had to take it apart to clean with deoxit. Since I had it open, I took a look at the work that was done in 1980 and tried to adjust the meters.

The only thing that looks worked on is the darlington power pack area. There are 2 sand block resisters on each circuit. One of the resisters on one of the circuits is much larger that the other 3. I don't know why, availability perhaps? Not sure if this is why the output meters move at different rates or not.

I did find 2 pots (white plastic) near the meters, so I threw all caution to the wind and started twisting. I was able to get the meters to match up at a set volume, however when I change the volume the meters go off sync. There are 2 more pots (blue plastic) on the board, so I twisted those and nothing happened so they don't appear to be part of the meter circuit.

It seems that the white pots will adjust the offset, but the "gain" is off so they don't stay in sync.

My question is, how do I adjust the output meters so they match right and left throughout the entire volume range?

Also, is there a schematic available for this unit somewhere on this site?

TIA, James


Attachments:
File comment: In this picture you can see the 4 pots and the 2 different size white sand block resistors on the bottom near the Darlington power pack.
IMG_5509.jpg
IMG_5509.jpg [ 76.86 KiB | Viewed 630 times ]
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 Post subject: Re: Pioneer SX-780 output meters not balanced
PostPosted: Mar Tue 03, 2020 3:59 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 17575
Location: Utica, NY 13502 (USA)
User manual, service manual, and schematics are available for free at HiFiEngine but you need to register for free to download them. I find the site an essential resource.

https://www.hifiengine.com/manual_libra ... -780.shtml

Have you measured the receiver's outputs with a tone to see if the meter or its circuit is actually at fault?

A small-value electrolytic coupling cap with high ESR was causing this problem in the meter circuit of my Carver amp.

Dave


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 Post subject: Re: Pioneer SX-780 output meters not balanced
PostPosted: Mar Tue 03, 2020 10:52 pm 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4912
Location: Sunnyvale CA
Dave Doughty wrote:
Have you measured the receiver's outputs with a tone to see if the meter or its circuit is actually at fault?


+1 on that one, feed it a mono sine wave signal at low frequency (like 500 Hz), and measure the output AC voltage on each channel.

Just listening can easily be off by a factor of two (3db audio, 6 db voltage gain) without being obvious. A normal load should be applied (speakers or 8 ohm resistor) but don't get crazy with the volume levels.

If so, correcting it may not be that easy.

Brett


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 Post subject: Re: Pioneer SX-780 output meters not balanced
PostPosted: Mar Wed 04, 2020 3:19 pm 
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Joined: Mar Mon 02, 2020 12:59 am
Posts: 4
Thanks for the links to documents and suggestions. I followed the procedure in the service manual for DC power balance of amplifier and meter drive circuit adjustments.
I still have unbalanced meters when I change volume. I can match the meters at a set level, but they don't match when I change level.
I measured the value of the larger resister and it matches the smaller resisters, so I think it's just a higher wattage and that should not affect the circuit. Oddly enough a watched a you tube video repair of this receiver and it had the same larger resister on one of the channels, but it was the opposite Chanel.

I checked the darlington hybrid IC's and they are the same part number. I suspect that is what was replaced under warranty and that is when this problem started so maybe there is something different with the IC's or the technician screwed up something during the repair.

I have read that the sound can be improved by replacing the darlingtons even if they are working OK. I also read that originals are not available and all the ones offered on ebay are reproductions and not as good as originals, so it is unclear to me if anything can be gained by replacing them other than finding out if that is the cause of the non-linearity of the meters.

The meters have been off for 40 of the receivers 41 year life. I may just balance it at my average listening level and live with it.


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 Post subject: Re: Pioneer SX-780 output meters not balanced
PostPosted: Mar Wed 04, 2020 4:33 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 597
Location: Smithfield, Pa. 15478
Unsure what you are referring to about the balancing. if you put a mono signal into the amp, set the balance to the middle, and monitor the output voltage with an external meter, do both channels track with each other as you adjust the volume up or down? If they do, the output Darlington is not the problem.

If however they don't track with each other you have an amp problem, not a VU meter problem. You will need to determine from that which way to approach troubleshooting.


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 Post subject: Re: Pioneer SX-780 output meters not balanced
PostPosted: Mar Wed 04, 2020 5:02 pm 
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Joined: Mar Mon 02, 2020 12:59 am
Posts: 4
Output of both channels track.

Meter deflection is off by a factor of about 10. In other words, it's very noticeable.


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 Post subject: Re: Pioneer SX-780 output meters not balanced
PostPosted: Mar Thu 05, 2020 2:16 am 
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Joined: Mar Mon 02, 2020 12:59 am
Posts: 4
well after cleaning up and putting it all back together I was listening to some vinyl Black Sabbath VOL. 4 side 2 song 1 and blew out the right channel which is the original darlington.

Any recommendations on vendors or just luck of the draw on ebay?


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 Post subject: Re: Pioneer SX-780 output meters not balanced
PostPosted: Mar Thu 05, 2020 2:33 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 17575
Location: Utica, NY 13502 (USA)
Sorry for your little set-back. Maybe someone can recommend a source for the Darlington. It's a nice receiver and worthy of repair.

The meter drive circuit is non-linear and is designed to match the power scales of the meters. This is accomplished using D16 in series with R334 for one channel and D19 in series with R335 for the other channel which reduces the drive to the meters after the amp's output reaches a pre-determined power level. This allows for a very wide range of power levels to be measured using a single meter scale.

Attachment:
Pioneer SX-780 Power Meter Crcuit.JPG
Pioneer SX-780 Power Meter Crcuit.JPG [ 70.99 KiB | Viewed 512 times ]


Because of this non-linearity, the circuit must be calibrated as described in the service manual in order for the meters to track properly. I would replace the four 4.7 uf caps in the circuit and also check all diodes and the 270 ohm resistors. Then follow the calibration procedure (after you get the right channel working again, of course). Notice that this is done without speakers or loads connected.

Attachment:
Pioneer SX-780 Power Meter Calibration.JPG
Pioneer SX-780 Power Meter Calibration.JPG [ 90.97 KiB | Viewed 512 times ]


Dave


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