Quad 34 low and bad quality output

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15-Jul-2019 04:58 PM

Pfcprc

Pfcprc

Posts: 8


Hi - I have recently aquired a 34 which was supposted to be fully working, unfortunaly the output was low and very bad quality (and not working at all on the disc output). On inspection I noticed the LED in the PSU area was not working.

I have replaced the LED and the volume is better, but still very bad quality audio.

I have ordered the 34 revision kit already as i planned to do this anyhow, is it best to fit the kit and see if the new caps cure the problem or repair first??

One thing i have noticed is that the 34 has a sticker on it saying output modified (increased) for Meridian 25 amp? Because of this i was expecting the output to be high on my leak 20? 

  

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16-Jul-2019 04:12 AM

EJP

EJP

Posts: 1677

Much too high for a Leak Stereo 20. It only wants 125mV. Have a look at the service manual for how it was modified to achieve this, and what to put it back to. You might also consider decreasing the sensitivity of the Leak to something more normal like 500mV: contact me off-list for how to do that.

The LED may not be the only problem in the PSU. Check the rail voltages. The kit will quite possibly fix any PSU problems but if not have a look at the 7815 and the TL071.

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16-Jul-2019 08:51 PM

Pfcprc

Pfcprc

Posts: 8


Thanks thats great, i will do the upgrade and lower the 34's output at the same time as you have kindly included that info in the upgrade manual :) 

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22-Jul-2019 02:39 PM

Pfcprc

Pfcprc

Posts: 8


Ok fitted the revision kit and the problem is still there. I have replaced the TL071 in the PSU circuit. I have ordered a replacement 7815, not really sure what to expect reading wise on the 7815, at the moment I can see 17.8v from it, i guess it should be closer to 15v but assume 17.8v ok? 

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23-Jul-2019 02:48 AM

EJP

EJP

Posts: 1677

My apologies, it is a 7918, but you must have figured that out if you have about 18 volts, which is correct. You should have +8.4V at pin 7 of the single opamps, pin 8 of the duals, and -9.5V at pin 4 of both, not counting the one in the PSU itself. The rails are slightly asymmetric due to R123/4 being slightly different values. This was a precaution against latch-up in the TL071s in the audio change. Strictly speaking if you no longer have those you can equalize those resistors to say 10K each and then you should have +/-9V.

You will have to probe to see where the distortion first appears. Check pin 6 of each single op-amp in turn from the input to the output until it goes bad. Also have a look for broken resistors near the op-amps, quite easy to do when changing them.

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