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QED A 240 CD Can Anyone Pls Help With Repair
QED A 240 CD Can Anyone Pls Help With Repair
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Old 5th March 2018, 05:04 PM   #21
Rol G is offline Rol G  United Kingdom
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Thanks again but 1. I don't have current limited supply,
2.no idea how/where exactly to measure low dc offset at outputs - what is low dc offset?
3. I'm unsure which points to check between output resistors and emitters of output transistors
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Old 5th March 2018, 05:17 PM   #22
Mooly is offline Mooly  United Kingdom
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QED A 240 CD Can Anyone Pls Help With Repair
DC offset is simply the DC voltage as seen across the speaker terminals. It should ideally be zero although historically values of up to 100mv (either + or -) were considered acceptable.

The bias current is determined using ohms law from the measured voltage across the known value of 'emitter resistor' connected to the output transistors. On my diagram they are not marked

QED say you should see between 10 to 20 millivolts between the two emitters as shown here.

The current limited supply is really to prevent damage when working on a faulty amp. The bias is set at full operating voltage.
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Old 5th March 2018, 05:32 PM   #23
Mooly is offline Mooly  United Kingdom
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QED A 240 CD Can Anyone Pls Help With Repair
There is no risk in checking the bias but do be careful with the meter leads as a slip and accidental short could be disastrous.

I also have concerns over the circuit diagram as drawn as it seems more like a prototype than a fully functional design.

For example the feedback network for the power amp isn't shown, also there is no 'zobel network' on the output, something that is normally an essential for stability.

The bias preset (while fully functional as drawn) is configured in a way that is against good practice. If the preset failed or went intermittent then full bias current would be developed, something that could be destructive.

Good practice would have the preset and the resistor below it in reversed positions i.e the resistor at the top and preset at the bottom. Failure of the preset would then give a safe minimum bias current.
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Old 5th March 2018, 06:22 PM   #24
Rol G is offline Rol G  United Kingdom
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Thanks, do you mean attach a mV meter across R45/74 and R47/76 and adjust the preset nearby pot until I get 10-20mV ?
BTW. What does the 1V4 in the oval mean ?
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Old 5th March 2018, 06:40 PM   #25
Mooly is offline Mooly  United Kingdom
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QED A 240 CD Can Anyone Pls Help With Repair
Your meter measures between the two arrow points in the diagram I posted above. Where is says 10-20mv in an oval. That oval will be your meter.

The 1V4 is approximate voltage between the two arrow points when the circuit is operating normally.

Check the voltage but don't turn the preset until you are happy with the method. It can be a fairly fine adjustment and it will vary with temperature.

Check the original unrepaired channel first to see what kind of reading to expect.

Also confirm there is no appreciable DC offset present between the speaker terminals.
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Old 5th March 2018, 06:42 PM   #26
Mooly is offline Mooly  United Kingdom
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QED A 240 CD Can Anyone Pls Help With Repair
Like this:
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Old 7th March 2018, 03:16 PM   #27
Rol G is offline Rol G  United Kingdom
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Hi again,
just over 0.2V is the lowest reading possible with the pots turned fully anti clockwise, (closed i presume) before i turned them the heatsink was very hot, now it runs very cool. Some of the small resistors around the pots look like thay have got hot at some point. should i check them? thanks
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Old 7th March 2018, 03:42 PM   #28
Rol G is offline Rol G  United Kingdom
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My last post refers to the measurement between r84/85 and 84/85. that should be 10-20mV ??
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Old 7th March 2018, 03:43 PM   #29
Rol G is offline Rol G  United Kingdom
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sorry r54/55 and r84/85
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Old 7th March 2018, 04:11 PM   #30
Rol G is offline Rol G  United Kingdom
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i readjusted the pots so both channels read 1.4v between r47/76 and r45/74

sorry, can't understand the 10-20mV adjustment described in 3d
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