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Old 21st August 2014, 04:39 PM   #1
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Default QSC 1700 ( not series one )

I am looking for a correct schematic for the QSC model 1700 but my product is not covered by the series one schematic .

The simple 1700 has different numbering on many of the parts so advices given on the troubleshooting do not apply

More or less though and except a trimmer and a couple of other parts amplifiers are pretty much a like ..

The trouble is that amplifier works perfectly but bias is far from high so its getting hot ...

It is supposed to measure about 80mv over 6R8 driver resistors but measures 400 ( measuring this quickly to avoid trouble ) and seems that full turn of the trimmer has very little things to change ...

No driver or output is shorted , all zeners an diodes in the bias network together with 1K resistors have been replaced as a precaution with no effect .
In between the amplifier fails to built 15+15 v for the OP amp which is actually working perfectly if you remove the bias network ( so no zeners in the 15 v area there shorted )

Any ideas ?
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Old 21st August 2014, 05:15 PM   #2
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some additional info

failure to bring up the 15 +15 volt means also that this is happening symmetrical Voltage goes deep symmetrically ...I will have to presume that there is an issue in the bias .

I assume that if for example any of the base of outputs is having some issue that will cause excessive current draw on this side of the amp but nothing in the other side ...which is not happening both sides of the amp seem to draw more current than supposed to be ...

here is the schematic of 1700 series one for reference
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File Type: pdf qsc_1700_[ET].pdf (869.9 KB, 67 views)
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Old 21st August 2014, 06:27 PM   #3
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ok ... getting there

I removed totally the output stage by cutting the common node of bases of all output transistors

Out of the blue both bias seems to adjust in normal level and between the range specified . While after that 15+15 volts built up properly So my first idea is wrong ...this not a bias issue

I thought about connecting the bases of the transistors one by one to check and see if any of the bases might have an issue

Connect one N fine , connect one P fine connect the third one and then oscillation appears while probably the Op amp latches to something and output is a mess If i disconnect the supposed to be faulty transistor the output remains at oscillation

So its got to be oscillation but why ?? where is it coming from ? none of the parts existing inside appears to be faulty What might drive the amp to oscillation ???

Help guys ?
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Old 21st August 2014, 07:23 PM   #4
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Found a capacitor broken ... feedback chain C4 one of the legs was Rusty and apparently cut off the body of the capacitor ...

Now here is the fun part the original cap was 270 pf in the schematic though 27pf is stated ... is QSC messing with me ?
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Old 21st August 2014, 07:36 PM   #5
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problem solved oscillation was present by the absence of C4 after capacitor replaced the amplifier works perfectly !!!

Problem is that the schematic is either fake made like that from QSC to prevent copy from anyone or the one that drawn it is a certified loonie .....

Anyone the owns any of them should look at it

Kind regards
Sakis
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Old 21st August 2014, 07:53 PM   #6
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I will continue measuring and perform extensive tests but i would really like to find out what happens in real life and what is the difference between normal 1700 and 1700 series one ...

Even though look alike there is a bunch of errors in the schematic all of them in critical points ...

--C6 and C5 are in the drivers area stated in the schematic as 0.0033 but in reality in the 1700 10nf exists on the upper side and 47pf exist on the lower side
--C4 27pf on the schematic but 270pf in the 1700
--Z4 3.9 V on the schematic ( obviously hand written ) but 4.7 inside the 1700
--Numbering of parts according to both service manual and the schematic between the 1700 and the 1700 series one is total nightmare
--D11-12 TR3 doesn't exist on the 1700
--There is a reference on the service manual about adjusting a TR 5 but it doesnt exist inside the schematic of the 1700Series one ,or the real 1700 amplifier

And probably there is going to be a few more that i haven't traced so far

Kind regards
Sakis
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Old 21st August 2014, 08:15 PM   #7
wg_ski is online now wg_ski  United States
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Quote:
Originally Posted by east electronics View Post
Now here is the fun part the original cap was 270 pf in the schematic though 27pf is stated ... is QSC messing with me ?
The use a 'resistor color code' - 2, 7 followed by "0" zeros, or 27pF. The 270 might in fact work, but hurt HF performance. A 270 pf would be labeled 271.
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Old 21st August 2014, 08:21 PM   #8
wg_ski is online now wg_ski  United States
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This thing is essentially the same as the USA1310, which I have several. In addition to the cap legs rusting off (and the amp running hot) you've got to watch out about E2 and E3 going bad. If that happens the amp clips at arpound +/-15V output. You can sometimes pick them up for a song with that failure mode (which won't show up till you bring it home and crank it, and the seller knows that).
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Old 21st August 2014, 08:25 PM   #9
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been replaced already no probs familiar with that ...
I had in mind that 270J means 270 PF but will check on that ...

I will also check the usa 1310 model to see

Thanks !!!
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Old 21st August 2014, 08:37 PM   #10
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The schematic of the 1310 looks identical with 1700 series one but still different with model 1700

Just a quick one : Does your 1310 features a trimmer for the ripple null? Cause if it does its not in the schematic ... so this is a mess
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