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 ?
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 ?
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
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
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 ?
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 ?
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
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
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
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
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.
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).
J means 5% tolerance. It's 27 pF.
27pF on the USA1310 and 1300 too. The 1300 used the D424/B554 output pair and actually had a 2 ohm rating. Transformer still didn't like the load - but those Toshibas would take it. They probably switched back to Motorola when the supply of D424's dried up in the early 90's. Now you can buy them on E-bay - how many do you want?
27pF on the USA1310 and 1300 too. The 1300 used the D424/B554 output pair and actually had a 2 ohm rating. Transformer still didn't like the load - but those Toshibas would take it. They probably switched back to Motorola when the supply of D424's dried up in the early 90's. Now you can buy them on E-bay - how many do you want?
In the 1700 series one schematic there is a hand drawn zener (Z4) probably some correction stated 3.9 V
The same correction and the same hand writing is also in the 1310 schematic ...Is there a chance ? absolutely not
So there is either intention from QSC to hide something or to mess things around or something else we miss here ...
I will search my PA friends tomorrow for either a USA 1310 or a 1700 series one to see if they have any just to put them next to each other
The same correction and the same hand writing is also in the 1310 schematic ...Is there a chance ? absolutely not
So there is either intention from QSC to hide something or to mess things around or something else we miss here ...
I will search my PA friends tomorrow for either a USA 1310 or a 1700 series one to see if they have any just to put them next to each other
They probably just made minor adjustments based on whatever returns they had coming in from the field. Hand drawn schematics were a lot more common back then.
service info is also wrong
They suggest to trim the ripple null for every model but at least according to the given schematic that actually belongs to the 1700 series there is no such a thing in the 1310...neither in the 1700 still its stated as service info many times in the service manual
So its a mess
They suggest to trim the ripple null for every model but at least according to the given schematic that actually belongs to the 1700 series there is no such a thing in the 1310...neither in the 1700 still its stated as service info many times in the service manual
So its a mess
Believe it or not the function of that trimmer is to adjust the time constant on the current limiting mechanism. They did a lot of crazy stuff on those amps and they worked surprisingly well. In my opionion, they rely too much on device characteristics (including secondary ones) and thus could be finicky when they get old.
first, get rid of the idea that QSC is hiding things and trying to be mysterious. These technical documents are the ones they provided their service centers, that is how I got mine.
Series one means simply the models that were just numbers (no letters) and they started with the digit 1. All you are seeing is different revision levels. For example C6, C5. On the 9/4/84 schematic they are 0.022 and 0.033uf, while on the 12/11/84 version (Export version) they had changed to 0.01uf and 470pf. Even later versions, such as the one you linked, had changed those caps to 0.0033uf. Nothing secret, nothing underhanded, just the evolution of the circuit. There are other more involved changes, and if a USA series drawing fits, then use it.
I agree that 270 on the part means 27pf like the schematic says.
When you have an amp oscillating, always check R27,28 over on the output for open condition.
Series one means simply the models that were just numbers (no letters) and they started with the digit 1. All you are seeing is different revision levels. For example C6, C5. On the 9/4/84 schematic they are 0.022 and 0.033uf, while on the 12/11/84 version (Export version) they had changed to 0.01uf and 470pf. Even later versions, such as the one you linked, had changed those caps to 0.0033uf. Nothing secret, nothing underhanded, just the evolution of the circuit. There are other more involved changes, and if a USA series drawing fits, then use it.
I agree that 270 on the part means 27pf like the schematic says.
When you have an amp oscillating, always check R27,28 over on the output for open condition.
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