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New to me Luxman SQ707
- Thread starter bhairavi
- Start date
Lets do the math.
36vac X 1.414 = 50.9vdc
minus the diode drop of 1.2vdc = 49.7vdc.
So, you are smack dab in the ballpark.
Honestly. I don't see any issues.
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Well that voltage is fine, I made a three-legged double 1N4008 rectifier just for kicks to check and go the exact same output (48.5vdc).
But really the only reason I was questioning any of this is because the voltage on the collector of the 'lower' transistor of the quasicomplementary pair of outputs should be half that of the higher? I see more like 22.5vdc. Am I making too much of 2v?
The rail is also low further back, shows 15vdc when it should be 18vdc, and 7vdc where it should be 9vdc
Yes. The amplifier output is capacitor coupled so the 2v doesn't pose any issues.
leesonic
Captured on security camera...
I couldn't really make out from the pictures, but have you removed the output transistors and re-greased them? I would definitely definitely do this before running it up to any serious power. The reason being, the old heatsink grease dries out, and doesn't transfer the heat away from the output transistors to the heatsink as efficiently as it should.
You know I did that with a Sansui 222 that needed it.. this one on the other hand seems to have an excess of grease, the entire mounting face was greasy, as well as the legs of the outputs in the other side. And not at all dry. I know that doesn't necessarily mean that the grease is where we actually want it... But I suspect it is.
But doesn't it mean that the waveform is being clipped on one side since 'midpoint' is not really at midpoint?
Haven't got my head around how Quasi works.
Looking at the schematic, the midpoint voltage appears to come from R172, R170, Q16be, R178.
I'd be taking a close look at the resistor marked "*" next to D102(SV-03), is this a VR?
Another vote for these amps, had some budget items, SQ-700X, still got a SQ-505X, sound good, can be had for small change (almost)
Looking at the schematic, the midpoint voltage appears to come from R172, R170, Q16be, R178.
I'd be taking a close look at the resistor marked "*" next to D102(SV-03), is this a VR?
Another vote for these amps, had some budget items, SQ-700X, still got a SQ-505X, sound good, can be had for small change (almost)
The asterisk marked R is attached to the underside, the only component to be there, and I believe its value is decided last in assembly based on measurements? Does it directly affect bias?
Anyway in this case it is 820 ohm and I measured both, they were on spec.
I'll check the other Rs... Q16 is a 2sc853. But what's odd is that all voltage measurements are the same on both channels.
---
R172 4.7k
R170 2.2k
R178 220
All measure within a couple %
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N2552
Super Member
That is some serious gauge intern wiring
I think it just looks that way, these carry two wires each, one insulated:That is some serious gauge intern wiring
Maybe replace it with a 1k VR preset to 820ohms then give it a nudge and see what happens to you center voltage. If bias is already spot on then forget about it.
Bias typically 40-50mA but could be as low as 20-25mA, measure across Q20 emitter resistor (1ohm) wanting to see at least 20mA, if you have an oscilloscope then maybe any crossover distortion will be visible, better to use something like QA403 or similiar for distortion measurement.
Finally getting around to the bucking transformer.
I ordered a Hammond187D20, which is rated 1.5A. For this amp which is rated 90W max at 100V, which means 0.9A, I think the transformer will suffice.
I have experience wiring a bucking xf in the Luxman L80, so it's not really a Q of whether it will work - the Q is where am I gonna put it.
In this case it'll have to be mounted on the R wall by drilling a couple additional holes in the chassis and then first mounting a piece of wood that skirts all the input wiring that runs on that side. Luckily, this wall of the chassis is easily removable as shown earlier in this thread. Then the next Q will be how much interference will I get from the transformer, and will I be able to mitigate it using a ferrous metal sheet.
I ordered a Hammond187D20, which is rated 1.5A. For this amp which is rated 90W max at 100V, which means 0.9A, I think the transformer will suffice.
I have experience wiring a bucking xf in the Luxman L80, so it's not really a Q of whether it will work - the Q is where am I gonna put it.
In this case it'll have to be mounted on the R wall by drilling a couple additional holes in the chassis and then first mounting a piece of wood that skirts all the input wiring that runs on that side. Luckily, this wall of the chassis is easily removable as shown earlier in this thread. Then the next Q will be how much interference will I get from the transformer, and will I be able to mitigate it using a ferrous metal sheet.
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