(80V - 2V)/2mA=39K
Pdiss=2mA^2 x 39K=156mW , so just use plain MF resistor , and plain ones are 600mW , so yup .
1/4W nope ....... even if 50% margin is usually OK , in that position I like greater derating factor
Pdiss=2mA^2 x 39K=156mW , so just use plain MF resistor , and plain ones are 600mW , so yup .
1/4W nope ....... even if 50% margin is usually OK , in that position I like greater derating factor
Question on bias setting...
Ok, next question:
The Service Manual says to set the initial bias (top of Page 3, steps 1-4) we shoud use the setup from the first paragraph of the Test Setup section (middle of Page 2, which says we should use a 600 ohm signal source).
Does that mean we need a 600 ohm plug for the input (RCA jack bridged with a 600 ohm resistor) or should it be shorted (or open)? I thought I had read in the forums that the inputs should be shorted when setting the bias, but now I'm not sure. Could "NO SIGNAL IN" mean input still attached (at 600 ohms), but not generating a signal, or does it mean open input?
What am I missing or misunderstanding, or am I just confused from reading too many threads?
Ok, next question:
The Service Manual says to set the initial bias (top of Page 3, steps 1-4) we shoud use the setup from the first paragraph of the Test Setup section (middle of Page 2, which says we should use a 600 ohm signal source).
Does that mean we need a 600 ohm plug for the input (RCA jack bridged with a 600 ohm resistor) or should it be shorted (or open)? I thought I had read in the forums that the inputs should be shorted when setting the bias, but now I'm not sure. Could "NO SIGNAL IN" mean input still attached (at 600 ohms), but not generating a signal, or does it mean open input?
What am I missing or misunderstanding, or am I just confused from reading too many threads?
600 ohms generator means they're having conditions like that ( industry standard)
which means , if you want to do the same , any decent signal source will do
no signal means no signal ; either with gene still connected (and muted) or input grounded - condition is - no garbage in.
which means , if you want to do the same , any decent signal source will do
no signal means no signal ; either with gene still connected (and muted) or input grounded - condition is - no garbage in.
Change the resistors on the brown board to 10k 3watt - only 1 required instead of two. The resistor is just a safety bleeder to remove the charge in the capacitor when you turn the amp off or blow a dc fuse.
That way you won't continue to degrade the board
That way you won't continue to degrade the board
I understand about the signal generator, but 600 ohms and a dead short are two different things. Are they close enough for the purpose of this test that it doesn't matter?
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Also, as you're a moderator, is there any way to move this thread to the solid state forum? While this is a Pass design, I noticed that all the other Adcom threads were in Solid State. It just seems a little out of place in the Pass forum.
exactly
actually , I am , but not in this forum
btw. nothing is out of place , in Papaland
Next data points and a couple questions:
I couldn't get voltage or resistance readings (E-C) on output transistors, so I used a diode tester. All came in between 0.68 and 0.70v, so I presume they are all good. All were showing open when reverse biased, so that's good. I didn't check the one non TO-3 transistors (Q11&12) - what's the chance that it is bad? It's an oversight on my part, I can check over the weekend, if anyone thinks it's useful.
While the diode tester was out, I checked D5 & D6, and they were both just over 0.7v (each one on each side), so that looks good for Q3 bias.
I got a capacitor-meter, and the big one under the burnt board read 14,543uF on a 15,000uF spec, do that's good. Both resistors (3K9) on the burnt board read 3K5, or slightly more than 10% low (not bad for a 5% part), resulting in just under 1.9 watts for a 2w part…
That's as far as I got with everything assembled and power off. I wanted to put R4 in, but can't figure out how to do it, as all the wires were soldered on after it was bolted in place. Is there an official "best practices" for getting the main board out?
I hope (R4 or not) to power it up this weekend and do some quick D.C. offset measurements at the outputs, and then measure the bias voltages per the manual. If all looks well, I might push 50mv in and watch the scope (no load) to see what it looks like. If bias looks bad, adjust bias per the manual and look at a small-signal on each side to see what I have.
Any comments on what I have done or plan to do?
I couldn't get voltage or resistance readings (E-C) on output transistors, so I used a diode tester. All came in between 0.68 and 0.70v, so I presume they are all good. All were showing open when reverse biased, so that's good. I didn't check the one non TO-3 transistors (Q11&12) - what's the chance that it is bad? It's an oversight on my part, I can check over the weekend, if anyone thinks it's useful.
While the diode tester was out, I checked D5 & D6, and they were both just over 0.7v (each one on each side), so that looks good for Q3 bias.
I got a capacitor-meter, and the big one under the burnt board read 14,543uF on a 15,000uF spec, do that's good. Both resistors (3K9) on the burnt board read 3K5, or slightly more than 10% low (not bad for a 5% part), resulting in just under 1.9 watts for a 2w part…
That's as far as I got with everything assembled and power off. I wanted to put R4 in, but can't figure out how to do it, as all the wires were soldered on after it was bolted in place. Is there an official "best practices" for getting the main board out?
I hope (R4 or not) to power it up this weekend and do some quick D.C. offset measurements at the outputs, and then measure the bias voltages per the manual. If all looks well, I might push 50mv in and watch the scope (no load) to see what it looks like. If bias looks bad, adjust bias per the manual and look at a small-signal on each side to see what I have.
Any comments on what I have done or plan to do?
Sorry the other resistor for the long-tailed pair, under Q2, in the same place R3 is to Q1. There is a provision for it on the circuit board, but a wire jumper is in its place.
leave it as is
you're allowed to mess there only if you know what you're doing
in worst case you can worsen amp stability margin
you're allowed to mess there only if you know what you're doing
in worst case you can worsen amp stability margin
I'm kind of confused. The resistor in question is in the design (but omitted in the build) and a normal part of a long-tailed pair. How does putting something back in which was omitted going to cause problems with the stability margin? This amp's schematic was the first time I saw such a design (but I'm not an expert) omitting one of the resistors.
On another note, do you have any thoughts in what I was planning on doing next? Any steps to be done before I put the fuses back in and check bias? Any word on DC analysis, to help give the proverbial "warm fuzzy feeling" before going and lighting the amp up?
As always (and far too often I forget to mention it), thanks for the help and insight.
On another note, do you have any thoughts in what I was planning on doing next? Any steps to be done before I put the fuses back in and check bias? Any word on DC analysis, to help give the proverbial "warm fuzzy feeling" before going and lighting the amp up?
As always (and far too often I forget to mention it), thanks for the help and insight.
well , having just one of these in LTP easily can be designer's choice
I can't see it in schematic , so who cares why it can be on pcb ......... and it really isn't
regarding pre-tests , I think it's time to flip that power switch
use bulb tester , just in case
I can't see it in schematic , so who cares why it can be on pcb ......... and it really isn't
regarding pre-tests , I think it's time to flip that power switch
use bulb tester , just in case
on circuit, that's fine. no worries.
good with pre-tests.
if I recall, you use a bulb instead of a varistor. I have a varistor, so I'll use that, if you don't mind (and presuming I understood your comment about the bulb tester... unless we're talking Christmas lights...). 8)
good with pre-tests.
if I recall, you use a bulb instead of a varistor. I have a varistor, so I'll use that, if you don't mind (and presuming I understood your comment about the bulb tester... unless we're talking Christmas lights...). 8)
Update - small signal test worked well and so the setting of the bias was run, and all looked well. Minor adjustments to get 50mV on loss of signal, will have to check what the shorted inputs look like coming up from cold (to see if it's still in the 16mV range).
Time to find a pre-amp and toast my good luck. And find speaker protection circuits. 8)
Time to find a pre-amp and toast my good luck. And find speaker protection circuits. 8)
So I have a couple kits for speaker protection kits (based on the UPC1237) on the way, and hope to have them built and installed this weekend, with a pre-amp arriving middle of next week. So far, so good.
Update: finished the speaker protection kit and integrated a small toroidal transformer, and built a mezzanine shelf (about 6.5" long, 1.5" wide and 4" tall) over the main power toroid. Need to finalize the wires and run speaker wires in and out. Photos when it's done (hopefully next week). Next up, an Adcom GTP-500... Fun!
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