Products > Test Equipment
Fool for the 8656A Sig Gen
Tony_G:
I believe that the 2N3528 can be replaced with an NTE5455 but you should compare the datasheets to make sure.
Now that Q3 is part of the 5V crowbar circuit and not specifically related to the attenuator (from my quick reading of the schematic/service sheet) - If it is being activated then it should cause a fuse to blow.
I would recommend that you get a tester - I use one of these https://www.amazon.com/Atlas-ADVANCED-SEMICONDUCTOR-ANALYZER-Tracing/dp/B00O0BMNX2 but you can really use any of these testers (including the ones off eBay).
Let us know how it goes.
jrharley:
Thanks for the cross reference Tony_G! I'll check out the NTE number and store it for future reference should I need it.
Very good point about the SCR action! I'd already checked the fuses associated with it just for curiosities sake, and they're good. The circuit makes more sense now.
When still hot off the board, I dropped the SCR into a cheap, classic, fish8840 tester just to see what I could see. It seemed to indicate that I had 2 diodes. The tester is supposed to test thyristors, but you never know for sure - it's a cheap tester, operator error, solar flares.......
Oh, for a suite of those Peak testers.......... Nice kits.
The B&K power amp issue turned out to be just gummed up output level controls. I love it when that happens. So now I'm back to the HP for more terrifying technical explorations.
As always, I really appreciate the feedback.
JRH
Tony_G:
Good to hear - Good luck with the unit - Look forward to hearing what you next discover.
Just to refresh my memory, you checked the voltages per service sheet 22 and did so by removing all the boards not used in SS22? This would tell you that the rails themselves are good (though it is possible that once a rail is loaded down it slumps in voltage) and let you add a board back in at a time to see if that board causes the rails to droop.
Again, look forward to hearing the progress.
TonyG
jrharley:
Hey Tony_G,
I see where you’re going with that. Good point.
I haven’t systematically isolated all the other boards, but I did isolate the Audio/Power Supply section on board A10 by disconnecting connectors J2 and J4 entirely. I tested most of the components within the isolated assembly, like the SCR Q3, and I’m reasonably convinced the draw down doesn’t originate in that portion. You can see that there are test points for all three main rails on either end of the rails. I tested the voltage at these points, (running the unit off the 120V mains), once with the #2 wire from connector J2 out, and again, with it in the circuit. First number is with the J2 wire out of circuit.
Testing the pins in test socket J6:
“Left side”
TP 11: 21.8V, 22V
TP 9 : 12V, 13V
TP 10 : -23.4V, -23.3V
“Right side”
TP 13 : 14.9V, 15V
TP 3 : 5.4V, 1.8V
TP 2 : -14.7V, -14.7V
Under each of the voltage call outs for each rail on the “Left side”, there are small “hand written” voltages. These voltages, whatever they are supposed to represent, are much closer to my measured voltages than the rails are designated to be. A different “state” perhaps. No idea at this point, but find it hard to believe it’s coincidence. I need to re-visit the theory of operation info if I can find it on this section.
I’ll study the schematic and see if isolating other boards provides any clues. Thanks!
Onward…….
JRH
Tony_G:
Sorry bit of a brain fart on my behalf - I was imagining that there was a motherboard but these units didn't have that.
Apologies if this caused lost time.
Did you get a chance to put a scope on those test points? Some of them have very tight ripple tolerances (eg TP13 should only have a 10mV ripple on it with the DC value between 13V-17V)?
Detail is in Table 1 on SS 22.
TonyG
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