Let's Get Technical > Troubleshooting Problems
Microtube 200 with no output
calan:
I have an original ADA Microtube 200 (circa late 80's/early 90's), that has no output. I'm not sure when it died, as it's been in storage for years. But, I remember how nice it sounded, and decided to bring it out and use it for a little stereo 8" cab I'm building...if I can get it to work again.
It powers up fine, drops out of protect mode after a minute or two, and the fan is running. Both tubes are glowing (replaced just for the hell of it with no change), all three fuses look good (main power and each channel), and the neon lamp is also glowing.
It just doesn't have any output sound or LED indications, no matter which inputs are used, volume, etc. It's as if it never goes out of standby. I've worked the knobs, standby switch, and jacks to clean any residue or gunk, but no change. I can't find anything obvious with a visual inspection; nothing burnt or discolored, no loose connectors or wires, no funky smells, etc.
Whatever it is, it seems common to both channels. Any ideas, or a known issue that causes these symptoms?
I can get around a circuit with a multi-meter and can read a schematic (which I downloaded), but I know nothing about the theory of these amps or where to start in troubleshooting. Any help greatly appreciated, as I'd really like to get this amp cooking again.
MarshallJMP:
Could be a bad R34 (220k 0.5W) resistor in the power supply.Check the the high voltage,should be around 170V.If R34 is busted it will be a lot lower.Check the voltage before and after the stanby switch just to make sure the switch isn't busted.
calan:
Cool. I'll start poking around.
Where is the best place to reference ground? Just use the chassis, or a specific point in the PS?
Thanks much!
calan:
Some testing (with voltmeter grounded to the back of channel A pot):
1. Across the standby switch, I see 240v on one side, and the other goes from 0 to 240 as I flip it. So the standby switch is working.
2. On the hot side of R34, I see 0v when in standby, and 228V when "on". Somewhat interestingly, as I touch the VM lead to the hot side of R34, the level indicators flicker on.
3. On either side of R36 (62ohms), I see -1.4v when in standby, and -1.1v when "on". If I'm reading the schematic correctly, I should see 170v at that resistor...correct?
EDIT:
After powering down and bleeding off all residual voltage, I measure an open across R34 (or at least higher than 2m ohms). I know in-circuit resistance checks are iffy at best, but I should see a max of 220k across it, right?
Just for giggles, I tried measuring the resistance of R40 (also 220k, right next to R34), but I'm getting too much reactance across it with the capacitors and transformer for it to mean anything.
EDIT #2:
I measure 0v at the junction of R32 and D21 when not in standby, so it appears that Q8 is never turning on...which does support R34 being open. Correct?
*****
If R34 is the culprit, what is the recommended replacement? Same value or something with a higher wattage rating? What causes that resistor to go bad so often? That seems a little odd for a single resistor to fail, much less as a common problem.
calan:
IT'S ALIVE!
Pulled R34 and sure enough it was wide open, with no signs at all that it was bad. Weird.
Anyway, replaced it with a 1 watt 220k resistor that's been laying in the parts box for the last 3-1/2 decades, and it works great. (After several minutes of cleaning the nasty pots).
It sounds even better than I remember. Thanks so much for the help!
Navigation
[0] Message Index
[#] Next page
Go to full version