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Author Topic: Microtube 200 with no output  (Read 9562 times)

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calan

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Microtube 200 with no output
« on: Time Format »

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.
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MarshallJMP

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Re: Microtube 200 with no output
« Reply #1 on: Time Format »

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.
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calan

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Re: Microtube 200 with no output
« Reply #2 on: Time Format »

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!
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calan

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Re: Microtube 200 with no output
« Reply #3 on: Time Format »

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.
« Last Edit: Time Format by calan »
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calan

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Re: Microtube 200 with no output
« Reply #4 on: Time Format »

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!
« Last Edit: Time Format by calan »
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calan

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Re: Microtube 200 with no output
« Reply #5 on: Time Format »

On a side note, I pulled out the tubes that I had replaced, and reinstalled the originals that were in it. When I powered it up, it would never drop out of protect mode. I put the "new" tubes back in it, and it works fine. Is it possible that the original tubes were what caused the issue with R34?

I'm not sure if they are the ones that came in it from the factory, but they are Phillips JAN12AX7WA, with blue lettering.

The ones I replaced them with are some generic NOS 12AX7A that I had in a box... red lettering and no "WA" at the end. Will that matter?
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MarshallJMP

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Re: Microtube 200 with no output
« Reply #6 on: Time Format »

Well these break down sometimes in the MT200,it's the same with R913 in the MP-2,they also break down a lot.I'm also starting to see the 100k anode resistors in the mp-1 tubeboard to break down a lot.They all go up in value.All of these are used in the high voltage circuit.

These carbon resistors  don't seem to like high voltages,that's why i only use metal film.It will be fine for now but if you get the chance ,replace it with a metal film.Maybe also check R40,desolder one side and measure it.

Now about the protect led,it only should go on for max a few seconds (maybe other MT200 owners can support this?).One to two minutes is too long.You need to replace the NE-1,it's a small neon light that is part of the protection circuit with a new one.Because in the end you will get into more problems with the protection circuit.It's a "NE-2H" bulb.

ADA never used Philips JAN tubes,they started out with National tubes (only found in the early MP1's) and then used chinese tubes with 12ax7 in red printing.So i guess the tubes that you put in now are the originals.
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calan

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Re: Microtube 200 with no output
« Reply #7 on: Time Format »

Awesome. Thanks for the info.

That would make sense on the tubes. I scored a few pairs of various NOS 12ax7's many years ago at a surplus sale, and probably installed the JAN tubes during some experiment or something.

I actually have a bunch of metal film resistors I just ordered from Digikey, but they are only 1/4 watt. I'll get a couple more with a higher wattage rating and a new bulb.   ;)
« Last Edit: Time Format by calan »
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MarshallJMP

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Re: Microtube 200 with no output
« Reply #8 on: Time Format »

Okay keep us updated !!
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calan

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Re: Microtube 200 with no output
« Reply #9 on: Time Format »

Is the C2A neon lamp the same as NE-2H?

Will this one work?

http://www.digikey.com/product-detail/en/C2A/C2A-ND/3150384
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MarshallJMP

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Re: Microtube 200 with no output
« Reply #10 on: Time Format »

According to the datasheet it seems to be the same bulb.

http://www.mouser.com/ds/2/423/Neon%20Indicator%20Lamps_7___8-552457.pdf
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calan

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Re: Microtube 200 with no output
« Reply #11 on: Time Format »

I looked and didn't even see that cross reference.  :)

Thanks!
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MarshallJMP

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Re: Microtube 200 with no output
« Reply #12 on: Time Format »

No problem.
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GonadiusPrime

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Re: Microtube 200 with no output
« Reply #13 on: Time Format »

Digikey does have actual NE-2's available.  I would have to check when I get home, but I know I got mine from Digikey.
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calan

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Re: Microtube 200 with no output
« Reply #14 on: Time Format »

While playing around with different pickups in my guitars, I noticed that I kept hearing a not-so-pleasing, ice picky distortion. After swapping to an old stereo amp, I realized that it's coming from the MT 200.

I'm guessing it's the old tubes going bad and causing that microphonic type distortion. Are there any inexpensive tubes I could swap in to smooth it out?
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