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Vercus
July 31st, 2009, 11:56 AM
Hello Guys,
Tradde generously gave me a 19" rack and two RL02's. I don't have a PDP yet, so I can't spin them up, but we did plug them in to AC at his house just to see what they did. One appeared to power up okay, while the other one buzzed fairly loud. Any ideas where to start checking? I got the tech manual from bitsavers, but don't really know where to start as I've never opened one of these up before.

I spun the rear fan manually and it's free, as well as the spindle motor inside. Could it be something in the power supply? I haven't powered it up again, as I'm afraid doing so might damage something if there's a short somewhere.
Any thoughts would be appreciated...
-Jon

Lou - N2MIY
July 31st, 2009, 03:10 PM
Were any of the front panel lamps lit?

Lou

Vercus
July 31st, 2009, 09:18 PM
Hey Lou,
I didn't get to see the front panel, as it was pointed away from me when we turned it on. We only ran it for about 2 or 3 seconds. It made it very apparent that there was something wrong.
-Jon

Vercus
August 1st, 2009, 11:01 AM
I powered it up today, and the buzzing didn't seem as loud as I remembered in the past. The rear cooling fan spins fine, and the fault light comes on (no controller). After a short time, the load light turns on too. I'm trying to use my ears to figure out where the buzz is coming from, but it's hard to localize.

-Jon

Vercus
August 1st, 2009, 11:11 AM
Okay, discovered where the buzzing is coming from. It's coming from the head assembly. When I opened the cover, the buzzing was MUCH louder, and when I touched my finger to the plastic shroud around the heads, I could feel an *intense* vibration. It feels as if the head seek motor is trying to move the heads further in, and they can't go any further, thus it is buzzing / vibrating. Does this make any sense?
What is the best way to open it up and investigate further?

-Jon

tradde
August 1st, 2009, 12:24 PM
Okay, discovered where the buzzing is coming from. It's coming from the head assembly. When I opened the cover, the buzzing was MUCH louder, and when I touched my finger to the plastic shroud around the heads, I could feel an *intense* vibration. It feels as if the head seek motor is trying to move the heads further in, and they can't go any further, thus it is buzzing / vibrating. Does this make any sense?
What is the best way to open it up and investigate further?

-Jon

Can you move the heads manually in and out? Maybe they are locked down? I don't
remember if an RL02 has a head lock. I know the RK05s do.

Vercus
August 1st, 2009, 12:34 PM
Hey Tim,
The heads do move in and out. In fact, I moved them out a bit by accident. When I did, the heads clicked together and touched. I gently slid them back, and they opened up again. Did this ruin the heads?

I'm going through the tech manual now, and printed out a list of voltages to check on the DC Servo board. I'll use my DMM to see what they're at. I don't have an oscilloscope, so I can't check some of the more advanced parameters unfortunately.

-Jon

Vercus
August 1st, 2009, 01:21 PM
Hello again,
I lifted the DC Servo board, and disconnected the 6-pin cable that goes to the servo motor so I could check voltages without it buzzing.

Sure enough, when I turned it on, there was no buzz. I checked the 5 test points on the servo board listed in the Tech manual and here are the results:

Voltage
+V unregulated: +15.0 Volts (+14V to +18 V acceptable)
-V unregulated: -10.5 Volts (-14V to -18V acceptable) * OUT OF SPEC*
+5V: +5.15 (+4.85V to +5.35V acceptable)
+8V: +8.0V (+7.7 V to +8.3V acceptable)
-8V: -6.1V (7.7V to -8.3V acceptable) *OUT OF SPEC*

As you can see, two voltages are out of spec. According to the manual, the voltages are not adjustable. If a voltage is out of spec, the faulty "FRU" should be replaced.

I'm assuming FRU means "Field Replaceable Unit". In which case, DEC is simply saying replace the DC Servo board? I don't exactly have spares lying around..lol. I'm guessing it's probably one or more of the Fixed Voltage Regulators on the underside of the board.

At least I feel like I've made some progress in determining the problem.

Any advice on how to proceed would be much appreciated!

Thanks,
Jon

Lou - N2MIY
August 1st, 2009, 01:31 PM
Jon,


Check the power supply board and all of the voltages on there. I have two old RL01s in my garage, and one of them has a bad power supply board. RL01 and RL02 use the same power supply. I remember that it was a linear supply. The bad PS I have has a bad 7808 (8V) in a TO5 case.

Are those voltages low with and without the servo board attached? (Is there somethng wrong on there pulling it down?)

In addition to finding a pdp-11, priority two should be a scope. A Tek 465 would do you well. Seen at hamfests in good shape for as little as $100....


Lou

Vercus
August 1st, 2009, 01:39 PM
Hey Lou,
So far, I haven't tried measuring voltages with the servo board attached, as I was afraid of damaging it. I have a revision 2 power supply board which, like you mentioned, would work in either an RL01 or RL02. However, the original RL01's with a Rev. 1 board won't work in an RL02.

I'm thinking of ordering a few 7808's just in case. My other concern is this: the -15V says "unregulated", which I am assuming means it is not controlled by the 7808 fixed voltage regulator. If that is the case, than what IS it controlled by? I noticed that the test point for -15Vu and -8V are directly under the same 7808. This leads me to theorize that the -15Vu is indirectly affected by the same 7808. However, what confuses me more is that the MC7808CK is a POSITIVE voltage regulator, and it is my NEGATIVE voltage that is out of spec. Odd...

-Jon

Vercus
August 1st, 2009, 01:45 PM
Nevermind, I misread it. I thought it said 7808, but upon looking closer I realised it is actually 7908, which is the -8V regulator.

Lou - N2MIY
August 1st, 2009, 01:59 PM
Jon,

Ok, I should really get out the print set myself at this point, but I won't quite yet. The unregulated supply means that it comes directly off a transfomer winding, rectified and filtered, of course. I can easily believe that the -8V regulator gets it's juice from the unregulated -15V supply.

Something may be pulling that -15V unregulated supply down. It could be a flaky 7908. Is the case of the 7908 hot (thermally)?

I have no clue which rev power supplies are in my RL01s. They got me so angry after trashing my only two packs, I banished them to a corner of the garage and gave away my RLV21. Giving away the RLV21 was pretty stupid, I wish I had it now (but at least the new owner is happy.)

Lou

Vercus
August 1st, 2009, 02:53 PM
Hey Lou,
I don't have the tech manual in print, just on PDF. It's annoying to go through compared to the real thing, but certainly a lot cheaper. So far, I haven't been willing to print it out, as it's 173 pages long.

I didn't think to check the case of the 7908 to see if it was hot. I'm planning on ordering a few, just to keep a few spares on hand. I closed it u p for now, but once the 7908's come in, I'll give it a go and see how it works.

I agree about the 'scope, I sure could use one. Unfortunately, I just don't have the money right now. I've been trying to sell a set of speakers I don't use any more, but they're not moving, even though everyone says I'm asking a great price.

The economy is definitely felt....
-Jon

Lou - N2MIY
August 1st, 2009, 05:24 PM
Jon,

The tech manuals are helpful, but equally as important are the field maintenance print sets (FMPS, the schematics). When I work on a piece of equipment, I plot out a hardcopy of the FMPS, or at least the relevant sheets. I trashpicked a broken HP inkjet plotter (power supply, fixed it) and use it to plot the prints B size. I find it very helpful to be able to write on them as I troubleshoot.

Maybe you can trade your speakers for a scope? That might be a fair trade. Personally, I only have one scope, and good speakers are lost on my ears (messed up from never bothering with hearing protection around machinery.)

Lou

tradde
August 2nd, 2009, 03:53 AM
Hey Tim,
The heads do move in and out. In fact, I moved them out a bit by accident. When I did, the heads clicked together and touched. I gently slid them back, and they opened up again. Did this ruin the heads?

I'm going through the tech manual now, and printed out a list of voltages to check on the DC Servo board. I'll use my DMM to see what they're at. I don't have an oscilloscope, so I can't check some of the more advanced parameters unfortunately.

-Jon

It's good the heads move. No you did not ruin the heads by having them touch. As to your power supply issue, I have an RL01 here too, that never worked. I can check what rev board it has to see if that might be useable by you? The more I can get out of
my basement the better for me for someday (hopefully soon) when I move.

As Lou said finding a older scope is a wise move. I have one that has come in very
handy, even though I just use it for the basics.

Vercus
August 3rd, 2009, 09:13 AM
Well, it looks like I can forget buying a 7908. The supply houses seem to have a $200-$400 minimum order, which is ridiculous. One even said he'd sell me 4 of 'em for $200! So, it's obvious that once again, big faceless corporations care nothing for the independent little guy.

Anyone know of a good place to look?

-Jon

tradde
August 3rd, 2009, 12:31 PM
I see I have a partly opened Rl02 and RL01 still downstairs if you want to check those.
The RL02 makes no noise at all, meaning the fan doesn't even run. Not sure on the
RL01 or if it's late enough to have the same rev power board. But they are here.
Tim

Vercus
August 3rd, 2009, 01:47 PM
Hey Tim,
I would be interested in taking a look at the PS boards on your RL's to see if one of them would work. I certainly appreciate the offer. You have been very generous, and I appreciate it.
-Jon

Lou - N2MIY
August 3rd, 2009, 03:40 PM
Jon,

I too can look at the RL01 power supply boards in those nasty beasts in my garage if need be (if Tim's don't get you far enough.)

As for low order quantity parts sources, I personally like Jameco. However, I just yesterday decided to try Futurlec to buy the replacement 2N6547 I need to fix a VT100 power supply. However, they don't have your 7908 in a TO-3 package, only TO-220.

Lou

tradde
August 3rd, 2009, 04:53 PM
Someday (hopefully soon) I am going to move and I need to get rid of lots of this stuff
(spares). I'd rather see them go to someone who can use them than just toss them out.
It's amazing how over time how much junk one can accumulate. :)

Vercus
August 3rd, 2009, 07:55 PM
Jameco doesn't have them either. So far, the only places that do have a $200 minimum order. Who would have thought in the 21st century things would still be so backward and underdeveloped? The internet is truly worthless for many things yet...

Lou - N2MIY
August 4th, 2009, 04:27 PM
Jon,

I've had two thoughts on this so far today. One, if the load is only 1A, than maybe a TO-220 case 7908 is worth a try. However, if you need the full 1.5A that the TO-3 version gave, you could use an LM337 in a TO-3 case and program it for 8V. But, you cannot just plug the 337 in where the 7908 was, you will need to mount it remotely and wire it back.

For the heck of it, I looked and was not able to locate any 7908s in TO-3 cases anywhere either.

I just looked at my notes scrawled on the top of the RL01 with broken power supply, and it was the 7908 (-8V supply) that is toast. So, with two data points, I would now definitely suggest the LM337 route.

Lou

Vercus
August 4th, 2009, 05:40 PM
Hey Lou,
So, it sounds like these 7908s are a fairly common failure point on the RL0X's. As far as a 337, I don't know what that is, or how to "program" it.

I was really hoping for something pretty straightforward- replacing with a like unit, but I'm not sure if that will be possible.

-Jon

Lou - N2MIY
August 4th, 2009, 06:29 PM
Jon,

The LM337 is the negative voltage cousin of the common LM317 positive voltage regulator. It is an adjustable device, with the output voltage "programmed" with two external resistors. See here: http://www.fairchildsemi.com/ds/LM%2FLM337.pdf .

Drop in replacements are nice, but on one out of ten projects I run into failed "unobtanium" devices that I have to work out some kind of replacement for. It's part of the fun of working on "vintage" equipment.

Lou