Author Topic: Tek 485 repair and restoration (Read 23962 times)

c4757p · « **on:** November 13, 2015, 05:16:14 pm »

So, I just got an old Tek 485 in need of some TLC. Hopefully with some effort this can become my new primary analog scope, a nice pair to my DS1kZ. I'll keep this thread updated as I go.

It's a pretty nice scope. Two channels at 350 MHz bandwidth, dual timebase, nice sharp CRT (seriously fuckoff high voltage going into this!), not too big, and has a nice and neat construction. Oh, and it's gorgeous, with that 70s Tek metallic look.

Here it is from the front as I got it. Some of the controls took a good whack - the vertical scale factor knobs are busted and missing the concentric variable knob, and invisibly from outside, the delay time pot is either loose or broken. Otherwise it's not awful, no connectors banged up, nothing seriously mangled.

front.jpg

It won't trigger, so no sense in taking a picture of it displaying a signal (I can barely get one to kinda-maybe stand still by fiddling with the holdoff). Here's the CRT flood-filled by a triangle wave though - nice, even brightness throughout, except for a burn-in of the center ground line - doesn't bother me.

crt-lit.jpg

And a preview of the sexy internals to come later....

b-trigger.jpg
crt-housing.jpg

I just ordered some "standard-replacement" parts from Mouser for it - with something this old, there are a few things I like to replace even if they're not yet broken: any caps working at 1kV or greater, tantalum caps, high ripple current aluminum caps, lamps, LEDs, and neon tubes. All of those age even under proper use.

More to come soon.

tautech · « **Reply #1 on:** November 13, 2015, 06:44:11 pm »

Nice, hope this goes well for you.

Sphere normally has a good selection of parts for Teks.
http://www.sphere.bc.ca/

c4757p · « **Reply #2 on:** November 13, 2015, 06:50:29 pm »

Thanks, I forgot about them. So far I have sources for everything I'm aware of needing, though we'll see how long that remains true

Quick update: yep, the pot's wrecked. The housing came off, and the moving wiper went missing in the process. Not sure how that happened, as there doesn't otherwise seem to be any internal trauma. Damn pot will cost about $30 to replace

I'll probably put a regular pot in there for testing until the next batch of replacement parts is ordered - I'm sure I have a pile of 3/4-turn 50k pots around here.

tautech · « **Reply #3 on:** November 13, 2015, 06:59:37 pm »

There's also the Tek CRO troubleshooting guide in the links sticky of this board.

Have you got an online manual link so we can all play along ?

c4757p · « **Reply #4 on:** November 13, 2015, 07:07:45 pm »

Sure, here's my version to which I added a PDF TOC:

https://misc.c4757p.com/tek-485.pdf (31MB)

I'll probably post schematic excerpts if I dig into any interesting bits.

BravoV · « **Reply #5 on:** November 14, 2015, 12:32:42 am »

subbed, as I also have an offering locally here, but haven't make any decision yet.

Keep it coming Chris, thanks.

c4757p · « **Reply #6 on:** November 19, 2015, 07:31:13 pm »

Okay, first round of preemptive replacements, on the horizontal amplifier board. The following parts were replaced:

Reference	Original	New	Part
C1099,1130,1131,1139	Tantalum, 15uF 20V	Aluminum, 15uF 100V	Nichicon UPW2A150MPD
C1133,1134,1137	Tantalum, 22uF 15V	Aluminum, 22uF 50V	Nichicon UPW1H220MDD
C1611,1633,1646,1651,1652,1663,1664,1667	Ceramic, 1nF, 4kV	Ceramic, 1nF, 6.3kV	Murata DECE33J102ZC4B
C1612	Ceramic, 6.8nF, 5kV	Ceramic, 10nF, 6kV	Vishay 564R60GAS10
DS1668,1669	Neon, 60-90V	Neon, 60-90V	VCC A1A

This board has the CRT's HV section, which is built in those silver-metallized ceramic strips Tek used to use to build whole scopes with. These require soldering with silver-bearing solder to avoid stripping the metallization.

hv_before.jpg

Failures of high voltage ceramic capacitors in these circuits aren't extremely common, but they do happen, and tend to take out a lot of other things too, so I replaced all the ?1kV ceramics in this circuit. Neons most definitely age, so the two clamping neons in here were also replaced:

hv_after.jpg

Nothing in the horizontal amplifier itself needed replacement. This board also has the B trigger circuit, though, which had plenty of tants - got those:

btrigger.jpg

The 15uF caps are oversized just because I merged two BOM lines - no particular reason they're 100V-rated, other than that I needed 100V ones elsewhere.

Here's a full view of the whole board after completion:

fullview.jpg (off-forum because it's big)

c4757p · « **Reply #7 on:** November 19, 2015, 09:55:02 pm »

Not many replacements on the sweep board.

Reference	Original	New	Part
C861,902,911,962	Tantalum, 22uF 15V	Aluminum, 22uF 50V	Nichicon UPW1H220MDD

More interesting perhaps is the metallic-looking dust that was covering this board. Bit worrying... It got a good washing to clean that up.

c4757p · « **Reply #8 on:** November 20, 2015, 01:44:42 am »

Inverter board. This is just the switching, monitoring and resonant circuit - the controller and output side are on a different board.

Reference	Original	New	Part
C1825,1827	Ceramic, 2.2nF 2kV	Ceramic, 2.2nF 250VAC class-Y2	Vishay WYO222MCMBF0KR
C1834,1841	Aluminum, 4.7uF, 35V	Aluminum, 4.7uF, 50V	Nichicon UPW1H4R7MDD
C1848	Aluminum, 2.2uF, 150V	Aluminum, 2.2uF, 350V	Nichicon UPW2V2R2MPD
DS1824	Neon, 90V, 300uA	Neon, 90V, 300uA	VCC A1B

Finished board:

inverter.jpg

And here is why I replace neons. This one's gone and metallized its whole glass envelope. It's being used in a relaxation oscillator, and the usual circuit (single R, single C, single neon) abuses the lamp somewhat with large pulses of current, making it prone to failure. It's in a noncritical circuit (it's just a safety indicator, it doesn't perform a real circuit function) so I didn't modify their circuit for longevity. If I wanted to, I could have added a series resistor with the lamp to protect it.

badneon.jpg

The gigantic screw-mount rectifier reservoir capacitors are bloody expensive to replace, so I didn't - I tested them instead. Nominally 430uF, with a test frequency of 1kHz, they read 522uF/50milliohm and 462uF/80milliohm capacitance and ESR. With a test frequency of 120Hz the resolution on ESR isn't even high enough to get a reading, they just read zero. Think I'll keep these.

That'll be enough for tonight

BravoV · « **Reply #9 on:** November 20, 2015, 03:18:37 am »

Thanks for the update.

At the inverter board, there are two TO-3 trannies, secured on the white block thingy that I guess as a heatsink, is that ceramic ?

I thought ceramic is bad to dissipate the heat, eg: compared to metal heatsink like aluminium ?

c4757p · « **Reply #10 on:** November 20, 2015, 03:22:47 am »

It is indeed ceramic, either alumina or beryllia. Its purpose is to transfer the heat to the chassis, not to dissipate it directly - and they are extremely thermally conductive. They feel very cold to the touch. (More than alumina usually does, I think they might be beryllia - don't want to scratch them

)

Those transistors are running at full rectified mains voltage, so I guess they didn't want to play around with the electrical isolation there.

Here's the schematic for this board - they are Q1834 and Q1844.

BravoV · « **Reply #11 on:** November 20, 2015, 03:29:58 am »

Ahh I see .. Beryllium Oxide ceramic, just be careful.

c4757p · « **Reply #12 on:** November 22, 2015, 05:34:18 pm »

Fuck this IC.

SeanB · « **Reply #13 on:** November 22, 2015, 05:46:10 pm »

Aside from the mangled threads and the trimmed leads, what is wrong with that hybrid.

c4757p · « **Reply #14 on:** November 22, 2015, 05:54:09 pm »

Nothing, it's just a sadistic design.

xrunner · « **Reply #15 on:** November 22, 2015, 05:59:13 pm »

Quote from: c4757p on November 22, 2015, 05:54:09 pm

Nothing, it's just a sadistic design.

LOL - yea really.

SeanB · « **Reply #16 on:** November 22, 2015, 06:09:15 pm »

Mmmm, I need to get this small circuit to work reliably, over a wide bandwidth. Discrete parts are not easy and are too big. Think I will call the hybrid division and give them this, they already are doing 20 others for this model range.......

c4757p · « **Reply #17 on:** November 22, 2015, 06:15:51 pm »

I'm just kidding, dude

SeanB · « **Reply #18 on:** November 22, 2015, 06:31:35 pm »

I used to repair them decades ago, as they had some common failures in them in that design. Plus was the lids were soft soldered on, and most of the faults were relatively easy to change underrated transistors where you could solder in a new metal can packaged device and still have it work. Another was a DAC thick film network which would have the thin wires corrode off from ambient exposure, there it was replace the unit with a "second source" part.

Later on VW Golf/Rabbit/Bora ignition modules, which are quite famous for a diode inside failing open circuit, and where you could bodge in a short leaded part or use a MELF package diode to repair it.

c4757p · « **Reply #19 on:** November 22, 2015, 09:47:26 pm »

Well, there's why channel 2 doesn't work in 1M...

u48.jpg

Don't see it? Here, a bit closer...

lm301.jpg

This attenuator module was serviced - it has an official "Serviced at factory service center" sticker on it. Guess the service guy wasn't too careful. I wonder how that got through the tests I'd hope they would do. This chip is rather important in the 1M buffer:

1mbuffer.png

KE5FX · « **Reply #20 on:** November 22, 2015, 10:25:37 pm »

Quote from: c4757p on November 22, 2015, 09:47:26 pm

This attenuator module was serviced - it has an official "Serviced at factory service center" sticker on it. Guess the service guy wasn't too careful. I wonder how that got through the tests I'd hope they would do. This chip is rather important in the 1M buffer:

Is there a date on the attenuator service sticker? That IC has a 1992 date code, so it would have been replaced long after the 485 itself was manufactured, and possibly after the attenuator was serviced.

The attenuators in some of the 485s had some annoying intermittent contact issues, so it's good that yours has been back to the factory for service. Hopefully they upgraded it or otherwise brought it up to date.

Another tip: I see a lot of >100K carbon-comp resistors on that inverter board. Could be a good idea to measure them to see if they've drifted out of tolerance.

c4757p · « **Reply #21 on:** November 22, 2015, 10:31:11 pm »

The service was also performed in 1992, perfectly matching the opamp.

I'm not yet aware of any Tek gear that doesn't have intermittent contact issues.

Yeah, I've been having a general poke around at things like that. So far everything's good.

lowimpedance · « **Reply #22 on:** November 22, 2015, 10:44:07 pm »

Just enough pressure to make contact to pass the retest.... perhaps

With just a little bit more care with inserting the IC they may have felt something was not right. Still it only has 8 legs, its not a 40 pinner, how hard can it be !.

c4757p · « **Reply #23 on:** November 23, 2015, 12:48:39 am »

Did some more routine replacements, power supply this time. Nothing particularly exciting:

powersupply.jpg

These are the output capacitors for the SMPS. I'm not sure exactly what type they are - they are much heavier than they look and appear to have glass seals; could be wet tantalum, or just really good aluminum. They're in excellent condition and test significantly better even than the replacements I chose for them, so I'm keeping these in:

goodcaps.jpg

Some diverse capacitors in here... I don't see the flat tantalum very often - those are 100uF.

colorfulcaps.jpg

c4757p · « **Reply #24 on:** November 23, 2015, 12:52:59 am »

The high voltage section of the transformer board provides some CRT bias levels. As before, I changed out anything exceeding 1kV as a matter of principle. The original ceramic caps had cracks in the outer insulation that I didn't much like the look of.

hv.jpg

Line filter board. The design of this is a bit annoying, there's not much of a way to work on it away from the chassis.

linefilter.jpg

Not a lot to do on the timing switch board, except at the back where the fan motor sits:

timing.jpg

Thank the lord for modern DC fans with built-in drivers...

fandriver.jpg


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Author Topic: Tek 485 repair and restoration (Read 23962 times)

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