Last time 'round on this topic I designed a heatsink that contacts only the metal tab parts of U800, not the plastic body. This is the intended (I think) means of getting the heat out of the chip.

The drawing of this heatsink is in the Tekscopes files page on the Yahoo Groups web site. Look for a folder titled "U800". Its in there.

Dave

-=-=-=-

On 20 January 2013, at 2:05 PM, Jeff Machesky wrote:

Is all of this heat related? One of the first things I did when getting
my 2465BDM was strap a very large heat sink to the U800 chip with good
quality thermal compound and very secure mounting. It's larger then the
chip. It runs just over room temperature with the case on. I

measured it

with a type K probe through the vents and touching the middle of the
heat sink with a dab of thermal compound over a half hour or so time

frame.

I do notice mine drifts a tiny tiny bit on warm up, one or two mm to

the

left. It later snaps back about 1mm to the right.

Jeff

Is all of this heat related? One of the first things I did when getting
my 2465BDM was strap a very large heat sink to the U800 chip with good
quality thermal compound and very secure mounting. It's larger then the
chip. It runs just over room temperature with the case on. I measured it
with a type K probe through the vents and touching the middle of the
heat sink with a dab of thermal compound over a half hour or so time frame.

I do notice mine drifts a tiny tiny bit on warm up, one or two mm to the
left. It later snaps back about 1mm to the right.

Jeff

I am repairing a 184 that has had transistors removed from the countdown board.

I am looking at;

Q104

Q114

Q134

SM says "selected from 2N3251"

I can get 2N3251A's local.

I looked at the data sheets. Specs are close. Looks like the A has higher ratings.

Does anyone know if they will work or do I need the non A part?

TIA

B

The price would add up fast if he was looking to replace all 33 of the
2N3605/2N918 transistors in his 184. Most of the other small signal
NPN transistors could be replaced as well.

Jelly bean parts are produced by multiple manufacturers in large
quantities and are almost always the most economical. New 2N918
transistors only come from Central Semiconductor and everything or
just about everything they make is for the replacement market at a
high premium although not as high as NTE. The original 2N3605
Tektronix used is not manufactured any more.

The 2N3904 is hardly a questionable substitution for the original
2N3605.

On Sun, 20 Jan 2013 23:55:47 -0000, "taylorvandy"
wrote:

OP was looking for a more available part - not a jelly bean part, whatever that is. If the original is still available, why the subs?

And no, most of us buy our parts in onesie-twosie fashion. What cheaper? 3 bucks for the originals, or $3.50/100 for a questionable sub?

Not trying to flame, just honoring the OP's intentions.

Cheers,
Taylor

--- In TekScopes@..., David wrote:

Under $2 is not cheap for a jelly bean part. 2N3904 transistors are
like 3.4 cents each. Even an MPSH11 which is a closer match to the
2N918 can be had for 8 cents each.

Note that the above prices are for quantities of 100 since I never buy
jelly bean parts like that in smaller quantities.

On Sun, 20 Jan 2013 15:29:44 -0800 (PST), Taylor Vandergrifft
wrote:

Why all the subs?? 2n918's are still very available, and at least tow places have them under $2 each.?? Hard to rationalize subbing an available, cheapo transistor.
?
Cheers,
Taylor

----- Original Message -----

From: i814u2.geo

To: TekScopes@...

Sent: Sunday, January 20, 2013 3:45 PM

Subject: [TekScopes] 475 progress but another road-block

?

So, with the help of the folks here and some troubleshooting, I've gotten my 475 closer to being usable again.

However, I'm stuck at a new spot now. I am getting almost perfect voltages at each test point except for the 110V TP which shows roughly 112.4 (which is still within tolerance, just not ideal).
The odd thing is that when I test resistance to ground, only about half of the points are within tolerance. 110, -15, U50, and 105 all show much more resistance than they should. Especially -15 which is showing several K of resistance (book shows it should be 480 ohms).

At some point, I also found a trace that was broken and made the repair. I now get a display if I use the beam finder button. However, I do not get a display without using that button. When I adjust knobs on the front, that "beam found trace" does move (mostly as I would expect). It doesn't seem to stay perfectly still in all cases, but I'm assuming that is because something is still bad and causing issues (including the resistance variances).

I do not have another scope in order to test ripple. I've been trying to follow the troubleshooting flow chart in the manual, but I may be getting confused. If I press the beam finder and adjust the trace to the center, but then release the beam finder button and still see no trace, should I be disconnecting the delay line as my next test?
Or have I gotten ahead of myself?

-Josh

Under $2 is not cheap for a jelly bean part. 2N3904 transistors are
like 3.4 cents each. Even an MPSH11 which is a closer match to the
2N918 can be had for 8 cents each.

Note that the above prices are for quantities of 100 since I never buy
jelly bean parts like that in smaller quantities.

On Sun, 20 Jan 2013 15:29:44 -0800 (PST), Taylor Vandergrifft
wrote:

Why all the subs?? 2n918's are still very available, and at least tow places have them under $2 each.?? Hard to rationalize subbing an available, cheapo transistor.
?
Cheers,
Taylor

Why all the subs?? 2n918's are still very available, and at least tow places have them under $2 each.?? Hard to rationalize subbing an available, cheapo transistor.
?
Cheers,
Taylor

No, not really. The 2N2222 has a higher power rating than the others
and the 2N4401 has a higher current rating than the 2N3904 but none of
that will matter in this application.

As far as performance, what could be improved? It is a time mark
generator. I would not replace the transistors unless I was fixing a
problem or some had already gone bad presumably because of age.

The 2N3904 is probably the best choice and the least expensive.

On Sun, 20 Jan 2013 23:02:12 -0000, "andersen_bill@..."
wrote:

That was my typo on the 2N918.

Of the 3 you list is one offer any better performance?

2N2222, 2N3904, and 2N4401

Thanks

--- In TekScopes@..., David wrote:

It is 2N918 and not 2N198. The 2N918 was a very common early VHF
transistor. My 184 has 2N3605 transistors as well.

The 2N918 is:

900 MHz @ 4 mA
30 Vcbo 15 Vceo 50 mA

From the Tektronix parts book the 2N3605 is:

300 MHz @ 10mA
18 Vcbo 14 Vceo 200 mA

The ubiquitous and inexpensive 2N2222, 2N3904, and 2N4401 should all
work as replacements.

On Sun, 20 Jan 2013 21:56:17 -0000, "andersen_bill@"
wrote:

Now that I think about that is the 2N198 missing a digit?

They are all listed in the SM as 2N198, semms like the last digit is missing?

--- In TekScopes@..., "andersen_bill@" wrote:

I have another transistor question on this one.

The unit is up and running as stated and looks to be within specs.

I was checking the rest of the transistors on the countdown board and their are quit a few of the same type listed in the SM. Q193, for example. The service manual states for these, "replaceable by 2N198".

All of the ones in my unit are marked 2N3605. They appear to untouched and original.

Is this correct? Is their a more available replacemnt for this type?

Maybe with better performance?

Thanks

It is 2N918 and not 2N198. The 2N918 was a very common early VHF
transistor. My 184 has 2N3605 transistors as well.

The 2N918 is:

900 MHz @ 4 mA
30 Vcbo 15 Vceo 50 mA

From the Tektronix parts book the 2N3605 is:

300 MHz @ 10mA
18 Vcbo 14 Vceo 200 mA

The ubiquitous and inexpensive 2N2222, 2N3904, and 2N4401 should all
work as replacements.

On Sun, 20 Jan 2013 21:56:17 -0000, "andersen_bill@..."
wrote:

Now that I think about that is the 2N198 missing a digit?

They are all listed in the SM as 2N198, semms like the last digit is missing?

--- In TekScopes@..., "andersen_bill@" wrote:

I have another transistor question on this one.

The unit is up and running as stated and looks to be within specs.

I was checking the rest of the transistors on the countdown board and their are quit a few of the same type listed in the SM. Q193, for example. The service manual states for these, "replaceable by 2N198".

All of the ones in my unit are marked 2N3605. They appear to untouched and original.

Is this correct? Is their a more available replacemnt for this type?

Maybe with better performance?

Thanks

Doubtful it's heat related. The chip runs at ~117F with no heatsink, that's pretty low.

Search for U800 you'll find hundreds if not a thousand posts on the subject.

Be careful with how large a heatsink you attach to the studs. A little bang on the bottom of the scope and you'll short out the -5V supply.

--Victor

--- In TekScopes@... <mailto:TekScopes%40yahoogroups.com>, Jeff Machesky wrote:

Is all of this heat related? One of the first things I did when getting
my 2465BDM was strap a very large heat sink to the U800 chip with good
quality thermal compound and very secure mounting. It's larger then the
chip. It runs just over room temperature with the case on. I

measured it

with a type K probe through the vents and touching the middle of the
heat sink with a dab of thermal compound over a half hour or so time

frame.

I do notice mine drifts a tiny tiny bit on warm up, one or two mm to

the

left. It later snaps back about 1mm to the right.

Jeff

On 1/20/2013 2:00 PM, victor_j_silva wrote:

Hi Chip,

That was me! I do remember because you were very nice with the refund
even though I had left Pos FB already. Not many would do that.

I have seen three main failure modes on U800:

1. The trace shifts to the left (operator's left when looking at the
CRT) as the scope warms up, usually 5 to 10 minutes. These are the
ones I baked and revived!
2. The chip is very noisy, by this I mean there is a lot of jitter on
a very fast step response so that on every retrace the horizontal
position of the rising waveform will vary by 500ps to 1ns. Unlike a
good U800 which will show the retrace at the same position.
3. Outright dead.

The U800 you sold me was failure #2 type.

--Victor

--- In TekScopes@...

<mailto:TekScopes%40yahoogroups.com> ,

"random.path" wrote:

I sold an apparently working U800 to a list member from an old

2445.

Though the scope seemed to function just fine on initial testing, he
later emailed this back:

"I left FB after doing just a quick test at 1ms/Div. After more

testing I discovered that it is very noisy at 10X when time base is
faster than 5ns/Div after a quick 2 minute warmup. This Horz Amp is
not usable."

So I refunded his money. What I take this to mean is that the U800

can be failing or not meeting specs even without total failure. Just
something to keep in mind.

Chip


--- In TekScopes@...

<mailto:TekScopes%40yahoogroups.com>

, "victor_j_silva" wrote:

Sounds very much like the U800 issue.

About six months ago a posted about baking some U800s that had

failed this way and was able to rejuvenate them.

I put one of the U800s in a test scope and it's still working to

this day. I'm amazed.

--Victor

--- In TekScopes@...

<mailto:TekScopes%40yahoogroups.com>

, "cmjones012003" wrote:

Hello all,

I haven't posted to this list in a long time but have just been

lurking, I'm afraid.

[SNIP]

Thank you in advance
Chris
Cambridge, UK

--

Is all of this heat related? One of the first things I did when getting
my 2465BDM was strap a very large heat sink to the U800 chip with good
quality thermal compound and very secure mounting. It's larger then the
chip. It runs just over room temperature with the case on. I measured it
with a type K probe through the vents and touching the middle of the
heat sink with a dab of thermal compound over a half hour or so time frame.

I do notice mine drifts a tiny tiny bit on warm up, one or two mm to the
left. It later snaps back about 1mm to the right.

Jeff

On 1/20/2013 2:00 PM, victor_j_silva wrote:

Hi Chip,

That was me! I do remember because you were very nice with the refund
even though I had left Pos FB already. Not many would do that.

I have seen three main failure modes on U800:

1. The trace shifts to the left (operator's left when looking at the
CRT) as the scope warms up, usually 5 to 10 minutes. These are the
ones I baked and revived!
2. The chip is very noisy, by this I mean there is a lot of jitter on
a very fast step response so that on every retrace the horizontal
position of the rising waveform will vary by 500ps to 1ns. Unlike a
good U800 which will show the retrace at the same position.
3. Outright dead.

The U800 you sold me was failure #2 type.

--Victor

--- In TekScopes@... ,
"random.path" wrote:

I sold an apparently working U800 to a list member from an old 2445.

Though the scope seemed to function just fine on initial testing, he
later emailed this back:

"I left FB after doing just a quick test at 1ms/Div. After more

testing I discovered that it is very noisy at 10X when time base is
faster than 5ns/Div after a quick 2 minute warmup. This Horz Amp is
not usable."

So I refunded his money. What I take this to mean is that the U800

can be failing or not meeting specs even without total failure. Just
something to keep in mind.

Chip


--- In TekScopes@...

, "victor_j_silva" wrote:

Sounds very much like the U800 issue.

About six months ago a posted about baking some U800s that had

failed this way and was able to rejuvenate them.

I put one of the U800s in a test scope and it's still working to

this day. I'm amazed.

--Victor

--- In TekScopes@...

, "cmjones012003" wrote:

Hello all,

I haven't posted to this list in a long time but have just been

lurking, I'm afraid.

[SNIP]

Thank you in advance
Chris
Cambridge, UK

--

You can often pick up a "for parts" P6042 for the price he is asking for just the sensor. The only problem with that approach is that you might end up with a few P6042s and no time to fix them...
Klaus


--- In TekScopes@..., "Alex" wrote:

That's why I said it's possible. Looks the same to me. Jam that guy in there and it should work.
The P6042 is captive to the main unit, and the A6302 is the same probe but with a connector and a different amp.
If the p/ns are different it's because the newer part uses a different potting compound AFAIK.

--- In TekScopes@..., David DiGiacomo wrote:

On Sat, Jan 19, 2013 at 6:10 PM, Alex wrote:

261114631650

Why do you say it's a P6042 part? The seller is claiming it's for an
A6302, is that incorrect?

I thought the P6042 used the 120-0464-00 and 120-0464-01.

I bought mine used from a local reconditioner several years ago and it has had several years of light use since then.

It has three flaws (other than these, it appears to work just fine and I have enjoyed using it):

1. Just recently, the green shift key failed to respond. While I believe it should be simple to fix and someone else can probably do the work quite easily and cost effectively, it is beyond my skill level to repair the problem.

2. There is a small crack in the RF Atten knob.

3. The surface of the screen has some discoloration. This is slightly visible in pictures, but it is not apparent when using it unless you look carefully for it.

Otherwise the display is bright and clear, and the control knobs are in great shape. It comes with the operating manual on CD. I also have the companion TR503 Tracking Generator for sale.

I have pictures I can share.

I would prefer to sell via the group. Please contact me off line.

Gordon, KA2NLM