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Hi,

When I look at the Nexperia 74HCT112 data sheet I would guess that it will be fast enough. You may have to buy a few and hand select. But, and it's a big but, they look to only be available in surface mount. No DIP part. :-( Going for a 74S112 from a NOS vendor would be the best solution I reckon. And. just in case a reminder is needed. eBay and various Chinese vendors sometimes end up delivering re-marked parts (whether this is deliberate or inadvertent is a discussion for another day), so testing/validating the part on the bench is essential IMHO. You don't want to be installing a re-marked LS part that will fade out as the temperature goes up, if it even managed it at the start.

Cheers, Brian.

On 09/21/2020 3:19 PM tgerbic <tgerbic@...> wrote:


On Mon, Sep 21, 2020 at 10:54 AM, ron roetzer wrote:

The 74HCT112 is only guaranteed for 24MHz clock. The 74S112 is rated for 80MHz minimum. The 74F112 is rated for 100MHz min.

Makes sense to look at the rated speeds. Tek picked the S type for a reason, and it is clock speed. If the oscillator is replaced, the original part numbers would work as designed, as they did for the last 30+ years. Pick F if you cannot find S. Digikey and Mouser have both types in stock. I did not see them at Allied, RS or Farnell.

There are only 4 actual pins, but their positions are mapped onto a 14 pin DIL package?

Yes oscillators often come in 8 or 14 pin footprints, so the pins are referred to by DIP numbering.

Regards
Tony

I managed to remove the crystal without any damage to the board using the SMD heat gun. I tested is as suggested with a 470Ω load resistor. The output was around 3.5V and stable for several minutes. Without the load resistor the amplitude was over 4V. Frequency is just a smidgen under 50MHz (49.999xxx) on the FC. I hooked it back up to the board using jump leads, powered up the 5V rail but there was no output. I hooked up the sig gen again and got output from both 74S112s. Could it simply be that the sig gen is handling the sink current better than the crystal? Or does the almost 0.8V voltage drop under load suggest that the output from the oscillator is weak? I'm still uncertain whether to replace both ICs and the crystal for good measure?

To test, connect pin 14 to +5, pin-7 to ground and pin 8 is the output. Pin 1 is no connect.

So there is a very easy albeit brutal sounding way of dealing with removing DIL type chips:

My copy of the manual has U128 and U230 as 74S122 (not LS122), my memory says that most LS devices are rated to around 40MHz which is why I suggested HCT as a possible alternative.

Hi Jared,
Congratulations on finding a TM503B. "B" versions of a product are uncommon
for Tek to make. They are an indication that the product sold very well. Tek
was also convinced it would continue to sell well. There were two main reasons
to create a "B" version of a product.
1) To update it to meet current market requirements such as new electrical
standards, safety standards, international environmental standards (such as
being free of lead or mercury), energy efficiency goals, etc.
2) To reduce manufacturing costs, reduce parts count, reduce assembly time,
improve product performance, improve product reliability, reduce the total
cost of ownership for the customer, etc.

I have seen enough different TM503s that I am certain Tek made many versions
of the TM503. I posted a 4 page paper on a TM503 that I came across that had
every indication of being a very late model TM503 judging from how simplified
Tek's production engineers redesigned it for quick assembly and low
manufacturing cost.

This particular TM503 was unusual for many reasons:
* There were no markings on indicating what it was but you could tell
immediately that it was a TM503 by looking at it.
* It had GPIB connectors for each slot like a TM5003 would have.
* The power supply was a conventional transformer and filter caps analog
design, and not a switching power supply like the TM5003 has.
* It produced higher TM5xx DC voltages than the TM5003 and TM5006. TM500x
voltages are lower than the TM5xx unregulated voltages because TM500x voltages
are regulated by the switching supply they use.

You may find something that applies to your TM503B in the unusual TM503 I
found and modified. It is at:

/g/TekScopes/files/Modification%20of%20an%20Unusual%20Three%20Wide%20TM%20Mainframe%20into%20an%20ordinary%20TM503/Unusual%20Three%20Wide%20TM%20Mainframe.pdf.
You may have to paste that link back together to use it.
Alternatively you can simply do a search for
Unusual TM503
in the TekScopes files section and find it that way.
Dennis Tillman W7pF