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Old 8th Feb 2019, 9:29 am   #1
tonyvirago
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Location: Haslingden, Lancashire, UK.
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Default HP T141 Display section repair

I have repaired my unit a few times over the 2 or 3 years of ownership. Usual fault is...no display.
Beam finder is inop when spec analyser modules are installed.
Unit is easy to work on.
1) power supply rail fail. Check non-std value resistors around volts setting pot (why have a potential divider using funny sizes, then have a pot in the middle?)
I have replaced R's in x2 supplies here.

2) The EHT 6.6kv and x2 low voltage "mesh" voltages get to the very front of the CRT on a 3 strip flexy strip. the EHT goes wild after 30-40 yrs on here. I bodged up the EHT with a thin silicone wire which worked for a while, but left the strip in situ. A year or so later, same issues. I removed the strip and replaced the conductors with thin quality wire. I insulated them as thick as possible but there is little room to get from the front of the CRT back into the case (prob why its a flexy in the first place). Rubber boot over the edge of the front of th CRT is tricky to keep in place when sliding the CRT back in after repair. (you can slide it out an couple of inches still with the base connections on) I cleaned up and lightly glued the rubber shroud in place.

Got is working again NO more living in the shed for this beastie. See photos. Tony G4NGV.
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Old 8th Feb 2019, 11:01 am   #2
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Default Re: HP T141 Display section repair

They likely are standard resistor values

The nice thing about standards as they say is that there are so many to choose from

HP standardised on 1% resistors in its instrument divisions. Other tolerances were used in special circumstances.

1% resistors have 96 values per decade for the +/-1% tolerance bounds to just meet up with no gaps or overlap. This way, every resistor made can be flogged as one value or another!

96 values per decade are too many for general use, so they picked the ones in the E96 family closest to the common E24 series

1.00
1.10
1.21
1.33
1.47
1.62
1.78
1.96
2.15
2.37
2.61
2.87
3.16

You can generate the E96 values by taking the 96th root of 10 and then keeping multiplying it into a geometric series. Round to two decimal places. For HP's preferred values, skip three out of every four up the E96 series.

2% resistors fit the E48 series
5% jobs got the E24 series (24th root of 10)
10% jobs got the E12 series.

For confusion, some resistor suppliers make 1% resistors (because accuracy is now a lot easier to achieve) but in the E24 2-digit numbers because some customers were confused.by the extra band on the colour code... oh, well.

Cheers
David
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Old 9th Feb 2019, 8:15 am   #3
tonyvirago
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Default Re: HP T141 Display section repair

Cheers David.
Thought I would contribute a post myself. Other peoples posts have helped me in the past.
Tony
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Old 9th Feb 2019, 10:10 am   #4
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Default Re: HP T141 Display section repair

Good post!


There are plenty of those storage frames out there, but sadly many have mesh burns.

David
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Old 9th Feb 2019, 6:16 pm   #5
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Default Re: HP T141 Display section repair

HP seemed to change to using the 1% tolerance resistors sometime in the 1960's, which is why you find a lot of odd values used, even when an earlier build of the same instrument may use standard 5% or 10% types instead.

Were the failed resistors the metal film ones? I found they can fail high in value or open circuit, sometimes without showing any visible signs of distress.
The metal film resistors HP used in the 1960's & 1970's do seem to be slightly less reliable then the carbon composition ones, despite the latter having a worse reputation in vintage stuff.

I bought myself a 141S SA last year, the power supply for that uses the same main PCB as the older 140A/B scopes & non storage 140T display section, the 141T looks very crammed by comparison.
The plastic covers for the adjustment pots in my 141S seemed to have reacted badly to something, it's the first time I've seen any crazed, cracked of missing, I don't know if this is a common problem or a minor problem with small batch of them due to the type of plastic used.

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David

P.S. shouldn't the title refer to the 141T not the T141?
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Old 9th Feb 2019, 10:13 pm   #6
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Default Re: HP T141 Display section repair

I've seen plenty of 140 series frames and never seen that happen to the trimmer bank before. Some of the hard plastics we used went friable if exposed to certain solvents like the inhibisol aerosols.

David
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