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Old 22nd Jan 2022, 4:38 pm   #1
Bufo Bill
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Default HP 182T 15V rail

Hi guys, I have a problem with the trigger not working on my various time bases. I checked the voltages of the rails, and the 15V rail measures just over 8V. I have checked the electrolytics in that part of the circuit, the big 2100uF can was leaky and replaced with a 2200uF, but this made no difference.
I would like to solve this one myself, but my lack of experience is showing. I think I will check transistors next, but they are mostly very small and spaced at the edge of the board where room is very tight, so I propose removing the transistors and testing on a multimeter. Is this okay or can you recommend a better way? Never dealt with transistors before so any advice greatly appreciated here guys.
Cheers from Bill.
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Old 22nd Jan 2022, 6:04 pm   #2
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Default Re: HP 182T 15V rail

I couldn't find a 182T manual on-line with schematics, the best I could get on a quick search was the 182A. In the case of the 182A the +15V supply is erferenced to the +100V so you need to check that +100V is OK.

The schematic for the low voltage regulator on the 182A shows a current sensing resistor between the base and emitter of Q6. If there is a similar regulator for the 182T you can measure the voltage drop across this resistor, it will go into current limiting when the voltage gets to somewhere around 0.65V - 0.7V. If this is the case then you need to find what is drawing too much current - it will probably be hot! If the supply is not current limiting then check the voltages on the bases of the two transistors described as differential comparator and we can go from there.

Best of luck,

Roger
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Old 22nd Jan 2022, 6:42 pm   #3
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Default Re: HP 182T 15V rail

Hi, many thanks Roger that sounds like a better idea. I can post the relevant part of the 182T circuit diagram if necessary. The +100V seems fine.
Will get back to you later.
Bill.
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Old 22nd Jan 2022, 7:11 pm   #4
Chris55000
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Default Re: HP 182T 15V rail

Hi!

A bit faint but readable 182T book can be obtained from xdevs here:–

https://www.google.com/url?sa=t&sour...XUseeeJhNJmwgN

. . .or a darker and more blotchy one, but not quite as grotesque as K.S.'s usual offerings, from:–

https://www.google.com/url?sa=t&sour...KnY9TmLuWKzwj7

Chris Williams
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It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!
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Old 22nd Jan 2022, 8:05 pm   #5
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Default Re: HP 182T 15V rail

Hi, we have a voltage drop of 27.1mV on that resistor (R13), so I have measured the bases of Q7 and Q8 (differential comparator) and they are Q7 = 8.57V Q8 = 5.72V.
Cheers from Bill.
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Old 23rd Jan 2022, 9:46 am   #6
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Default Re: HP 182T 15V rail

The base of Q8 is about two thirds of the output voltage, kind of what I would expect as its from the adjustment voltage divider. The base of Q7 seems a bit low, could R14 have drifted up in value? Also what are collector & emitter voltages for Q8?

David
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Old 23rd Jan 2022, 12:50 pm   #7
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Default Re: HP 182T 15V rail

Bufo Bill
Would the attached HP182T circuit diagrams be more clear than those you have found.
I have the complete manual if there are other pages wanted.
Think you have found already that the version T is the same as version A except that T has auxiliary outputs on the rear panel for use with a spectrum analyser plug-in, circuit diagram 3.
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Old 23rd Jan 2022, 2:41 pm   #8
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Default Re: HP 182T 15V rail

Interesting the regulator board in my German made 181T looks the same as those in the 182T manuals, need to check the board part number now.
And the 181T manuals on the web only show the older regulator board, with the row of eight transistors across the middle of the board. Typical of what I've found with the German HP stuff, it often differs from the manuals and the serial prefixes sometimes don't make sense.

David
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Old 23rd Jan 2022, 4:11 pm   #9
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Default Re: HP 182T 15V rail

Sorry to be so slow in getting back to this, I can blame it on a mixture of senility and something else I can't remember!

With the base voltage of the (NPN) transistor Q8 less than the base voltage of Q7, Q7 ought to be drawing lots of current, Q8 should be cut off and its collector voltage should be high. So the crucial question is 'what is the collector voltage of Q8?' . If Q5 and Q2 are working OK then the collector voltage of Q8 should be about two Vbe drops (around 1.7V) above the output voltage. If the collector of Q8 is much higher than this, say above 10V then either Q5 or Q2 is open circuit (this is probably the most common failure) check the voltage on the base of Q2 to decide which is the culprit.

If the collector voltage of Q8 is around 10V then probably Q2 and Q5 are OK and the likely culprit is Q7 or Q8 so measure the collector voltage of each and the emitter voltage (which is the same for the pair). One or other should show a collector-emitter voltage Vce inconsistent with the base emitter voltage Vbe.

Failure of the linear voltage regulators is quite common.

Regards,

Roger
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Old 23rd Jan 2022, 5:35 pm   #10
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Default Re: HP 182T 15V rail

Hi guys, thanks for taking an interest.
Bill, thanks for the pdf, I have a schematic now.
Oops, was measuring PNP not NPN, embarrassing but as I said, totally new to transistors.
Remeasured results:-

Q7 Base 5.04V
Collector 8.48V
Emitter 9.14V

Q8 Base 5.04V
Collector 9.7V
Emitter 5.7V
Cheers from Bill.
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Old 23rd Jan 2022, 6:18 pm   #11
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Default Re: HP 182T 15V rail

Bill,

Sorry to be a pedant but I think your unfamiliarity with transistors is still showing!

Q7 emitter and Q8 emitter are connected together. Q7 collector is the same as the output voltage. So I strongly suspect that what you have is :

Q7 emitter -> 5.04V Q8 emitter -> 5.04V
Q7 base -> 9.14V Q8 base -> 5.7V
Q7 collector -> 8.48V Q8 collector -> 9.7V

This is consistent with Q8 Vbe being 0.66V which is reasonable, Q8 collector is about 1.22V more positive than the output, again quite reasonable and Q7 Vbe being 4.1V which is not reasonable (anything more than about 800mV would give a catastrophic current through the base emitter junction) and it has probably failed open-circuit.

I would suggest unsoldering Q7 and checking it either with a transistor tester if you have one or just checking the base-emitter and collector-base junctions as diodes. Q7 has a diode to protect its base-emitter junction against reverse bias (CR4) so while Q7 is out of circuit check that CR4 has not failed open circuit.

I don't have a parts list but from the circuit it would appear that Q7 could be substituted with a 2N3904 or almost any small signal NPN transistor with a Vce rating of 25V or more.

Hope all goes well.

Roger
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Old 23rd Jan 2022, 6:39 pm   #12
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Default Re: HP 182T 15V rail

Sorry Roger, confusion reigns for me right now! I understand what you are saying, and we shall see how we get on after dinner. Many thanks for your help.
Bill.
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Old 23rd Jan 2022, 8:49 pm   #13
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Default Re: HP 182T 15V rail

Q7 has a Triode magnification value of 0. I presume we have an O/C transistor?
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Old 23rd Jan 2022, 9:20 pm   #14
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Default Re: HP 182T 15V rail

Repeat post, sorry!
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Old 23rd Jan 2022, 10:01 pm   #15
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Default Re: HP 182T 15V rail

I suspect it's open too, had one of those small transistors fail open in the 180A I repaired last year and another quite literally blown apart in another a few years back. The manual said it was a selected from 2N3704, so I selected one at random from my spares.
Just watch the transistor pinouts as sometimes the custom labelled parts were made by different suppliers & can have a different pinout for the same part.

Oh and my 181T has regulator board 00184-66509 (shown in my last previous post), same as listed in the 182T manual for the A1A2 regulator board.

David
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Old 23rd Jan 2022, 10:19 pm   #16
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Default Re: HP 182T 15V rail

Hi David, thanks for posting. I will definitely be careful with the pin-out after the debacle of measuring the Base Voltages earlier. There is a little bead around one leg, is this ferrite? Is it for shielding the signal from interference?
Cheers from Bill.
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Old 24th Jan 2022, 9:09 am   #17
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Default Re: HP 182T 15V rail

I have to admit the only reliable way I have found for measuring physically small transistor voltages in situ, when you don't know the arrangement of the lead out wires, is to trace what they are connected to. You only need to work out two of them! It is also usually much safer to put a test prod on a resistor or other component rather than risk shorting two closely spaced wires directly on the transistor.

I suspect the ferrite bead is to reduce the likelihood that the feedback amplifier will oscillate at high frequencies, most of the common small signal NPN transistors have cut off frequencies around 300MHz.

Regards,

Roger
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Old 24th Jan 2022, 3:43 pm   #18
Bufo Bill
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Default Re: HP 182T 15V rail

Thanks for the tips Roger, your help is most welcome. I have ordered some transistors and I will let you all know how I get on.
Cheers from Bill.
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Old 25th Jan 2022, 7:51 pm   #19
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Default Re: HP 182T 15V rail

Roger, good spot on the diode CR4, it was also O/C!
Put in an order with RS.
Will let you chaps know how I go on.
Cheers from Bill.
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Old 26th Jan 2022, 2:42 pm   #20
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Default Re: HP 182T 15V rail

Hi, I have some Schottky diodes rated 30mA/ 40V, would one of these be a suitable replacement for CR4?
Cheers from Bill.
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