HP 141T Spectrum analyser (Latest model) +248 not adjustable ??!! - Page 1
Forum section: electronics / repair

author: NewWave
author_rank: Newbie
timestamp: May 11, 2019, 10:46:16 pm
content: Hello all here on this forum.I'm from Holland and i'm radio amateur. Worked 25 years in consumer electronics as technician.Recently I have an HP 141T spectrum analyzer (Latest model.. i guess) that does not work.With diagrams downloaded from the internet, I have restored the power supplies so far. -12.6, -100, +100 volts.The problem i have is with the +248 volts, here is almost 300 volts on it and not adjustable.the Power transistor had a Collector emitter of 300 Ohm and replaced it with a 2N 5240 NPN version.All resistors measure the correct value within 2%. Capacitors have been replaced but not successful.On the (middle) output of the potentiometer I do not measure any difference with turning, on the underside of the potentiometer to the resistor R12 (150 KOhm) I measure a varying voltage while turning.Driver transistors are 100%, with replacements in it, the same problem.the -100 Volt is present everywhere, also on the R12 150 KOhm resistor.All other voltages can be set normally.who recognizes this problem?Greetings from the Netherlands ... Hans

author: bob91343
author_rank: Super Contributor
timestamp: May 11, 2019, 11:07:18 pm
content: With a voltage regulator, the feedback is such that the wiper (middle) of the control is constant. To make that happen, the output voltage must change as you turn the control. The error amplifier compares the wiper voltage to a reference voltage and causes the circuit to make that difference nearly zero.If you could post a diagram we could help you troubleshoot.

author: NewWave
author_rank: Newbie
timestamp: May 11, 2019, 11:26:54 pm
content: Hello Bob,I was already surprised that the voltages on the power transistor and drivers are within 5%, as indicated on the diagram.I'm going to upload a schedule so that you can make clear to me what I am not measuring correctly ...I found it very strange, all values of the parts were good, the diodes, all good ...With switching on in the beginning the -12.6 was -18 Volts, but that has been restored ...(Power transistor C-E short) replaced by a 2N3055 and works great.Thanks already... Hans

author: bob91343
author_rank: Super Contributor
timestamp: May 12, 2019, 01:23:53 am
content: I am looking at the schematic diagram now. The pass transistor is Q1 I think. Q3 is the sensor transistor that compares the wiper voltage to a zener diode VR2. When you turn the control, the 248 V output is divided in an adjustable way from the -100 Volt supply so that Q3 can compare the wiper voltage with the zener diode and change the 248 V accordingly.You say the 248 is not adjustable. How much is it? Too low or too high? The problem is likely with that rather simple circuit. My diagram is from a military manual and may not be similar to yours. It uses very old components such as PNP transistors, probably germanium. It's probably more than 50 years old. If the 248 is too high you may have a shorted pass transistor Q1 or its driver Q2. If it's too low you may have bad electrolytic capacitors such as the 430 microfarad filter C1 I think.Not long ago I repaired a very old HP counter with PNP germanium transistors. A 2N404 had failed short and luckily I had one so that was not a difficult repair.

author: NewWave
author_rank: Newbie
timestamp: May 12, 2019, 07:14:27 am
content: Hi Bob,There are several versions, HP has changed during the years the Layout from this board.I ordered the Artec CD, my version is on it. It is the latest version with the potentiometers horizontal on the power board.First have to install the platine because I have replaced capacitors.The board is not placed service friendly.All wires must be removed on one side. Fortunately with sliding plugs. have made a picture first so i can see all colors of these wires.Q1 is in series with the output of Q2 (stabilized + 100V)Q1 controls the +100 Volts back to 48 Volts relative to ground. In the Colletor output of Q1 there is a 200 Volt rectifier in series, so that the total voltage becomes 248 Volts. .. This way I read the diagram.All voltages are correct, also on the drivers. Exact +100 Volts on the emitter input of Q!.I removed the drivers for exact measuring, these are good, C-E are 100% open in both directions.The output of R11 varies 0.1 - 0.2 Volts only, during full rotation of R11, the Voltage here is about 100 Volt, also on the Base of sensor amplifier of Q2The R12 150 KOhm (1/2 Watt) is within 2% exact. The -100 Volt is on the other side of R12. All seems Ok...The original transistors have their own HP type number, but I assume it is not Germanium ... Although ...Q1 Power output is an NPN Type and have found for this a nice replacement, the 2N5240 (300 Volt-5 Amp.) Also TO3 version.I have worked in this profession for over 25 years, but I have never experienced anything so strange. I don't give up ,,, Never !!

author: bob91343
author_rank: Super Contributor
timestamp: May 12, 2019, 04:01:37 pm
content: Well okay you seem to know what you are doing so my advice is probably superfluous. Having said that, it's just a matter of seeing what may have failed, and that could be as simple as a broken wire or cold solder joint rather than a failed component. As mentioned in another thread, make sure the diodes aren't leaky and the zeners have the correct voltage. All transistors need to be forward biased unless they are overload protection.The worst part is gaining access to the circuit without damage and probing while it's apart.Yes, HP made that product for a long time and certainly made many changes along the way. One thing they didn't change is the size and weight of this boat anchor. I wonder how many strained backs can be blamed on the HP designs. I own a VNA that was labeled by the company owning it 'two person lift only'.

author: NewWave
author_rank: Newbie
timestamp: May 13, 2019, 09:04:38 pm
content: Hi Bob,it's a simple power Supply with three transistors... Had already three times a look on the back of the board, but all seems ok, no strange solder connections so far i can see... Sometimes we have these strange situations... It's all in the game..for free.The weight is incredible, but the free-running oscillator(s) that needs a stable housing. Such devices must be able to work under all circumstances.I have a new Rigol DSA 815 with tracking generator, but I would like this analog ship anchor to work. i also have the 8443A tracking generator from HP with all cables to connect the 141T on it. The CRT is in good condition as I see it on the brightness ...i will connect at least a +100 volt separate regulator to the base from Q3, let's see what happens... Have all Fuses in stock !!Hans.