Replacement caps for Tektronix 434 Power supply board.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Here you go......

http://w140.com/tekwiki/wiki/434
This is the site

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Hi gents sorry for the mix up.
These are the voltages at present
324Vdc at P119
324Vdc at Q1080 collector
324Vdc at Q1030 and Q1032 collectors.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

OK then, now we have the correct manual. D.c. volts on the collector of Q1080 is correct for 220VAC operation. Apparently, your switching power supply is not switching. My earlier comments about startup still apply. Manual page 3-20 Headings "Start Circuit" and "Turn-on Circuit". Note that the startup pulse cicuit uses 2 neon lamps...so typical for early Tek and HP gear, but reliable.

Keep in mind that startup happens as a result of the startup circuit receiving a healthy voltage pulse when the power switch is pushed. If the startup circuit is working, you will see (using a working scope) a pulse at the cathode of CR1032. At that point the Turn-On circuit must operate to turn on the switcher, Q1080. Some startup voltage is provided by a small winding on T1085 at the far right - due to the turn-on pulse. C1095 is the 10uF filter for that voltage. Worth checking.

You asked about C1081. This 1000pf ceramic or mica is across all the inductances in the collector of the switching transistor. I'm sure it is a tuning capacitor or maybe a parasitic supressor which is needed to tweak the switching waveform. While it is unlikely that C1081 is faulty, if shorted it would certainly prevent any switching operation. If the capacitor is discolored or suspicous, de-solder one leg and do a resistance check of the capacitor. Most of the power-off checks in this area can be done in-circuit using either resistance or diode settings on your meter. Watch for flashing of the 2 neon tubes when pressing the power button.

Just as a personal note, feast your eyes on output transformer T1085. Every secondary output plus the switching windings are all contained in that one part. Amazing!

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

When the supply is working, the collector of Q1080 will have some high voltage spikes. Not recommended to place multimeter probes there. Instead, look at pulses on the secondary side of T1085 (on another drawing).

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Hi I will desolder C1081 tomorrow and check.This is an old dog bone ceramic.
I turned the scope on today and the neons flash but not for the same duration.
One flashes for about 15 seconds the other for 1 second then back to same sequence. When I had the boards removed also the main cap and T1085.
I thought to myself hope there is no problem with the Transformer because the proximity of all the caps and diodes would be a nightmare if any are shorted.
Incidentally I am using a I/T while checking voltages.
I have read the sequence of events but find it a bit heavy ha ha.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

Handle C1081 with care since it is a rather critical part. Tubular ceramics are hi-Q specialty caps.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Just to let you know I have a Hameg 203-6 scope with component tester so I can test for signals it's just knowing what to look for.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

Ok, then. Now we are getting somewhere. I would start by looking at C1095 when the power switch is pressed. You are looking for -50 volts d.c. Note that the "ground" reference for the regulator circuit is the positive side of C1095, the mains ground. Anywhere that is marked "1" inside a triangle is ground for that circuit. The emitter of the switcher, Q1080 is connected to the same ground return. Just solder a pigtail anywhere on this trace. Use it for the scope and meter "ground".

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Longbow I desolder end ceramic cap c1081 checked capacitance was 1273nF.
Attached Dmm com probe to + side of C1095
Where do you want me to probe for the -51vdc?

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

Longbow I desolder end ceramic cap c1081 checked capacitance was 1273nF.

It should be 1000pF, not 1000nF. Or for those who hate pf, it is 1000 mmF. That's fine, put it back.

Attached Dmm com probe to + side of C1095
Where do you want me to probe for the -51vdc?

The plus side of C1095 is your ground reference. The 51 volts should appear on the (-) side of the same cap as soon as the power button is pressed. Then just follow the "Startup" and "Turn-on" explanations in the manual.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Couple of photo's.
I measured across c1095 no voltage.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

Beautifully built equipment. Thanks.
Well, you will have to do a bit of searching and reading. At this point we know that the supply is not starting up...great we knew that last week. At this point I would look carefully at the start circuit which is elegantly explained in the paragraph labeled "Start Circuit". Hameg scope time. It says that the lamps should be flashing at about a 5 second interval. Those flashes <should> be generating +50 volt positive pulses at Q1032 emitter and then CR1032 cathode.

If not, troubleshoot the start circuit. For example, have a close look at C1020 which sets the period of relaxation oscillator. There are some high value resistors in that circuit well worth checking. This is a simple circuit. Let us know what part has failed.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Neons are flashing but not at 5second intervals more like 15sec on one then 1second on the other.
I don't have 51vdc every 15 secs there is a pulse about 0.6vdc at cr1032.
I noticed that R1032 was hot after about 5mins.
Turned off measured resistance was 10kohm +
Going to Newcastle in morning for resistors and caps don't have wattage locally.

Resistance results when cold.

Value measured. --- Spec.
R1032----3.105kohm. 1.8kohm
R1034----1.719kohm. 10kohm
R1030----39.54kohm. 39kohm
R1028----1.37 Mohm 1Mohm
R1026----100-ohm 100-ohm
R1025----98.8kohm. 100kohm
R1024----2.4Mohm. 2.2Mohm
R1020----25Mohm. 22Mohm

Which resistors capacitors are used for timing ?

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

I will assume that the resistors were measured with one leg desoldered? If so, my comments are: you have discovered the next biggest pain in electronics to electrolytics and that is carbon resistors changing value. Your photo shows a mixture of carbon and film resistors.

Of the ones you listed, R1032 and R1034 need to be replaced. Don't try to buy new carbon resistors, use film. Interesting that they are both in series with Q1032. Be sure that transistor Q1032 and CR1032 are both OK. Also C1020, since the neons are not flashing correctly.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

Neons are flashing but not at 5second intervals more like 15sec on one then 1second on the other.

Confirm 100 vdc at C1020 (+ terminal). This is a simple RC circuit consisting of the neons acting as switches, and associated resistors charging/discharging C1024, the timing capacitor.

I don't have 51vdc every 15 secs there is a pulse about 0.6vdc at cr1032.

What could be a better clue that this? Q1032 and Q1030 are not receiving positive-going pulses. Q1025 or VR1025 might be shorted. See what happens after replacing the faulty resistors.

I noticed that R1032 was hot after about 5mins.

That might be ok, given the oscillator is probably turning it on longer than normal. Once the supply starts, Q1032 no longer conducts and thre won't be any current going through R1032. Be sure that Q1032 is in good shape.

After getting the values around the neons correct, you will have a pulse at the collector of Q1025 going from 0 volts up to 51 volts in time with the neons.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Thanks Longbow, been to Newcastle today but bit under the weather bug going around.
Don't want to keep removing psu board and secondary board to replace components.
Awaiting for some components from Farnell then will replace all at once, reason transformer pins and delicate wiring on connectors.
No I did not desolder one leg but after turning off scope, took resistance reading and the readings where reducing as resistors cooled miles out of spec especially r1032 and r1034.
Q1032 is a Tek part cannot find equivalent anywhere if required.
Tested ok
Will check again tomorrow.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

There isn't any way to really test resistor value in-circuit, especially when you know this circuit is faulty. Other stuff is in parallel with the part. The only thing you might know is, for example, if you have a 10K resistor. Other parts may parallel this resistor, so it may read lower than 10K in-circuit, including ZERO if there is a coil. But if the reading is above 10K in-circuit you know something is wrong. For now, check your carbon resistors with one lead desoldered.

Q1032 is not a critical Tek part. If it is faulty, there are lots of subs that will work. Esssentially, you have a Darlington pair delivering +50 volt pulses to the turn-on circuit to get the power supply running.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Well Longbow never rains but it pours.
Set up my Hameg 203-6 after about ten minutes noticed trace just a dot in middle of screen
Been trying to trace fault while waiting for components from Farnell then this morning received email from Farnell saying there was a problem with the transaction and to contact them. So order will be late expected it today so back to the Hameg.
As Dave Jones says on EEVBLoG Murphy will get you every time wish he would take a day off. Everything I go to use I end up doing some sort of repair.

Regards, John.

[Agent24]

Re: Replacement caps for Tektronix 434 Power supply board.

It's not just you! I lost the trace on my Metrix OX860 last week!

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Disaster, replaced R1032, R1034.
Turn on and R1032 smoked.
Turned off rechecked all connections all ok.
What could have caused this????