[KrisBlueNZ]

Perhaps I wasn't clear enough about where to connect the negative multimeter lead. Try this.

1. positive lead to Q1 base; negative lead to 0V (available at bottom end of R6)
2. positive lead to Q1 emitter; negative lead to 0V (available at bottom end of R6)
3. positive lead to Q2 base; negative lead to 0V (in other words, measure across R6)
4. positive lead to Q4 base; negative lead to right end of R2 (in other words, measure across R4)
5. voltage across R2.

 

[tryppyr]

The problem isn't the lack of clarity in your instructions. It's in my ineptitude at translating a point on a logical diagram to a point on a physical circuit. I need to study things a bit more before posting the next batch of readings.

 

[KrisBlueNZ]

OK. I can translate those measurements into descriptions with reference to the picture you posted in post #60.

First we need to fix the markings of Q2 and Q3 on that picture - you have them swapped over - and add markings for the other components. Can you post an unedited copy of the original picture? I will mark up the component references and repost it, along with instructions for where to measure.

 

[tryppyr]

Thanks for understanding. Here's a link to the unedited image.

 

[KrisBlueNZ]

OK. Here are the measurements I need.

1. Positive lead to R1 east; negative lead to R6 west. (Q1 base wrt 0V.)
2. Positive lead to R3 east; negative lead to R6 west. (Q1,2 emitters wrt 0V.)
3. Voltage across R6 (positive to east end). (Q2 base wrt 0V.)
4. Voltage across R4 (positive to north end).
5. voltage across R2. (positive to north end).

 

[tryppyr]

I took each measurement twice, second time using low voltage mode for higher precision.

1. Positive lead to R1 east; negative lead to R6 west. (Q1 base wrt 0V.)
First reading was 1.6, second reading 1.93

2. Positive lead to R3 east; negative lead to R6 west. (Q1,2 emitters wrt 0V.)
First reading 1.4, second fluctuated between 1.35 - 1.36

3. Voltage across R6 (positive to east end). (Q2 base wrt 0V.)
First reading 0.7, second reading 0.75

4. Voltage across R4 (positive to north end).
First reading 0.2, second 0.207

5. voltage across R2. (positive to north end).
First reading 0.1, second 0.16

 

[KrisBlueNZ]

Hmm. Those voltages don't entirely make sense. It looks like something is pulling the voltage on Q1 and Q2 emitters down, but it's pulling it towards 0V, not towards -160V which is what I would expect if Q1 or Q2 were shorted or leaky.

Can you make a few more measurements please?

1. positive lead to R3 east; negative lead to R5 east. (Q2 base-emitter.)
2. positive lead to R3 east; negative lead to R4 north. (Q2 emitter-collector.)
3. positive lead to R1 west; negative lead to R6 west. (15V rail incoming.)

 

[tryppyr]

1. positive lead to R3 east; negative lead to R5 east. (Q2 base-emitter.)
0.602

2. positive lead to R3 east; negative lead to R4 north. (Q2 emitter-collector.)
182.8

3. positive lead to R1 west; negative lead to R6 west. (15V rail incoming.)

14.6

By the way thanks for your help. I do appreciate it.

 

[KrisBlueNZ]

No problem.

Well, I'm not sure what's going on with the circuit. But this back-and-forth is a very slow way to figure it out, so this is what I suggest.

First, R1 could be faulty. You can replace it with a 33k 5% resistor; it doesn't have to be 33.2k and it doesn't need to be highly accurate.

If that doesn't help, I suggest you just replace all of the small semiconductors on suspicion, since they're pretty cheap. The replacements are:

A16CR8: 1N914
A16Q1: MPSA92
A16Q2: MPSA92
A16Q3: 2N3904
A16Q4: MPSA42

The 2N3904 is extremely common and should be available from Radio Shack or whoever, but the MPSA42/92 are not so common. If you have a specialist electronic parts shop nearby, try them. Otherwise you might need to mail-order them, and the freight charges can be pretty high. If you have to go that route, get a few extra of each type, and a few 2N3906es as well.

As for Q2 (off-board), there's no evidence that it's faulty, but power components are more likely to fail than small components. According to the manual it's either TIP758 or TIP75B. Neither of those is available now, but Googling TIP75B got me a replacement - NTE51, and some specifications. A suitable replacement is http://www.digikey.com/product-detail/en/KSC2333YTU/KSC2333YTU-ND/1052438 but you may be able to find something locally that's suitable. It needs to be a silicon NPN transistor in a TO-220 package with a maximum Vce (collector-emitter voltage) of at least 250V and a maximum collector current of at least 2A. Pretty much anything that meets those specs will do. That's if you decide to replace it on suspicion.

 

[tryppyr]

Thanks, Kris. I do have a Vetco store near where I work and they usually have most of what I need (and a lot of other things to spend money on). It's become a regular stop for me of late. I'll pop by there and see what they have from the list.... after checking my box of supplies first.

Update: Yes, Vetco has all the parts needed, so no shipping charges will be necessary.

 

[KrisBlueNZ]

Hah! We have a Vetco here in New Zealand, but they are... veterinarians!

I don't see MPSA42 and MPSA92 on their web site though. Maybe they just aren't listed there. They do list the NTE51 so you could replace Q2 offboard if you want.

 

[tryppyr]

I picked up the parts from Vetco yesterday. After doing a little more research I'm becoming a bit confused about whether they are proper replacement parts, so I'm slowing things down until I get comfortable with that.

I did remove R1 and test it. It checked out at a perfect 33K, so I put it back on the board.
I removed the off-board Q2 and tested it. According to my meter it has an HFE of 7, whereas the replacement part read 12 (though the package stated typical HFE should be 25). Not sure if that's a meter error or an operator error. In ant case the NTE51 is now installed, and that alone didn't change anything in terms of the function of the unit.

One of my concerns is CR8. The replacement part I got was NTE519, which is described as "DIODE SILICON PRV=100V IF=0.3A DO-35 CASE FAST TRR=4NS FAST SWITCHING" . However, when I look up the original part description it says "DIODE SWITCHING 30V 50MA 2NS DO-35".

The 2N3904 (NTE123AP) and 2N3906 (NTE159) replacements should be okay, since they are so common.

The part recommended as a replacement for the MPSA92 (NTE288) concerns me, as does the complimentary component MPSA42 (NTE287). As they were so cheap I went ahead and bought them, but it would be nice to get confirmation that they are suitable replacements.

 

[KrisBlueNZ]

CR8 is not critical. The NTE519 will be fine.

The NTE287 and NTE288 look OK as replacements for MPSA42/92. They're probably actually identical, just rebadged.

 

[tryppyr]

Great. Thanks for the boost of confidence.

I just installed the new parts. Everything seems to have gone well. Next, I need to reassemble and test. That will take a while.

 

[tryppyr]

Okay, it is reassembled. Upon power up it is now evident that the 120V rail is active again. Still no sign of life on the CRT, but a few seconds after hitting the power switch the neon on the card that connects LVPS to HVPS lights up and starts buzzing. Not the tic-tic-tic sound it used to make, but more of a steady buzz.

Prior to the start of the buzzing the 120V test point reads in at 92V. Once the buzzing starts it reads 121+. The 15V test lead reads pretty close to 15V too. The 48V test point seems to not rise about about 32.7V. I can't let it run too long before a strong ozone smell starts to arise from the unit, at which point I power it down, let it cool for a minute and then restart the test.

This is certainly progress.

 

[KrisBlueNZ]

OK, first try disconnecting the "HV MULT" unit from the board. This will be an external block with a thickly insulated wire emerging from it that goes to the "ultor cap" on the side of the bulb of the CRT.

Try powering it up in a darkened room and look for a purple corona anywhere on the HVPS board. It may just need to be cleaned.

 

[tryppyr]

Will do. In the meantime, I took measurements at the rail heading from LVPS to HVPS.

+156 now reads 119V
+15 UN GND now reads - 0.01
+15 UN is now +22.4
+15 now fluctuates rapidly between 15.7 and 17.2
-15 now hits -7.2
+120 comes in at 11.99
+48 is sitting at 27.0

 

[tryppyr]

No corona on the HVPS. I so see some evidence that the CRT heater is now working, as the corners of the screen have a VERY faint glow. Only visible in a totally dark room. That's probably the source of the smell.

All of the neons on the HVPS are lighting up, and two of them are brighter than the other three, with almost a purplish color to the tips.

 

[KrisBlueNZ]

You said the +120V rail is 11.99V but you meant 119.9V, right?

Interesting that the 156V rail is now equal to the 120V rail. This implies a problem in the circuitry around CR3 and Q11. Can you measure the voltages on both ends of R18 (relative to the 0V rail, i.e. the chassis)?

Interesting that the 48V rail is low. Is this rail used in the HVPS? The HVPS schematic is scanned at such a low resolution that it's very hard to follow, and I can't see a better version of it anywhere else in the PDF.

If you want, you can try replacing the small transistors in the +48V regulator - Q5~8 - just as you replaced Q1~4. That is:
Q5 and Q6 same as Q1 and Q2, which are MPSA92 or NTE288;
Q7 same as Q3, which is 2N3904;
Q8 same as Q4, which is MPSA42 or NTE287.

Did you disconnect the wire from the HV MULT unit? Did that have any effect on the smell?

When the smell is present, is there a sizzling or hissing sound anywhere? The smell must have a corresponding corona somewhere in the unit. It glows a dull purple colour and makes a sizzling sound. See if you can find it.

Just out of interest, can you try disconnecting the base connector from the CRT and seeing whether anything changes?

I have a suspicion that there may be a problem with the main transformer (T1) on the HVPS. Is it possible to break the connections from pins 9 and 10 to the rest of the circuit? For example, can you suck the solder off those pins and make sure the connection from the pad to the pin is broken? If so, check resistance (not continuity or diode test) from pin 9 to the other pins of the transformer. You should measure open circuit (probably shows as OL) to all other terminals. If there is any reading other than OL, there's a problem with the transformer.

 

[tryppyr]

You said the +120V rail is 11.99V but you meant 119.9V, right?

Yes that's correct. The 120V rail now seems to be in spec.

Interesting that the 156V rail is now equal to the 120V rail. This implies a problem in the circuitry around CR3 and Q11. Can you measure the voltages on both ends of R18 (relative to the 0V rail, i.e. the chassis)?

I was planning on doing a fairly comprehensive set of measurements soon, so I'll add that to the list.

Interesting that the 48V rail is low. Is this rail used in the HVPS? The HVPS schematic is scanned at such a low resolution that it's very hard to follow, and I can't see a better version of it anywhere else in the PDF.

Not sure whether HVPS uses the 48V rail. It isn't mentioned directly in the circuit theory text. I'll see if I can come up with a better schematic.

If you want, you can try replacing the small transistors in the +48V regulator - Q5~8 - just as you replaced Q1~4. That is:
Q5 and Q6 same as Q1 and Q2, which are MPSA92 or NTE288;
Q7 same as Q3, which is 2N3904;
Q8 same as Q4, which is MPSA42 or NTE287.

I had that thought too, but it also occurred to me that when I took the readings straight from the transformer output plug it was low there as well. What are the odds that the transformer itself is faulty?

Did you disconnect the wire from the HV MULT unit? Did that have any effect on the smell?

No, I never located that wire.

When the smell is present, is there a sizzling or hissing sound anywhere? The smell must have a corresponding corona somewhere in the unit. It glows a dull purple colour and makes a sizzling sound. See if you can find it.

No sizzling or crackling sounds, or if they are present they are drowned out by the buzzing of the neons. I'll look more closely at the transformer this time to see if anything is happening beneath the board where it's harder to see.

Just out of interest, can you try disconnecting the base connector from the CRT and seeing whether anything changes?

No. That's an easy enough test though. I'll add it to the list.

I have a suspicion that there may be a problem with the main transformer (T1) on the HVPS. Is it possible to break the connections from pins 9 and 10 to the rest of the circuit? For example, can you suck the solder off those pins and make sure the connection from the pad to the pin is broken?

I'll check on that. Last time I looked I saw no way to do that in a reversible manner.