Fixing a 7603 Power Supply


 

I've got a 7603 that I received free with a 7D01 plug-in. It powers on (the power indicator lamp comes on), but the CRT does not show anything. Checking the voltages I find that the +130 V rail is way out of spec at +150 V. Checking the components in the +130 V regulator I find that Q852 is not recognized by my component tester, and the readings I get with the multimeter make it look like a resistor network (current flows both ways through all junctions).

The Tek part # is 151-0276-00, but the part has 2N5087 printed on it, so I'm guessing that's what it is. I don't have any 2N5087s in stock, but it looks like some other parts I have to hand might do the trick while I wait for 2N5087s to arrive (they are cheap and plentiful, but shipping seems a bit slow).

Here are the specs of the 2N5087 and the candidate parts:

2N5087: PNP, Vce = 50 V, Vcb = 50 V, Veb = 3 V, Pd = 625 mW, hFE = 250-800, fT = 40 MHz
2N2907A: PNP, Vce = 60 V, Vcb = 60 V, Veb = 5 V, Pd = 400 mW, hFE = 100-300, fT = 200 MHz
2N2904: PNP, Vce = 40 V, Vcb = 60 V, Veb = 5 V, Pd = 800 mW, hFE = 40-120, fT = 200 MHz
2N3906: PNP, Vce = 40 V, Vcb = 40 V, Veb = 5 V, Pd = 625 mW, hFE = 100-300, fT = 250 MHz
BC327-25: PNP, Vce 45 V, Vcb = 50 V, Veb = 5 V, Pd = 625 mW, hFE = 150-400, fT = 250 MHz
A1015: PNP, Vce = 50 V, Vcb = 50 V, Veb = 5 V, Pd = 400 mW, hFE = 70-400, fT = 80 MHz

I think that the salient spec in this circuit is Vce, and the 2N2907A and A1015 are the best fits , but their total power dissipation is low. The hFE specs are all lower than that of the 2N5087, but the Common Design Parts Catalog only indicates an hFE of 250 for the 151-0276, so several of these parts will probably satisfy that spec.

Any advice on what might work in place of the 2N5087 would be appreciated.

-- Jeff Dutky


Tom Lee
 

Hi Jeff,

I commend you on your thorough research of candidate alternatives!

I'd choose a 2907(A) here. Specifically, I'd select a PN2907A. It is counterintuitive, but true, that the plastic package permits a higher dissipation (one that matches the 2N5087's) than does the 2N2907A's metal package. The reason is that the 2N2907A (and its cousin, the 2N2222S, by the way) has the die mounted on a thin header whose thermal resistance is surprisingly high. The epoxy package here actually allows 50% greater dissipation.

It's nice that the PN2907A is also cheaper and more widely available. Sounds like a win to me.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 7/9/2021 00:30, Jeff Dutky wrote:
I've got a 7603 that I received free with a 7D01 plug-in. It powers on (the power indicator lamp comes on), but the CRT does not show anything. Checking the voltages I find that the +130 V rail is way out of spec at +150 V. Checking the components in the +130 V regulator I find that Q852 is not recognized by my component tester, and the readings I get with the multimeter make it look like a resistor network (current flows both ways through all junctions).

The Tek part # is 151-0276-00, but the part has 2N5087 printed on it, so I'm guessing that's what it is. I don't have any 2N5087s in stock, but it looks like some other parts I have to hand might do the trick while I wait for 2N5087s to arrive (they are cheap and plentiful, but shipping seems a bit slow).

Here are the specs of the 2N5087 and the candidate parts:

2N5087: PNP, Vce = 50 V, Vcb = 50 V, Veb = 3 V, Pd = 625 mW, hFE = 250-800, fT = 40 MHz
2N2907A: PNP, Vce = 60 V, Vcb = 60 V, Veb = 5 V, Pd = 400 mW, hFE = 100-300, fT = 200 MHz
2N2904: PNP, Vce = 40 V, Vcb = 60 V, Veb = 5 V, Pd = 800 mW, hFE = 40-120, fT = 200 MHz
2N3906: PNP, Vce = 40 V, Vcb = 40 V, Veb = 5 V, Pd = 625 mW, hFE = 100-300, fT = 250 MHz
BC327-25: PNP, Vce 45 V, Vcb = 50 V, Veb = 5 V, Pd = 625 mW, hFE = 150-400, fT = 250 MHz
A1015: PNP, Vce = 50 V, Vcb = 50 V, Veb = 5 V, Pd = 400 mW, hFE = 70-400, fT = 80 MHz

I think that the salient spec in this circuit is Vce, and the 2N2907A and A1015 are the best fits , but their total power dissipation is low. The hFE specs are all lower than that of the 2N5087, but the Common Design Parts Catalog only indicates an hFE of 250 for the 151-0276, so several of these parts will probably satisfy that spec.

Any advice on what might work in place of the 2N5087 would be appreciated.

-- Jeff Dutky




 

Thanks for the feedback Tom.

What I have to hand are metal can 2N2907As that I bought, I think, to effect repairs on a DM501. I've got one of those in the scope at the moment, and it cleared up the +130 V rail quite nicely. It should hold (fingers crossed) until the proper replacement parts arrive in a couple weeks.

I had a thought, after getting the power supply squared away, that the reason I might not have had any image on the 7603 when I powered it on was because I didn't have any plug-ins installed. I used one of the 7A18As and the 7B53A from the 7623A (which is still not working, and I have shelved until my skills improve enough to tackle it again), and tried again. There is still no image on the CRT, but I did get a couple of faint flashes while turning the intensity knob through its full range of motion.

My presumption is that there are multiple failures in this unit, and that fixing the LV power supply was not going to directly fix the CRT image issue. Still, it was worth a try.

My next step, according to the flow chart, is to install plug-ins and check for horizontal deflection saw-tooth, which sounds pretty easy. Of course it would help if I were certain that any of my plug-ins actually worked, but where's the fun in that?

-- Jeff Dutky


Dave Peterson
 

Jeff,

Interestingly I reassembled my 7934 last night to test the PS with load (see the recent thread noting the need for a load).

After adding plug-ins, I had the same behavior: no display, but flashes during power cycles. After a horizontal plug-in swap and switching to the B horizontal slot I got a horizontal readout (only). Viola! The CRT was alive! Continued fiddling also resulted in horizontal traces intermittently flashing. Ultimately I got a stable horizontal trace from all plug-ins.

Along the way I've realized that there are a multitude of problems going on: some of my plug-ins are not in great shape (knew that), I strongly suspect the plug-in connectors are in dire need of cleaning, and I know that the vertical and horizontal mode switches are in dire need of cleaning. Swapping plug-ins resulted in changing behavior - never consistently. Repeated switching of mode switches also resulted in inconsistent, but somewhat improving behavior. Sadly it currently appears stuck in some kind of a quasi-alt horizontal mode. I had a good A and B horizontal trace, then after I tried ALT I now have a trace at the bottom of the CRT that is always there, no matter what other settings or adjustments are made.

It was late, I have other projects to attend to, but now I know that this scope has sat too long and needs a going through with contact cleaner and fader lube. Hopefully this is also true of your 7603.

I was considering asking this group about processes and procedures for plug-in connector cleaning, but I suspect I should search the messages first, and a bit of common sense applies too. Just carefully work on cleaning plug-in contacts, and gently swab the connectors in the scope with IPA.

Dave

On Friday, July 9, 2021, 2:07:19 AM PDT, Jeff Dutky <jeff.dutky@gmail.com> wrote:

Thanks for the feedback Tom.

What I have to hand are metal can 2N2907As that I bought, I think, to effect repairs on a DM501. I've got one of those in the scope at the moment, and it cleared up the +130 V rail quite nicely. It should hold (fingers crossed) until the proper replacement parts arrive in a couple weeks.

I had a thought, after getting the power supply squared away, that the reason I might not have had any image on the 7603 when I powered it on was because I didn't have any plug-ins installed. I used one of the 7A18As and the 7B53A from the 7623A (which is still not working, and I have shelved until my skills improve enough to tackle it again), and tried again. There is still no image on the CRT, but I did get a couple of faint flashes while turning the intensity knob through its full range of motion.

My presumption is that there are multiple failures in this unit, and that fixing the LV power supply was not going to directly fix the CRT image issue. Still, it was worth a try.

My next step, according to the flow chart, is to install plug-ins and check for horizontal deflection saw-tooth, which sounds pretty easy. Of course it would help if I were certain that any of my plug-ins actually worked, but where's the fun in that?

-- Jeff Dutky


Zentronics42@...
 

I have done a few of the 7603, interestingly enough I had the EXACT same issue with my +130 Vdc rail. The driver for the pass element was dead. Starting the unit with no plugins expected behavior looks like a dead CRT. No display, no readout, no sweep nothing. To get a trace you will need at minimum a time base or a vert amp with a sine wave in the HORZ port. That should be enough to unblank the trace and get some signs of life.

Zen

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Dave Peterson via groups.io
Sent: Friday, July 9, 2021 10:05 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Fixing a 7603 Power Supply

Jeff,

Interestingly I reassembled my 7934 last night to test the PS with load (see the recent thread noting the need for a load).

After adding plug-ins, I had the same behavior: no display, but flashes during power cycles. After a horizontal plug-in swap and switching to the B horizontal slot I got a horizontal readout (only). Viola! The CRT was alive! Continued fiddling also resulted in horizontal traces intermittently flashing. Ultimately I got a stable horizontal trace from all plug-ins.

Along the way I've realized that there are a multitude of problems going on: some of my plug-ins are not in great shape (knew that), I strongly suspect the plug-in connectors are in dire need of cleaning, and I know that the vertical and horizontal mode switches are in dire need of cleaning. Swapping plug-ins resulted in changing behavior - never consistently. Repeated switching of mode switches also resulted in inconsistent, but somewhat improving behavior. Sadly it currently appears stuck in some kind of a quasi-alt horizontal mode. I had a good A and B horizontal trace, then after I tried ALT I now have a trace at the bottom of the CRT that is always there, no matter what other settings or adjustments are made.

It was late, I have other projects to attend to, but now I know that this scope has sat too long and needs a going through with contact cleaner and fader lube. Hopefully this is also true of your 7603.

I was considering asking this group about processes and procedures for plug-in connector cleaning, but I suspect I should search the messages first, and a bit of common sense applies too. Just carefully work on cleaning plug-in contacts, and gently swab the connectors in the scope with IPA.

Dave



On Friday, July 9, 2021, 2:07:19 AM PDT, Jeff Dutky <jeff.dutky@gmail.com> wrote:

Thanks for the feedback Tom.

What I have to hand are metal can 2N2907As that I bought, I think, to effect repairs on a DM501. I've got one of those in the scope at the moment, and it cleared up the +130 V rail quite nicely. It should hold (fingers crossed) until the proper replacement parts arrive in a couple weeks.

I had a thought, after getting the power supply squared away, that the reason I might not have had any image on the 7603 when I powered it on was because I didn't have any plug-ins installed. I used one of the 7A18As and the 7B53A from the 7623A (which is still not working, and I have shelved until my skills improve enough to tackle it again), and tried again. There is still no image on the CRT, but I did get a couple of faint flashes while turning the intensity knob through its full range of motion.

My presumption is that there are multiple failures in this unit, and that fixing the LV power supply was not going to directly fix the CRT image issue. Still, it was worth a try.

My next step, according to the flow chart, is to install plug-ins and check for horizontal deflection saw-tooth, which sounds pretty easy. Of course it would help if I were certain that any of my plug-ins actually worked, but where's the fun in that?

-- Jeff Dutky


 

Dave,

It did occur to me that, along with all the other unknowns, I had no idea if the mode selector switches were making good contact. They're not nearly as sticky as some Tek pushbutton selectors I've encountered, but that's no guarantee that they're not dirty.

I'm not sure that my methods are advisable, but for cleaning the card edge connector on the auxiliary board for my 5103N I applied DeOxit to a paper towel and wedged it into the card edge connector with the wooden end of a long cotton swab (the wood, I hoped, would be rigid enough to provide some lateral force for cleaning, but not so much as to possibly break anything). I then applied more DeOxit to the card edge pads themselves using the cotton end of the swab, and reinserted the auxiliary board. For the card edge connectors in a 7000-series scope I would be more careful about the process, just because I know that the plug-in connectors are known points of catastrophic failure, but otherwise do something fairly similar.

I wonder if it would be enough to apply DeOxit to the male card edge before inserting the plug-in. The wiping action of the card edge connection interface is supposed to clean the contacts on each insertion and removal (or, at least, that is my received wisdom; though I'm not sure where I received it from). It's certainly less abusive than sticking some foreign object into the connector.

Also, aren't the card edge contacts (in the connector and on the mating board tongue) gold plated? Shouldn't they be immune to oxidation?

-- Jeff Dutky


 

Zen,

thanks for the verification. I have been told that you can freely interchange vertical and horizontal plug-ins between vertical and horizontal bays on the 7000-series scopes, and I think I did such a thing while testing the 7623A.

It didn't occur to me, at first, that I needed a plug-in in order to get anything on the CRT simply because most of my experience is with portable scopes, where everything is integrated and all you need to get a trace is to turn on the power (assuming everything is working correctly). I'm a little surprised that the 7000-series scopes blank the screen with no plug-ins installed. I guess they didn't want to risk burning a spot in the center of the CRT?

-- Jeff Dutky


Tom Phillips
 

Not to hijack but what thread was the 7934 power supply? I need to diagnose the PS on one I was given. Someday!

-Tom P.


Zentronics42@...
 

Jeff,
An "A" amp in the Vert and Horz slot is how to turn a 7000 scope in to X-Y mode. In terms of the dot on the scope it is there it is just blanked. You need a signal in the Z axis to turn off the blanking and get it to actually turn on the tube. In terms of cleaning the plugins I have used alcohol and a qtip you will pull a bit of tarnish off the connector. But it might just be dirt and grime. One sore spot in the entire 7000 line is the plastic covers for the connectors in the frame. Those are starting to fail and are susceptible to breakage. Be gentle on them. I cant say I have had much issue with the frame contacts bit I have definitely cleaned the plugins. And I have definitely run in to issues with the cam switches in the plugins.

Zen

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Jeff Dutky
Sent: Friday, July 9, 2021 11:59 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Fixing a 7603 Power Supply

Zen,

thanks for the verification. I have been told that you can freely interchange vertical and horizontal plug-ins between vertical and horizontal bays on the 7000-series scopes, and I think I did such a thing while testing the 7623A.

It didn't occur to me, at first, that I needed a plug-in in order to get anything on the CRT simply because most of my experience is with portable scopes, where everything is integrated and all you need to get a trace is to turn on the power (assuming everything is working correctly). I'm a little surprised that the 7000-series scopes blank the screen with no plug-ins installed. I guess they didn't want to risk burning a spot in the center of the CRT?

-- Jeff Dutky


Dave Peterson
 

The impetus to reassemble the PS/scope was: https://groups.io/g/TekScopes/message/184001

The 7934 and 7854 PSs are effectively the same. My 7934 blew the main fuse on first power on. Not one to consume fuses just to verify that the scope effectively consumes fuses, I took to debugging. Following the SM I did my best to learn and follow the troubleshooting guide. As I am bench space limited I pulled the PS entirely from the scope. I was under the impression it should work unloaded. Lesson learned.

There were a few posts (and pictures) by Karin Johnson dealing with 7934 power supplies:
https://groups.io/g/TekScopes/message/182650
https://groups.io/g/TekScopes/message/182720

I was following these threads and was encouraged to keep going on the debug. It languished for a while as I dealt with other projects. Getting the 7934 off the floor and reassembled was part of the shop cleaning/clearing I've been trying to do. I never did find anything wrong with the PS. I even had Q34/Q40, Q43/Q45, and VR45 out of it. All tested good on a curve tracer. I suspect I either: had a bad plug-in (I know I have one that makes my 7854 chirp), there was just a large start up transient after sitting for many years (old electrolytics?), and/or the ramp up with a variac as prescribed in the SM troubleshooting provided a modicum of reforming of the bulk line electrolytics that reduced the initial transients. For completeness: I checked the ESR on the bulk electrolytics and they were at 1100uF and 0.04 ohm. So they seem ok. Replacement would involve even more aggressive teardown. I'm not enthusiastic about doing that, especially given that it's working now.

Best of luck in your efforts. Feel free to reach out for any further notes or assistance in your debug.

Back to the original thread?
Dave

On Friday, July 9, 2021, 9:10:08 AM PDT, Tom Phillips <bumpstart21@gmail.com> wrote:

Not to hijack but what thread was the 7934 power supply? I need to diagnose the PS on one I was given. Someday!

-Tom P.


 

My remark about working plug-ins turns out to have been prophetic rather than merely sarcastic: the 7B53A that I obtained "for parts, not working" is, in fact, not working. The big daughter board (670-1864) is dirty and badly corroded along the top inter-board connectors, and the corrosion is also present on the main board ((670-2257) along the mating pins for the same connectors. It's actually some of the worst damage I've seen in a scope so far, and that includes a 5103N that was literally filled with dog food. I'd even say that this is worse than the 5103N that arrived with the CRT shattered in shipment.

I verified that the horizontal plug-in was not working by tracing back from the vertical deflection pins on the CRT through the vertical amplifier.

At the moment that only leaves me with the 7B53A that came with the 7623A, which had suffered some severe kinetic trauma. I could probably rebuild that unit with parts from this one (or vice versa), but I've already got another horizontal plug-in on the way (as part of an entire scope that is known to be working). I may still try to disassemble this 7B53A and see if a) the damage is as severe on the main board, and b) if I can make one working 7B53A from two damaged ones.

I could also put one of the vertical amps in the horizontal bay and feed it a sawtooth. It's worth a try, at least.

It does not feel like it, but I think that this is progress.

-- Jeff Dutky


 

This is somewhat off-topic, but I'm setting out to make one working 7B53A from two damaged and dead ones. This actually shouldn't be too difficult as the damaged one just needs to have it's TIME/DIV switch assembly, and one slide switch replaced, and those don't seem to be damaged on the poor, corroded 7B53A that I'm already taking apart.

While taking the corroded 7B53A apart, however, I noticed that the push buttons (other than the single-sweep reset button) don't seem to have any illuminating lamps. They do, however, seem to be build to have lamps, both because the back end of the plunger is beveled to redirect light along the clear plunger, and because the button housing seems to have mount points for a light cage/baffle.

I've experimented with shining a laser pointer down the back of the back of the switch assembly, and the engaged pushbutton lights up quite beautifully. I expect that I could wire up and mount some LEDs at the top or bottom back of the switch assembly to illuminate the engaged buttons, and that half a dozen diodes will not be a significant current drain on the +5V rail, at least not compared to incandescent lamps.

Without the cage/baffle to keep the light from spilling all over the interior of the plug-in the transparent skirt on the TIME/DIV knob also illuminates in an especially subtle and pleasing way.

One 5mm LED is able to illuminate all four buttons in a column very nicely.

-- Jeff Dutky


 

I have added a photo album with images of the dead 7B53A and my experiments illuminating the pushbuttons using a laser pointer: https://groups.io/g/TekScopes/album?id=265929


 

SUCCESS!

The 7603 "works" after a fashion. The reason that I don't see anything on screen is that the INTENSITY pot is very, very dirty and not making contact through most of its range of motion. If I rotate the knob rapidly back and forth I am able to get the CRT to show a trace in fits and flashes, enough that I can see that I'm getting signal and that the sweep is working.

When I went to unmount the pot from the panel, after I removed the knobs, I found that there was some kind of grease leaking around the outer shaft. The inner shaft also appears to have been straightened. This is a standard Bournes modular pot with three elements. My understanding is that these should be well sealed, and I do not expect to find grease leaking around the shafts, so I'm guessing a previous owner tried to recondition the pot. I might simply have to order a replacement.

-- Jeff Dutky


Eric
 

Jeff,
Excellent news Jeff. If you have trouble locating the POT let me know. Ill see what I can do to help there.

Eric

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Jeff Dutky
Sent: Sunday, July 11, 2021 9:32 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Fixing a 7603 Power Supply

SUCCESS!

The 7603 "works" after a fashion. The reason that I don't see anything on screen is that the INTENSITY pot is very, very dirty and not making contact through most of its range of motion. If I rotate the knob rapidly back and forth I am able to get the CRT to show a trace in fits and flashes, enough that I can see that I'm getting signal and that the sweep is working.

When I went to unmount the pot from the panel, after I removed the knobs, I found that there was some kind of grease leaking around the outer shaft. The inner shaft also appears to have been straightened. This is a standard Bournes modular pot with three elements. My understanding is that these should be well sealed, and I do not expect to find grease leaking around the shafts, so I'm guessing a previous owner tried to recondition the pot. I might simply have to order a replacement.

-- Jeff Dutky


 

Eric,

Thanks. I found one on the auction site, and I’m going to disassemble the pot I have in hand and see what I can do to clean and repair it while I wait for the replacement part to arrive.

The part I have in hand is one of the mod pots that is assembled with screws rather than rivets, so it will be easy to take apart. The cleaning and lubrication has been discussed previously and I’m reading through those posts now.

I’m also looking at the power switch, which is very hard to turn on (it’s a pull style switch). QService has a replacement for sale, but I’d like to fix the one I have, if that’s possible.

— Jeff Dutky


 

ACTUAL SUCCESS!

First I traced the INTENSITY control signal through the logic board, and checked Q90, Q108, and U99 (a C3046 transistor array). All of those checked out fine. Then I traced the gate signal from the logic board, through the main interface board, and to the Z-axis amp (it seems that I am fated to fix Z-axis amps). Once at the Z-axis amp I verified that the gate signal coincided with the signal I was triggering against (the front panel probe calibration signal) and then simply checked every transistor on the Z-axis board using my trusty component tester.

I found that Q1148 was bad. That's just a 2N3906, which I have plenty of, and replacing that brought traces to the CRT! Easy as pie.

I was also able to fully rejuvenate the INTENSITY pot by dripping Faderlube down the internal shafts of each pot section. I was never able to figure out how to get the pot sections open to clean the wiper and track, but that doesn't not seem to matter, as the Faderlube seems to have done the trick: the intensity pots are much smoother, and there's not more scratchiness or dead spots. I also applied DeoxIT to the switch at the end of the mod pot, which also helped make the switch action smoother and crisper.

Now it's on to "removing" Option 1 (aka installing a readout board). I have readout board, and the necessary Peltola cables, I just need to make the ribbon cables. I am studying the schematics so that I'm sure I understand all the connections that are needed.

It's a pretty good deal, getting a working 7603 for the price of two transistors (the PN2907A and the 2N3906).

-- Jeff Dutky


Dave Peterson
 

Nice job Jeff!

Great way to start the weekend. Save those dead transistors. Might make for interesting examples for the curve tracer when you get that up and running.

Dave

On Friday, July 16, 2021, 10:25:05 PM PDT, Jeff Dutky <jeff.dutky@gmail.com> wrote:

ACTUAL SUCCESS!

First I traced the INTENSITY control signal through the logic board, and checked Q90, Q108, and U99 (a C3046 transistor array). All of those checked out fine. Then I traced the gate signal from the logic board, through the main interface board, and to the Z-axis amp (it seems that I am fated to fix Z-axis amps). Once at the Z-axis amp I verified that the gate signal coincided with the signal I was triggering against (the front panel probe calibration signal) and then simply checked every transistor on the Z-axis board using my trusty component tester.

I found that Q1148 was bad. That's just a 2N3906, which I have plenty of, and replacing that brought traces to the CRT! Easy as pie.

I was also able to fully rejuvenate the INTENSITY pot by dripping Faderlube down the internal shafts of each pot section. I was never able to figure out how to get the pot sections open to clean the wiper and track, but that doesn't not seem to matter, as the Faderlube seems to have done the trick: the intensity pots are much smoother, and there's not more scratchiness or dead spots. I also applied DeoxIT to the switch at the end of the mod pot, which also helped make the switch action smoother and crisper.

Now it's on to "removing" Option 1 (aka installing a readout board). I have readout board, and the necessary Peltola cables, I just need to make the ribbon cables. I am studying the schematics so that I'm sure I understand all the connections that are needed.

It's a pretty good deal, getting a working 7603 for the price of two transistors (the PN2907A and  the 2N3906).

-- Jeff Dutky


 

Also, a comparatively minor success, but noteworthy: I also fixed a 7A18A that I was using to test the 7603, but which did not have any signal on Ch 2, nor any ability to adjust the vertical position. This fix is noteworthy because it is the third time I have found a malfunction in an instrument merely by visual examination (the other two times were the repair of my father's 475 which had a leaking wet tantalum cap on the sweep board, and the restoration of the vertical signal on both channels on the same scope that was the result of having unseated the channel switch IC while removing the case). In this instance it was a tiny resistor that joins the BNC connector to the position pot that was just dangling in the breeze. It took more time to warm up the soldering iron, and remove the metal covers on the front end than it did to reconnect the resistor. Now both channels on the 7A18A are working perfectly.

To my mind this emphasizes the importance of close visual inspection for diagnosis and repair. Sure, lots of things are not visible to the eye, but some are, and you can save a lot of time and effort by doing the visual inspection first.

-- Jeff Dutky


Tom Lee
 

Congratulations, Jeff! It's always nice to hear that perseverance pays off, and doubly nice that a couple jelly bean parts were all it took.

Cheers
Tom

Sent from an iThing; please forgive the terseness and typos

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On Jul 16, 2021, at 22:24, "Jeff Dutky" <jeff.dutky@gmail.com> wrote:

ACTUAL SUCCESS!

First I traced the INTENSITY control signal through the logic board, and checked Q90, Q108, and U99 (a C3046 transistor array). All of those checked out fine. Then I traced the gate signal from the logic board, through the main interface board, and to the Z-axis amp (it seems that I am fated to fix Z-axis amps). Once at the Z-axis amp I verified that the gate signal coincided with the signal I was triggering against (the front panel probe calibration signal) and then simply checked every transistor on the Z-axis board using my trusty component tester.

I found that Q1148 was bad. That's just a 2N3906, which I have plenty of, and replacing that brought traces to the CRT! Easy as pie.

I was also able to fully rejuvenate the INTENSITY pot by dripping Faderlube down the internal shafts of each pot section. I was never able to figure out how to get the pot sections open to clean the wiper and track, but that doesn't not seem to matter, as the Faderlube seems to have done the trick: the intensity pots are much smoother, and there's not more scratchiness or dead spots. I also applied DeoxIT to the switch at the end of the mod pot, which also helped make the switch action smoother and crisper.

Now it's on to "removing" Option 1 (aka installing a readout board). I have readout board, and the necessary Peltola cables, I just need to make the ribbon cables. I am studying the schematics so that I'm sure I understand all the connections that are needed.

It's a pretty good deal, getting a working 7603 for the price of two transistors (the PN2907A and the 2N3906).

-- Jeff Dutky