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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

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

-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Dave Peterson via groups.io
Sent: Friday, July 9, 2021 10:05 AM
To: [email protected]
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@...> 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

-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Jeff Dutky
Sent: Friday, July 9, 2021 11:59 AM
To: [email protected]
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

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

-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Jeff Dutky
Sent: Sunday, July 11, 2021 9:32 PM
To: [email protected]
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

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

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