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Old 30th Nov 2020, 4:42 pm   #1
vintage_8bit
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Default Tektronix 2445

Hi gents.
Scope was in good working order until last switch on.

I have just a blue dot on screen. I have 10 front panel led's permanently on.
Only the Focus and readout pots are functional. The "Find beam" moves the spot towards the centre.

All the regulated supplies and 10V ref are there. What I have found is there is no activity from the processor on board A5. The clock is there and there is a high on reset pin 40, 6, 2, 3, & 4.

I have re-seated processor and ROM chips.

Any ideas gratefully appreciated.
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Old 30th Nov 2020, 6:19 pm   #2
RogerEvans
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Default Re: Tektronix 2445

The manual has some information on the power-on tests and indicators. The relevant section starts at page 6-9 'Diagnostic Routines' in my manual. The front panel lights should flash as the processor goes through the tet sequence.

When you say the readout intensity control functions what do you actually see in the readout areas of the CRT?

Regards,

Roger
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Old 30th Nov 2020, 6:49 pm   #3
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Default Re: Tektronix 2445

Thanks for you reply Roger.
The front pannel lights just come on and stay on. The readout intensity has a null point where the blue dot is at its dimmest. I have set the focus as far out as possible to minimize damaging the tube. There is no text on the screen only the blue dot. So no X or Y amps running.

I'm thinking the processor may be duff or a fault elsewhere on the A5 board. Apart from the clock there are only fixed 0's or 1's on all other pins on the processor.

I will have a look at the points you mentioned in the manual.

Colin
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Old 30th Nov 2020, 8:54 pm   #4
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Default Re: Tektronix 2445

Had a look at 6-9 as suggested. As the front pannel controls are all inoperative there is no chance of entering diagnostic mode. I assume with the processor board not running diagnostics would not work anyway.

Colin
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Old 1st Dec 2020, 9:59 am   #5
Craig Sawyers
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Default Re: Tektronix 2445

The big grey heatsunk hybrids in there are very prone to failure. I believe that the silicon in there is OK, but for some reason a bond wire fails. There used to be someone out there who would take the thing out, fix it, and reassemble.

If I were you I'd join the Tekscopes mailing list on groups.io . There will be plenty of help on there on diagnosis and solution.

Craig
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Old 1st Dec 2020, 10:32 am   #6
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Default Re: Tektronix 2445

Colin,

Reading a bit further down the manual pages you find that if the processor ever started its kernel checks before failing then you would get a flashing 'triggered' light and an error code on the ch1 .. ch4 LEDs. So it looks like you have a very low level processor problem as you suspect. Can you confirm you have a 5MHz clock on pin 39 and a 1.25MHz clock on pin 37? If you are missing the clock out signal on pin 37 it can either be a failed 6802 CPU or a failed TTL device connected to the E clock line. The smaller devices are hardly ever socketed so that is a pain to track down any further.

I second Craig's suggestion that you ask on TekScopes, they will always suggest that you begin by measuring DC and ripple on all the low voltage supplies. A high level of ripple on +5V could well be the cause of an inactive processor board and 24x5 power supplies are prone to need recapping with age.

I see there is one 6802 on eBay at present with bids at £1.99, ending later today, and another in Italy for about £6. It probably ought to be a 68A02 (rated to 1.5MHz rather than 1MHz) but I would be tempted to try the standard part first.

Regards,

Roger
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Old 1st Dec 2020, 11:47 am   #7
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Default Re: Tektronix 2445

Thank you Craig and Roger. I will be in the workshop latter so will have a further look around. I will look at the Tek forum as well as I didn't know about that. When I checked supplies they were all spot on but I didnt look at ripple.

Colin
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Old 1st Dec 2020, 1:15 pm   #8
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Default Re: Tektronix 2445

The 5V line on the processor looks clean, I will recap though when working.
I have a 5MHz clock on P39. There is just a high on pin 37. I isolated pin 37 and it still sits just high on pin 37. Am I right in thinking there should always be a clock here under any condition if pin 39 is clocked?

Colin
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Old 1st Dec 2020, 1:46 pm   #9
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Default Re: Tektronix 2445

Colin,

I think the answer to 'any condition' is yes, but to be more certain I would say with some certainty 'after the processor is released from reset'. I would gamble a small amount of money on a replacement 6802 (the parts listing is 68A08, the schematic is 6802). Given your choice of user ID on the forum I may be telling you the obvious but the 6808 was Motorola's way of selling 6802s that had a failure in the onboard 128 bytes of RAM. The RAM can be disabled anyway by pulling RE, pin 26, to ground and the 2445 CPU board does do this by default.

Regards,

Roger
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Old 1st Dec 2020, 3:05 pm   #10
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Default Re: Tektronix 2445

Roger. No I didnt know that so thanks for the info. My name came from my interest in the commodore 64, which is when I joined the forum. My main interst now is valve based radios and equipment. The Tek scope is used for fault finding or at least was. I can't clame to be very knowledgeable about digital things. Anyway, I think I will go for a replacement processor and report back. Having just retired I had been using that series of scope for many years in my work and feel comfortable using them.

Colin
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Old 1st Dec 2020, 3:34 pm   #11
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Default Re: Tektronix 2445

Just ordered a 68B02 for £5 so hope thst will be ok. Will let you know.
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Old 1st Dec 2020, 4:47 pm   #12
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Default Re: Tektronix 2445

I have a 2465 (very similar architecture) that had a dead 6802 CPU. Replaced it & then started working my way through the rest of the faults -- still got a couple left that I need to go back to. These are great scopes when working...........


Phil
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Old 1st Dec 2020, 6:12 pm   #13
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Default Re: Tektronix 2445

When I was working, we were spoilt for choice with scopes. Mostly LCD digital storage types, going up to GHz. I always went for the Tek if I could.

Colin
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Old 8th Dec 2020, 3:28 pm   #14
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Default Re: Tektronix 2445

It's fixed. A new processor was all that was needed. Just got lots of caps to change now, just to be safe. Thanks for all of your advice.

Colin
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