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Indeed, my suspicions were also heading in that direction, but its good to have another view. HOLD is connected directly to GND and I confirmed a ground connection there. READY is connected to 5V via

Hi Tony. First I want to thank you for your kindness in answering me. Both scopes show code 3000 in the POST. I have the service manual. Reading pags 5.36 and 5.37 of the manual, they tell you to

John, I think suspicions may be moving from ROM rot to a bad CPU or at least a need to clean the CPU pins. Before condemning the CPU check that the voltage swing on the clock input, pin 1, comfortably

One technique to "trap" a bad microprocessor is to have a series of jump or branch instructions surrounding a halt instruction.? If the mandatory jump or branch failed, then the processor would halt,

Chuck, thanks. I will have a look at the crystal driver. Will need to remove that board. Regarding the processor, I get what you are saying. I put the jumper in reset mode which holds the RESET line

The scope should have no problem running 24-7 with the case off and no fan. You have just found a problem to fix. Crystals that get hot have defective drivers that are hitting them with too much

Hmm, stuck the LA on it today but I see no processor cycles whatsoever. The LA has 16 channels but I have only 8 clips, so I connected the control lines only: RESET S0 S1 ALE RD WR TRAP INTR and GND

Looks good to me, and I don't have to go online to see your posts now... Jay

Chuck, John, The insight into 8085 programming wisdom is very illuminating. I am guessing that the 160-0459 ROM is mapped to $0000 and there are many HALT instructions (hex $76) scattered through the

Does anyone know of a link for the download of the Tek468.zip file, which contains the service ROM bin file? Thanks Tony

Heitor, What are the symptoms of the two scopes and what have you already identified as problems? Perhaps that would help us figure out ways we can help you. Please keep the symptoms of each scope

Harrison, Yes, but that action does not eliminate the rest of U550. There are other circuits in U550 that are powered through that same secondary winding of T550. That winding has 3 taps, so Pin 10

Roger, So I'm sending this letter from my PC rather than the phone. Wondering if this looks any better. I have seen my own letters from the phone since I send to all and thus getting a "Copy" Well

Alright, found a spot for -5V and checked it with both a meter and a scope, came out to -5.05V with some funky high frequency noise/ripple around 40 mV for the main body and nearly 100mV counting the

Harrison, As I recall, U550 was rather easy to remove. All the pins are somewhat obvious from the trace side of the board since they're slightly larger diameter pins. They desoldered rather easily too

I'm trying to restore a TDS6604 to it's former - hours of fun for the whole family! After a few setbacks (Overheating CPU and a flat battery on the PPC board) I have the scope working 99%. The

Hi Michael I confirmed in my shop notes, I did separate U550 and T550 by breaking the connection at T550 pin 10. After doing so F558 blew upon powering up the scope. I was looking at pulling U550.

There is one command that halts, 0166 octal, and 255 commands that don't. As I mentioned earlier, a wise programmer salts the unused memory with halts, and makes the 7th interrupt vector contain a

Hi Harrison, After reading your latest post, a light bulb went on. I did a quick search, and found that I still have a NOS U550, P/N 152-0635-00 that I bought on eBay a couple of years ago to fix a

Harrison, The problem with U550 is that it does not only contain the 1.5X HV multiplier, it also contains the DC restorer circuits and some other relevant circuitry within the module. If you look at