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Chromedome45
April 10th, 2011, 05:52 PM
So I have a recently aquired PCjr from Maverick1978 and it will not start up. I have good voltages on the board as far as I can tell. 5 Volts on all major IC's including the 8088. This was just taken out of the box and was still sealed. We did find a leaky capacitor on the power supply board and I replaced it. But as I said all voltages appear to be good. I need some help... Mike B you there!!! :)

mbbrutman
April 10th, 2011, 06:24 PM
So I have a recently aquired PCjr from Maverick1978 and it will not start up. I have good voltages on the board as far as I can tell. 5 Volts on all major IC's including the 8088. This was just taken out of the box and was still sealed. We did find a leaky capacitor on the power supply board and I replaced it. But as I said all voltages appear to be good. I need some help... Mike B you there!!! :)

This was new in the box? Interesting.

I don't come across too many machines that are just complely dead. Here is what I would do:


Remove the 64KB memory expansion card
Move the diskette controller card
I would have left the diskette drive in because the tray is fragile, but it sounds like you have already removed it.
Examine the rear connectors for pins that might be bent and touching
If the right side expansion cover has been removed, look at the I/O expansion pins for bent pins that might be touching
Look for anything stray in the machine that doesn't belong.
Reseat the system ROMs (two chips near the cartridge slots)
Look for burn marks ;-0


Have you checked *all* of the voltages coming from the power supply card? Which version of the power supply card is it - is it a short card, or is it long enough to stretch from front to back? Is the card properly aligned in the slot?

Are you getting any beeps out of it? Have you tried looking for video both on the RBG connector (marked 'D'isplay) and the composite output? How about putting an external speaker and amp on the 'A'udio connector, just in case the onboard speaker is not working for some reason?

Is your 8088 socketed or soldered? (Just curious ..)


Mike

Maverick1978
April 10th, 2011, 07:58 PM
Hope Frank doesn't mind me jumping in here, but I know that he's in bed currently, and in the interest of moving things along... :)

All of the bulleted list was done, all checked out ok. *edit* Actually, as I think back, I don't think we checked the expansion connector pins, though since the cover was on it from the factory, that SHOULD have been ok...

All voltages were checked as completely as we could figure without having the tech ref handy (I didn't think to bring mine - didn't think it would be needed as the machine was NOS - DOH!). Alignment of the PS in the slot was good - but the leaking cap did get some goo inside the power slot. That was scraped and cleaned as best as could be done without drilling the riveted connector and pulling it - he verified that the chips were seeing correct voltages on the power pins of each chip, and that ground on each chip checked out. Note that the goo cleaned quite easily from the motherboard and PS card, showing no visual damage afterwards.

We're not getting any power beeps whatsoever, nor are we seeing lights on the floppy drive when it's connected. Video output to the montior does change when power is turned on and off. We got the same result with the memory card/fdc installed, and without - the expansion cards being present made no difference.

We didn't check screen output via the composite output, nor did we connect a speaker to the external connector.

The 8088 is a machine socket and an Intel chip (unlike my Jr which has a socketed AMD).

Note that I also have the Hardware and Maintenance manual for the Jr complete with the set of dongles... again, I didn't think to bring this with me when I loaded up to go to Frank's house. Hindsight is always 20/20....

I believe that Frank is leaning towards replacement of the 64kb memory on the motherboard at this point (I'm assuming that he'll test with piggy-back chips prior to snipping?) - It'll probably be mid-week next week before I can meet up with him to borrow my two manuals...

Chromedome45
April 11th, 2011, 03:04 AM
I did check the Video on the composite output and still the same. No display. Didn't think of checking the audio coming out of the back. I plan on cutting each RAM chip out and socketing in a new one. Seems to be where the problem is in the RAM. And as Maverick noted I did reseat all socketed chips allready. And the PS card is the Long version. All voltages coming off of it are good. Any more suggestions are welcome.

mbbrutman
April 11th, 2011, 05:15 AM
Wait before you touch the RAM. Unless somebody rubbed the insides of the machine with a cat, the chances of soldered RAM going bad are pretty low.

Chromedome45
April 11th, 2011, 05:04 PM
Replaced RAM chips and as you said Mike no difference. But my feeling is that the problem is with the RAM addressing or DATA. I checked the 8088 clock input at pin 19 and I read 4.77 with a freq counter. I also went ahead and replace the 8259 and the 8253. Still no luck. Wish I had the Hardware reference manual with schematics so I can trace the Address and Data buses to the RAM.

Chromedome45
April 11th, 2011, 05:20 PM
Oh and when I did hook up the audio all I got was a loud hum. Kinda like 60hz.

Maverick1978
April 11th, 2011, 06:00 PM
Frank - my Foxit PDF Reader is crashing out on this one, but here's another link to a Jr Tech Ref. (ftp://ftp.oldskool.org/pub/tvdog/tandy1000/documents/jrtech.zip) I'm downloading Adobe Reader now to see if there are schematics in this one. If not, it might be worth busting one of these up to insert the schematics....

*edit - COOL! That version linked has the schematics you're looking for, unlike the retroarchive.org one we found last night.. They start at section B1 (page 498 ).

mbbrutman
April 11th, 2011, 06:07 PM
The frequency counter is a good sign. 60hz hum on the audio seems like a bad sign.

You mentioned that you replaced one leaky cap. Electrolytic caps age, especially the aluminum ones. You might need to replace them all if the poor thing was never powered up. (I'm not sure if they are supposed to sit unused for 25 years.)

I'm not well versed in the 8088 pinouts, but is something like "reset" being asserted?

This is a longer term solution, but right now on ePay the following are available:


Technical Reference
Hardware Maintenance and Service
Sams ComputerFacts


The pricing on the first two is $20 each from a seller with 100% good feedback. The Sams is more expensive, but might be useful.


Mike

Chromedome45
April 12th, 2011, 02:51 AM
The leaky Cap was on the power supply board. I ohmed all the small elctrlytics on the board and all showed resitance. No shorts. So I am guessing they are good. Looking at the schematics the addressing for the Memory comes from the 6845 through 2 LS258 ships. I have spares for these and might be the next chips I replace.

Chromedome45
April 12th, 2011, 05:13 PM
I think I know what the problem is. I dragged out my 40+ year old O'scope and started probing around. The address bus looks to be good but the Data bus is pretty much non-existant. Looking at the outputs of ZM66 the first LS245 after the cpu has good inputs but no outputs I replaced it but still no go. I ohmed the Data bus to ground and have very high resistance so nothing appears to be pulling it down to ground at least. Still trying to track it down.

Chromedome45
April 14th, 2011, 04:27 PM
Got it working with the help of Maverick1978. Turned out to be bad BIOS ROM part number 1504037. I have ROM images and will burn a 28c256 EEPROM. Hopefully will work out.

mikey99
April 14th, 2011, 05:56 PM
Hmm, maybe the ROM chips are prone to failures too just like memory chips. Have heard of quite
a few memory chips going bad in 5150's lately. I had a 16K chip die on my 5150 16-64KB motherboard
awhile back, Fortunately it was in the second (socketed) bank.