Commodore 1084ST repair  - Page 1
Forum section: electronics / repair

author: Sanxion
author_rank: Contributor
timestamp: July 10, 2023, 09:19:52 pm
content: Hi all,I need some guidance in how to establish the fault in my lovely 1084ST monitor.When I received it, the monitor did not power on nor was there any activity. The internal fuse had also not blown.I therefore watched two YouTube videos of people repairing their 1084STs which had the same symptoms - no power. Both individuals replaced the TDA 4605 IC - a common point of failure apparently - and the 12K 2W resistor near the IC...I therefore decided to do the same.Upon switching the monitor on, rather than being greeted with an illuminated green LED and active screen, I heard a distinct "pop" from the back where the fuse is. Now I am not certain the fuse has blown as I never checked, but it certainly seems that way.So, after replacing the TDA and resistors, I had a brief moment of activity, but not the kind I wanted. My question is, what do I do next? The main board does not have that many components but I can't realistically replace them all.I welcome your suggestions!

author: james_s
author_rank: Super Contributor
timestamp: July 10, 2023, 11:29:43 pm
content: It would be helpful if you would describe the activity you had that is not what you wanted. Also if you have a link to a schematic it would be helpful to post that here, the 1084ST monitor is old enough that relatively few people will be directly familiar with it. Then you need to get in there and start testing components, test transistors and diodes looking for shorted parts, test resistors looking for open ones. If the fuse is blown then that strongly suggests something in the power supply is shorted, the horizontal output transistor is a common culprit.

author: decoder_82
author_rank: Regular Contributor
timestamp: July 11, 2023, 04:32:43 am
content: I have repaired many commodore monitors but not this exact model. Usually power supply is on a separate board. Always start there and check for correct output voltage.The two most common issues I seen were shorted horizontal output transistor (HOT) and bad flyback transformer.

author: inse
author_rank: Super Contributor
timestamp: July 11, 2023, 07:33:32 am
content: But those faults shouldn’t blow the primary fuse…

author: Sanxion
author_rank: Contributor
timestamp: July 11, 2023, 07:38:17 am
content: The activity that I did not want is:nothing happens at all. It appears as dead as before.Unfortunately, there doesn't seem to be any schematics for this version of the monitor.I did however check the fuse this morning and found that it had not actually blown. Unfortunately, I discovered something else. The two 12K resistors I fitted show a value of 0 resistance on my meter. I then checked the two I replaced (the old ones) and one showed 0 and the other 12.09k.The "new" resistors I bought - a pack of ten 12 Ohm - all show 0k when I test them. Have I bought a batch of bad resistors or are these WW type different to what I need?

author: eurgenca
author_rank: Regular Contributor
timestamp: July 11, 2023, 08:35:26 am
content: On the pic you have a 0.12ohm resistor in the post you mention 12K resistor. What is the original value of the resistor?

author: Sanxion
author_rank: Contributor
timestamp: July 11, 2023, 08:47:07 am
content: The original value is 12K Ω.

author: eurgenca
author_rank: Regular Contributor
timestamp: July 11, 2023, 08:59:58 am
content: Theh you insert wrong resistors. I believe that one on the pic. aren't 12K resistor

author: Sanxion
author_rank: Contributor
timestamp: July 11, 2023, 09:06:26 am
content: Yes, you are correct. I was sold 12(Ω) resistors as opposed to 12K(Ω).I will now have to check to see which other components have been damaged including the TDA 4605 I fitted.I will not make that mistake again!

author: Sanxion
author_rank: Contributor
timestamp: July 15, 2023, 10:45:43 am
content: Resolved!Thank you for your help