Transformer failure ? - Page 1
Forum section: electronics / repair

author: moss99
author_rank: Contributor
timestamp: August 06, 2020, 11:04:21 am
content: Hi guys , i have this site radio which works with the 18 volt battery but not on mains ac power , i suspected the ac transformer (240 Vac to 16 Vac ) , as when i take the transformer out of circuit and supply the 16 v with my bench supply it works ok .However when i test the transformer out of circuit it seems to work fine , also when i connect the transformer to ac source off the board and connect the transformer output to the board the unit all works fine ..I am getting 240 AC on the terminals that the transformer connects to as long as the transformer is not connected ... I have tested all the components and replaced the 2 larger caps on the primary side as the esr seemed a bit high ..Any input would be appreciated ..

author: SMdude
author_rank: Frequent Contributor
timestamp: August 06, 2020, 01:09:51 pm
content: I would say that the big black relay has a high resistance contact, when you measure voltage with no load, it seems fine, when you put a load on it(ie transformer) the voltage drops. You could do a volt drop test on the relay, or just measure the voltage on one side of the relay switch vs the other. Be careful though!

author: moss99
author_rank: Contributor
timestamp: August 06, 2020, 02:00:59 pm
content: Hi , i did test the relay , not sure if i did it correctly though , i took it out and put dc voltage across it , it measures open with no voltage and 0.2 ohms with voltage ..I also de soldered the 2 relay pins on the hot side .

author: bob91343
author_rank: Super Contributor
timestamp: August 06, 2020, 05:06:50 pm
content: Best way is to make measurements. What are the transformer input and output voltages when it's not working?

author: moss99
author_rank: Contributor
timestamp: August 06, 2020, 05:30:03 pm
content: With the transformer disconnected i get 241 Vac across the main voltage in terminals , 230Vac across the transformer input terminals , with the transformer connected i get still 241Vac across the main voltage in , 0.55 Vac across the transformer input terminals and .03 Vac across the transformer output ,

author: SMdude
author_rank: Frequent Contributor
timestamp: August 07, 2020, 02:38:36 am
content: When you press the power button, do you hear the relay click?If the relay closes and the contacts are ok(your resistance reading seems correct) the transformer should work.Perhaps there is a fault with the standby circuit.

author: moss99
author_rank: Contributor
timestamp: August 07, 2020, 07:26:14 am
content: There is no relay click when i switch it on , isn't the standby circuit powered by the transformer ?

author: SMdude
author_rank: Frequent Contributor
timestamp: August 09, 2020, 08:50:12 am
content: No, the standby power is from the rest of that circuit. The transformer only comes on once that relay is closed.

author: moss99
author_rank: Contributor
timestamp: August 10, 2020, 10:14:06 am
content: I found the problem , the inductor between the first 2 caps had blown , all working again now , thanks for your help..

author: SMdude
author_rank: Frequent Contributor
timestamp: August 10, 2020, 10:21:12 am
content: Nice! And hiding under the white gunk too.If it goes again, then you might have something else going wrong with the cct, but perhaps there was a small power surge?Glad you got it working.