VIZ WA-504B/44D Bad Transformer? - Page 1
Forum section: electronics / repair

author: tplayne
author_rank: Newbie
timestamp: March 05, 2024, 05:56:50 pm
content: Ive been trying to repair a VIZ WA-504B/44D audio generator that is misbehaving. I been tracking issues back and through the circuit and I found something on the output of the transformer(while still connected to the circuit) that I cant really explain, which Ive included below. Looking at the schematichere, the output of the (center tapped) transformer is piped directly into a textbook example of a 2 diode full bridge rectifier, right out of "The Art of Electronics". I am at a total loss to explain the wildly asymmetric droop on the transformer output. This droop also corresponds to a strong negative bias of the generated waveforms, that can hardly get above 0v, but is pulled well below the expected -10v on the swing.Is it possible that this is just a bad transformer? If not, what could explain such a aggressive droop.PS. Im in USA, so this is appears to be a 120 to 240 step up transformer.

author: m k
author_rank: Super Contributor
timestamp: March 09, 2024, 02:02:24 pm
content: Transformer alone can't do what you described.What you see over C109?

author: Vicus
author_rank: Regular Contributor
timestamp: March 10, 2024, 05:19:40 pm
content: Probably something shorted to ground and the transformer can't push higher voltage.

author: tplayne
author_rank: Newbie
timestamp: March 12, 2024, 04:13:17 am
content: Out of the diodes pair and thus the high side of C109Aa Im getting somewhat wibbly 112V DC. Then at the high side of C109b I have 92v DC, while at highisde of c109c there is 44v dc. I measured the relevant resistors for the power area and they seem within spec. So the cap (or single conjoined cap in this case) seem to be doing their job as smoothing and rectifying elements. I suppose they could have significant leakage current, but I haven't pulled it from the board yet, so I cant test that theory yet. Im not even sure I can test it with equipment on hand. Assuming it is leakage current, it doesn't seem to come in to play until +112v DC. My DC power supply will only do +64v in parallel mode, which obviously wont put me into the failure domain.Tom

author: Jason Henry
author_rank: Contributor
timestamp: March 12, 2024, 08:46:58 am
content: The asymmetric droop on the transformer output may indicate a faulty diode in the full bridge rectifier. Check and replace the diodes to see if it resolves the issue before considering the transformer as the culprit.

author: m k
author_rank: Super Contributor
timestamp: March 12, 2024, 03:33:53 pm
content: Power supply part seems to be as it should, but can't find the actual voltage values.(C109A is not enough as a reservoir then R115 is dropping the voltage a bit and then C109B is smoothing the rest)Probe is 10X so scope should be just fine.What voltages RMS DMM is giving from the transformer?

author: tplayne
author_rank: Newbie
timestamp: March 12, 2024, 09:03:19 pm
content: Ok I tested them and they are toasted. Over 1v drop in both directions. I thought I had already tested them but I guess not. Well 1n4007s are cheap and easy to get. I might still swap the big Mallory cap, since CE isnt making this exact 40/40/100 combo anymore. Each measures ok on my lcr meter but... who knows-Tom