Need help with powered studio monitor speaker amp! - Page 1
Forum section: electronics / repair

author: silar
author_rank: Contributor
timestamp: July 23, 2022, 04:00:55 pm
content: Been a long time stalker on this forum but haven't really posted anything. I have a set of AKAI RPM-800 studio monitor speakers and one that just all of a sudden stopped working. before, if the speaker was turned on, you could hear an audible hiss that's present on both both speakers, but now this one doesn't have any hiss or audio outputting. It does have power going to the transformer and the led light is on.. I have a basic workbench and am still learning so please forgive me. I do have a Instek GDS-1054b coming. I was going to go for the Rigol 1054z but this was a little cheaper and seemed to have a much better FFT section.I have tested the preamp section (which is on a separate board) and it does seem to be working. I just used my cheap signal generator with a sine wave at 15khz to test the output of the preamp to the input of the power amp board and it came through fine, even adjusting the level using the volume potentiometer looks to be good.It does have 2 TDA7294 DMOS amps. One for the tweeter and one for the woofer.I'm worried it may be the transformer which has a dual secondary of 22VAC*2 and 13.5VAC*2 as it looks like it's feeding 2 separate full wave bridge rectifiers to output DC. I don't think I'm testing this thing correctly.I'm not sure I'm testing the transformer properly and wanted to make sure I am. Like I said, I'm still learning and haven't really had much experience.I would take it to an electronics shop, but 1 - got the set new for $200 which is a steal and have had them for years, 2 - There are no local repair shops around my area, and 3 - I really wanted to use this as way to learn more about electronics. Any help would be appreciated.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 23, 2022, 04:36:20 pm
content: First and easiest thing to check is you have voltages on all 4 large electrolytic capacitors.The small ones will be for the pre-amp (which you say is working), the large ones will be for the 2 TDA power amp IC's.Maybe this is missing.

author: silar
author_rank: Contributor
timestamp: July 23, 2022, 04:50:44 pm
content: Quote from: Audiorepair on July 23, 2022, 04:36:20 pmStrange thing I'm trying to wrap my head around is the output of the 2 bridge rectifiers is feeding the 2 large caps.Should I just be able to measure the AC voltage on the secondary's of the transformer?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 23, 2022, 05:00:59 pm
content: Quote from: silar on July 23, 2022, 04:50:44 pmI doubt that is the case.The 13.5v AC will go to one bridge rectifier into the 2 small caps, then to probably +15v and -15v regulators to power the preamp.The 22v AC will go to the other bridge rectifier and then the large caps, to power the TDA chips.You could measure the transformer AC output voltages to see if they are present, but this may not be as simple or as safe as measuring the voltages on the capacitors.If you try to measure the AC at the bridge rectifiers, for instance, you run the risk of inadvertently shorting the pins out and blowing stuff up.We've all probably been there and done that as a newbie, which is why we don't do that any more, but take the safer option instead.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 23, 2022, 05:15:54 pm
content: Also, if you check the transformer AC voltages and they are OK, you are then going to have to check the voltages on the capacitors anyway, to see if the bridge rectifiers and capacitors are ok, both of these being common points of failure.Transformer secondary failure is actually quite rare, so I'm suggesting this from experience as opposed to single step logic.

author: El Rubio
author_rank: Regular Contributor
timestamp: July 23, 2022, 05:18:35 pm
content: Measure the supply voltage feeding the chip-amp and trace the audio signal through the amplifier in the same manner you checked the preamp. Look for your signal on the input to the amp and the output. There is a pin to mute the amp. Check that to make sure it isn’t shorted. Compare voltages with the good speaker.

author: silar
author_rank: Contributor
timestamp: July 23, 2022, 05:24:34 pm
content: Think I found the issue. while testing at the transformer, I decided to remove the foam covering over the wires. I'm glad I did because I found a fuse holder on each one of the 22VAC lines running to the connector. one fuse was blown. I'll have to find a replacement fuse to further troubleshoot. both fuse holders are shown to require a T2AL250V fuse, but in the one that was blown had a F1AL250V fuse.Looks like the transformer secondary's are good.

author: silar
author_rank: Contributor
timestamp: July 27, 2022, 12:51:37 am
content: Just wanted to mention I feel like a complete idiot. It was just the fuse. All is well.