SMPS JBL 6328P Voltage higher after repair - Page 1
Forum section: electronics / repair

author: shoeman_g
author_rank: Newbie
timestamp: February 06, 2025, 12:55:13 am
content: I have a pair of JBL 6328p studio monitors. One of them recently stopped working so I pulled it apart to see what the issue was. I am documenting this as I was unable to find anything on the net that would help me.On inspection one of the 470uF caps on the switch mode power supply board had blown, presumably due to old age, I have had these for 11 years. I replaced this and a bunch of other caps caps at the same time as well as the fast blow 6.3A fuse that was just powder. This time with a fuse holder instead of directly on the board. All seemed well once I put it together with all the delicate easily dislodged plugs correctly in place (several attempts to get it right).Sadly after a couple of days use a buzzing noise appeared. Pulling out the ps board the new capacitor had developed a case of sunburn, small bubbles or blisters all over it.I replaced it and began an investigation.I am not much of an electronics person, I dabble a bit but mostly arduino stuff. Switch mode power supplies scare me, I hate working with 240volts (or more) and as an old bugger I value the time I have left. With my pcs when I have ps problems I just replace them. With these JBLs however that was not easy. I enquired from the Australian distributor service agent and they told me the boards were no longer available. I looked at purchasing amplifier power supplies on Ali Express but none were equivalent. The board has three dual output supplies +-70v, +-36v, +-15v and as well as that a muting circuit. At best I would have to use two power supplies and add a muting circuit. However, after much searching on the internet to no avail, I found a kind person who gave me circuits for all three boards in the speakers, Power, Input and Amplifier boards.The ps one came with a parts list and an overlay diagram for the top and underside of the board. The circuit was a little hard to read as it was a copy of a copy sort of thing.For safety I only ran the ps through a variac (I don’t have an isolating transformer), made sure I wore shoes and set up clips to my new Uni-t 61E multimeter and stepped away when turning on gradually to check measurements. To make it non destructive I only connected the leads via 10k resistors.This resulted in a slow process, but on the upside I had two boards, one to compare against (at least at first).The upshot was that there were two caps in sequence (C1 and C3 470uF each see diagram below) and they should have about 160 volts across each instead there was 290 volts across C1 and nothing across C3. I’ve shown this part of the circuit and assume this is fair use of the diagram. If not let me know and I will remove.After spending several hours exhaustively metering all components and not finding anything obvious I started just replacing components. The opto coupler IC4 a 4 pin opto Triac TLP 161J, Q3 an SOT23 0.1A NPN transistor and finally SCR1 an 8 amp 600v D2pak triac. That last one did the trick. Back to 160v across C1 and similar across C3. One of the things I noticed on my travels was that R47 a 9.1 Ohm resistor was actually 15 ohms. I replaced it with a couple of low value resistors .to give me 9.2 closest I could get but it made no difference that I could see.During the process I tried hooking up an oscilloscope to see what was going on, didn’t help me much but in the process of comparison I blew up the sister board when I connected to pin 7 of ic2. Not sure why, the oscilloscope was battery operated and unplugged from mains earth.The sister board now had a different issue to the first one, with no output voltage at all. To fix this one I applied a blanket approach (parts are cheap the speakers cost me $4000 back in 2014, a replacement set of the later model 708s would have cost me $8000) I replaced the opto, q3, IC1 an S0-8 half bridge IR21531S, ic2 an LM293 and ic3 an SOT363 NPN/PNP transistor. Having done all that, the second power supply was now back up. However on checking the output voltage on bot supplies was up by 10% on the +-70 and +-36v lines. I’ve documented all the parts I replaced here with the descriptions in the circuit diagram for anyone who needs it in the future but am happy to pass on the circuit diagrams I acquired if requested. The parts that were changed common to both were the Opto and the npn transistor. One of these parts appears to be functioning more efficiently than the original, although the specs were the same, and giving me a higher output.I think I have to change one or other of the 1k resistors on the Q3 transistor R17 and optocoupler R29 or the 10k resistor at R19. Any advice would be appreciated.

author: fzabkar
author_rank: Super Contributor
timestamp: February 06, 2025, 01:19:55 am
content: IC2-B, IC4 and SCR1 autoconfigure the unit for an AC input of either 120VAC or 240VAC. If the mains voltage is low, the SCR1 latches on and connects the midpoint of the two 470uF bulk capacitors to mains neutral. The PS then becomes a voltage doubler.Since you are in a 240VAC locale, you can disconnect that entire branch of the circuit by opening the connection to the junction of C1 and C3.

author: shoeman_g
author_rank: Newbie
timestamp: February 06, 2025, 02:13:02 am
content: Wow. Thank you. I couldn't figure out what the hell that was for. Makes sense now. I'll give it a fly and see if it solves my problem.

author: fzabkar
author_rank: Super Contributor
timestamp: February 06, 2025, 03:09:41 am
content: I would break the connection at the circled spot (not yours, mine).

author: shoeman_g
author_rank: Newbie
timestamp: February 06, 2025, 04:22:09 am
content: Thanks doing that. As luck would have it there's a couple of cuts and a link involved as the pcb trace isn't quite as conveniently laid out as I'd like.

author: shoeman_g
author_rank: Newbie
timestamp: February 06, 2025, 11:27:38 pm
content: Have tried that but still getting +10% voltage on the output +70V is at +78V + 36V at 40V. Should maybe disconnect somewhere else as well?

author: fzabkar
author_rank: Super Contributor
timestamp: February 07, 2025, 12:33:00 am
content: I confess that I'm having a hard time trying to understand how the circuit regulates. Can you identify the markings on IC1?

author: shoeman_g
author_rank: Newbie
timestamp: February 07, 2025, 12:47:09 am
content: S0-8 half bridge IR21531S Mouser part NoRoHS1942-IR21531STRPBF. Data sheet attached.

author: shoeman_g
author_rank: Newbie
timestamp: February 07, 2025, 12:59:31 am
content: R7 is 11k and C7 is 1000pf

author: fzabkar
author_rank: Super Contributor
timestamp: February 07, 2025, 01:06:48 am
content: AFAICT, the circuit is unregulated. The output voltage is proportional to the mains.

author: fzabkar
author_rank: Super Contributor
timestamp: February 07, 2025, 03:02:12 am
content: BTW, SCR1 is a TRIAC, not an SCR. :-?

author: shoeman_g
author_rank: Newbie
timestamp: February 07, 2025, 03:12:01 am
content: Going to put it back in the speaker and see what happens. Bit confused as to why my original readings were lower.Yes I said the SCR1 is a triac in my initial post but it is labelled SCR1 on the circuit diagram and the parts list so I used that as a reference.

author: shoeman_g
author_rank: Newbie
timestamp: February 07, 2025, 03:12:56 am
content: Thanks for all your help. I'll let you know how it goes after the weekend.

author: shoeman_g
author_rank: Newbie
timestamp: February 09, 2025, 10:46:49 pm
content: Have put them back together and they provided nice music all weekend. Many thanks for your help. I learnt a lot from this exercise including how to handle smd components, (lots of flux, solder wick and a digital microscope helped), I now understand the basic principles of SMPS, (there are some good videos on youtube), and also the voltage doubler principle, thanks to you. Great to have a place like this to come to.

author: fzabkar
author_rank: Super Contributor
timestamp: February 09, 2025, 11:54:34 pm
content: Congratulations. BTW, I'm an old bugger, too.