TEAC BX-330 amplifier repair - Page 1
Forum section: electronics / repair

author: Ashhhhh
author_rank: Contributor
timestamp: August 04, 2018, 03:26:25 am
content: Hey all,Hoping for a bit of assistance with repair of this amplifier. I'm a rank amateur with no formal training but still enjoy repairing these things.My lack of knowledge stings me sometimes though and I'm unable to find the fault, this is one of those occasions.This had a number of problems and a number of blown components, I won't go into all the detail (unless needed) but I'm now able to correctly adjust the midpoint voltage, the idle/bias (whereas originally there was no power due to some blown diodes and a dual FET 2sk item) BUT there is no sound, on any input.The voltage at various points is a little low when compared to the schematic too. For example 34v where it should be more like 38.I've tested various components and scratched my head for ages, just can't crack it.Any advice appreciated, I've got access to a scope, signal generator etc.I'll attach the manual too.Cheers,Ashhttp://cloud.tapatalk.com/s/5b651c5b010b3/hfe_teac_bx-330_550_service_en.pdfSent from my Pixel using Tapatalk

author: bitseeker
author_rank: Super Contributor
timestamp: August 04, 2018, 04:33:00 am
content: If power supply voltages are far from where they should be, resolve that first. Then, if the lack of audio persists, put a signal on one of the inputs and follow it to see where it's interrupted. Since you indicated that none of the inputs makes any difference, choose the input with the least complicated path to follow and/or probe.

author: cncjerry
author_rank: Unknown
timestamp: August 05, 2018, 05:36:46 pm
content: Having fixed who knows how many amps, it is almost always a blown output transistor or power device. This usually takes out the driver as well. There could also be a protection circuit that fails but usually that is a peripheral failure, not the primary. The fact that you can set the bias only tells me that the outputs are no longer loading the bias circuit, not that anything is any good. Usually when an output fails they short all over the place internally causing the bias circuit to be loaded down (as well as the power supply).The voltages will be low due to loading in the outputs. I would try to get one channel working first by lifting the outputs on the blown side.You should be able to easily track the input signal through the path. If you don't have a scope, clip the connector off some old headphones and ground one side while using the other as a probe.Jerry

author: David_AVD
author_rank: Super Contributor
timestamp: August 05, 2018, 11:50:44 pm
content: Is there any signal appearing at the REC OUT sockets?Is there any signal output from U602 (pins 1 and 7) ?Can you hear any hiss / hum from the speakers?Is the output relay engaging after a short delay at power up?Occasionally the DC detection circuitry can be damaged when an output transistor fails.

author: Ashhhhh
author_rank: Contributor
timestamp: August 06, 2018, 12:21:58 pm
content: Thanks guys for the responses, not ignoring and will pick this up again shortly and work through the advice/checks.Sent from my Pixel using Tapatalk

author: Ashhhhh
author_rank: Contributor
timestamp: August 11, 2018, 11:11:19 am
content: OK, finally got some time to get back into this..Ive found that the volume control was also faulty and causing the signal to vanish when P401 was connected to the main PCB...I had a spare which fixed that problem.The issue as it stands, a 1khz signal fed into the Aux input makes it to R115 on the main PCB (as a nice sine) and then breaks down. R118 shows a clipped sine and R126 further along shows a mess.The other channel shows the same thing at the same test points. (pictures shown of this degradation at each point mentioned)Not sure whats going on...thoughts?No hiss or hum David, nothing at all, dead silent.If its relevant, R304 and 305 are pretty hot, dont know if this is a problem or normal as they are high wattage and stood off a long way.There is still no output and the relay does not click. (I assume its meant to)Hope you can help, this one has been a real hair tearer!CheersAsh

author: David_AVD
author_rank: Super Contributor
timestamp: August 11, 2018, 12:22:49 pm
content: R304 and R305 are for the +/- 15V rails and will get hot. Are both the +15V and -15V rails present?It sounds like the protection relay is not engaging (DC on outputs). What voltages do you read at TP-5 and TP-6 ?You replaced Q103 and/or Q203 ? Was it an exact replacement and did you install the correct way around? Some dual FETs have reversible pinouts and some do not.

author: Ashhhhh
author_rank: Contributor
timestamp: August 11, 2018, 01:13:59 pm
content: Yep +15.2 on R304 and -14.7 on R305.0V on TP5 and 6 because its set that way with R124 & R224.Yeah I found a scrapped BX300 which has very similiar components including the correct 2SK109. (the original was marked K109-06D and the replacement K109-8XD)These are reversible because the centre pin is NC.They not new parts but they beep out fine. Ive also tried the one good original and its the same. Plus the issue is identical between channels.I dont think the FET's are the issue?

author: David_AVD
author_rank: Super Contributor
timestamp: August 12, 2018, 01:27:40 am
content: If the outputs are indeed 0V, the relay should be engaging. Hence my querying about those test points.Maybe check around U303 (C308 low in value could cause this) to see why the relay is not engaging.

author: Ashhhhh
author_rank: Contributor
timestamp: August 13, 2018, 08:33:58 am
content: Yeah it's not (engaging), hence my confusion, haha.C308 is brand new, as are all electrolytics.Really killing me..Quote from: David_AVD on August 12, 2018, 01:27:40 amSent from my Pixel using Tapatalk

author: David_AVD
author_rank: Super Contributor
timestamp: August 13, 2018, 09:27:37 am
content: There's plenty of marked voltages, so it should be that hard to find out why it's not engaging.

author: mzacharias
author_rank: Frequent Contributor
timestamp: August 13, 2018, 11:02:54 am
content: Have you tried replacing capacitor C308 associated with the protection IC? 10uF at 16V.It's function is to provide sharp cut-off at power down so there are no pops when you turn the set off.If it's open circuit, the relay will never come onI don't think the clipped waveform is your issue; the relay would come on at zero volume if that was the case.

author: Electro Detective
author_rank: Super Contributor
timestamp: August 15, 2018, 11:22:57 pm
content: FYI to people having a go at these BX series amps:At the bottom of the main board (hidden underneath) there is usually a cap or two secured by that disgusting amber glue (my personal favourite electronics HATE item)In most cases the affected board tracks may be almost totally eaten away and or intermittent temperature dependent 'bridge work' is going on to cause audio hissing n crackles, and other unexplained headachesi.e. I would strongly suggest to sort this out first before moving on to the next troubleshoot/repair proceduresfwiw the gunked greencaps are usually ok and reusable after a clean up, and solder some wires or whatever on the eaten tracks.Check the amp for other 'innocent looking'glue affected areas too, and sort them out asapI have saved a few this way without troubleshooting any further for issues,and own a couple, nice cheap decent linear performance amps at low to moderate loudness levels with efficient loudspeakersand not too shabby waveforms on the oscilloscope

author: Ashhhhh
author_rank: Contributor
timestamp: August 18, 2018, 06:02:14 am
content: Yeah that cap is brand new. Surely that waveform is an issue?Yeah that glue...I haven't noted any issues from it on this particular amp but it's definitely a pain!Still working through this, no updates as yet, welcome advice.Sent from my Pixel using Tapatalk

author: David_AVD
author_rank: Super Contributor
timestamp: August 18, 2018, 07:38:28 am
content: The relay not engaging when the outputs are both 0V is worth chasing up. Finding the cause for that may solve other issues. Missing voltage rail, cracked joints, etc.