Teac A-BX10

[dave87]

Hi everyone,

I've aquired this amp recently... It sounds (sounded) very good, and it's built like a tank. However, it had an intermittent fault that's now become permanent - it would cut out whilst making a chattering clicking sound, which I'm fairly sure was coming from one of the relays. It's now only working intermittently - most of the time there's no output at all, not even a hiss when the volume's at max.

Does this sound like a fault with a relay? I've had a look inside and there are 4 relays, presumably one for each channel; when the amp is powered on, they "click" on, one pair after the other. I can see that one looks "sticky" - the part that moves to make the connection moves later than the others do, but I can't actually see anything moving when it's making the chattering noise.

How easy would this be to fix? I'm fairly handy with a soldering iron, but it's a nice amp and I don't want to destroy it if it's affordably fixable...

 

[MisterFishey]

There are also 4 relays right behind what is presumably the volume pot... those 4 sealed black boxes look like relays to me from your picture. My initial guess is a cold solder joint (possible, since the relays are so close to the output transistors), so I would just go through and reflow all the joints on the 4 output relays near the speaker outputs and the 4 relays behind the volume pot.

Also, nice looking amp! :thmbsp:

EDIT: Also looks like there are yet 4 more relays (again in the sealed black boxes) right near the source inputs. Reflow the joints on those too, if you have the board out, for good measure.

 

[dave87]

Thanks! I didn't realise those were relays as well (they definitely are - I checked the part numbers) - I was doubtful that the clicking noise was coming from the 4 at the back - those sound more likely. I'll go over the joints once I get the board out and post the results! Unfortunately one of the hex screws holding the back on is rounded, so I'll have to cut a groove in the head to get it out with a flat-blade screwdriver.

 

[ryuuoh]

Wow, that is rather nice looking.

 

[dave87]

An update: I've traced the fault to a single relay - the small black one nearest to the big capacitors... When it fails to work, a gentle prod (with something non-conductive, obviously) fixes it, albeit temporarily. I'm going to completely resolder it this evening, and hopefully that should fix it! Otherwise I can get hold of a new one cheaply.

Just curious, how did those of us here who know anything about electronics learn it? It's something that's always interested me, but I can't make head nor tail of a circuit diagram...

 

[MisterFishey]

An update: I've traced the fault to a single relay - the small black one nearest to the big capacitors... When it fails to work, a gentle prod (with something non-conductive, obviously) fixes it, albeit temporarily. I'm going to completely resolder it this evening, and hopefully that should fix it! Otherwise I can get hold of a new one cheaply.

Just curious, how did those of us here who know anything about electronics learn it? It's something that's always interested me, but I can't make head nor tail of a circuit diagram...

Good! If resoldering doesn't fix it, I would just replace the relay, they are pretty cheap to get. If you take a close up picture of the top of the relay with all the data on it, I could find a replacement on Mouser or Digikey for you if you want.

If the relay replacement doesn't work, you will certainly need a service manual, if you don't have one already.

How did I learn about electronics? I'm fortunate that my father is an electrical engineer, so he was able to show me the basics of reading schematics, soldering, diagnostics, etc. However, I learned most of what I know from lurking around AK, asking questions in threads like these, and from first hand fixing of stuff. Circuit diagrams especially are imposing at first, but once you look at a few, you can generally tell the purpose of the circuit (especially in audio equipment) by the voltages and components, symmetry or lack thereof, and any inputs/outputs/controls in the circuit. An intimate knowledge of the exact workings aren't really necessary for most problems. Just getting stuff and fixing it is the best way to learn, IMO. Poke around AK, proceed with educated caution. :yes:

 

[dave87]

Another update: I've replaced the four relays in the middle of the PCB, but the problem persists! I noticed that when I turn the amp off at the wall (rather than putting it on standby), it makes a frantic clicking sound for a few seconds before the power LED goes out. This is likely to be a problem with a relay, right? I know it's a long shot, but I found a service manual with a circuit diagram - could anyone tell me if there's anything that immediately stands out as being a potential cause? I've replaced relays K5, K6, K10 and K11... I have one replacement relay for K100, K101, K400 or K401, as those are the ones I originally thought might be at fault, but looking at the circuit diagram, it seems that one of those failing should only cause one channel to drop out, rather than both. Thanks!

 

[sregor]

I'll throw in my 2 cents (YMMV I have no direct experience with these) . The small black relays are 48 volt relays running between the + and - 24 volt supplies. The selctor switch turns them on through transistors. They also handle the balanced input and the tape monitor functions. Each relay is 2 pole so both signals go through them. I would check the + and - 24 volt circuits first - if one of these is dropping down, would cause relay chatter and dropping out. Good luck with it.

 

[dave87]

I've just got this amp back from the technician - it was a problem with one of the 24v supplies - apparently the circuit track passes from one side of the board to the other at one point, via an unrelated transistor's leg - this was never fully inserted through the board and subsequently the connection was very dodgy. I suppose any slight movement caused by the amp heating up must have broken the connection. The guy who fixed it said that it was one of the most difficult faults to track that he'd come across!

But it was worth it - having got used to a perfectly decent Marantz amp the past couple of months, this is in another league altogether (and the Marantz is going on ebay to finance the repair, which wasn't cheap)!

 

[jsixis]

"Just curious, how did those of us here who know anything about electronics learn it? It's something that's always interested me, but I can't make head nor tail of a circuit diagram..."

I started off taking things apart in grade school. Went in the US Army and took their Fuel & Electrical course. While in the Army met a guy who was a whiz at electronics and just sort of picked his brain. In the 90's I joined the OANG and took one of their Electrical Courses, (wasn't Avionics, sure wish it had been) way more theory then I needed as a parts changer airman but great course. I forgot 99% of it.
Since then I just read read read and play.

I can't tell you how it works or why it works but I know a bad solder joint, resistor or cap when I see it. The first things I started to fix was the old commodore 64's and 128 computers. 18 pin Ic's teach you how to remove solder and traces on those boards are about the size of a hair. I lifted a few (and fixed them).

As for schematics, find a simple one, get some color markers and just trace them out.
I studied a Shure M65 pre amp for a solid month before it made sense and since then I can usually figure out all the ones I needed to.

I have a long way to go but the people here are fantastic for offering help and advice.