Teac A-H300 Amplifier x 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Outrun_uk · 25th Mar 2016, 1:14 am

Hi all,

I really liked the look of the micro sized Teac Hi-Fi systems that were around a few years ago but at the time they were way out of my price range. I put a few bids on eBay for working systems which always ended on silly money but eventually won a faulty system for a reasonable price.

The seller had the full system with 2 of the amplifiers, both dead. Tuner needs a backup battery, CD needed a loading belt and could probably use a new laser and the tape deck was OK so I turned my attention to the amps.

It turned out both of them had o/c primary windings on the mains transformer. A quick call to the UK company for Teac revealed that they wouldn't supply a transformer but would repair the unit but I shudder to think at what cost...

So with nothing to lose the transformer was removed, the mu-metal shield removed and then (carefully) some of the plastic was snipped away. Eventually got to the thermal fuse which was as you have probably guessed o/c. Nearest I could find was RS 176-9407 which was one degree lower than the original. Fitted, heatshrinked and transformer re-assembled with it's shield and some epoxy resin. Measured about 22 ohms on the primary so gave it a (very) quick test and all was good.

As I was still a bit dubious about both failing the same way I took it into work and gave it a good run while monitoring the temperature of the transformer with a thermal imager. Nothing suspicious and has been working just fine since. The second unit had the same problem and again working ok!

I'm thinking either bad design, under-rated transformer on the original owner had been running them at excessive power for long periods??

As a side note I'm glad I didn't win any of the working systems for the price I was bidding - the performance is not what I would have expected. Will be fine for the spare bedroom though...

Cheers,

Kev

Radio Wrangler · 25th Mar 2016, 8:05 am

I suspect a lot of small amplifiers die at parties. A few hours of playing heavily compressed music wound up to 'twelve' will produce heat in the transformer, abetted by heat from everything else around it for long enough for its thermal mass to get above the cutoff temperature.

The same fate befalls many tweeters. Tweeters are sized to handle the relatively lower power in their part of the spectrum. Run an amplifier hard to the point where there is significant clipping and a lot of extra energy is created in the tweeter's part of the spectrum and two burned out tweeters result. Of course, at the party, no-one notices the tweeters, but they do notice the thermal fuse opening.

I'd classify it as bad design.

A protection device that stops the amplifier being burned out (perhaps literally) but does so by making the amplifier beyond economic repair is not good enough. Such a thing should be a last-ditch protection which should only act if other protections have failed.

David

julie_m · 25th Mar 2016, 10:09 am

Yes, unfortunately, many amplifiers have power transformers that are not rated to supply full power continuously -- speech and music typically contain short high-energy bursts separated by longer, lower-energy periods, so as long as the amplifier is not going into distortion on loud passages, the average power dissipation in the transformer will be within acceptable limits. Of course, once the amplifier is driven into distortion, not only is there more power being used in the "quiet" bits (so the transformer gets hotter), but the output signal is full of sharp transients, which really do no favours to loudspeakers either ..... It sounds bad for a reason!

Jimmyhaflinger · 26th Mar 2016, 12:46 am

these thermal fuses fitted in many "modern" transformers are notorious for going o/c for no particular reason, maybe just for spontaneous internal corrosion

AC/HL · 26th Mar 2016, 2:12 am

Perhaps a touch pejorative, but the reference to parties would seem to be apt. These are domestic amplifiers, not PA systems. However, an external device would have been preferable.

vosperd · 27th Mar 2016, 5:34 pm

Hi,
I have recently purchased from Ebay one of these amps to go with my Garrard turntable. Fits nicely into my converted wind up gramophone cabinet. Unfortunately long since lost the accoustic bits!
This amp also was sold as faulty with one of the tape channels not switching off. Looks like these amps are not too bad to repair but for the moment the fault doesn't bother me.
Great amp and in this case a reasonable price. Looking for a CD player to go with it but no hurry.
Regards
Don m5aky

25th Mar 2016, 1:14 am	#1
Outrun_uk Heptode Join Date: Aug 2009 Location: Liverpool, Merseyside, UK. Posts: 705	Teac A-H300 Amplifier x 2 Hi all, I really liked the look of the micro sized Teac Hi-Fi systems that were around a few years ago but at the time they were way out of my price range. I put a few bids on eBay for working systems which always ended on silly money but eventually won a faulty system for a reasonable price. The seller had the full system with 2 of the amplifiers, both dead. Tuner needs a backup battery, CD needed a loading belt and could probably use a new laser and the tape deck was OK so I turned my attention to the amps. It turned out both of them had o/c primary windings on the mains transformer. A quick call to the UK company for Teac revealed that they wouldn't supply a transformer but would repair the unit but I shudder to think at what cost... So with nothing to lose the transformer was removed, the mu-metal shield removed and then (carefully) some of the plastic was snipped away. Eventually got to the thermal fuse which was as you have probably guessed o/c. Nearest I could find was RS 176-9407 which was one degree lower than the original. Fitted, heatshrinked and transformer re-assembled with it's shield and some epoxy resin. Measured about 22 ohms on the primary so gave it a (very) quick test and all was good. As I was still a bit dubious about both failing the same way I took it into work and gave it a good run while monitoring the temperature of the transformer with a thermal imager. Nothing suspicious and has been working just fine since. The second unit had the same problem and again working ok! I'm thinking either bad design, under-rated transformer on the original owner had been running them at excessive power for long periods?? As a side note I'm glad I didn't win any of the working systems for the price I was bidding - the performance is not what I would have expected. Will be fine for the spare bedroom though... Cheers, Kev

25th Mar 2016, 8:05 am	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,899	Re: Teac A-H300 Amplifier x 2 I suspect a lot of small amplifiers die at parties. A few hours of playing heavily compressed music wound up to 'twelve' will produce heat in the transformer, abetted by heat from everything else around it for long enough for its thermal mass to get above the cutoff temperature. The same fate befalls many tweeters. Tweeters are sized to handle the relatively lower power in their part of the spectrum. Run an amplifier hard to the point where there is significant clipping and a lot of extra energy is created in the tweeter's part of the spectrum and two burned out tweeters result. Of course, at the party, no-one notices the tweeters, but they do notice the thermal fuse opening. I'd classify it as bad design. A protection device that stops the amplifier being burned out (perhaps literally) but does so by making the amplifier beyond economic repair is not good enough. Such a thing should be a last-ditch protection which should only act if other protections have failed. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

25th Mar 2016, 10:09 am	#3
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Teac A-H300 Amplifier x 2 Yes, unfortunately, many amplifiers have power transformers that are not rated to supply full power continuously -- speech and music typically contain short high-energy bursts separated by longer, lower-energy periods, so as long as the amplifier is not going into distortion on loud passages, the average power dissipation in the transformer will be within acceptable limits. Of course, once the amplifier is driven into distortion, not only is there more power being used in the "quiet" bits (so the transformer gets hotter), but the output signal is full of sharp transients, which really do no favours to loudspeakers either ..... It sounds bad for a reason! __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments. Last edited by AC/HL; 25th Mar 2016 at 1:41 pm. Reason: OT aside edited

26th Mar 2016, 12:46 am	#4
Jimmyhaflinger Heptode Join Date: Feb 2007 Location: La Spezia, Italy Posts: 834	Re: Teac A-H300 Amplifier x 2 these thermal fuses fitted in many "modern" transformers are notorious for going o/c for no particular reason, maybe just for spontaneous internal corrosion

26th Mar 2016, 2:12 am	#5
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Teac A-H300 Amplifier x 2 Perhaps a touch pejorative, but the reference to parties would seem to be apt. These are domestic amplifiers, not PA systems. However, an external device would have been preferable.

27th Mar 2016, 5:34 pm	#6
vosperd Hexode Join Date: Oct 2007 Location: Bristol, UK. Posts: 446	Re: Teac A-H300 Amplifier x 2 Hi, I have recently purchased from Ebay one of these amps to go with my Garrard turntable. Fits nicely into my converted wind up gramophone cabinet. Unfortunately long since lost the accoustic bits! This amp also was sold as faulty with one of the tape channels not switching off. Looks like these amps are not too bad to repair but for the moment the fault doesn't bother me. Great amp and in this case a reasonable price. Looking for a CD player to go with it but no hurry. Regards Don m5aky