Trio KA1500 Amp Reapair

[Arpangel]

Actually, with kit of this sort of vintage, I'm very reluctant to make 'improvements'. In the first place, to me, it makes it a fake, but more significantly there is the issue of unintended consequences - unless you really understand the original design concept.

I just want it repaired, no modifications.
Trio made some classic amps, some of the best IMO, their LO series were phenomenal, this goes beyond personal opinion, if you’ve heard one, you’ll know what I mean.
A friend back in the early 80’s had the LO series pre-power combination with these speakers,

https://www.ebay.com/itm/186131583481? ... R-qTzcmEYw

Fed by a Michell Gyrodeck and Namamich Dragon, that system was a complete freak-out to listen to.

My little KA1500 is lovely, it has life, depth, and you’d pay a lot today to get a similar sound.

PS, Dave, yes, I’ll look for bulging caps.

[ef37a]

Actually, with kit of this sort of vintage, I'm very reluctant to make 'improvements'. In the first place, to me, it makes it a fake, but more significantly there is the issue of unintended consequences - unless you really understand the original design concept.

I can understand your reservations with things like venerated guitar amplifiers that can fetch 1000s Will but mid range Japanese audio that was pumped out in the tens of thousands worldwide? There are some iconic designs by Quad, maybe Cambridge Audio and others that certain collectors might covet but not many. The actual audio performance of that gear is now an order or more worse than the best we can get now and has never been cheaper."Digital" also shows it up.

Power amplifiers especially have improved greatly. Add to that the fact that repairing some of those old AB designs is fraught and the repair quite likely to "blow up in your face" makes them uneconomic to fix. My approach years ago when faced with a burned out PA containing maybe a dozen transistors was to rip it out and replace it with a suitably rated power amp module. I used the ILP range (still got a 2x 60W chassis that works!) but depending on the power needed and customer's pockets, Maplin MOSFET or Sinclair Z 50s!

I cannot be doing with 'quaint' audio electronics.

Dave.

[Arpangel]

Actually, with kit of this sort of vintage, I'm very reluctant to make 'improvements'. In the first place, to me, it makes it a fake, but more significantly there is the issue of unintended consequences - unless you really understand the original design concept.

I can understand your reservations with things like venerated guitar amplifiers that can fetch 1000s Will but mid range Japanese audio that was pumped out in the tens of thousands worldwide? There are some iconic designs by Quad, maybe Cambridge Audio and others that certain collectors might covet but not many. The actual audio performance of that gear is now an order or more worse than the best we can get now and has never been cheaper."Digital" also shows it up.

Power amplifiers especially have improved greatly. Add to that the fact that repairing some of those old AB designs is fraught and the repair quite likely to "blow up in your face" makes them uneconomic to fix. My approach years ago when faced with a burned out PA containing maybe a dozen transistors was to rip it out and replace it with a suitably rated power amp module. I used the ILP range (still got a 2x 60W chassis that works!) but depending on the power needed and customer's pockets, Maplin MOSFET or Sinclair Z 50s!

I cannot be doing with 'quaint' audio electronics.

Dave.

Qaud and any other British stuff around at that time, the Trio was in a different league and is still sought after, you’d have to spend a lot today to a get a similar performance, that’s why there’s a resurgence in vintage hi-fi, people are realising despite all the "progress" that’s supposedly been made, "some" of the older stuff still sounds good or better, and it’s a lot cheaper to buy than current so called high end gear, the downside is maintenance, it helps if you can do it yourself.
My friend back then had an unlimited budget, he could have bought anything he wanted, and I remember auditioning lots of things with him, Quad, Naim etc, after listening to the Trio LO series we wondered why we even bothered listening to anything else.

[MarkOne]

...Maplin MOSFET...

Ah yes, that was a go-to solution to many a broken amp.

And featured in my first homebuilt keyboard amp/speaker combo

[Hugh Robjohns]

'Back then' most hi-fi had very little fidelity compared to now. Vinyl replay was generally rubbish, cassettes were popular... but rubbish, and even the real aficionados could only muster quarter track r2r at 7.5ips. The 'quality' threshold was pretty low and nowhere close to what came out of the studio.

There's something to be said for nostalgic warm and fuzzy — and I've just bought a 1970s Thorens vinyl player for my office, so I get the pleasure... but let's not confuse reminiscing pleasure with technical fact.

The KA1500 has a THD spec of 0.1%... Good for its age, but two orders of magnitude worse than modern high-end gear. So it's definitely adding (musical?) harmonic content to all that passes through.

[ef37a]

'Back then' most hi-fi had very little fidelity compared to now. Vinyl replay was generally rubbish, cassettes were popular... but rubbish, and even the real aficionados could only muster quarter track r2r at 7.5ips. The 'quality' threshold was pretty low and nowhere close to what came out of the studio.

There's something to be said for nostalgic warm and fuzzy — and I've just bought a 1970s Thorens vinyl player for my office, so I get the pleasure... but let's not confuse reminiscing pleasure with technical fact.

The KA1500 has a THD spec of 0.1%... Good for its age, but two orders of magnitude worse than modern high-end gear. So it's definitely adding (musical?) harmonic content to all that passes through.

Quite and I'll bet that 0.1% is the PA only. The RIAA stage will easily add another 0.1% at 5mV IF yer lucky!

Dave.

[tea for two]

On another thread fairly recently where we were discussing such gear, I mentioned I read some glowing comments on current tube hifi amps with RIAA input in the 150-250 price badged as Aiyima, Auna, Douk, Fosi, Nobsound lol how these made their speakers Quads, Rogers sound darned decent.
I have yet to get one. Also the robustness of these tube amp components I would want to find out.

[Nazard]

I just want it repaired, no modifications

By far the best approach and likely to prolong the life of the unit far longer than any 'improvements'.

There are some exceptions, though. When I repaired a valve Leak Troughline tuner, making it safe, adding an IEC socket, double pole switch, etc., the stereo decoder output was poor by any standard. So I added a multiplex output and designed a new decoder around a uA758, which is outboard, keeping the original inside the case.

[Folderol]

People seem to get really hung up on THD, 0.1% is -60dB. That's quite a long way down, and likely to be inaudible under most circumstances.

I'd also add that measuring THD is a sledgehammer to crack a nut! It takes no account of which harmonics and their relative amplitude. During the late 1960s when I worked for Hacker Radio, during training on the HiFi kit we measured the first fifth individually. In normal operation this was principally 2nd and 3rd. As a demonstration our instructor also showed the relative amount of 7th harmonic, which is the first really nasty sounding one - it was insignificant on a fairly ordinary valve amplifier.

What nobody seems to check these days is intermodulation distortion, which can sound really horrible at quite low levels.

[Hugh Robjohns]

I agree, 0.1% THD is not a disaster. We used to accept 3% at peak level off tape, after all!

But distortion is added content not in the original, and 60dB down is perceptible in the right conditions.

THD is indeed the total distortion which gives an overview. The balance of individual harmonics is very important too, of course.

The AP system I use displays a bar chart showing the relative or absolute levels of the first 11 (I think, from memory) harmonics. The second and third are normally the strongest by far.

And I always check IMD when I'm bench testing....

[James Perrett]

Thanks all, yes, it’s both channels that are affected, and James, I’ll check those things out today, I just need the schematic.

I downloaded it from Electrotanya

https://elektrotanya.com/kenwood_ka-150 ... nload.html

but it is also available from all the other usual places (Hifi Engine etc).

[Arpangel]

Thanks all, yes, it’s both channels that are affected, and James, I’ll check those things out today, I just need the schematic.

I downloaded it from Electrotanya

https://elektrotanya.com/kenwood_ka-150 ... nload.html

but it is also available from all the other usual places (Hifi Engine etc).

Thanks for that James, I’m looking at it today.
Been busy doing other things, like learning my MX61 (been up since 4am) and buying a Christmas tree.

[ef37a]

People seem to get really hung up on THD, 0.1% is -60dB. That's quite a long way down, and likely to be inaudible under most circumstances.

I'd also add that measuring THD is a sledgehammer to crack a nut! It takes no account of which harmonics and their relative amplitude. During the late 1960s when I worked for Hacker Radio, during training on the HiFi kit we measured the first fifth individually. In normal operation this was principally 2nd and 3rd. As a demonstration our instructor also showed the relative amount of 7th harmonic, which is the first really nasty sounding one - it was insignificant on a fairly ordinary valve amplifier.

What nobody seems to check these days is intermodulation distortion, which can sound really horrible at quite low levels.

Whilst I agree Will that 0.1%THD is 'good enough' in most cases the amplifier market is obsessed with numbers! Then, although IMD can be important, if all harmonics are at a very low level then by definition the 'system' is very linear and all forms of distortion will be low.
It has been possible, for well over a decade, without great trouble to make transistor power amps with THD in the third decimal place and even so low that they compare to the residual distortion in even the best analysers. Indeed such amplifiers have to be tested at 5kHz because the distortion at the more usual 1kHz is so low!

The rise of digital audio meant analogue devices have had to 'catch up' Noise levels need to be much lower and it is a bit daft to have a power amp that produces 100 times more distortion than the rest of the repro' chain!

For decades amp makers went in for 'specmanship'. "Our PAs produce more watts than yours at x% lower distortion!" The hi fi press at the time (the sane part!) had the task of testing these claims. These days, PAs are built into the speakers and so we never get any information as to their quality. Must be OK though or Mr Ward would have said something?

Dave.

[Nazard]

These days, PAs are built into the speakers and so we never get any information as to their quality. Must be OK though or Mr Ward would have said something?

I did a little bit of research into this a few months ago when deciding on some new amps/speakers. Apologies for any errors, but in summary:

All ATC active speakers, consumer and pro, are Class A/B
All PMC active studio monitors, are Class D
Neumann are a mix, the older ones, e.g. 301 and 420 are Class A/B and the newer DSP models, e.g. KH 80, 120 II and 150, Class D.

In the HiFi world, Class D is gaining a strong following and I don't remember anything other than a good review for any Class D amp in the last five years. I think we are at the stage where a power amplifier of any class is definitely not the weak link in the audio chain.

[Hugh Robjohns]

Some of the low budget active speakers used single chip amps which can be a bit iffy in terms of specs... but by and large none of the electronics in any part of the audio chain have been 'weak links' for a decade or more.

The only remaining weak links are by far the room acoustics and the practical recording skills/ musical performances.

[Arpangel]

The only remaining weak links are by far the room acoustics and the practical recording skills/ musical performances.

Never before in the history of recorded music, have we had so many amazing tools available to us, and never before has there been such a dearth of what I would call, seminal, amazing music, music that will go down in history, or at least start a movement.
I’m not hearing anything, nothing, zilch, and I’m not seeing anyone coming up that will step into the shoes of the greats that went before them, no one.
If there were I’d be listening to them, going to their gigs, and buying their music, all the music I buy is to fill gaps in my collection, from the past.
It’s very sad, or I’m very sad, but whatever way you look at it, that’s the way it is, and that’s what I’m seeing, and what I’m hearing.

[ef37a]

The only remaining weak links are by far the room acoustics and the practical recording skills/ musical performances.

Never before in the history of recorded music, have we had so many amazing tools available to us, and never before has there been such a dearth of what I would call, seminal, amazing music, music that will go down in history, or at least start a movement.
I’m not hearing anything, nothing, zilch, and I’m not seeing anyone coming up that will step into the shoes of the greats that went before them, no one.
If there were I’d be listening to them, going to their gigs, and buying their music, all the music I buy is to fill gaps in my collection, from the past.
It’s very sad, or I’m very sad, but whatever way you look at it, that’s the way it is, and that’s what I’m seeing, and what I’m hearing.

That's as it should be Tony. "They" are not writing music for you/us. I am quite content in the time I have left to be exploring the vast outputs of the Bachs, Mozart, Beethoven and some others. Much I have heard before but there is always a sonata or some such on R3 that is unfamiliar. I like most forms of jazz but have not listened to very much of it...and Floyd, Zep and the rest are always there for good mad hour or so!

Dave.

[Nazard]

I disagree, great music is to be found where you may not be expecting to find it. For example, some of the writing for film and games, is wonderful. Take "Everybody's Gone To The Rapture" by Jessica Curry, for the game of the same name. A magnificient score with first rate composition, singers, orchestra and recording.

[Nazard]

There is so much new and new to be discovered music out there.

Ellen Andrea Wang, their Closeness album is wonderful: the best cover version of 'Nobody Knows' and one to test any speaker/monitoring system.

My old friend Jonathan Plowright, whom Gramophone recently said was one of the finest twenty living pianists, (we had the same piano teacher for a number of years), promotes and plays many 'forgotten' works. E.g. by Gablenz, Michal Bergson, Rozycki. You should listen to him! His Brahms recordings of the complete piano works are fantastic.

[Hugh Robjohns]

I've said it before, but you see (and hear) what you want to see.

I hear things on R3 more or less every week that lead to me buying a CD or investigating a new performer, and friends, family and professional acquaintances introduce me to new music in other genres every month, too.

But the fact is that most new bands and performers aren't aiming themselves at folk with one foot in the grave....

[ef37a]

I've said it before, but you see (and hear) what you want to see.

I hear things on R3 more or less every week that lead to me buying a CD or investigating a new performer, and friends, family and professional acquaintances introduce me to new music in other genres every month, too.

But the fact is that most new bands and performers aren't aiming themselves at folk with one foot in the grave....

OOOooooooo! Bit harsh, even for you Hugh!

Dave.

[Arpangel]

I've said it before, but you see (and hear) what you want to see.

I hear things on R3 more or less every week that lead to me buying a CD or investigating a new performer, and friends, family and professional acquaintances introduce me to new music in other genres every month, too.

But the fact is that most new bands and performers aren't aiming themselves at folk with one foot in the grave....

I've got one foot in, and almost two, because of some peoples failure to realise exactly what’s going on.
I see and hear the world through my eyes and my ears, shaped by my experiences and the things I’ve done, that applies to all of us, and that's the way it is.
All I can say is that I was extremely lucky to have been born when I was, and I haven’t got to put up with this world we’re living in now for much longer.
I’ve witnessed the rise, and the fall, of art in general, the future has nothing to do with creating it, it will be of no concern, art will be just a historic unexpected byproduct of a species that no longer has a purpose.

[Folderol]

Folks round here really need to curb their unbridled optimism and joy

[Arpangel]

Folks round here really need to curb their unbridled optimism and joy

I’m happy, I’ve been up yet again, since 4am this morning, learning the MX61, it’s a joyful thing, I’ve never been happier, I actually saved a voice this morning.

[jjlonbass]

Anyway, back to diagnosing the fault.

Cm22 is a very likely candidate for the cause of the fault as it decouples the supply quoted as 11.4V that is applied to the first transistors Qm1 and Qm2 in both channels of the phono preamp.

Other likely capacitors are Cm23 that decouples the 28V supply to both channels of the phono preamp and Cm24 that decouples the -18V supply to both channels.

Also a likely cause is the 18V zener diode Dm7 as others have stated. If this is the cause, the replacement should be rated at 1.3W or greater as the zener will dissipate over 630mW in this design. A BZX55C18 would be suitable.

The 31.5V rail from Qm6 needs to be present and correct as the 28V supply is generated from it and it is also used in both channels of the power amp.

John