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PostPosted: Sun Apr 21, 2019 1:05 pm 
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Location: It's not easy being a dolphin.
Yeah, BITD they always seemed on the same playing field as NAD. Not super class but good value.

As for the root cause of the blown diode, here is the long answer:

I was a bit of a HiFI nerd years ago and while I studied Electronics I'm very rusty and forgot a lot, plus I
sold all my test equipment years ago. Using only a Lidl soldering iron and multi-meter at the moment! Managed to download
the circuit diagram which was a massive help. From the audio fault I zoomed into the problem area
fairly (and surprisingly) quick. On the PCB there is some black gooey stuff around a cap (but it may actually be
glue to keep the fairly large caps in place....?). The cap is to help stabilise the reference voltage on the base of the
power supply transistor for the two op-amps in the pre-amp stage. The voltage is -18v, but the cap is only rated at 25v. So I've ordered some 35v caps and will replace both for the -ve and +ve rails at the same time. I'll stick with the ½W zeners first and
see if they pop.

While there is 'separate' L + R power rails for both the main amp and pre-amp, oddly the reference voltages are taken from only the L channel power. I'm suspecting a leaky cap and/or high voltage transient "on-off" spike blew the zener; there is no 'spike' protection capacitor across it. Could also be down to age or over heating. The rest of the circuits seem all good; no internal
fuses blown - I'm confident it will be working fine.

In all honesty a good upgrade would be to modernise the whole pre-amp power supply with it's own
transformer and regulators, and possibly swap out the op-amps for something more recent.

I'm kind of enjoying the trip down memory lane fiddling with amps - especially with those that
would have been well over budget BITD!


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PostPosted: Mon Apr 22, 2019 12:16 am 
Old School Grand Master
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Joined: Sun Mar 14, 2010 2:33 am
Posts: 3744
Location: Kentcestershire
Quote:
In all honesty a good upgrade would be to modernise the whole pre-amp power supply with it's own
transformer and regulators, and possibly swap out the op-amps for something more recent.


I tried some op-amp "rolling" in a headphone amp a while back. I really like the LM4562; very clean and neutral sounding to my ears.

The LME49720 is the same item and there is a LME49710 if you need a single channel version.

Some say the TO-99, metal can, versions sound better: https://www.avnet.com/shop/emea/product ... E?aka_re=1


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PostPosted: Mon Apr 22, 2019 9:33 am 
rBoTM Winner
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Joined: Fri Jul 29, 2016 11:31 am
Posts: 638
Location: North Kent Coast UK
Woz wrote:
Yeah, BITD they always seemed on the same playing field as NAD. Not super class but good value.

As for the root cause of the blown diode, here is the long answer:

I was a bit of a HiFI nerd years ago and while I studied Electronics I'm very rusty and forgot a lot, plus I
sold all my test equipment years ago. Using only a Lidl soldering iron and multi-meter at the moment! Managed to download
the circuit diagram which was a massive help. From the audio fault I zoomed into the problem area
fairly (and surprisingly) quick. On the PCB there is some black gooey stuff around a cap (but it may actually be
glue to keep the fairly large caps in place....?). The cap is to help stabilise the reference voltage on the base of the
power supply transistor for the two op-amps in the pre-amp stage. The voltage is -18v, but the cap is only rated at 25v. So I've ordered some 35v caps and will replace both for the -ve and +ve rails at the same time. I'll stick with the ½W zeners first and
see if they pop.

While there is 'separate' L + R power rails for both the main amp and pre-amp, oddly the reference voltages are taken from only the L channel power. I'm suspecting a leaky cap and/or high voltage transient "on-off" spike blew the zener; there is no 'spike' protection capacitor across it. Could also be down to age or over heating. The rest of the circuits seem all good; no internal
fuses blown - I'm confident it will be working fine.

In all honesty a good upgrade would be to modernise the whole pre-amp power supply with it's own
transformer and regulators, and possibly swap out the op-amps for something more recent.

I'm kind of enjoying the trip down memory lane fiddling with amps - especially with those that
would have been well over budget BITD!

It's fun isn't it? And does keep the old grey matter ticking over. I did the same with turntables in the 90's when everyone else was ditching vinyl so I've now got some very exotic turntables that I couldn't really justify spending the money on even now.

Similarly I did a HND in electronic engineering when I was an apprentice and then never used it as I went into industrial remote control which meant I had to learn about 3 phase AC and 400V DC power on things like cranes, and as I reckon I've probably got a few years on you I've had more time to forget a lot more.

I do still fiddle with electronics but my focus now is valves which I learned cock all about as an apprentice but it's been fun learning and thus far I've managed to build 2 pre amps, a phono stage and an electronic crossover.

As to your rogue zener I think your right about high transients being the cause and a leaky cap not doing its smoothing being the cause of that, luckily the zener is peanuts so if you have to keep digging it won't cost the earth. Unless you go a very long way up the food chain (I'm talking 5 figure exotica here) every bit of hifi is built to a price and that means there are compromises. The biggest exponent of this apart from Mr Sugar who took it to extremes was the early Richer Sounds made Cambridge Audio kit, some very effective designs that were literally strangled by accountants and which have been 2nd hand bargains for years that with a bit of electronic knowledge you can build a great system out of for little money.

And yes as @xerxes says op amp rolling can be fun and it can produce similar results to what the Americans call tube (valve) rolling and completely change the sound of the audio stage you do it in, I favour the Burr Brown OPA series devices myself


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PostPosted: Mon Apr 22, 2019 10:02 am 
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Joined: Thu Jun 23, 2011 11:25 pm
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Location: It's not easy being a dolphin.
Thanks for the input allenh and Xerxes as I'm well out of touch; I did a BTEC and just got into polytechnic to
do electronics - I took options in a new fangled thing called digital electronics but had an interest in HiFi; one
of my first Saturday jobs was in a good HiFi shop so had play access to some good gear like Linn etc. Never worked
in the industry so my only practical experience is repairing really. I reckon my last audio fiddling was a good 25
years ago baby sitting a Sherwood CD player until certain death; tapping the laser carriage along with a pencil
from time to time :lol:

Oh how times have changed; looks like I can use a PC for a signal generator with
free software and have just ordered a Hantek 6022BE PC oscilloscope to check deeper if necessary.

A far cry from the Farnell, Tektronix and Phillips PM3214 test equipment I had requiring muscles, large
desk real estate, constant contact cleaning and a lull in solar flare activity to use.


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PostPosted: Mon Apr 22, 2019 12:39 pm 
rBoTM Winner
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Joined: Fri Jul 29, 2016 11:31 am
Posts: 638
Location: North Kent Coast UK
Woz wrote:
Thanks for the input allenh and Xerxes as I'm well out of touch; I did a BTEC and just got into polytechnic to
do electronics - I took options in a new fangled thing called digital electronics but had an interest in HiFi; one
of my first Saturday jobs was in a good HiFi shop so had play access to some good gear like Linn etc. Never worked
in the industry so my only practical experience is repairing really. I reckon my last audio fiddling was a good 25
years ago baby sitting a Sherwood CD player until certain death; tapping the laser carriage along with a pencil
from time to time :lol:

Oh how times have changed; looks like I can use a PC for a signal generator with
free software and have just ordered a Hantek 6022BE PC oscilloscope to check deeper if necessary.

A far cry from the Farnell, Tektronix and Phillips PM3214 test equipment I had requiring muscles, large
desk real estate, constant contact cleaning and a lull in solar flare activity to use.

Yep they certainly have, I still have most of my test gear (you need a lot with valves and that Avo valve tester weighs a ton)) and have brought some of my Dads back from the US (his company were thowing out some beautiful stuff becasue times had changed) but yes I recently built a little colour screen scope and a sig gen from little kits I bought from ebay for pennies and they're actually really good.

I'm sure that Rotel will be singing pretty soon so you'd better treat it to the rest of the system, i.e. decent big speakers can turn up cheap for collection only on ebay


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PostPosted: Mon Apr 22, 2019 3:46 pm 
Old School Grand Master

Joined: Mon Feb 11, 2008 9:42 am
Posts: 3491
Ha. My old rotel amp completely s**t its guts. Blew a line of components across the board and smoked out the living room.

Really not repairable!


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PostPosted: Tue Apr 23, 2019 6:27 pm 
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Joined: Thu Jun 23, 2011 11:25 pm
Posts: 2392
Location: It's not easy being a dolphin.
allenh wrote:
I'm sure that Rotel will be singing pretty soon so you'd better treat it to the rest of the system, i.e. decent big speakers can turn up cheap for collection only on ebay


The Rotel is indeed singing again :!: :)

Caps and zeners replaced along with a few squirts of contact cleaner in the selection switch and tone control switch and
all seems good. When the oscilloscope arrives I'll have a final check of channel balance etc. I won't be able to test
the phono stage properly as I don't have a turntable - I'll probably ground the phono inputs at the back.

WIll be on the look-out for a mid-size set of speakers, either skinny column floor standing or something on a decent stand; something low-sensitive and warm / rounded sounding as it will eventually be used to listen to FLAC rips of my CD
collection in a medium sized room with not much acoustic absorption.

I'm still on the look-out for a fairly decent USB DAC if anyone as any recommendations.


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 Post subject: Re:
PostPosted: Wed Apr 24, 2019 2:54 pm 
Old School Grand Master
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Joined: Sun Mar 14, 2010 2:33 am
Posts: 3744
Location: Kentcestershire
Quote:
I'm still on the look-out for a fairly decent USB DAC if anyone as any recommendations.


How much were you thinking of spending?


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PostPosted: Wed Apr 24, 2019 9:33 pm 
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Location: It's not easy being a dolphin.
Good question, I don't know really, what is the gamut like?


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 Post subject: Re:
PostPosted: Thu Apr 25, 2019 11:09 am 
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Joined: Sun Mar 14, 2010 2:33 am
Posts: 3744
Location: Kentcestershire
Well, you can get something pretty good for a few hundred pounds up to tens of thousands.

I have an HRT Music Streamer II+: https://www.soundstagexperience.com/ind ... treamer-ii, this was quite highly regarded in it's time, but it's quite old now and I imagine comfortably bettered by things at a similar price point currently.

The HRT Music Streamer replaced the £1,500.00 Cyrus CD player I had previously and when playing uncompressed WAV files from a PC with JRiver Media Player, https://jriver.com/, to provide bit perfect data to the DAC, I reckon it at least equalled the CD player. I subsequently sold the CD player and ripped all my CDs to a hard drive.

For best results, uncompressed music files, such as WAV and not compressed, "lossy" MP3 type files, and bit perfect playback, which bypasses the operating systems sound processing, are best.

Bit perfect playback: https://www.thewelltemperedcomputer.com ... erfect.htm

I also have an neat little Audioquest Dragonfly: https://www.whathifi.com/audioquest/dra ... v15/review, this works with my Android phone and sounds pretty good with a decent set of heaphones, much better than via the phone's own headphone output. You plug it into the USB port and it replaces the internal DAC with a better one and also provides additional amplification for harder to drive headphones. You could also plug it into a PC and use a mini 3.5mm headphone jack to two phono plug adapter cable and plug it into an ordinary amp, but I've never actually tried it.


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