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Mr.Sneis

Melos SHA project troubleshooting

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Dear HC crew, I'm getting desperate and could really use some input on troubleshooting...

First off I've been plugging away at this old thing for a while now, was always afraid to post up here for various reasons but at this point I don't have much to lose anymore.  I do recognize I am still relatively new to DIY but I've been doing what I can with what I've got.

I have a long winding thread with chronicle of the madness while it was working great; more than happy to remove the link if the admins do mind.  There are links somewhere in there to the original schematic but it won't be of much help.

http://www.superbestaudiofriends.org/index.php?threads/melos-sha-gold-2016-project-long-post.1745/

 

The TLDR without going into excruciating detail here's what I've done/changed on the amp, it has taken a lot of manhours on my behalf:

Silicon Carbide Schottky Rectifier bridges to replace the HV and LV bridges
Replaced all electrolytic caps and Sonicap film caps on the pre-amp out replacing the 6uf MKP's
Sonicap for inputs and the bypasses
Big honking 100uf 400v Solen PB to replace the "last" power supply electrolytic
Left-Channel mod/reroute
Remote locate TL783 regulator from pcb to chassis for heatsinking
Swapped JRC NE5332N for a Signetics NE5332N
Add a miller cap to darlington mosfets as seen on other Melos amps
Rewired DPDT to Headphone mute
Replaced Photentiometer with Tortuga LDR3x v 2.1 board
Swapped out resistors on the tubes for the plates, heaters, grid stoppers and cathodes. Cut traces and relocated some resistors closer to tube pins
Removed relays for warmup timer and Pre-mute when HP is plugged in

 

 

I believe that about covers it, the amp worked GREAT with all of the above up until I got restless and then the latest round of futzing around and ended up with a big roadblock.  Right now I can only get sound out of the left channel (both headphone and pre-out) and I've already sunk some heavy hours into troubleshooting!

Here's what I wanted to do at first:
Re-wiring for thicker gauge wiring for amp board inputs and outputs (16ga teflon to match the stock wiring)
Re-glued final pre-amp caps with GC electronics silicone rather than hot glue
Remove VU meter lamp wiring
-22db passive attenuation resistor network on pre-out jacks

Powered it up and boo, no right channel.  My observed notes are as follows:

No smoke, no smell
Thought I might have over-heated a diac sharing the R headphone out pad, replaced with a new part L and R and no change
Tried adding back in the VU wiring, no change
Removed passive attenuation resistors no change
Triple checked input output wiring no change
Removed ground wire from amp to dpdt Mute switch; it seems to work the same with or without the ground wire
Disconnected and re-soldered all transformer wiring
Touched up/reflowed nearly every solder joint on the backside of the board
HP jack checks out OK when just wiring Left channel directly to L and R poles, the DPDT switch seems OK too

Tortuga volume board in passive configuration works OK, grounding to the board seems OK

Amp board right hand MOSFET lights light up like clockwork

Continuity of RCA input wires to rotary switch seems fine, especially considering the volume board in passive mode is OK
Continuity of input and output wires on amp board seems fine, grounding seems fine

 

Tubes light up like normal but I need to check the voltages.  My next thought is to remove/refit the preamp film caps, maybe try running without them but I don't think they have any bearing on the headphone out.  I'm tired and frustrated to have gotten too far invested to be taken out like this.  Thank you for any help you guys can give!!

 

 

 

 

Edited by Mr.Sneis

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Waxing nostalgia?  Wanted to get my feet wet into the amp years ago but never got around to it!  I know I done f'd up.

Edited by Mr.Sneis

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1 hour ago, Mr.Sneis said:

  I'm tired and frustrated to have gotten too far invested to be taken out like this.

Sunk Cost Fallacy.

The thing to do at this point is to use a scope* to follow the signal path. You'll find your break that way. My suspicion would be that soldering thick wire onto the flimsy PCB pulled up a trace somewhere.

* if you don't have a scope, run a 60Hz tone through it and test with a DMM on AC.

Edited by dsavitsk
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Thanks Doug. 

Speaking of.  I just set the dmm on the plates for the right tube... looks like 105vdc and 88vdc :(  Left tube looks normal 62vdc and 59vdc.

What's my next step?

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F*ck yeah!  Well that was a lot easier than I thought.  Something in my lead arrangement for the bypass cap that runs across the plate resistors must have been causing a short for the 18k resistor so the tube was only seeing 1k resistance on pin 1.  What in the hell, 3 very long nights down the drain!!!!!

Edited by Mr.Sneis
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Now, put it on fleabay - and get a GS-X.

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Back a decade or three I was CTO of Wharfedale, and we acquired a bust company called Lynx for a tiny sum of money.  Run by two New Zealanders; they made a really sweet range of products based around an FET power amplifier.  Sounded very nice indeed (even if it was a bitch to manufacture). This is an example  http://www.canuckaudiomart.com/details/649106536-super_rare_lynx_nebula_mosfet_integrated_amplifier/images/694258/ .

Anyhow, we were sat one day when a return came in - with a letter saying from a guy in Hong Kong saying it had burst into flames and demanding some outrageous compensation.  We unpacked it, and sure enough it was a charred mess inside.  Smelling strongly of petrol.  We got the Chemistry lab at Leeds University to do some forensics and write an expert report.  We sent that to the guy in HK - and that was the last we heard from him.

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18 hours ago, Mr.Sneis said:

The TLDR without going into excruciating detail here's what I've done/changed on the amp, it has taken a lot of manhours on my behalf:

 

IJWTS I'd hate to see the long version, if that was the short version.

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2 hours ago, Craig Sawyers said:

Back a decade or three I was CTO of Wharfedale, and we acquired a bust company called Lynx for a tiny sum of money.  Run by two New Zealanders; they made a really sweet range of products based around an FET power amplifier.  Sounded very nice indeed (even if it was a bitch to manufacture). This is an example  http://www.canuckaudiomart.com/details/649106536-super_rare_lynx_nebula_mosfet_integrated_amplifier/images/694258/ .

Anyhow, we were sat one day when a return came in - with a letter saying from a guy in Hong Kong saying it had burst into flames and demanding some outrageous compensation.  We unpacked it, and sure enough it was a charred mess inside.  Smelling strongly of petrol.  We got the Chemistry lab at Leeds University to do some forensics and write an expert report.  We sent that to the guy in HK - and that was the last we heard from him.

I actually remember the Lynx brand - probably back in the 90's. The dealer I worked with (and I trust his taste for good sounding audio gears) carried it. One of my cousins may have purchased one with his recommendation IIRC. 

It has not combusted  to my knowledge :-) 

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It was a great, beautiful  and good sounding product with two problems.  

First, the laser machined, triple silk screen printed, and highly innovative (and expensive!) perspex front panel was fixed to the metalwork with double sided tape, and there was no datums to assist, nor any jigs.  So it was applied by eye, and needed to be in the correct place and square to better than half a millimeter.  There was a massive wastage - until we got a jig designed and made.  

Second, I discovered that the Lynx designer (and company founder) was dyslexic, so on the (hand drawn) engineering plans instead, for example, of 124.5mm it was marked up at 125.4mm.  So there were tantalizing minor fit problems all over the place.  In fact, the first time they went bust in New Zealand was because the blind tapped holes in the heatsink for the power transistors were not deep enough for the screw length (so the screws bottomed, and the transistors were not in contact with the sink), so every last product blew up when it got used - I strongly suspect that was also a dyslexic issue - sort of "drill and tap 3.5mm depth" when it should have been 5.3mm.

Like many audio products it died not because of lack of passion, hard work and innovation, it died because of minor and avoidable technical issues.  And after being acquired by Wharfedale, it died as a result of the near death experience of Wharfedale in the early 90's consumer recession.

Edited by Craig Sawyers

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4 hours ago, Craig Sawyers said:

Back a decade or three I was CTO of Wharfedale, and we acquired a bust company called Lynx for a tiny sum of money.  Run by two New Zealanders; they made a really sweet range of products based around an FET power amplifier.  Sounded very nice indeed (even if it was a bitch to manufacture). This is an example  http://www.canuckaudiomart.com/details/649106536-super_rare_lynx_nebula_mosfet_integrated_amplifier/images/694258/ .

Anyhow, we were sat one day when a return came in - with a letter saying from a guy in Hong Kong saying it had burst into flames and demanding some outrageous compensation.  We unpacked it, and sure enough it was a charred mess inside.  Smelling strongly of petrol.  We got the Chemistry lab at Leeds University to do some forensics and write an expert report.  We sent that to the guy in HK - and that was the last we heard from him.

I told that dude to use napalm, but he cheaped out..........   Serves him right..........   :rolleyes:

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1 hour ago, Craig Sawyers said:

It was a great, beautiful  and good sounding product with two problems.  

First, the laser machined, triple silk screen printed, and highly innovative (and expensive!) perspex front panel was fixed to the metalwork with double sided tape, and there was no datums to assist, nor any jigs.  So it was applied by eye, and needed to be in the correct place and square to better than half a millimeter.  There was a massive wastage - until we got a jig designed and made.  

Second, I discovered that the Lynx designer (and company founder) was dyslexic, so on the (hand drawn) engineering plans instead, for example, of 124.5mm it was marked up at 125.4mm.  So there were tantalizing minor fit problems all over the place.  In fact, the first time they went bust in New Zealand was because the blind tapped holes in the heatsink for the power transistors were not deep enough for the screw length (so the screws bottomed, and the transistors were not in contact with the sink), so every last product blew up when it got used - I strongly suspect that was also a dyslexic issue - sort of "drill and tap 3.5mm depth" when it should have been 5.3mm.

Like many audio products it died not because of lack of passion, hard work and innovation, it died because of minor and avoidable technical issues.  And after being acquired by Wharfedale, it died as a result of the near death experience of Wharfedale in the early 90's consumer recession.

Thanks Craig! Very interesting history.

I had to look up what "dyslexic" means. And after reading the information, it makes me wonder if that's the problem I have :wacko: 

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