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sergeauckland

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I wrote above about buying a Quad II amplifier for restoration. It arrived with very little wrong with it, so have done what's necessary. Cosmetic condition could be better but short of removing everything and having the chassis and transformer covers resprayed, I'd rather keep it original, warts and all.
1) Replaced the ungrounded 2 pin mains connector with a properly grounded IEC.
2) Replaced the main smoothing capacitor (16+16uF) with a new one as one of the sections of the original was marked as having failed. It's quite a bit smaller than the original.
3) Matched the EF86s (I had a few in stock) which reduced distortion from 0.9% to 0.15% at 12 watts 1kHz. I could fiddle around trying closer matching of the valves and resistors, but I don't see it as worth the effort.
4) It gives out 15 watts just prior to clipping, nice square waves with only a little (around 10%) overshoot, no sign of instability and noise around -80dB relative to 12 watts, around 20 watts at 1% distortion.
5) Frequency response flat to a needle's width on my meter 20-20kHz at 12 watts output with no visible distortion on the 'scope.

Here are some pics:- The last one was just for effect...
Quad II outside.jpg

Quad II inside.jpg
Quad II lit up.jpg


S.
 

kiwifi

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IMG_20221113_110830_217.jpg

Marantz PM7003: A friend brought me this to take a look at because it was tripping into protection mode at power up. I am happy to say that I was able to find and fix the problem.

This is a well constructed A/B amplifier with a discrete transistor output stage and not a single surface mount component in sight. A pleasure to work on!
 

Ricardus

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Right now I am working on some Neumann KM84 clones. The boards fit into donor bodies from another mic, and then you need to find quality capsules. Is it a Neumann? No. But with good transformers and good capsules you really have something quite good with this circuit.
 

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kiwifi

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Just replaced the linear 120vac power supply in my M&K MPS-2810 (MX700 Pro) subwoofer with a 100-240vac switching power supply.
IMG_20221120_083046_113.jpgIMG_20221120_082706_089~2.jpg
 

kiwifi

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I've wonder about those power supplies... how they perform?
It was working well, but was on all day today and has started buzzing! Some kind of heat related issue I expect. It will require more investigation tomorrow...
 

restorer-john

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It was working well, but was on all day today and has started buzzing! Some kind of heat related issue I expect. It will require more investigation tomorrow...

How are you heatsinking the SMPS PSU? It will need some serious cooling.
 

kiwifi

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How are you heatsinking the SMPS PSU? It will need some serious cooling.
No additional heastsink at the moment, but cooling is always going to be a problem inside a sealed cabinet full of (insulating) stuffing. I had hoped that the high conversion efficiency of the switch mode supply would have been sufficient, but it looks like that was wrong. I can try adding a heatsink, but it will still be trapped inside a sealed box.
 

restorer-john

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No additional heastsink at the moment, but cooling is always going to be a problem inside a sealed cabinet full of (insulating) stuffing. I had hoped that the high conversion efficiency of the switch mode supply would have been sufficient, but it looks like that was wrong. I can try adding a heatsink, but it will still be trapped inside a sealed box.

It will need to be mounted on a finned heatsink with access to external air or it will fail/shutdown or just burn up the components prematurely.

Their efficiency may be good at near to full load, but at low loads, they can be poor.
 

kiwifi

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It will need to be mounted on a finned heatsink with access to external air or it will fail/shutdown or just burn up the components prematurely.

Their efficiency may be good at near to full load, but at low loads, they can be poor.
Thanks! Hopefully I can avoid mounting the PSU outside of the cabinet, but it would be doable. The good news is that the buzzing has stopped this morning now that things have cooled down, so I can look at adding a finned heatsink.
 

kiwifi

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It was working well, but was on all day today and has started buzzing! Some kind of heat related issue I expect. It will require more investigation tomorrow...
Looks like the buzzing was due to a dodgy XLR cable. I fitted a finned heatsink to the PSU anyway and everything is working well now.
 

JP

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I found it ironic that I needed a keyboard and monitor to fix my stereo while discussing lacquer cutting with a mastering engineer.

IMG_1371.jpeg



Next up for the holiday break is to build as much of this 3D printer as I can.

IMG_1376.jpeg
 
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I have just repaired the mechanical keyboard with KAILH switches (Titanwolf Imperial). I bought it by eBay for €21 and got today. It should be B-Ware "refurbished". But one Key was death :-(. One diode was loose and fell down when the housings were unscrewed. Now works fine again. This was my first SMD soldering job :cool:.

DSC05050.JPG



DSC05052.JPG
 

Doodski

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I have just repaired the mechanical keyboard with KAILH switches (Titanwolf Imperial). I bought it by eBay for €21 and got today. It should be B-Ware "refurbished". But one Key was death :-(. One diode was loose and fell down when the housings were unscrewed. Now works fine again. This was my first SMD soldering job :cool:.

View attachment 250331


View attachment 250329
Nice try. :D It appears 1/2 of the solder job looks OK and closest solder looks a bit, "Cold." What you need is some liquid rosin flux. It makes soldering easier to do and the end result will look much better and be a better contact too. Cost is maybe $7 to $15 depending on the size of container that you buy.
 
OP
pma

pma

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class D filter for noise and distortion measurements

Though I do not have special problems with soundcard saturation from class D amplifiers during measurements, I have decided to make a simpler version of something like AP AUX-0025 filter. I made an RLC filter that is possible to use with both single ended and BTL output class D power amplifiers, in 20Hz - 20kHz band.

It looks like this, and it has air-core inductors and MKP capacitors:

View attachment 171316

and this is the frequency response:

View attachment 171317

Not ideal, but usable.

This filter did not have a big effect if used with entry-level soundcard ADC like Focusrite 2i2, however there is a huge effect if used with E1DA Cosmos ADC. Please see below measurements of SMPTE IMD of my PMA-NC252MP amplifier, with and without the LC filter. It was measured at 77W/4ohm.

PMA-NC252MP SMPTE LCfiltr anone.png


Though IMD number is almost same in both cases, TD+N is of 5dB better with the filter, and the noise floor difference is about 12 - 15dB at middle frequencies.
 

Steven Holt

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Well guys, I really don't have a workbench right now, but I do have a nice piece of plywood that I put on my washing machine and right now what is on the top of it is a 2009 Yamaha RX397 that I just bought at the local Goodwill for 6.99US. I'm going to do a complete, through cleaning inside and out, and then use my analog multimeter to check the voltages. It looks clean and well-cared for, but you never know. If I start to bite off more than I can chew, it's always good to learn new things.
 

dsmartboxat

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Various items under NDA, so the picture is a clean workbench instead.
 

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