Technical Assistance/Advice Thread

[Pars]

Just a quick and stupid question: when testing amps/psus not in a chassis, do you tie any heatsinks to safety ground just in case? Just thinking that might be a good idea, particularly on HV stuff.

[simmconn]

Yes that’s what I would do.

[mwl168]

Need advice about safely powering up an all-transistor CD player that has not been powered up for many years. .

I have a Marantz SACD/CD player that has been sitting in its box for many years. Is it a good idea to use my variac, like the one in the picture below.  to gradually power it up? 

If so, what's the proper process? 

Thanks in advance.

 

 

 

 

Edited November 27, 2023 by mwl168

[MLA]

Not my field of expertise, but I would i imagine the start sequence is designed to start from full power, so if it was me I'd rather just flick the switch and see what happens as I would be worried that gradually ramping up voltage might leave parameters out of spec at the wrong time... But that's mostly a guess :).

 

[kevin gilmore]

depends on what kind of power supply is inside. if its a switcher, it might benefit the input caps for a 30 second slow ramp. if its a linear supply, probably does not matter.

[mwl168]

Thanks MLA and Kevin for your help.

I got the player up and running yesterday first using the variac to slowly ramp-up. Later tested it without the variac and, fingers crossed, it's working fine so far. 

The device in question is a Marantz SA-1 SACD/CD player which uses linear power supply based on what I can see in the service manual. 

To expand on a long story made short, it powered up but was unresponsive when I first took it out of the box. After consulting with the service manual I dissembled the unit and found a blown fuse inside. Instead of the factory spec'ed 1A/250V slow-blow fuse, whoever last serviced the unit put in a fast-blow 2A/250V fuse. Which likely explains why it's blown upon turn-on. I replaced it with a proper slow-blow 1A/250V fuse. This is when I posted my question thinking I should play it safe given the age of the unit and suspecting why the last person serviced it put in an out-of-spec fuse.

By the way, this thing is beautifully built like a tank and getting to the fuses take some work. A few photos from the web and of the exact unit.  

It's wonderful sounding especially with the SACD.

 

 

Edited November 29, 2023 by mwl168

[astromaddie]

I’ve got a mostly-completed CCS project for a Stax T1S driver… I’m comfortable enough doing the soldering but not confident enough to adjust the variable resistors, and don’t really understand how to actually add it to the T1S itself. Could someone give me some detailed hand-holding assistance? I can share photos of the board as it is right now if it would help. (I don’t think this deserves its own thread but let me know if it should be.)

[n_maher]

Ok, so it's been a good long time since I've worked on much in the DIY world but I took on the project of trying to rehab Knuck's Millett hybrid and for the most part it went well.  The amp arrived with issues with the power supply capacitors, which were spec'd as 25V units but with the +/- 15V power supply have the distinct chance of seeing more than that and three out of the six had failed.  I replaced all 6 with 35V units to provide some future resiliency and while they don't fit the PCB as well, they fit.  Then I ran into the unexpected problem of the power switch being flaky.  I think it's got an internal issue and I'm tempted to simply bypass it and turn this thing into an "always-on" scenario because I can't imagine successfully desoldering, removing and replacing the unit.  That's assuming I could source the correct replacement.  So, before I screw something up royally, I decided to ask for some advice.  If I wanted to "bypass" this switch, which appears to be a DPDT unit, anyone have an recommendations on how best to accomplish that?  

And yes, I know the joints look like hot garbage.  I tried to reflow them a couple of times hoping the issue was a cold joint or something like that but a small amount of pressure to the left on the switch body is all it takes to correct the issue and no amount of resoldering had any effect.

[dsavitsk]

I think I'd clip each of the leads coming out of the switch close to the switch body. They should be easy enough to pull from the board one by one at that point. Then you can jumper the pads to make it always on. Alternatively you can add some small jumper wires to the switch leads but that's a lot more fiddly.

[Helium]

Is electrolytic capacitors reforming needed? What is common attitude to this procedure?

I've got quite a few pcbs that I completed 8-10 years ago. Most projects are from amb and KG. I soldered PCBs, washed them and put aside on the shelf. But life is life, time has passed and finally I thought that this year it's a good time to complete them.

Pcbs were never switched on. Caps I used are all good if not the best brands: Panasonic, rubycon, nichicon. Never frozen, never heated.

However I'm confused how short the shelf life is according to manufacturers specs: 2 years, maybe 3. I remember receiving caps from mouser that are almost 2 years old, I'm curious what they do after 2 years, dispose them?

Anyway, do I need to do anything about caps? Do they really degrade, or is it just a BS from manufacturers to stay on safe side? Maybe 8-10 years are nothing to worry about? I really would avoid desoldering and replacing everything if possible.

 

Edited Wednesday at 03:57 PM by Helium

[spritzer]

Capacitors aging is definitely a real thing as they are filled with electrolyte and it will leak/evaporate over time.  I do recap a lot of Stax stuff so I can track how the caps are doing.  I have no idea of the usage (aside from unit overall condition) but most of the HV caps hold up really well.  Never seen one that is leaky (that is made in the 80's or later) but they are clearly aging though when being measured.  They are also all in either pure C filter setup or a C-R-C setup so they are easy to compare. 

As for reforming... I'm more skeptical.  Maybe if you are dealing with very old caps but anything made with modern manufacturing tech it shouldn't apply. 

Now for caps soldered in and sitting for a decade... too many variables really.  How were they soldered would be my biggest question as some seals could have been damaged, compromising the inside. 

[Craig Sawyers]

There are stories both ways about electrolytic capacitors. As an example, I have an AVO transistor tester (just for historical curiosity; an early 1960's beast). The large circuit board is rammed full with Plessey axial electrolytics, and had not been powered for decades. They all measured just fine with my PEAK ESR meter (it also reads capacitance too). So I just fired it up - works perfectly.

But the dreaded power supply and output electrolytics in the old Quad 303 power amp were arranged so the vent faced downwards towards the circuit boards. Quad got steady returns as a result of rotted circuit boards because of electrolyte venting, eventually doing what they should have done originally and put the vents upwards.

And the old Sprague "twist-lock" electrolytics eventually disconnect themselves because of the outer foil being crimped into a steel closure - so the foil suffers from dissimilar metal corrosion.

Modern electrolytics are physically much smaller for a given voltage, capacitance and ripple rating. This is really a mix of more highly etched foils and improvements in electrolyte chemistries. But they tend to be more highly stressed - and lifetime goes as the Arrhenius equation. The lifetime of the capacitor halves for every 10C increase in temperature. The flip side is that if you conservatively rate the capacitor, its lifetime will exceed the lifetime of the engineer.

As an example, suppose the capacitor lifetime at 85C is 2000 hours (about 3 months). If it is run at 35C, being 50C under the maximum increases the lifetime to 2^5 x 2000 hours, or 64,000 hours, or 8 years if turned on permanently. If it is turned on say 3 hours a day, the lifetime of the capacitor should be 64 years. By which time I would be 123 years old and long being with the choir eternal.

Of course this is a idealistic calculation, and rubber seals will have degraded enough to cause the capacitor to fail much earlier than that.