Meridian Unplugged - Powered by MrTechGuy.co.uk Forums Discussion Meridian - Systems DSP life expectancy

Big house, George, big house!
Actually I thought maybe you'd secretly reformatted your G68 to produce 12 discreet channels!!!
S.

Speaker failure is a nightmare scenario for me as I don't think I can physically remove them from behind my screen without some major (and very careful) manoeuvring. I have 7 x 5500 in my room, with the ones behind the screen bought in 1999. So far only tweeters have failed and they were simple enough to replace. It's heartening to read that not only are they made to last, but that if things do go wrong with the electronics then it's easy enough to pull the guts out for repair. Still, I'm hoping they just keep running...

Fidler; you could sell your 10 DSP5000s and treat yourself to a new fiddle!!

Excuse me for digging this up from the past, but just couple of points in the interests of future readers, it being a wiki entry.

In the wiki, it says two 680 ohm resistors but looking at an earlier 5k amp board, it looks like there's one 680 ohm and one 330 ohm.

The replacement suggested in the wiki is a "typical" resistor, whereas earlier 5ks has wire wound ceramic types. A smiley face 5k seems to have a "typical" resistor but it doesn't look like 680 to my untrained eye. The ceramic is 4W and suggested replacement looks like 2.

Any comments from the EEs out there (apart from get an expert to do it...)?

Was the 330ohm used in front of the negative regulator?

In my M60's (different PCB layout) the drop across resistor R9 is about 33V. The drop across R1, which feeds the positive regulator is about 21V. R9 gets pretty damn hot, and my three M60's had overheated R9 resistors (crispy!) and burnt PCB spots under R9. Around R1 gets warm, but not like R9.

I suspect your 330ohm resistor was used in later models using similar regulator circuits to my M60's to adjust the voltage divider created by R9 and the rest of the negative regulated circuit. The 330ohm resistor seems to shift more volt drop to the regulator and the rest of the negative regulated circuit.

Apparently, the regulators and/or the rest of both regulated circuits have different impedances that were not correctly accounted for. Using 680ohms for both R1 and R9 makes R9 run very hot.

Just about to embark on preventative maintenance of the resistors.

According to the schematics, R1 is in front of the +ve regulator and R9 the -ve regulator.

The schematics have also been updated by being written on to show R1 as 330R and R9 as 680R as per the first picture in TinTin's post.

I currently have both R1 and R9 as 680R, should I change to be as per the schematics?

Is anyone able to answer the question in the previous post ?

I didn't want to risk making an expensive mistake so I replaced like for like.

When I opened them up I had two 680 ohm resistors (R1 and R9) so put the same back in, however I did increase the rating to 5W.

As the speakers were manufactured in 1994 (I think), I would be happy if the resistors lasted another 23 years.

Both of the resistors in both of the speakers were crumbling so I would recommend everyone with a speaker of similar age to at least check the resistors. You may save a larger repair bill if you catch them before they fail.

Tarik, thanks for the informative reply. Much appreciated.

I just checked and I did change R9 to 330ohms so it runs cooler. This shifts some power to the regulator, which is not a big deal since it's bolted to a large pad. Just in case I glued small heat sinks to the regulators and R1 and R9. I changed all electros to Panasonic FRs, except electros in the signal path, which I changed to a higher end film type that I can't remember. There was an improvement in sound, which makes sense. A lot of the removed caps were obviously brown (especially around R9) and tested to only a fraction of their rated capacity.

Sounds like you are all good to go! Nice work on the DIY recap.

I was swapping "Cabinets" with a friend (rose for cherry) and inspected the infamous resistors and they were cooked on both 24/96 sets (all 4 24/96 backs).

2 were "Smiley" 300xxx S/N neither ceramic. Assuming 2 680's (cooked)
2 were "7 Segment" 200XXX S/N neither ceramic. Assuming 2 680's (cooked)

I also recently bought another smiley pair and inspected one of them and took pictures for reference and they looked great.

1 "Smiley" 300xxx S/N 1 is a 330 Ceramic the other not Ceramic.

I also have another ancient 18bit pair and those look great also (only one). 100xxx S/N both non ceramic. They are both 680's.

So we were trying to figure what is the best resistors to use. He is a very experienced EE guy. Between us we have about 16 DSP5000's and we plan to do this maintenance to most of them.

For grins I pulled up the photo of most recent pair that looked good and noticed that in ADDITION to the 330 Ceramic. There is another Mod that was done in that area. Take a look at the photo below.

There was mention of schematics (old and new). Could someone PM/Email me those schematics.

Also just lowering the 680 to 330 could RAISE the heat the resistor has to dissipate not lower it.

The 330 might need the rest of the mod.

I'll check exact serial numbers to double check what is newer than what.

Also this voltage regulation does NOT feed the digital board.

You have a lot of work ahead of you!

On my M60s the overheated resistors from the power rails to the regulator power input create a voltage divider with the regulator and dissipate some of the power the regulator would normally take.

The regulator pulls constant current to feed the connected circuit, so when you reduce the value of those resistors the power dissipated by them is reduced and added to the regulator.

On all three of my M60s the negative regulators had a lower drop across them than the positive regulators. They are different parts. Lowering the value to 330ohms shifted power to the negative regulator.

My PCBs were toasted and I didn’t want that to deal with resurrecting them again. I recommend replacing with the same value you have at a higher power rating. It will still get too hot, but should last at least another 10 years assuming your PCB is in good shape.

If not good shape, you might want to consider 330ohms. Obviously, you should check voltages before and after and understand what you are doing before you do it. These were all hand made boards and there seems to be some variation in design and parts used, but they seem similar.

-R