Monster Amp Meltdown

[George S.]

I looked for a DCME data sheet, no luck. There are DCM data sheets, so the E must be a subset.

 

[Gepetto]

Strange, I was on the Mouser site and could find the lower voltage caps (18,000 ufd 75V)

They have 15KuF at 80V

 

[Peter S]

Thanks Joe and George for looking. It doesn't look like there's too much choice for replacements anyway. Mouser has a 10,000 ufd 100V that fits. It 85 C temp, 7.5 A ripple, 2000 hours. If I relocate the caps, Mouser has a 12,000 ufd 100V cap 105C, 12.6A ripple, 7000 hours, and lower ESR.
With more ufd or lower ESR, would this be a stress on the soft start resistors?

 

[Gepetto]

Thanks Joe and George for looking. It doesn't look like there's too much choice for replacements anyway. Mouser has a 10,000 ufd 100V that fits. It 85 C temp, 7.5 A ripple, 2000 hours. If I relocate the caps, Mouser has a 12,000 ufd 100V cap 105C, 12.6A ripple, 7000 hours, and lower ESR.
With more ufd or lower ESR, would this be a stress on the soft start resistors?

We do not know the design intent so hard to guess...

 

[George S.]

Peter, I'm thinking those caps were made in the 2nd and 3rd week of 1999. I went back to earlier Cornell Dubilier websites on the "Way back Machine" and can't find that series listed anywhere.
Perhaps they were custom made for Crest.
I see no reason the current DCMC series wouldn't work, but pricey.
Perhaps a new White Oak Series ONE would be a better investment.

 

[Peter S]

I will have to investigate the White Oak caps, thanks George. BTW, I saw "1999" hand written on the power supply pcb. I was also pondering a Kemet ALS70A203KF100. Very long-life and 20 Amp max ripple! It's 20,000 ufd so I would only 4 for the high rails and 4 for the low rails. My next step is to cut some thin plywood plates to see how things fit together before I cut aluminum plates. It seems the final amp will be 6 rack spaces (10-1/2" tall)
My design mod requirements are;
--make the amp passively cooled by adding sufficient heat sink
--mods should be reversible, to restore it for sale or pro use if I decide that a 1000 Watt per channel subwoofer amp might not be a good fit in the seniors home if/when the day comes.

 

[Peter S]

It's a little tough to get motivated on this project. The result will be one ugly 6 rack-space lump. I have another dead Crest 5-1/4" cabinet to cover the top half of the front. I will need to take the plunge on the caps soon, so I can test for temperature rise, idling and with music.
The wood plate is a mock-up of 4 aluminum plates attached to the sides of the amp modules. The 2" .extension above will allow attachment of more heat sinks. I am open to suggestions from all the experienced amp builders here and can easily change direction at this point.

 

[George S.]

Looks like a good start and idea about opening it up for more airflow by increasing its height.
You can probably relocate any hard to reach test point boards for easier service.
Remember to pressurize the case with your fans rather than exhaust it.
Something to consider is what Dynaco offered long ago for their then monster amps.
Perhaps you could mount the caps in the unused chassis or something else and keep the original Amps dimensions.
https://forums.phxaudiotape.com/threads/dynaco-c-100-energy-storage-system-capacitor-bank.7863/

 

[J!m]

That's right... I had a big Dynaco amp for a short time and that had the connection for the optional external capacitor bank. Not a bad idea and if you built it above, you could accommodate the heatsinks as well as keep the connection short between the two units.

 

[Gepetto]

Looks like a good start and idea about opening it up for more airflow by increasing its height.
You can probably relocate any hard to reach test point boards for easier service.
Remember to pressurize the case with your fans rather than exhaust it.
Something to consider is what Dynaco offered long ago for their then monster amps.
Perhaps you could mount the caps in the unused chassis or something else and keep the original Amps dimensions.
https://forums.phxaudiotape.com/threads/dynaco-c-100-energy-storage-system-capacitor-bank.7863/

You have to be concerned with wiring inductance when outboarding capacitor banks...

 

[J!m]

I knew there was a reason.. a top side connector in the amp could mate to a bottom-side connector in the cap bank and the enclosures stack in the rack. That should be a reasonably short run.

I seem to recall designing something like that before, but I don't remember what it was...

 

[Peter S]

I am thinking about locating the caps in the original (soon to be referred to as "Lower Chassis"). That way the caps would be as close as possible to the 3 bridge rectifiers. They would also be below the largest heat source. If the caps are 20,000 ufd, only 8 would be required (four on each side).

 

[Peter S]

Change of direction; so this is as far as I got with my passive heat sink project before I started to see the error of my ways. I have decided it might better to address the fan noise issue rather than build a 10-1/2 " monster with no resale value (and have to fit the ugly beast into the rack)
So I'm back to finding caps that fit; 10,000 ufd 100 V Nichicon caps for the high rails are available from Mouser. The problem is the 18,000 ufd 75 V units.
Thanks joe, for pointing out that 15,000 ufd 80 V caps are available....
NOS from ebay or other sources in the US, are available in the required 18,000 ufd 75 V size. I have always found newer caps tend to be smaller than original caps of same value on vintage gear. I guess this amp is not vintage enough. I am restricted to 35 mm (1-3/8") dia x 120 mm L.

Could I expect hum or performance issues by reducing the low rail caps from 18,000 to 15,000 ufd? Would I better off buying NOS 18,000 caps and possibly doing a 'reforming' proceedure? Don't know how old the NOS stock is.

 

[Peter S]

As mentioned, I have decided to simply replace the caps and keep the amp fan cooled in it's original configuration. I have ordered NOS NIPPON CHEMI-CON 18,000 ufd 75 V caps a Chinese supplier, the only source of 35 mm dia. x 118 mm available.
Has anyone got a suggestion or link to a good capacitor reforming proceedure?

 

[Gepetto]

As mentioned, I have decided to simply replace the caps and keep the amp fan cooled in it's original configuration. I have ordered NOS NIPPON CHEMI-CON 18,000 ufd 75 V caps a Chinese supplier, the only source of 35 mm dia. x 118 mm available.
Has anyone got a suggestion or link to a good capacitor reforming proceedure?

Beware on those NOS capacitors from a Chinese supplier. Nippon Chemicon is the most common knock off out there. They have the branded shrink wrap and will give you anything you want...

 

[mlucitt]

Has anyone got a suggestion or link to a good capacitor reforming procedure?

I would test those capacitors first.

 

[Peter S]

Thanks, guys, I'm bracing myself for the disappointment. My meter can only measure capacitance up to 15,000 ufd, Hopefully it won't get a reading.
I see mouser has 15,000 ufd 80 V caps that would fit. I was concerned about consequences of going from 18,000 (called for) to 15,000 ufd.

 

[George S.]

I've found caps and other parts on Mouser and Digikey that would never populate or be shown by the search engine matrix by directly entering the part #.
Quantities are typically very low, but are in stock, and always ship.
Try entering the caps part #.

 

[Peter S]

I've found caps and other parts on Mouser and Digikey that would never populate or be shown by the search engine matrix by directly entering the part #.
Quantities are typically very low, but are in stock, and always ship.
Try entering the caps part #.

Thanks George. I have already ordered the 18,000 ufd caps from 'overseas' so I will wait for them to arrive to see if I have been bamboozled. I will keep that in mind. I suppose I could always phone Mouser or Digikey too.

I have previously replaced the fans with quieter, lower CFM units. This may have contributed the the failure. The amp module heat sinks seemed to run at a reasonable temperature but the caps are very boxed in. I am contemplating drilling vent holes in the top cover over the caps. Any thoughts?

 

[George S.]

Don't know. Maybe it had a hard life prior to you getting it.
I'd like to think it was designed correctly and just needs adequate air space around it.