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PostPosted: 18 Apr 2018, 01:11 
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Joined: 31 Jan 2013, 20:56
Posts: 5
My 900V amp is on its last legs and needs a rebuild. There is a service manual on Hifi Engine if people want to look at it. That said, I have a question about the power supply capacitors.

I have read people doing it, but is this amp designed well enough to handle an increase in capacitance of the PS caps? The stock ones are 71 volt, 8200uf, 20% operational range, and a 35mm diameter. I want to have them replaced with a Kemet slit foil cap with specs of 80 volts, 10000 uf, 20% range, and 35mm diameter. One tech suggest leaving the caps at 8200uf. He was concerned about inrush control, if this amp has that or not. It does have a delay when powered on, so I would assume yes on the inrush part?

Also other caps in the unit, someone suggested using Elna and Nichicon caps. He said when modding his 900V:

"The amp was completely recapped with premium capacitors including the six PS filtering ones. Their capacity was increased by 20% to 24.6 KmkF per channel which helped with bass response. In other locations, Nichicon or ELNA caps were used, with the preference for ELNA SILMIC II in the signal path.

The phono stage received most of upgrades; this elevated its performance to a new level. All signal path resistors were replaced with Vishay 1% metal film; all electrolytes were replaced with ELNA Silmic II or Nichicon premium caps (FG/FW/KZ). The output opamp, Mitsubishi 5218, was replaced with 8-pin socket and the LM4562 was installed with the addition of two coupling caps to prevent parasitic oscillation. The socket now allows opamp rolling."

Is he on the right track or did he overlook something in the design? Any other mods this amp would do well with? The connectors on the back aren't hard mounted very well, as they move a little when stressed. This includes the speaker terminals.

Also my selector for tape 1, source, tape 2 is a bit wonky. Not sure if cleaning would help or if it needs repair. Are these commons switches in this amp?


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PostPosted: 18 Apr 2018, 10:28 
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Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
Welcome aboard, mate! (you found me ;) )

As for the coupling caps, how do you want her voiced? I use both FG and Silmic-II.

The switches need cleaning. I'll do a clean and if they don't respond, a replace.

Looking forward to hearing everyone's opinion here.

Cheers!

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-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 18 Apr 2018, 10:45 
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Joined: 31 Jan 2013, 20:56
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Hi Gregg! Small world eh?

You'll be getting this amp of course, so I'm curious what others will say. I like the sound from the Rotel amps which use the BHC/Kemet Slit foil caps, which is why I'd like to use those. As for other caps, Elna and Nichocon are good caps to use and their tolerance is tight. The guy I referenced used those, so he liked what he heard.

But opinions from others here are welcome.


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PostPosted: 18 Apr 2018, 11:24 
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Joined: 31 Jan 2013, 20:56
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Gregg,

Another tech I talked to said he quickly checked the amp service manual. He thinks it doesn't have inrush control, which would mean added PS caps would stress the amp internals. Is it possible to add inrush control with an added circuit, or is that not possible to add in?


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PostPosted: 18 Apr 2018, 19:55 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
Yep!
As we discussed in person (repeated for others here), since she's a class-AB amp not drawing continuous current for a CL-series inrush limiter, a good old fashioned Sansui style power resistor bridged by a relay after three seconds may be called for ^_^

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 18 Apr 2018, 22:15 
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Joined: 31 Jan 2013, 20:56
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Gregg,

Your Sansui option- that isn't technically a thermistor is it? Other tech was poo pooing this approach.


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PostPosted: 19 Apr 2018, 03:33 
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Joined: 09 Oct 2012, 19:43
Posts: 301
Location: Vancouver Canada
What value of bridge diode or rec is/are in the power supply. They should certainly handle an increase from 8200 to 10000 uF without a worry about surge. The 80v is just more headroom so good. But ya the res and relay with delay is cool to build and adds to flavor.


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PostPosted: 19 Apr 2018, 06:34 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
Scott - No, the CL-series thingy I showed you is the thermistor. Good for class-A, not so good for AB.

Laurie - I'd be using a 50A bridge or Schottky diodes :)

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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