Recapping a Philips AH-6731

Porthos

Porthos

Active Member
I recently purchased a Philips AH-6731 (same as AH-673). Looking at the schematic I can see that there are 92 electrolytic capacitors. Of which:

10 are electrolytic in the power supply that must remain electrolytic
19 electrolytic outside the power supply >10uf
63 which are ≤10uf can be replaced by film capacitors that are small enough to fit

Ultimately I am wondering if replacing all 63 with film capacitors would be a colossal waste of cash vs sound quality. Or cause instability/hum. Also, which caps <10uf should I prioritize?

Service manual:
http://www.fmtunerinfo.com/AH673service.pdf

AH673inside.jpg
 
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Wholesale swapping small caps with films IMHO is a waste of time and money. Unless they are directly in the signal path. Stuff like metering, muting, etc don't care, and will not affect the sound or operation.
 
Go to FMTUNERINFO.com under under Jim & Bob's DIY Mods. Under Philips AH673 contributor John C. lists the caps he recommends to replace with the old/new values and the brands he recommends. I have used this particular recommendation on my AH6731 and it turned out very well. In fact I've followed John C's advice on other tuners and have never regretted it.
 
Thanks. In that case I'll just replace the smallest with film capacitors (cost of a McDonald's happy meal) instead along with the signal path. For the rest electrolytic.

Who is this John C. guy though and will these mods really improve an already superb tuner? The post neglects to state what measurable impact this has and if it compromises anything.

I'll definitely put the bypass caps but I notice this mod replaces a lot of caps in the signal path with a much larger electrolytic. A small film would have fit without a problem. Is this the sort of mod where I can keep the audio path stock and replace with film? I was thinking perhaps something like below. Green is where I retain stock, red is a no to the mod, and blue is where I follow it. For the signal path can this mod be partial to keep as much film as possible? Also, Punker X I will likely send this to you for alignment when I have the cash. Would you approve of these mods?

IF Board TN6340
Decoupling
C51 - 220/10 to 560/35
C60 - 47/25 to 120/50
C72 - 100/25 to 220/25
C8/22/39/61 - 10/35 to 22/50
C21 - 22/25 to 47/25

Composite (signal path)
C52 - 220/10 to 220/16 + 0.1 µF polypropylene bypass (limited space, check for fit)
C53/62 - 10/35 to 22/50 + 0.1 µF polypropylene bypass

Decoupling - Meters (Optional)
C2 - 1/50 to 2.2/50
C10 - 10/35 to 22/50
C17/30/34 - 10/35 to 22/35
C18 - 33/25 to 68/35

Meters, V/H Output (Optional)
C32 - 47/16 to 47/25
C23/24/40/45 - 10/35 to 10/50
C48 - 10/35 to 12/63

MPX Board TN6344
Decoupling
C2/78/60 - 1/50 to 2.2/50
C17 - 220/25 to 560/35
C5/7/38/39 - 47/16 to 120/16
C36/37 - 10/35 to 33/35
C59/61/62/63 - 10/35 to 22/50
C66 - 10/25 to 22/25

Composite/Audio (Signal Path)
C16 - 10/35 to 33/25 + 0.1 µF polypropylene bypass
C19/20 - 10/35 to 22/25
C34/35 - 10/35 to 22/25
C40/45 - 10/35 to 22/50
C50/51 - 10/35 to 22/50

Switch Board TN6323
Decoupling
C1 - 1/50 to 2.2/50
C17 - 220/25 to 470/25
C18 - 2200/16 to 4700/25 Nichicon HE

Audio (Signal Path)
Fixed Output
C8/11 - 2.2/50 to 10/100
C10/13 - 2.2/50 to 22/25
Variable Output
C2/5 - 2.2/50 to 10/25
C4/7 - 2.2/50 to 22/25

Touch Control (optional)
C1/2 - 100/25 to 150/25
 
Nothing wrong with these suggestions. I also prefer larger coupling caps with a film by-bypass. Improves dynamic range. ie better bass.
 
I'll just piggyback on Punker X's comments (another IMHO unimpeachable source on these sorts of questions). John C. I believe is a retired individual with a strong technical background. He may though I'm not sure be an EE. His circuit analysis/knowledge is above that of an amateur hobbyist. I've had occasion to query him on more technical questions (to bolster my limited knowledge) and he has always offered info that is supported by other sources I have (my brother is an EE etc.). John has posted numerous tuner mod 'recipes' on fmtunerinfo. This site is run and frequented by many hobbyists and several very technically qualified people. I've never read anyone there questioning John C. choices. That alone tells me a lot. There are several things that John and others frequently recommend in discussions of tuner mods. One is frequently upping values of key power supply e-caps and also key audio output e-caps to improve low frequency response. He's not a nut about boutique caps but IMHO offers good advice for PS caps (Panasonic FR, FM, FC and Nichicon HE) as well as fast response, lower noise diodes. In signal path Elna Silmics/Cerafine ... and Nichicon's audio grade caps (Muse KZ, ES ...) are solid suggestions.

As you might notice I'm a pretty big fan of John's. Being myself a hobbyist, I appreciate people like John offering their advice and opinions in these open forums. I for one have learned much from studying what they have to offer. As with anything on the internet initial skepticism about sources of info is valid. But, after enough input you get a pretty good idea who is the real deal. Also keep in mind even with legendary tuners, many of these, even the newest are decades old. Electrolytic caps do deteriorate over time, especially those in high temperature settings (PS caps). Capacitors have also improved in quality over those decades. Those tuner using opamps often can benefit sonically with newer opamps as these have improved over the decades. So even fully operating super tuners frequently can benefit from judicious mods.

Final note here: My last tuner project was a Sansui TU-X701 where I performed John's mods. Very good sounding tuner before, great sounding after. But, one thing I noticed was that the signal strength meter was frequently pegging even on signals I knew it should not be doing this on. John advised correctly as it turned out that there was a pot that could be adjusted to fix this without having any other system impact. John took the time to verify this from schematics and then post the advice. The effort was appreciated :-)

Bill C.
 
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