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Advent 400 table radio

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The Advent 400, for those who may not know, is a terrific FM-only radio from Henry Kloss after he left KLH. It is solid state of course but reminiscent of Kloss's first radio--the KLH Model Eight which featured an FM receiver with his signature vernier tuning dial in one box and a separate acoustic suspension speaker.

Like all Kloss radios, the Advent 400 is known for its excellent reception. I've owned about a half dozen of these and they're impressive. The 5.25" full-range driver is the biggest of any Kloss radio and at some point they apparently used the famous Phillips whizzer-cone driver (but those are rare). According to Fred Pinkerton, Audio Product Manager for Advent, the 400 cost over $90 to produce although they sold it to dealers for under $110 and it had an MSRP of $125 (no wonder they went bankrupt). The tuner section, according to the Owner's Manual, "uses techniques and parts . . . associated with far more expensive receivers."

Unfortunately, the cabinets are white polystyrene plastic that turns yellow with exposure to UV rays. The only way to restore the white is a painstaking process of sanding and polishing. One ebay seller paints them white but I was never a fan of the painted look or the possibility of scratching. YMMV.

Another issue with these is low volume. Apparently some of the capacitors used--10uF axial leaded ones in the amp section--are known to fail.

So.... this particular 400 worked but the volume was very low and the cabinets were just awful. I asked my favorite tech about the volume issue. I didn't want to take the radio to him because the repair would cost more than the radio is worth. He alerted me to the axial cap issue and implicated some other possible culprits. Also got some help over on AK.

I replaced the four 10uF 16V axial caps with 10/25 radials (they had to be bent over to clear the pots) but there was no improvement (btw they all measured ~14uF). Went back in and found a 10/16 radial and pulled that. It read open! Bingo! Problem solved. Just to be safe though I replaced all the electrolytics on the amp board. Put it back together and it now plays LOUD! btw, all the 'lytics were about 40% out of spec. Maybe should have done the FM board as well. I did not replace the 1000uF 35v filter cap. It's a Rubycon and the radio doesn't hum so I left it. Couldn't measure that value with my cheap LCR meter.

As for the cabinets, I've written before about using truck bed liner on old CSW speakers with the Nextel finish http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=7315

Thought I'd try that. It's truck bed liner, so it's rugged and there shouldn't be a problem with scratching.

Results shown below. That's a before shot of the same radio--yuck! Not a "restoration" but these radios don't have that much collectible value. IMHO the pebbled black finish looks good. Also replaced the power cord for 2 reasons: I can use the original white cord on a KLH Model Eight restoration, and the black cord looks better with the black cabinet.


edit: 2nd photo is new, showing truck bed finish 



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Great job - - that looks better in black now than it ever did in white, even when it was new! So great to see a small table radio that has tone controls, and all of the knobs appear to be identical to those used on the Advent 300 receiver. Also interesting to learn that some of these had the 5" Phillips whizzer driver. I always liked the early Advent electronics, especially that rugged 201 cassette deck, but this model 400 is one snazzy little combo.

Good description of the fix - - if you have pics of the insides, always appreciated.

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Thanks for the positive comments. I like the black too.

Sorry but I only took 1 shot of 1 cap. There's a pic here of the innerds: http://www.davidreaton.com/advent_model_400_fm_only_radio.htm

I'll be doing another and will be sure to take pictures!

You're right about the knobs, and especially painted black the 400 is sort of a "mini 300."

Ah... the 201 cassette deck! Here's a page: http://www.davidreaton.com/Cassette_Decks.htm

Back in the day (about 1970 or so) I bought my first tape deck. Advent had revolutionized the industry by adding Dolby and Chromium Dioxide tapes to what had previously been suitable only for dictation or other lo-fi applications. The 201 was a Wollensak unit widely used in schools (where they had to stand up to abuse), modified with the addition of the Dolby circuitry and CrO2 bias. At the time Advent and Wollensak marketed identical units (except for different knobs, meters and logos) but unlike Advent, Wollensak could be had at a discount.


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Hey, that's pretty neat you fixed it, that's always a great feeling. I would probably replace the filter cap but leave the tuner board caps alone, as you never know if that will affect the alignment, since otherwise it seems to be working fine.

I have the slightly more sophisticated 420s (the 's' may mean stereo, as it came with two speakers), it came in a grey plastic casing that has held up very well. It has phono and aux inputs, perfect for the hipster generation.

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Great design!

Can you use it without the speaker and as a (Stereo) Tuner attached to an amp.?

The 400 has a Mono FM out jack. You could use it with a mono amp or attach it to a stereo amp using a Y-adapter.

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  • 1 month later...
On 6/12/2015 at 9:41 AM, ra.ra said:

if you have pics of the insides, always appreciated.

OK. Here's another one, with some inside pics.

I took the other one to my local independent electronics supply store where the owner sells some old stuff on consignment. Both he and one of his customers went ga-ga over the black one and asked for another.

This one looked AND sounded pretty bad. Unfortunately there's nothing I can do about the faceplate. edit: Turns out there IS something I could do about the faceplate. See posts below. The faded knobs will benefit "a little" from some Armor-All.

I replaced all of the electrolytics on the audio board. Attached is a photo of the ones I removed. Have not measured them but there was a DRAMATIC improvement with the new caps.

The speaker cone had faded to gray and I remembered a tip from our friend John at M-Sound: To get a "wet look" he recommends thinning his white "WD" adhesive to skim milk consistency and painting it on the cone. I added Mixol Black to the thinned WD and it came out pretty well.

The original power cord was white so I replaced it with a black "NOS" cord set with non-polarized plug (the white will be used on a KLH Model Eight or Twenty-One restoration).

Trying to decide whether or not to paint the aluminum grille gloss black. The natural aluminum looks nice (see post #1) but the all-black could be cool.








Edited by JKent
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Kent, for the knobs, there's a product called Wipe New

that might restore their look. It's basically a thin acrylic

clear coat for restoring the finish on bumpers and cladding.

I've used a similar product to restore fogged plastic


Here's a Consumer Reports test (on a car :-))

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Hey Ken, thanks for the tip. I see in the link there's also a product called ReNu Finish that has a black dye. Maybe I'll check my local auto parts store. Never thought of using that kind of product but it's a great idea!

BTW-- I almost forgot (and these ARE the SPEAKER pages). The speaker has an LCR filter so the 1uF NPE was replaced with film.


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To tell the truth I had "assumed" it was assembled in Mexico, like the 300 receiver but you're right: Made in the USA. I don't "think" it was ever made in Japan but the subsequent model, the 410 was (so was your 420).

Here is an interesting personal communication from Fred Pinkerton:

Hello Kent,

My name is Fred Pinkerton. I worked at Advent Corporation from 1975 until 1981. From 1978 until my departure from Advent, I was the the Audio Product manager. I worked with Henry from 1975 until he left Advent, as well as Andy Kotsatos, who was the Audio Product manager. I was in charge of the development of the "Response" series of radios (Model 410M, 420S and 450S and the 402 separate speaker). Yes you are correct, Henry had left Advent by this point. So had Andy Kotsatos (to form Boston Acoustics).

The 410M was a direct replacement for the Model 400 radio. The 420S was a stereo version of the same "upright" center console, separate speaker design. The 450S was a "mini receiver" that came with two speakers, like the 420S. It had more power than the other two radios.

This line of radios was developed to replace the Model 400 radio. The M400 cost Advent well over $90 to produce, even though they sold it to retailers for under $110.

The electronic design of the Model 410M and 420S were done by Shin Shirasuna, an excellent Japanese company that also produced the bulk of Harmon Kardon's receivers of the late 1970's. The cosmetic design was done by the same industrial designer as all previous Advent products (John Price). The 450S electronic design was done by Dong Won electronics in Korea, again to my specification and John Price's design.

The Model 410M and 420S are excellent products, both mechanically and electronically. I consider them better overall products than the Model 400. My only reservation is with the speaker driver's longevity. I don't remember if it had a foam urethane surround (which would be useless by now) or a cloth/rubber surround. I have seen versions of these two products in the last five years or so and they still perform well, with the exception of noisy source selection switches.

The 450S was a different story. I left the company for Boston Acoustics before this product went into production. The only examples I have seen of this product had poor mechanical fit and finish.

The M402 speaker was simply the speaker system made for the 410m and 420S, packaged separately. It was acoustically similar to the Model 400 speaker, but a little more efficient and without quite as much bass extension.

Advent filed for Chapter 11 about a year after my departure. Many products were made using this brand name by entirely different companys. But the Response series was designed and marketed by the the original Advent Corp of Cambridge MA. The biggest difference was the place of manufacture. In the case of the M410M and the M420S, I would have to say the manufacturing facility was better than Advent could come up with in North America.

I have since worked at Boston Acoustics and Cambridge SoundWorks and have produced many products. Unfortunately, this makes it difficult to remember specifics about products I made thirty years ago. But I'll be happy to answer what questions I can about Advent products in the seventies.

Best regards,
Fred Pinkerton

Here are pics of the 410M (with a Rat Shack Minimus 7 speaker) and a 450S. The 420S looked almost exactly like the 410M. Here's info http://www.radiomuseum.org/r/adventcorp_response_420s.html



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Back to SPEAKERS for just a moment:

The 400 used a full-range 5.25" driver that has a rubber (IIRC) surround that does not need to be replaced.

SOME 400s used the ubiquitous Philips "whizzer cone" driver (the one used as a mid in Dahlquist and Rectilinear speakers).

The 402 speaker was a 2-way design and the woofers always need to be refoamed.

I have not done a side-by-side comparison but Mr. Pinkerton says the full-range in the 400 had better bass extension than the 402.


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What a shame to see the faceplate in such a bad condition :(

Did those Advent 400's ever made it over to Europe?

Yeah. I'm looking into having new ones made so stay tuned....

Don't know if they made it to Europe. They're 115v/60Hz of course. Really nice radios.


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On 7/24/2015 at 2:18 AM, dxho said:

Kent, for the knobs, there's a product called Wipe New

that might restore their look. It's basically a thin acrylic

clear coat for restoring the finish on bumpers and cladding.

I've used a similar product to restore fogged plastic


Here's a Consumer Reports test (on a car :-))

Thanks Ken! I tried it and it worked well. I also used it on some "black ash wood grain vinyl" on a subwoofer and it worked well. Also good for the black plastic knobs on my KLH Model Eight restorations.

Hope to have some new reproduction face plates made and will post pictures when this one is finished.


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Kent, you are quite welcome! I knew if it could light up faded auto side molding

(which nothing else but paint will do), I suspected it could help you. I hadn't

considered using it on vinyl siding, but it apparently shines up quite a few

different kinds of surfaces.

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After doing a little research, I see the Model 400 dates back to 1974 or so. There are a couple early IC's in the FM section, otherwise all discrete compnents.

By the early 80's a lot had changed. The 420 in comparison use several modern Sanyo IC's in the FM section, and a couple of Sanyo LA4250's for the power amp. These are rated around 6 or 7 watts. There are some nice photos here, http://www.radiomuseum.org/r/adventcorp_response_420s.html.

I pulled the case off of mine, it's packed like a sausage inside the casing. I'll post some pics in another thread later so you can compare the two models. The state of the art moved fast back then.

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  • 4 weeks later...

New faceplate!

It's not an exact duplicate--the original was dark gray and light gray on an aluminum background. I had some new ones made of laser-etched aluminum so they're just two-tone: Black and silver-gray. I like the color with the black cabinet and it looks much better than the chipped original.

I should order a minimum of 10 from the engraver so if anyone wants one let me know and we'll work out a fair price.

Knobs were treated with "Wipe New" (see Post #8. Thanks Ken!)

btw--for you Advent fans--I just bought an Advent 350 receiver. I had never heard of it. It's sort of a big brother to the 300. Unfortunately the one I bought has a bent faceplate and a damaged tuner cap. Drat!





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  • 10 months later...

An update on the 400, in case anyone plans to re-cap theirs:

There are 15 electrolytics (including the NPE in the speaker). They are gray jacketed and some are marked Unicon. They're 40 years old, so this time I replaced the ones on the amp board and the tuner board. Here is a list of the caps, along with the measurements of the ones I just pulled. When I replaced, I went up one step in voltage ratings (replaced 16v with 25v, 25v with 35v etc) and used 105 degree caps if I had them on hand. Replaced the speaker cap with a PIO. Those guys sure liked hot melt glue!

  • 1000uF 35v radial (measured 1081)
  • 220uF 35v radial (measured 290)
  • (3) 100uF 25v radial (measured 133, 135 & 139)
  • 47uF 16v radial (measured 66.4)
  • 10uF 25v radial (measured 13.13)
  • 10uF 16v (measured 14.46)
  • (4) 10uF 16v axial (measured from 14 to 18)
  • 2.2uF radial (measured 3.56)
  • 1uF 50v radial (measured 1.47)
  • 1uF 50v axial NPE speaker cap (tossed before measuring)


07 21 16_0740.JPG

07 23 16_0760.JPG

07 24 16_0766.JPG

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  • 3 months later...

I have an Advent 400 Table Radio. I'd like to give it to someone who is interested in fixing and restoring it. Problem is I now live in Israel so you'd need to pay shipping. I bought it in the early '70s, used it for decades (for a while as my only music source), and LOVED it so I hate to throw it out. At this point it's not really working anymore, extremely low volume and the knobs are not so accurate.  Please let me know if you're interested.

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  • 2 months later...
On 10/31/2016 at 6:20 AM, Shalom said:

I have an Advent 400 Table Radio. I'd like to give it to someone who is interested in fixing and restoring it. Problem is I now live in Israel so you'd need to pay shipping. I bought it in the early '70s, used it for decades (for a while as my only music source), and LOVED it so I hate to throw it out. At this point it's not really working anymore, extremely low volume and the knobs are not so accurate.  Please let me know if you're interested.

I'm interested but I don't think the shipping would be worth it.  Might you be making

any trips to the US in the next year or so?

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  • 1 year later...

I decided we needed to change up to a better radio in the kitchen and I had the Advent 400 powered receiver but still needed to find the matching speaker. Now the unit I have has been in storage almost 20 years and the white plastic casing has yellowed like an early Apple computer - pretty nasty. I have read that some people have had luck with re-finishing so I went at it slowly with sandpaper - 600, 1000 and then 1600 grit wet sanding and then two steps of polishing with my automotive polisher, clamped in the vise so I could manage the delicate plastic. It came out nice but if you look closely at the pic of the back, you can see the yellowed colour of the un-restored back panel. I found a speaker nearby (yay!) on line and had it sent up and, since this was also yellowed, I proceeded with the same sanding and polishing process to get a matching look. It looks great and sounds even better. Thanks to this page I also feel confident that there is still communication and support for older electronics, including some good info on changing components, like capacitors if the volume falls off or if a speaker needs new suspension. Analogue is always fix-able.



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  • 7 months later...


I am new to this group.  I have a low volume and buzzy Advent 400.  I started out with the table radio KLH model 21.  Great sound for such a small radio.

I love the look of the 400 and I want to get it up to spec.  I've whitened the case using beauty salon hydrogen peroxide and sunlight.  Worked like a charm.  I can replace the caps, no problem.  My next task is to take on the speaker.  The 5.25" speaker in my cabinet is damaged.  Possibly water damage.  The elastic surround is deformed.  I've replaced the gaskets on a pair of old speakers before.  This one looks like the gasket is attached to the back side of the speaker and is thus very hard to replace.  Q:  is there a good quality but not too expensive replacement for this?  Is there one that will match the existing screw pattern?  I can cut a adapter plate on the laser cutter here at work if needed.  Is the speaker 4 or 8 ohm?  Any ideas of which direction to take this is most welcome.  

I will try to shoot a few photos of my project and get back to you all.

Many thanks,


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This seems to be all I have.

Model 400 radio parts

FM Tuner board
Q4    NPN, 2N4996
Q5    NPN, 2SC1345

Audio Amp and Power supply

Q101, 102, 103, 105, 106, NPN, TIS97 or 2SC1345
Q104, 107, PNP, 2N5087

Output transistors – Use an NPN/PNP complementary pair
Q108, NPN, 2SC1212 or 2SD669 or NTE 373/184 or BD237/437
Q109, PNP, 2SA743 or 2SB649 or NTE 374/185 or BD238/438

Power Supply
Q110    NPN, MPSU02, MPSU06

Electrolytic capacitors: Audio amp/PS board
4 x 10uF 16V AXIAL
1 x 10uF 16V RADIAL
1 x 10uF 25V   “
1 x 47uF 16V   “
2 x 100uF 25V  “
1 x 220uF 35V  “
1 x 1000uF 35V  “


L = ?
C = 1.0 uF
R = 18 ohms 10%


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