April 30, 2005

Changes since July 4, 2002:

A new recipient of an inherited non-working 300 emailed me asking for a schematic, which of course in my case, is stuck to my unit. While I do have a service manual, I can't respond to the many requests I get for copies of it now that Sound Valves is apparently out of business. I do, however, copy my responses to such requests to my friend Chuck, who will copy his and sell it cheap with inexpensive postage. Most users take Chuck up on this offer, but it's not his business, so I don't post his name and/or email address.

This new recipient did, however, inform me of a website that DOES have schematics in PDF file format, as well as some limited repair information and decent photographs, as well as a unique trade-in service for working units in exchange for broken ones with a semi-suitable cash offset. So consider yourself informed.

What is, however, profoundly disturbing to me is that much of his repair information seems so close to mine that I have to believe it was directly pulled from this site. If this is not the case, he'd have to explain to me how he found, for example, MOYER ELECTRONICS by himself--independently--since they're basically local to SE PA, are not a common supplier, and have been mentioned on this site since roughly 1999.

I should point out that, as with my EICO ST-40/70 modpage, some so-called DIY websites have used information on this site that is fully copyrighted without my permission nor attribution, which is a clear violation of US Copyright law. It only bothers me because my SOLE purpose for writing and publishing this and all my other audio modpages in the first place was to draw traffic to my site where, should the reader be so inclined, he or she might learn about organ donation and how it not only saved my life but can literally save thousands of others, many of whom die each day. The need is extraordinary, and over the years I've dedicated my life to helping patients in need, and spoken to hundreds of transplant candidates, many of whom are sadly no longer with us because of this needless shortage. So this is, indeed, PERSONAL.

Therefore, in not attributing what they've stolen and presented it as their own, they do a disservice not just to me, but to all of us. I make NO money on my modpages--nothing. Not coincidentally it would seem, both offenders of which I am aware do, however, on their's. So you can decide who has their ethical compass properly calibrated.

As a result, from this point on, I must insist on the following:

ALL OWNERS OF DEAD ADVENT 300's SHOULD DIRECT THEIR REQUESTS FOR ASSISTANCE HERE:

http://www.davidreaton.com/Model_300_receiver_main_page.htm

He can have the traffic--no charge. At least you found it here. Thanks a lot.

Please be a tissue and organ donor and tell your family that you mean it.

July 4, 2002

Changes since June 3, 1999:

I made a few spelling corrections (one), added a new description of the amplifier bias adjustment, refined the frequency counter requirement and tuner alignment instructions, and adjusted some other small items of semi-interest. I also removed the www.winblows.com link that was disguised as a Notepad endorsement, removed my discrete email address, and adjusted the copyright statement.

June 3, 1999

Changes since June 2, 1999:

1. I dug out all the parts I pulled from the unit and the bag that the new stereo decoder chip came in so that I could better document what came out and what went in (see below).

June 2, 1999

Here's one of my favorite audio things--the (solid-state) Advent Model 300 FM stereo receiver.  Modest but elegant, and a high-quality performer at a reasonable price point. A few problems can creep into your relationship with your Advent 300, and I think I've experienced just about all of them. Expect to pay anywhere from $75 to $150 for one used, depending on the condition and history of the unit. It can make the basis of what is perhaps a perfect computer sound system, or it can provide continuous background music for an office, workshop, or store. However, it can higher aspirations as well--the design comes from the pen of Thomlinson Holman (currently of THX fame), and its preamp in general and its phono preamp in particular is of high enough quality to justify its use as a high quality front end to an external amplifier, as it has preamp outputs and power amp inputs on the rear panel. The tuner, while simple in appearance, aside from a little warmup drift is amazingly musical and pleasing, and holds its alignment well. Through use of a PLL stereo demultiplexer, it has decent separation and overall performance. At 15 watts per channel the 300 is no powerhouse, but for the uses described above, it's enough. Plus, it looks cool.

What's not so cool is that it has internal amplifier bias adjustments that can go out of adjustment over time and/or with the vibration of shipping. What's good about that is that the schematic is usually on the bottom of the unit, and not only that, but the details of the bias adjustment setting is silkscreened on the circuit board. What's again not so cool is that the piece is a victim of cost cutting of the worst kind--close-tolerance parts that will--repeat WILL--fail over time. I'm talking about 16vdc electrolytic caps on 15vdc regulated rails, and some 25vdc caps on 22vdc unregulated rails. Besides that, many of the transistors are hard to get, and if you can get them at all, you can't get the hand-selected ones they used in the amp which were in many cases originally selected for beta ratings for their specific purpose in the circuit (those red-painted guys in there). Also, one additional thing to remember--I've heard, and think I've seen that the circuit breaker in the Advent 300 can deteriorate over time, to the point where it is ineffective as a protection device in the event of a catastrophic failure. I advise--although haven't yet done it--replacing the circuit breaker with either another circuit breaker or a fuse holder and fuse of an appropriate value. Enough said. Ah, the joys of repairing solid-state equipment!

My Personal Advent 300 Story

I got mine in an auction I won up on eBay, from a creepy, dishonest seller from (sorry) Tennessee, who after failing to trap me into abhorrent shipping costs, just threw it in a box, never realizing or worse fully knowing tht the tuner had a dead channel and the stereo demultiplexer function was kaput. Not only that but the cover was resprayed extremely poorly (drips, runs, etc.), and the unit arrived loose in a box with some knobs off and little else going for it. The good news was the amp worked. So much for truth in auctioning ("works well").

After checking over everything, I decided that while the amp was OK, the tuner had serious problems, and it took me probably two to three weeks to finally straighten it out. One channel was dead, and the stereo light would never come on. At this point, I decided it was time to get smart, so I ordered a Service Manual from Sound Valves in Ohio. I could not fix this thing without it. The schematic on the bottom is a help, but given the nature of the opaque circuit boards used, you need not only the transistor pinouts and equivalencies, but the parts layout diagrams as well in order to find, much less replace any parts. Of this I am certain--don't buy anything you can't get documentation for (there may be some exceptions, but I can't think of any).

Repairing the Tuner

Here's the trick--what's wrong, and how do you fix it?  A dual-channel scope is a help, along with a signal generator capable of generating a 19kHz square wave, unless you have the requisite frequency counter which must be accurate at 19 kHz at a minimum. If your signal generator extends up to 10.7MHz, it will be even more useful, since you can peak the IF strip, as described in the service manual.

My unit's problems? A few things--a bad output capacitor (more on this later), a bad FET switch feeding the stereo decoder chip, and a bad CA-1310E stereo decoder chip itself.  Can you say surge? Who knows? Now the hard part--what, where? I called a parts house and got them to cross the decoder chip to an existing ECG Philips part, ECG 801(and send me an ECG catalog in the process--very useful!), but what they ended up shipping me was an RCA equivalent part (SK3160 IC FM Multiplex Stereo Decoder). No matter--it works great. This, as it turns out, is very common these days. For a supplier that has a website that you can contact that will do the same, see Moyer Electronics. They're in three locations in Eastern Pennsylvania, but are real cooperative and will help you out. After replacing these parts, and here's an interesting point I learned--according to the ECG book, the series FET switch feeding the decoder was equivalent to an MPF-102, which I grabbed up at Radio Shack. Also according to the ECG book, the Source and Drain leads are interchangeable, but guess what--not in this application! In fact, I had to reverse them from the lead orientation of the original part to get it to work. But I digress.

The only real adjustment to the tuner stereo section, once the parts are working, is to set the 19kHz oscillator in the chip. Here my approach is to hook one channel of the scope up to a 19.0 kHz signal generator generating a square wave, sync to it, and hook the other channel to the test point in the Advent 300 (see the book), and make them look the same. Works for me. I don't have a counter that reads down into the audio range, so I was stuck. Later I acquired a Sencore SG-165 AM-FM Stereo Analyzer, and with that I could make even finer adjustment for maximum separation. Early tuner boards, such as on the silver model, for example, do not have an output level control on the board, but mine does. Both have a separation control, but adjustment is rarely necessary. If you have a stereo generator, feel free to verify that it is optimized. Other than that, IF peaking and dial accuracy alignment are pretty straightforward, so if you're so inclined, follow the instructions in the book and you'll be fine.

Replace All of the Caps

OK. Here's the bottom line. After all this work, and believe me, working on a circuit board is not my idea of fun, although it can go pretty fast when you know what you're doing (solder wick and a solder-sucker are your friends)--my unit worked fine for maybe a year until one day the left channel dropped down about 10dB or more. I figured out it was in the preamp, as the tape outs and amp ins were fine, so what to do? That's when I decided that I'd check all the silicon junctions and voltages, and while doing so, somehow shorted the mounting tab of a Motorola MPS-U52 audio driver transistor that is actually used as part of a discrete -15vdc voltage regulator, and there I was back calling an ECG distibutor again (which is when I met up with Moyer Electronics). After getting that fixed--with another RCA SK part--I looked closer and made the decision that all of the electrolytic caps HAD TO GO. 16vdc caps on 15vdc regulated lines may work in theory, but it just bothers me. So up to Radio Shack I went and bought a crapload of 35vdc and a couple of 100vdc caps, mostly of original value, and shotgunned (replaced) them all. Problem solved. This is a MUST DO if you want to enjoy your Advent 300 for any significant length of time. Take my word for it.

Re-Bias the Amplifier Section

OK, time to set up the amp--warm it up and follow the instructions in the book, basically turning down the volume, disconnecting all inputs and speakers, and adjusting the two potentiometers near the output transistors so that the voltage drop across the two .15-ohm 1-watt resistors on each side of each channel pair is the same--.003vdc. Repeat for the other channel.  Then repeat both sides again (and again). Use a 3-1/2 digit DMM to be accurate. That's it. No offset adjustment or anything else. If you can't set it properly, make sure you disconnected all inputs and switched off the speakers. If you still have trouble, you have a sick output stage.

Sit Back and Let 'er Rip (first on a Variac):

Bring it up on a Variac, if you have one. Otherwise, pull the internal 22vdc line fuses, and check for smoke, observing all the other normal first-time turn-on precautions. Then plug in the fuses, bring up the amp and play some music. NOTE: Be sure that when you work on any amp that you have proper loads applied to the outputs at all times--use resistors unless you like to blow up speakers you like for no good reason.

Have fun and let me know how you make out.
 

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