Looking for info that isn’t provided in this article (like a detailed dive into the design of the AU-517)? Check out our other article on the AU-517 here!
The enclosure opened with the driver boards and 2 of the power capacitors removed.
We are back from a 3 month hiatus and I’ve got a special article for you guys/gals, this will eventually transition into at least one dedicated page, maybe multiples. This is a project of decently high complexity; one that I have been working on for a good while. When the AU-517 was originally donated (Dec 2017) it did work, partially. When you turned the volume up to beyond 60-70%, the entire unit would shut off. This is probably from too much AC ripple being allowed in with old primary power capacitors.
I put the amp on my list of things to fix and slowly began to acquire the parts required to restore this historic amplifier, a true DC design. When it wasn’t yet working, the unit was still worth about $100-$200, as it showed no signs of damage. Now that I’ve fully restored the unit, it should be worth $500-$700! When you consider I only had $75 invested in parts, that is a nice flip. I’ve invested a little more into the amp, to make it really last, replacing the 24V relay, as well as all the fuse-resistors on the driver boards, and all new variable resistors (trim-pots). The last replacement to do is the primary power transistors, but I don’t think I need that yet. The biggest improvement most people notice is the main 12,000 uF power capacitors, especially for bass performance and overall transient performance.
Here we see the original amplifier in stock form. You may notice what looks like leakage around the base of the capacitors, but that is actually a type of glue/cement used to hold the larger caps steady. This glue should be cleaned up, since it can become conductive as it ages and it can corrode metal (i.e..
Part Selection Overview
Replacement Electrolytic Capacitors:
- BC Philips 12,000 uF 63V
- Nichicon Fine Gold
- Nichicon FW
- Panasonic FM
- Rubycon RX30
- Nichicon Muse ES (bipolar)
- Nichicon Muse
Replacement Film Capacitors:
- Siemens Film Caps
- Seacor Film Caps
- EMZ Film Caps
- Vishay Film Caps
- Arcotronics Film Caps
- Panasonic Stacked PET Film Caps
- Dale IHD-1 1.5 uH 15% (High Current, High Frequency Inductor (for DC filter curve of signal output) (x2)
These 2W-5W resistors are found on the left and right channel driver boards as well as the Power Supply & Protector Circuit. They are notorious for causing noise issues as they age.
Sources for AU-517 Parts:
The majority of the smaller electrolytic capacitors used are from a kit by a store called “Audio High End“. You can find them either on EBay or through their website. My experience with them is mixed, perhaps just some bad luck (3 times though?). They do provide all the capacitors needed for restoration in the basic kit (except the four 12,000uF 63V power caps). My first issue was the fact that when my kit arrived, it was missing about 8 capacitors from the list I was provided.
Another wrinkle in the story with “Audio High End Store”. I told them I couldn’t find 10uF BP (bi-polar) capacitors used on the AU-517, then I found out they sent the 10uF Muse BP capacitors by mistake. So, that is why my tone control is acting so strangely. For now I’ll just keep the tone control switched on defeat until the replacement capacitors arrive. This will be the 3rd time they are sending capacitors out to me, I’m trying to stay patient. It helps that the amplifier works 100% (aside from the tone control circuit).
I contacted them 5 or 6 times to get the missing capacitors sent out and it took at least 2 months for me to get them. I’m not sure if they made a mistake when making the kit or if a few fell out of the box during transit. Either way, they got me what I needed and they are usually quick to respond to email. For the sake of transparency, I was provided a 20% discount in exchange for talking about my experience with their kit. I should also point out the fact I used multiple capacitors I already had, in place of their all Nichicon lineup. I swapped in Panasonic FM and Rubycon RX30 capacitors on the Power Supply & Protect Board. The rest of the capacitors I used are directly from their kit.
Aside from Audio High End, I sourced the primary 12,000 uF 63V capacitors from a well established EBay seller. I chose BC Philips over the more commonly seen choices, such as Nippon-Chemicon, Nichicon KG (modern equivalent to OEM Nichicon CEW part) and Rubycon. I purchased these BC Philips instead, from seller “the_electronics_superstore_2014” on Ebay. There are those who think buying any components on EBay is a bad idea, but I have been doing it for over 10 years and I’ve yet to receive a bad component. Switching gears to talking about the actual audio quality, during my testing the headroom for the restored AU-517 is phenomenal! Well beyond what I expect to ever need unless I use it as a PA at some point. Bass output is more pronounced and free of distortion.
Along side the new models, we have the original Nichicon CEW 12,000uF capacitors and their Nippon film 1uf capacitors.
Another shot of the amp before restoration. I had already replaced x4 220uF capacitors with Panasonic’s FM model, before I decided to do a full recap, rest is OEM parts.
Here you have the Power Supply & on the left and the Equalizer circuit board on the right, with the dual mono-block drivers in between.
I pulled from my own stock of Siemens, Arcotronics, EMZ, Seacor, Panasonic, Nippon, and Vishay film capacitors. I recommend replacing the original green Mylar films, if you are going to bother to remove all the PCBs. Replacing the remaining capacitors is optional as most of it is not in the signal path, most of the ceramics are coupling or decoupling of power rails to things like transistors.
Original Tone Control board before restoration.
Tone control board restored
Good shot of the primary issue for a lot of users, the Power Supply & Protect board. It’s possible to manually clean and polish the relay (contacts) if yours isn’t closing fully and causing drop out or intermittent issues, such as balance misalignment.
If you look closely at the board above, there are 4 sets of 4 diodes (black cylinders with a stripe on one end), the top left set of diodes (purple stripe) has two much larger diodes (white stripe) in the center of the set. This is because when I was first testing the amp on my trusty 2000VA Variac, these diodes would start smoking around 35V (AC). For those who can’t afford a dedicated variac, here is a guide to show you how to use a light bulb as a budget variac.
So I pulled out the PCB and checked the diodes, sure enough the two middle ones had shorted out and if I had thrown full power to the amp, it probably would have blown these diodes in pieces considering they smoked at about 1/4 of the full voltage. I didn’t mean to use such large replacements, but it was the only kind I had of the right type, that were at or above spec. Never de-rate a diode replacement. Many circuits require you to use the same type of diodes for proper function. For more information on AU-517/717 diodes problems and their modern replacement, see this great thread on AudioKarma.
Original Driver Circuit parts (Mono-Block Amplifier, 1 of 2)
One of the two restored/re-capped main driver circuits (mono-block).
Upgrade Part 2: Resistors and Inductors (Original on Right, New on Left
I did the upgrades in 2 stages so I could test the difference between the 1st stage (re-cap) and the 2nd (resistor/inductor/relay restoration). This way, I will be able to determine what upgrade changed what sound properties. If you upgrade everything at once, you can’t say for sure what upgrade did what. I also have five 1.5 uH Dale inductors.
Tone control board before restoration
Tone Control Board after restore: Siemens film, Nichicon Fine Gold and Vishay film capacitors.
Restored Tone Control Board from another angle
This project wasn’t without hiccups; one of my four 12,000 uF 63V BC Philips capacitors actually blew and vented! This discouraged me greatly the night it happened, but after just a few minutes researching during the following day I found the problem. I hooked up the metal rail (looks like a grounding rail at first glance) to the negative terminals of the 4 capacitors and then put the twisted pairs of colored wires to the positive connection on each capacitor. Thank God I bring up new equipment on my 2000VA variac, as this saved the other 3 capacitors. Had I thrown it into an outlet immediately and hit it with 120V, it would have most likely blown all 4 of them. By chance I had one spare BC 12,000 uF capacitor on hand. After correcting the wiring to what is shown in the diagram below, the amp worked immediately. I believe I literally danced around the room when this happened.
AU-517/717 Wiring Help:
Although the diagram below isn’t pretty, it gives all the information you need to hook up your power caps correctly (for the AU-517) and not blow them up like I did one of mine. Blowing up components is sometimes a part of the journey of learning in ECE, just don’t electrocute yourself. Always, always, always use fuses between the hot line and the positive power input on the device, especially on circuits that are DIY (fully or partially). On the note of safety, I can’t think of a smarter investment than a variac (variable AC transformer) for those working with either vintage electronics or DIY projects with brand new components. A variac allows you to bring up your device’s voltage slowly and safely. You can catch issues before they explode, like my smoking diodes. Variacs range in price from $20 used to $2000 and more for top of the line. Get only what you really need and spend that extra cash elsewhere. 2000VA is likely more than I’ll ever need.
Viewed from looking down on the bottom side of the amplifier (amp flipped upside down)
This amplifier is not easy to take apart or put back together if you want to actually remove the boards to give you easy access to the components for replacement. I managed my way by using a lot of pictures that I took of the amplifier before I ever took anything apart. I am sharing these photos in the hopes that it may help you if you are having wiring trouble.
When I thought I had killed my amplifier (when one of the main power capacitors started to vent), I was sure I just blew it (literally). Never give up, unless you are either exhausted/annoyed having to work on the thing so much, or if you are pouring more money into fixing it than it cost to buy a new one. Of course the AU-517 isn’t on the shelf at any Best Buy!
Bottom of Power Supply & Protect board
Power Supply & Protect Board’s center and some of the bottom wiring shown in place. Note: the red wire was already removed when I took the photo, but it’s near the hole it goes in.
Here you can see the bottom connections for the Power Supply & Protect board.
Found on the Equalizer Circuit Board One of 3 cables that is actually 3 shielded conductors in one (gray jacketed one).
The other two 3-in-1 cables and their respective locations (on EQ circuit board)
A few more from pictures I took before anything had been taken apart (aside from the 4 power caps).
A good shot of the Power Supply & Protect board’s bottom wiring
Overall wiring of any connections on the bottom. Don’t forget to right click image and view/open new tab to see full size.
Headphone jack, tone control and speaker select wiring.
Some of the driver board and some of the equalizer board connections, also shown earlier at a different angle.
I am extremely satisfied with the outcome of this initial restoration/recapping and the only things left to do are clean the volume pot, fix the tone controls and replace a few fixed and variable resistors, as well as the items below.
After receiving a well thought out and well written “Service and Restoration Info Guide” on the AU-517/717 from Ebay Seller “s9radio”, I decided to replace the relay all together. They are a common cause of noise and channel imbalance, especially as the contacts age. It’s possible to restore the original if you take the time to truly disassemble, clean and then polish each contact. The new relay looks a little different, but has the same performance specs. The model is the Omron MY2-02.
One other major shortcoming of the AU-517/717 is the old style spring loaded speaker terminals. Over the years the springs get old and have less clamping pressure, as well as 40 years of oxidation to the copper contacts. This is by no means a new idea and I am only following the lead of those who came before me and worked on this line of amplifiers. I went with Rhodium plated terminals, considered one of the best contact treatments/coatings for the mid-range crowd.
This is all for now, but if you have any specific questions, feel free to leave a comment below or contact me using the contact me tab. As always, thanks for reading Hallman Labs!