Heathkit AR-15 Receiver project

[Arty30]

Hello all. I recently found another old receiver to try to get going again. This time it's a Heathkit AR-15 from the late 60s, solid-state and pretty beefy at 50W/channel. It was really dirty so I cleaned it and I just powered it up for the first time after making sure the monster 8000uF filter wasn't fubar. It powers on with no issues but I was stopped in my tracks when I measured DC across the speaker terminals and saw 15V. clearly something's no right with the power amp. however another issue I discovered was a multitude of small green film capacitors are shedding their outer coating. I would assume this is bad and I've attached a picture. I would probably start by doing a full capacitor job including those film caps, but any other insights would be welcome. Thanks.

Arthur.

[Arty30]

forgot to attach pic.

[BrokenYugo]

Gross, looks like the epoxy coating went bad, bulk replacement is probably advisable.

You probably know this if you've already dealt with equipment from this era but don't miss the little plastic canned electrolytic in picture upper right. Also check resistors for drift, especially in high heat positions, they tend to drift up.

[Chris56000]

Hi!

Any help?

https://drive.google.com/file/d/1CI6RWwD-n3n9P2NMnZA5PHrqgalbJS7b/view?usp=drivesdk

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.vintage-radio.info/download.php%3Fid%3D527&ved=2ahUKEwib9anq36CBAxVZRkEAHVQqB3gQFnoECBkQAQ&usg=AOvVaw29TsMoS3eTQ7luDpN2mHVs

Chris Williams

[Arty30]

Thanks for the links. I have found that schematic as well as a complete 260 page long scan of the assembly manual. I don't know if this was factory assembled or not but it seems like the quality of work is good. Now I guess I need to go through and order all the caps I need to replace, probably the most I've ever done.

[Arty30]

I'm compiling a list of caps to buy from mouser and I need some clarification on the capacitor lingo. There's "Mylar", "disc", "Tantalum", and "Silver Mica". Electrolytic of course and Tantalum is lumped in with the electrolytic. I'm wondering what the modern names for these are. The little green ones pictured above are the Mylar, however there are also colored striped ones that may also be called Mylar. I would assume I can replace them with film capacitors? The disc caps usually don't fail, but I'm not sure about silver mica or tantalum. I'll see if I can get a better look at some to clear it up but I'm wondering if anyone else has figured out which ones to leave alone and which to replace other than the blatantly falling apart ones of course.

[Arty30]

The other dilemma I haven't decided on is what voltage rating to choose. it's not listed for most of the electrolytic, and none of the Mylar caps. I can at least look later at the electrolytics and figure out voltages, but I still don't know about the Mylars. I find it hard to discern from the schematic alone what voltage rating would be appropriate.

[Chris56000]

Hi!

Most of the modern small green Mylar capacitors sold today are 100 V minimum, but you might have to get a 250 V type for some of the smaller values to fit the holes in the board more easily!

0.22μF,  0.47μF, 0.68μF, 1.0μF, 1.5μF, 2.2μF and above are far more easily made and cheaper in plastic–film than mylar nowadays, and where "mylar" was quoted in  the original Heathkit Parts Lists, a modern polyester or plastic–film type of suitable lead spacing or pitch to fit the PCB is perfectly adequate for replacement purposes!

Some of the largest values are only conveniently available in 63 V or 100 V rating, but either of these ratings will be adequate for the AR–15!

Capacitors coded with colour–bands are more usually likely to be of the polystyrene type, which do occasionally develop open–circuits due to oxidation between the foil electrodes and the leadout wires – moisture ingress can cause this in equipment stored out of use for decades – if you have any problems with stages that are fitted with these colour–coded capacitors I would fit C0G ("Low–K") ceramic disc replacements if you can't get hold of polystyrene or mica capacitors!

Chris Williams

[TimFox]

Mylar^TM is a DuPont trademark for polyester (PET).
What used to be called Mylar capacitors are now listed as polyester.

[BrokenYugo]

With the electrolytics try to bump every voltage rating up at least one step, because you often find stuff of this era running caps near the limit. In the main filter positions of the power supply try to get the ripple current rating as high as you can. I've seen smaller modern parts run warmer than marginal 50 year old originals, given in a stable state they're built to handle, but remember lifespan of an electrolytic goes up well beyond the specified hour rating quickly if you run it far away from the temp and ripple limits.

[WimWalther]

Silver mica caps don't typically fail, they're as or more reliable than ceramic disc types.

Tantalum parts, OTOH, are like little time-bombs. I'd replace all of them as a matter-of-course without a further thought.

[Arty30]

Thanks for all the info, I'll definitely keep it in mind. I ended up getting 50-63V film caps just as an assumption although I found an original I could actually read and it's 100V like someone mentioned. perhaps someone can tell me why they're so high if the tuner for example is only fed with 15V? The electrolytics I replaced with modern equivalents of voltage ratings, and I only replaced the ones labeled "Mylar" (green flaky ones in picture). after spending darn near $50 and six hours replacing north of 100 caps I got a result I can work with. Before cleaning the controls I had the left channel sounding great, the right was weak and very tinny. I cleaned the controls and left channel that was fine is now completely gone, however I found that pushing on the control preamp board in the front of the receiver brought the weak right channel back. I don't think cleaning the controls had anything to do with that but the dead channel leads me to believe bad/flaky connections still.

[Arty30]

The weak/tinny right channel was an easy fix, small crack in the circuit board originating from a screw hole. now just the left channel, which outputs static that gets louder with volume so at least the main amp is working, it's probably on that control board still, transistor perhaps?

[Arty30]

Ignore that last part about the left channel being out, I completely forgot about the individual level controls for all the inputs. I was cleaning them and left the left pot all the way down, so now its fine. might be what you call user error.