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<\/a><\/p>\nWhen I got it, it ‘worked’.\u00a0 The indicator indicates, and sound played as expected.\u00a0 It had a bit of hum and noise – not unreasonable, but definitely room for improvement.\u00a0 At first, I thought I’d just replace any parts that were out of spec, but given how this is constructed, I decided a ground up re-build would be a better approach.<\/p>\n
Here’s the before pic – the component count isn’t so high that it’s difficult to trace out in it’s condition, but it’s not exactly easy on the eyes.<\/p>\n
Unbuilt Heathkits are getting fantastically expensive, but I figured if I did a complete tear-down, and ordered all new caps & resistors, I’d essentially get some resemblance of the new kit experience.\u00a0 So off I went ordering parts, and stripping the chassis.<\/p>\n Everything showed up as ordered, but I didn’t pay attention to the physical size of the resistors.\u00a0 Everything that showed up was 1\/2 W as ordered, but they’re suspiciously small – even though they’re probably ‘correct’, they look out of place. They look like 1\/8 W, they’re absolutely tiny.\u00a0 <\/span><\/span><\/p>\n The solid-core for the wire used throughout measures .025\u00a0 which puts it at 22 AWG. The OD w\/ insulation is .056.\u00a0 I have a bunch of 24 AWG solid core PVC jacketed hookup wire in various colors, and some cloth covered 20AWG from New Old Sounds.\u00a0 The PVC wire has a slightly smaller OD than the original stuff, whereas the cloth covered stuff is on the thick side; slightly thicker than the transformer leads.\u00a0 24 AWG is really fine for all of this, and I may use the cloth covered stuff for leads that pass through the deck.\u00a0 The highest current is probably the heaters.\u00a0 Each tube is 150 mA, and 24 AWG is good for 3.5 A, so we should be fine.<\/p>\n Most, if not all Heath gear of this era uses the same grey, solid-core wire.\u00a0 I’d like to impart some color coding, and a quick search unearthed this: Ground = Black I was able to strip out almost everything, with the exception of the transformers and a few of their connections.\u00a0 There’s really no slack on the transformer leads, so I didn’t want to risk it.<\/p>\n Here it is as disassembled as it was going to get – note I already started on re-stuffing the electrolytic capacitor.<\/p>\n Speaking of capacitors – this one was a bear to empty out.\u00a0 I think it tested OK, but I was trying to make it future proof.\u00a0 In my experience, really dried out, dead electrolytics seem to come apart rather easily.\u00a0 When I opened this one up, it was still a damp, packed, impervious mess.\u00a0 Picking, poking, & drilling did nothing, nor did soaking in water.\u00a0 Boiling did the trick – a poor mans double-boiler with a sauce can kept me from exposing my cookware to the nasty innards. Tube compliment: I generally followed the instructions, making a few modifications to component placement, particularly where smaller replacement components allowed for a neater layout.<\/p>\n Started with the twisted heater wiring<\/p><\/div>\n <\/p>\n Power & fuse wiring<\/p><\/div>\n The face-plate was installed after getting a polish.\u00a0 It’s not perfect, but it’s better.\u00a0 The screw-heads & jack hardware also got hit with a wire wheel – It’s easy, and I think makes a big difference.<\/p>\n Face-plate installation<\/p><\/div>\n B+ Wiring<\/p><\/div>\n Coupling capacitors & plate wiring<\/p><\/div>\n Shielded cable<\/p><\/div>\n Shielded cable – termination detail<\/p><\/div>\n Upper Deck<\/p><\/div>\n Wiring up the eye tube socket<\/p><\/div>\n Final Assembly<\/p><\/div>\n I’m pretty proud of how this came out.\u00a0 There was one guy razzing me on the Vintage Test Equipment group about how some of the runs were too long & unsupported, which would cause variations in capacitance when they vibrate, or there was a possibility of exposed leads shorting if the unit was dropped.\u00a0 I’m dismissing those as non-issues; I’m not going to sweat variations in picofarads on a device that tops out at a few kilohertz, and any drop hard enough to cause these components to come into contact would have much larger consequences anyway.\u00a0 It may be difficult to tell from these photos, but leads that look close to touching are either at the same point in the circuit, or are a few cm away from each other.\u00a0 These construction methods aren’t appropriate for high frequency circuits, but for audio, it’s just fine.<\/p>\n With the final assembly complete, all that’s left to do is power this up, slowly on a Variac.\u00a0 It came to life with little fanfare – no creaks, pops, crackles, or smoke.\u00a0 Once I saw the glow of the eye tube, but didn’t hear anything, I was convinced I’d screwed something up – I expected at least a little noise.\u00a0 I tried playing back some music through the 1\/4″ just for kicks, and was pleasantly surprised to hear clean audio reproduced.\u00a0 It’s sounds as good as can be expected for this tiny speaker, but it’s quiet and clean. I haven’t tested the wattmeter function – I may at some point, even though I’m confident it’ll never get used.\u00a0 The paint on the outer case isn’t in great shape, but I don’t have the means to strip it down, so that will be a future project – perhaps doing a whole batch in one go.<\/p>\n Success! Intro This is not my first signal tracer, but it’s the first one I’ve gone to this much trouble on.\u00a0 These have been getting pricey ever since musicians discovered they make cute little practice amps, so whenever I see one … Continue reading <\/a><\/p>\nParts<\/h2>\n
<\/a><\/p>\n<\/a>The one’s in question are:
\n47\u03a9 – on the output
\n10k\u03a9 – in the power supply
\n470\u03a9 – on a cathode
\nThere’s also the 1W 1k\u03a9 in the power supply (the one that was looking toasty in the original build). It’s bigger, but it still just doesn’t feel right.\u00a0 I’m going to go 2W on all these, somewhat for piece of mind, and somewhat for looks.
\n<\/span><\/span><\/p>\nWire<\/h1>\n
Color Coding<\/strong><\/h2>\n
\n<\/a><\/p>\n
\nFilaments = Brown
\nB+ = Red
\nControl Grids = Green
\nPlates = Blue
\nAC Line = Grey<\/p>\nTeardown<\/h2>\n
<\/a><\/p>\n<\/a><\/p>\n
\n<\/a> <\/a><\/p>\n\u00a0
<\/a><\/h2>\nSchematic<\/h2>\n
<\/a><\/p>\n
\n12C8 –
\n12SH7 –
\n12A6 –
\n1629 –<\/p>\nRebuild<\/h2>\n
<\/a><\/a><\/a><\/a><\/a><\/a><\/p>\n<\/a><\/a><\/a><\/a><\/a>Testing<\/h2>\n
<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"