Heathkit AA-14 troubleshooting

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TC Chris
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Heathkit AA-14 troubleshooting

Post: # 24353Post TC Chris »

I'm trying to diagnose a funky channel in the amp. One channel sounds fine; the other only has any output when the volume is cranked up, and even then it is distorted. This is for a Heath AD-19 console, but the amp is the same as the AA-14. The output stage is complementary. I pulled both output transistors for each channel on a little RD transistor checker, and all 4 measure OK and virtually identical.

Our site won't allow pdf files but you can see the output stage at page 4 of this article:

https://www.w6ze.org/Heathkit/Heathkit_074_AA14.pdf

The problem is that I am measuring 38 VDC on the emitters on the bad channel. It's supposed to be 19 V, and it is in the good channel.

On the schematic in that article, the "A" figure in the upper right is the 38 V tap in the power supply. The only way I can see to have 38 V on the emitters is for electrolytic cap C27 to be short-circuited.

Does anybody have other ideas?

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Re: Heathkit AA-14 troubleshooting

Post: # 24359Post electra225 »

As far as I know, any forum powered by PhpBB software won't allow PDF files to be posted on the forum. You can post them in Downloads so there can at least be some point of reference if that will help.

I am not as familiar with solid state equipment as I should be, but, from what I've read, one should always start by suspecting electrolytic coupling caps first. If you have a signal tracer, you can use it like you would on tube equipment to see where you are losing the signal. Good luck. :D
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Re: Heathkit AA-14 troubleshooting

Post: # 24364Post TC Chris »

It's pretty clear that the problem is having 38 VDC where there should be 19 V. The working channel has all the voltages in their proper place.

So I did replace the electrolytic that, if it were shorted, would put about 38 V on the emitters. And that had no effect, so I put the original one back.

So now the only way there could be 38V on the emitters is if there were an internal short in Q13, the TA2577A silicon NPN output transistor. That tested fine out-of-circuit on my little Radio Shack transistor tester, but that device is doing its work with one or two AA batteries. Perhaps when 38V are applied to the collector, it's breaking down and putting 38 V at the emitter too. That would also explain the 38V appearing on the other transistor in the pair, a germanium 2N2148 PNP.

I will see if I can post to downloads. That is usually beyond my pay grade.

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Re: Heathkit AA-14 troubleshooting

Post: # 24365Post TC Chris »

OK, I just uploaded the Heath AA-14 info to the downloads section (Up the down staircase?) under "amps and tuners."

In the schematic, the square box "A" in the upper right goes to the 38 VDC tap on the power supply. That puts 38 VDC on the NPN collector.

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Re: Heathkit AA-14 troubleshooting

Post: # 24366Post Conelrad »

Since 19 is half of 38, I'm thinking it might be a problem with a bi-polar supply.

Complementary pairs can fail, causing the "neutral" not to hold the rails divided.

Lots of times this will occur in a driver stage, causing the output Xsister to run high current and add distortion.

If one has a dual-trace scope, it can be used to "see" distortion: Hook one scope input to the amp input. Hook the other to the amp output. Set the amp output scope range accordingly to get the two traces to coincide as close as possible to a single trace. Any derivation is distortion...
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Re: Heathkit AA-14 troubleshooting

Post: # 24370Post TC Chris »

It's a B+ power supply referenced to chassis ground. The other channel (and preamp, and tuner) are all just fine, so the problem is somehow localized in one channel's output stage.

The full schematic that I have is a low-res download so I have been mostly looking at the clear but partial one in the article. I'll have to do some studying to see how the 19 VDC is developed at the emitters and maybe that will temm me how it has become 38 V.

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Re: Heathkit AA-14 troubleshooting

Post: # 24387Post TC Chris »

I made contact with the guy who wrote the AA-14 article via his ham club and has has graciously agreed to advise on my problem. I sent him a reply detailing the issue and will wait to see what he suggests.

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Re: Heathkit AA-14 troubleshooting

Post: # 24404Post Motorola minion »

Blow is JPG from amplifier text. This is a good example of solid state evolving as silicon devices are replacing germanium. This makes for tricky resistance and voltage checks. My first inclination would be to check the resistors including R69/71 (out of circuit) and that diode D1. If D1 or R69/71 were open, voltage at base would increase, biasing it on and conducting the 38v right through.

After everything switched over to silicon transistors, direct coupling circuits replaced capacitors, the advantage of this was better frequency response but again, troubleshooting became more difficult.
SS comp symm.jpg
SS comp symm.jpg (127.75 KiB) Viewed 649 times
One of the interesting features of this "hybrid" circuit is that it does not require a phase splitter (often interstage transformer) to drive the outputs at 180 degrees phase shift. One input from the preamp stage seems to simplify the diagnosis of a transistor biased on.

Good luck with the Author, maybe we can all learn something so THANKS for addressing this occasionally encountered solid state problem.
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Re: Heathkit AA-14 troubleshooting

Post: # 24418Post TC Chris »

The guy who wrote that article did reply to my inquiry, and I have emailed back twice. The second was tonight after I went out and did some voltage checking. It is looking like the diode shown as D1 on his diagram (D101 &102 on the Heath schematic) or the driver transistor, Q11 on his, Q111/112 on Heath's. He has been very gracious about providing guidance.

I am lucky--all of the output transistors check OK out of circuit. They are hard to come by. And it is interest that the output pairs include one germanium and one silicon.

As I told him, this may be the on-ramp to diagnosing the funk channel on my old Heath AA-22. I was looking at my AA-22 file the other night and found the graphs for its performance at McIntosh clinics in 1968 and 1977. For you young folks, in those days, a guy named Dave O'Brien traveled around doing these amp clinics for Mac. He would test any make. He would measure distortion at rated power (or below, if necessary) across the audio spectrum and draw a graph. It was great advertising because in those days most equipment did not meet advertised specs, but McIntosh equipment always did (or he would repair it on the spot). My little Heath breezed through, performing within specs all the way both times. Yay for a rare company that wasn't just advertising puffery.

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Re: Heathkit AA-14 troubleshooting

Post: # 24641Post TC Chris »

With the help of my online correspondent who worked on the AA-14, I have been moving ahead on mine (which is actually an AD-19, the console version). He sent me the circuit board diagrams, both sides, which are a huge help because the chassis is large and awkward and I was tired of flipping it back and forth all the time. Now I can just leave it and can locate the points where the components are soldered for voltage and resistance checking.

Indications are that it is the driver transistor that's bad, and the good news is, they are available and cheap. (Of course, the really good news is that the outputs are intact). Tomorrow I will pull the transistor and check it out of circuit but meanwhile I have ordered 2. Just in case....

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Re: Heathkit AA-14 troubleshooting

Post: # 24642Post Conelrad »

I did mention the drivers could be at fault in many SS final problems.

DG
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Re: Heathkit AA-14 troubleshooting

Post: # 24993Post TC Chris »

I've been frustrated by the Heath's failure to cooperate so I've been avoiding it. My internet helper on the project has emailed a helpful set of questions about voltages and resistances so tonight I wandered out to the shop, did some more testing, and emailed them off to the expert. I have a hunch that one of the output transistors tests good on the tiny Radio Shack tester (low voltage) but fails in circuit in the full 38 VDC. If so, it's the germanium half of the pair that's defunct (boo!).

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