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 Post subject: heathkit aa-15 replacement parts questions
PostPosted: Jul Wed 25, 2018 2:25 am 
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Joined: Mar Mon 05, 2012 6:35 am
Posts: 382
Location: Chicagoland, 60194
Almost every old amp I've ever worked on had tubes, not solid state so please bear with me.
I'm working on 1969 Heathkit aa-15 integrated solid state amp. 75w/channel. The manual states it uses a "quasi-complementary output circuit". The original complementary driver pair for each channel is one each 40409 (Q204) and 40410 (Q205). Depending on which way the ac is flowing they in turn drive a pair of 40411 (Q206 and Q207) output transistors TO-3 mounted on a big rear panel heatsink. The 40409 and 40410 are unlike anything I've seen before and are permanently attached to a wrap around metal heatsink as shown in the picture. They look like TO-39 cases, but are not separable from the heatsink.

Q1: The right channel has had the 40411 output transistors previously replaced by NTE181 substitutions. There is also a burned beyond recognition resistor on the right channel driver board immediately next to the 40409/40410 pair. I haven't done any further testing so I don't know if this incident wiped out the 40409/40410 pair yet but the burnt resistor in on the emitter of 40409 (Q204). Do you think the burnt resistor might be a result of substituting the NTE output trans? If I buy original 40411 on ebay they are like 50 years old. Are they subject to degradation if NOS? I'm not sure how reliable this amp was from the beginning so maybe I'm reading too much into it.

Q2: The Heathkit manual makes no mention of matched transistors in either the driver stage or the output stage. Each is listed by individual part numbers in the manual so am I correct to assume Heathkit did NOT match any of these....or possibly matched all of them so they could sell them as-is with the same part number?

Q3: (((edit: Forget this question. Snap-in caps are readily available and a whole lot cheaper))) The b+ supply is a bridge rectifier filtered through a 1/2 beer can size 8000uf 90v electrolytic. These are expensive to replace, but this thing is 50 years old. Buying a used electrolytic seems like wasted money. The original was 3" dia x 3 1/8" long w/solder tabs. I could probably fit a little longer cap in there say up to 4". Shopping by dimensions and more importantly what is in stock at the normal suppliers, I found an 18000uf 160v ESR 15.2mOhm Digi-key P/N 565-4850-ND for $40. It would fit well and exceeds the original values so other than price I think it would work fine. I also found a 10000uf 100v ESR 0.009 ohm 2" dia x 80mm length at Newark P/N B41456B9109M000 for $23. The Newark cap is rated for 12000 hours while the Digi-Key cap is only rated at 2000 hours. Any problem using the cheaper Newark cap?....and why the dramatically different rated hours?

Sorry for long post.
Matt


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aa15_driverPCB.jpg
aa15_driverPCB.jpg [ 89.37 KiB | Viewed 653 times ]
aa15_quasi_complementary_pair.jpg
aa15_quasi_complementary_pair.jpg [ 57.43 KiB | Viewed 650 times ]
driver_pcb_components.jpg
driver_pcb_components.jpg [ 108.92 KiB | Viewed 649 times ]
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 Post subject: Re: heathkit aa-15 replacement parts questions
PostPosted: Jul Fri 27, 2018 2:10 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1187
Don't really have much to offer, but...

The AR-15 receiver probably contains the AA-15 amp, if you know what I mean. The only online paperwork for my AR-15, that I can find, is a schematic... so you probably already have better paperwork. But the AR-15 was / is pretty popular, so a search for it might bring up some relevant related info / tips for your AA-15. Also, there might be some online group that specializes in Heathkit audio gear. Best of luck.


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 Post subject: Re: heathkit aa-15 replacement parts questions
PostPosted: Jul Fri 27, 2018 4:51 am 
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Joined: Mar Mon 05, 2012 6:35 am
Posts: 382
Location: Chicagoland, 60194
Greg Dan wrote:
Don't really have much to offer, but...

The AR-15 receiver probably contains the AA-15 amp, if you know what I mean. The only online paperwork for my AR-15, that I can find, is a schematic... so you probably already have better paperwork. But the AR-15 was / is pretty popular, so a search for it might bring up some relevant related info / tips for your AA-15. Also, there might be some online group that specializes in Heathkit audio gear. Best of luck.


Thanks and yes the AR-15 receiver contains the same AA-15 amp. The AA-15 came out in '68 and I believe the AR-15 came out a year earlier. My manual is dated 12-19-69 so I assume there must have been either changes to the manual or possibly a circuit modification. I have virtually zero experience with solid state amps so I decided it was best for me to stick with the original 40409/40410 drivers and 40411 output transistors although I have since found MJE253/243 drivers probably would have worked fine, been more robust, and saved me $10 or so. And I could have saved another $10 using MJ802G for the output transistors. If I ever acquire another one I'll have learned a little hopefully. There is a guy on youtube with a video about what a pain it was to restore an AR-15 for a young customer. He shows it hooked up to a THD meter and his results aren't near what Heathkit published when new. Hopefully I can do better. He was doing it as a business and I'm doing it mostly for fun. I also have the matching AJ-15 FM tuner and it came with a nice set of Dynaco A25 speakers. Probably been together since new and maybe a one-owner. Cosmetically they are very nice with hardly a scratch on the wooden cabinets. They have the cool Black Magic front panels and should light up nicely when finished.

Matt


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 Post subject: Re: heathkit aa-15 replacement parts questions
PostPosted: Jul Mon 30, 2018 1:56 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4390
Location: Sunbury, Ohio 43074
I had an AR-15. Foolishly got rid of it when in my late teens. It blew several of these transistors, and of course the outputs and the resistors along with them, until Heathkit finally decided it was the output sockets. That finally fixed it, and the AR-15 blew out my windows for years to follow with my AR2AX speakers.

At any rate, the transistors attached to the heat sink can be substituted by a similar spec transistor with a separate heat sink... these were just welded to the sink at the factory and sold that way, eliminating one step in assembling units such as this. If you cannot find these exactly, just match up the specs with a replacement, and t hen find a heat sink that can dissapate the same amount of power. THAT may be the trick .... this package transistor might give you a challenge to find an aftermarket heat sink that will be sufficient. Hence the welded assembly you've got. I suppose you could always use a lesser heat sink and add a fan, but that might bring other issues........ thermal drift being one, not good in a push-pull arrangement

happy hunting

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 Post subject: Re: heathkit aa-15 replacement parts questions
PostPosted: Jul Tue 31, 2018 9:29 pm 
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Joined: Mar Mon 05, 2012 6:35 am
Posts: 382
Location: Chicagoland, 60194
Barry H Bennett wrote:
I had an AR-15. Foolishly got rid of it when in my late teens. It blew several of these transistors, and of course the outputs and the resistors along with them, until Heathkit finally decided it was the output sockets. That finally fixed it, and the AR-15 blew out my windows for years to follow with my AR2AX speakers.

At any rate, the transistors attached to the heat sink can be substituted by a similar spec transistor with a separate heat sink... these were just welded to the sink at the factory and sold that way, eliminating one step in assembling units such as this. If you cannot find these exactly, just match up the specs with a replacement, and t hen find a heat sink that can dissapate the same amount of power. THAT may be the trick .... this package transistor might give you a challenge to find an aftermarket heat sink that will be sufficient. Hence the welded assembly you've got. I suppose you could always use a lesser heat sink and add a fan, but that might bring other issues........ thermal drift being one, not good in a push-pull arrangement

happy hunting


Output sockets....hadn't considered that. Thanks for bringing it up. I'll replace them. I would suspect kit builders sometimes messed up the mica insulators and thermal paste. Actually my set appears to have been meticulously assembled. The manual has the guys name in it so I googled him and discovered he was/is a Bacteriologist Phd at U of Wis. Great assembly or not, after 50 years new mica and paste would certainly be in order. On that subject, I've been looking around and discovered Bergquist Sil-pads (same as NTE Thermo-pads). There is also a guy selling ceramic insulators. Probably isn't any advantage from mica, but since I've come this far and have already spent way more than I'll ever get back I ordered a few of each to play with.


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