Heathkit Blues with AA-21 Amp

[chazix]

The manual does in fact say that if the results are unexpected, to try reversing the leads.

That sure seems like an indication that semiconductor junctions are involved in the measurements, so I think you will need to use an old-school multimeter.

The gradual increase in readings is very likely due to capacitance being charged up in the circuit under test. If my theory holds water, that capacitance would shunted by a semiconductor junction if the multimeter was causing that junction to conduct, and then the charge-up effect would not be noticeable.

So what VTVM did you get? The key thing, in this context, is that it needs to apply more than about 0.7V to its test leads when measuring resistance. You could check that with the Fluke! Just connect the Fluke (set to measure voltage) to the VTVM (set to measure resistance).

 

[P90]

@chazix,
I didn't get an actual VTVM, I went with the Simpson.

 

[chazix]

went with the Simpson

Sweet meter! Just note that, for measuring voltages, you will probably still want to lead with the Fluke because of the impedance business mentioned above.

And now, for measuring resistance, you will have the choice of including semiconductors (Simpson) or ignoring them (Fluke).

 

[P90]

@chazix Getting a VTVM too. Just need to find a good one on the cheap! Thanks.

 

[spicer]

Hi everyone.

I acquired a super-rare NIB un-assembled Heathkit AA-21 amplifier that I just finished assembling moments ago. Being that the kit is so old (even though the box was still sealed), I used new electrolytic capacitors and 1% metal film resistors instead of the originals. I didn't want to chance using the 50+ year old electrolytic caps and carbon resistors that were in the box.

Anyway, I haven't plugged it in yet because the pre-power-on resistance checks are...WRONG.
Resistance at all the test points is about double what it should be. Instructions say +-20% or don't plug it in.
I would really like to plug it in at some point, so...

I am uploading the schematic, along with the test points, target resistance, and what I measured at each point.
I'm new to troubleshooting circuitry, but have a great passion to learn and am on a mission to get this thing up and running.

I have a dim bulb tester, variac, Fluke 101, an inexpensive capacitor tester, an old Tektronix scope that typically works on Thursdays and Fridays, an audio function generator app, and an inexpensive TC-1 Multifunction tester for transistors and diodes.

I wanted to use very high res for the schematic, too big to upload, so here's the WeTransfer link to the full-resolution schematic and the resistance test. It expires in a week...

https://we.tl/t-XYet3OOnuO

Thank you in advance for any assistance you care to offer!

Gene

Wow, that's quite a find. I have an AA 22 in very good condition that I have repaired a couple times during the 28 years I've owned it, use it as an amplifier for my computer audio into a set of Infinity 5000 speakers and it sounds great. I do not have a schematic for the AA 22 but the circuitry looks very similar to the AA-21, wonder if it is same with minor variations. I think the 22 in the model designation refers to watts per channel. It has LEDs for the mode indicator which were new tech at the time of manufacture. Mine looks to be factory assembled.

 

[P90]

Wow, that's quite a find. I have an AA 22 in very good condition that I have repaired a couple times during the 28 years I've owned it, use it as an amplifier for my computer audio into a set of Infinity 5000 speakers and it sounds great. I do not have a schematic for the AA 22 but the circuitry looks very similar to the AA-21, wonder if it is same with minor variations. I think the 22 in the model designation refers to watts per channel. It has LEDs for the mode indicator which were new tech at the time of manufacture. Mine looks to be factory assembled.

I did some research when I first acquired these units, and the gentleman who designed the amp sadly passed just a month before I got it. Mine looks brand new, of course, but I can assure you, it doesn't look factory-assembled - LOL.

Does the AA 22 use germanium transistors and diodes like the 21? Many claim these sound better than silicon right up until they get old, grow whiskers and explode. That's the one thing I worry about on mine.

 

[P90]

That sure seems like an indication that semiconductor junctions are involved in the measurements, so I think you will need to use an old-school multimeter.

The gradual increase in readings is very likely due to capacitance being charged up in the circuit under test. If my theory holds water, that capacitance would shunted by a semiconductor junction if the multimeter was causing that junction to conduct, and then the charge-up effect would not be noticeable.

So what VTVM did you get? The key thing, in this context, is that it needs to apply more than about 0.7V to its test leads when measuring resistance. You could check that with the Fluke! Just connect the Fluke (set to measure voltage) to the VTVM (set to measure resistance).

Just got a Heathkit V5 VTVM. Basic unit but exactly what Dr. Heath prescribed. The more I research this the more I think the issue is with the Fluke, not the AA-21.

 

[P90]

Wow, that's quite a find. I have an AA 22 in very good condition that I have repaired a couple times during the 28 years I've owned it, use it as an amplifier for my computer audio into a set of Infinity 5000 speakers and it sounds great. I do not have a schematic for the AA 22 but the circuitry looks very similar to the AA-21, wonder if it is same with minor variations. I think the 22 in the model designation refers to watts per channel. It has LEDs for the mode indicator which were new tech at the time of manufacture. Mine looks to be factory assembled.

OMG! I just realized the AA-22 is the only other Heathkit I've ever built, back when I was a kid! Wish I still had it.
It's very different than the 21, though. 4 output devices vs 8, and a whole array of additional features on the AA-21.

 

[spicer]

I did some research when I first acquired these units, and the gentleman who designed the amp sadly passed just a month before I got it. Mine looks brand new, of course, but I can assure you, it doesn't look factory-assembled - LOL.

Does the AA 22 use germanium transistors and diodes like the 21? Many claim these sound better than silicon right up until they get old, grow whiskers and explode. That's the one thing I worry about on mine.

Appreciate the humor. I don't know if the transistors are germanium, my guess is no because of the LED indicators for mode, by that time everything was silicon as far as I know. My repairs were replacing failed power supply capacitors, replacing a poorly tracking channel to channel volume control and a couple of transistors whose placement in the circuit was not explored beyond diagnosing one as bad from voltage measurements on associated resistors, replaced in both channels using silicon replacements I had on hand, don't remember measuring base to emitter voltage as that would determine silicon or germanium, part numbers too but didn't check. It seems like I replaced a few capacitors as well but can't remember, it has been working fine for over ten years, quiet and good audio.

 

[spicer]

OMG! I just realized the AA-22 is the only other Heathkit I've ever built, back when I was a kid! Wish I still had it.
It's very different than the 21, though. 4 output devices vs 8, and a whole array of additional features on the AA-21.

More features on the AA-21! Wow. Was examining the AA-21 schematic and the controls are the same right down to the individual level controls for each channel of each input. One thing I don't see on the AA-21 schematic is a balance control, the AA-22 has one behind the drop down that covers the rest of the tweak controls. It also has the phase reversal switch. I'd like to compare schematics, my AA-22 has output devices mounted all over the chassis in various places, never counted them. It is transformer coupled to the output stage like the AA-21, outputs directly coupled to speaker terminals, a four ohm wire wound resistor in series for the four ohm speaker terminal, I use the eight ohm terminal for the Infinities.

 

[P90]

More features on the AA-21! Wow. Was examining the AA-21 schematic and the controls are the same right down to the individual level controls for each channel of each input. One thing I don't see on the AA-21 schematic is a balance control, the AA-22 has one behind the drop down that covers the rest of the tweak controls. It also has the phase reversal switch. I'd like to compare schematics, my AA-22 has output devices mounted all over the chassis in various places, never counted them. It is transformer coupled to the output stage like the AA-21, outputs directly coupled to speaker terminals, a four ohm wire wound resistor in series for the four ohm speaker terminal, I use the eight ohm terminal for the Infinities.

I stand corrected. The AA-22 isn't the one I had. It was the AA-14. Someone posted of photo of that one with the AA-22 designation. Sorry for the confusion. Indeed, it looks very similar to circuit-wise to the 21. The AA-21 provides independent volume for each channel on a dual-concentric pot, and the older-style front panel with overhead lighting from 4 bulbs.
Here's a photo of some of the controls that are hidden. https://www.springair.de/media/image/b0/5c/99/h51926-7-51926g.jpg

 

[spicer]

I stand corrected. The AA-22 isn't the one I had. It was the AA-14. Someone posted of photo of that one with the AA-22 designation. Sorry for the confusion. Indeed, it looks very similar to circuit-wise to the 21. The AA-21 provides independent volume for each channel on a dual-concentric pot, and the older-style front panel with overhead lighting from 4 bulbs.
Here's a photo of some of the controls that are hidden. https://www.springair.de/media/image/b0/5c/99/h51926-7-51926g.jpg

I see. My AA-22 has all the controls, for each channel of each input, and the dual concentric volume control on the AA-21 would function as balance. The AA 21 might be geranium, it's an older amp and in my opinion a much nicer, classic look. I like the lighting. It seems the AA-22 is what was at the time a more modern face put on a basic amplifier updated with silicon devices. I really like the period look of the AA-21, very appealing to me.

 

[P90]

I see. My AA-22 has all the controls and the dual concentric volume control on the AA-21 would function as balance. The AA 21 might be geranium, it's an older amp and in my opinion a much nicer, classic look. I like the lighting. It seems the AA-22 is what was at the time a more modern face put on a basic amplifier updated with silicon devices. I really like the period look of the AA-21, very appealing to me.

Oh, it's 100% germanium. But I know none of the devices are shorted. Whether they work or not is yet to be determined. If not, then my next project will be to put silicon in there with some trimmer pots to raise the bias voltage as needed without allowing the magic blue smoke to escape. But with the information you just gave me, I might even be able to use the circuit diagram from the 22 as a reference.

 

[drjim]

Heathkit was well-known for putting existing circuitry into updated cabinets, and calling it a "New" model, when it was a 100% cosmetic upgrade. They also went the other way, and made reduced cost models using existing circuitry from more expensive models.
Chuck Penson has written an excellent book about Heathkit Hi-Fi and audio products. From reading the book, at least 30% of their different model numbers used 99% identical circuitry. Same with their test equipment.

 

[P90]

Heathkit was well-known for putting existing circuitry into updated cabinets, and calling it a "New" model, when it was a 100% cosmetic upgrade. They also went the other way, and made reduced cost models using existing circuitry from more expensive models.
Chuck Penson has written an excellent book about Heathkit Hi-Fi and audio products. From reading the book, at least 30% of their different model numbers used 99% identical circuitry. Same with their test equipment.

His HiFi book looks very compelling. I can't find it for sale anywhere though.

 

[drjim]

His HiFi book looks very compelling. I can't find it for sale anywhere though.

Amazon or direct from him. He's on hold until July 1st for direct sales, but it shouldn't be that hard to find a copy.

 

[fromnowon]

I had that book in my Amazon cart for awhile but now I see that it's out of stock there. Hopefully it will be back in stock!

 

[spicer]

Oh, it's 100% germanium. But I know none of the devices are shorted. Whether they work or not is yet to be determined. If not, then my next project will be to put silicon in there with some trimmer pots to raise the bias voltage as needed without allowing the magic blue smoke to escape. But with the information you just gave me, I might even be able to use the circuit diagram from the 22 as a reference.

Keep us informed if you would and please post the AA-22 schematic should you find one. Comparing schematics would be interesting. I'd like to have a copy in case my AA-22 decides to be difficult. I must say the amp sounds good through the Infinity 5000s, twelve inch three ways from later seventies.

 

[rustnrot]

Heathkit AA-21 fan here. I just adore the look and recently acquired one in extremely good cosmetic condition.

Please show us some pics of yours!

From a safety standpoint, this unit, like many of the day uses a so-called "death cap", see C37 in the schematic. It's called a death cap because of a capacitor connected from the 120vac to the chassis ground. If the non-polarized plug is plugged in so that the hot prong is connected to C37 and then C37 fails shorts, 120vac will be present on the chassis. The best solution is to install a 3-wire power cord with the green conductor connected directly to the chassis. However, that can lead to ground loop issues. As a minimum, I'd install a polarized cord so the neutral can only be connected to C37. This is rather imperfect as then you are switching the neutral, instead of the hot, for power.

 

[fred soop]

If you are getting lower resistance readings that are later rising, the meter could be charging capacitors during the measurement.

The transistors are all germanium. I researched the whisker issue a while back and it appears to be limited to a few certain device types. Probably not an issue.