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Heathkit Blues with AA-21 Amp

drjim said:
I downloaded the file and unzipped it. I'm guessing the bold type are your readings. Yep, they're way off. Good call on NOT plugging it in.

Without knowing your technical level, and not wanting to insult you, all I can suggest at this point is to go through it again and carefully check that the parts and wiring are correct. Heathkit manuals are superb, but I've seen people get things one lug off, or one wire in the wrong place.

Having built and serviced well over 100 Heathkits, I've done it myself!
Click to expand...

Hey Dr Jim!

I brought it up with a variac and dim bulb tester.

The Good: Nothing exploded or even got hot. The dim bulb remains nicely dim.
The Bad: No test signal reaches the outputs from any input, and front panel controls have no effect.
The Ugly: When I bring it up on the variac, as the voltage is increased or decreased, a sound like not unlike revving a motorcycle engine is heard from both channels. Not a Harley. More like a Honda. It is a constant sound, which increases in both frequency (not by much) and amplitude as the voltage is increased. The panel lights also get into the groove, getting dim then bright in time with the pulses.

Only brought it up to 120v once, and for a very short time.

I do have an old Tektronix 2213 that typically works on Fridays if I tap on it.
Also have a Multi-Function Tester - TC1 for testing transistors and diodes.

Thank for any help you can offer.

- Gene
 
P90 said:
Hey Dr Jim!

I brought it up with a variac and dim bulb tester.

The Good: Nothing exploded or even got hot. The dim bulb remains nicely dim.
The Bad: No test signal reaches the outputs from any input, and front panel controls have no effect.
The Ugly: When I bring it up on the variac, as the voltage is increased or decreased, a sound like not unlike revving a motorcycle engine is heard from both channels. Not a Harley. More like a Honda. It is a constant sound, which increases in both frequency (not by much) and amplitude as the voltage is increased. The panel lights also get into the groove, getting dim then bright in time with the pulses.

Only brought it up to 120v once, and for a very short time.

I do have an old Tektronix 2213 that typically works on Fridays if I tap on it.
Also have a Multi-Function Tester - TC1 for testing transistors and diodes.

Thank for any help you can offer.

- Gene
Click to expand...

Sounds like it might be "motorboating". All I can say is now that it doesn't blow the fuses, bring it up and start measuring the DC voltages as listed in the manual. If BOTH channels exhibit the same behavior, it's something common to both, like a power supply or grounding issue.

Start with the simple things, like voltage and resistance checks (again), and don't worry about the fancy test gear. 90% of the problems I've found can be fixed with your DMM.
 
drjim said:
Sounds like it might be "motorboating". All I can say is now that it doesn't blow the fuses, bring it up and start measuring the DC voltages as listed in the manual. If BOTH channels exhibit the same behavior, it's something common to both, like a power supply or grounding issue.

Start with the simple things, like voltage and resistance checks (again), and don't worry about the fancy test gear. 90% of the problems I've found can be fixed with your DMM.
Click to expand...

Thanks. I'm thinking the same thing.
It has 5 circuit breakers that look like Christmas Tree lights. No fuses to blow, but I am not sure how those things work. Do they operate like a relay, and reset automatically after a period of time?
 
P90 said:
Thanks. I'm thinking the same thing.
It has 5 circuit breakers that look like Christmas Tree lights. No fuses to blow, but I am not sure how those things work. Do they operate like a relay, and reset automatically after a period of time?
Click to expand...

Yes. They open under heavy load, and reset after they cool. Should read close to zero Ohms when reset.
 
I don't think it is, but why would you want to, unless troubleshooting a problem?
 
I agree no damage will be done. But AC (signal) behavior of the driver circuitry will definitely be different, since a feedback loop will be broken. It looks like DC operating points will be different as well - I'm not sure what to expect.
 
drjim said:
I don't think it is, but why would you want to, unless troubleshooting a problem?
Click to expand...

That's exactly why, actually.

My thinking is to remove the outputs in each channel separately in order to break a feedback loop and isolate the issue with motorboating. My guess is that it's either a leaky device on one channel, or it's something that is occurring in the power supply circuit itself.

I am the first to admit that I am a novice, so this is at worst case going to be a learning experience. Best case is that this does help isolate the problem.
 
@drjim,
I haven't removed the output devices at this point. I decided to use my DMM first as you suggested.
(It may be helpful refer to the schematic I posted on the first page along with the drawing in the subsequent image for context here)
The voltage across the secondary of the power transformer is exactly as it should be (24V AC) between the middle tap and each end. Those connect to the 4 diodes (which are original). At the at the next point of measurement (on the far side of each of the circuit breakers), I should be seeing 35v DC, but I am only seeing 25v DC. So what would cause that kind of unexpected drop?

Thanks!
 
Update:
Well, I connected it directly to the mains instead of the variac, and now the voltages measure very close to correct to that point in the circuit. Still motorboating though. So moving onward to do further investigation with the DMM.
 
@fromnowon
Still a work in progress. I solved the motorboating issue. There was a missing connection to ground. As for the transistor...

I decided to replace a pair in order to avoid having a bad match.

I purchased a set of "new old stock" Motorola equivalents on eBay that were supposed to have been unused. One was bad. I think eBay is a minefield of expensive germanium junk.

So then I purchased NTE121 matched pair from Amazon, and one of those was bad right out of the bag if you can believe that. Glad I checked before installing them!
I called NTE and asked them if these are currently being manufactured or if they are using new old stock. They assured me that these are still being made, and said that my experience wasn't typical, especially with a match pair. They were very helpful. I am probably going to get another set to see where that leads me.
 
rustnrot said:
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.
Click to expand...

Here's a photo of the underside, before I replaced the death caps with some new 2KV safety caps, upgraded a few resistors and cleaned up some sloppy soldering. I didn't shrink the tubing on the shielded cables yet. I'll probably do that once everything is up and running.

IMG_1907.jpg
 
drjim said:
I downloaded the file and unzipped it. I'm guessing the bold type are your readings. Yep, they're way off. Good call on NOT plugging it in.

Without knowing your technical level, and not wanting to insult you, all I can suggest at this point is to go through it again and carefully check that the parts and wiring are correct. Heathkit manuals are superb, but I've seen people get things one lug off, or one wire in the wrong place.

Having built and serviced well over 100 Heathkits, I've done it myself!
Click to expand...

Well, kudos to you.
Just when I was thinking that there's no way I would made a mistake, fate stood up and said "Hold my beer." I missed a connection to ground in the power supply. Motorboating issue solved.
 
I think eBay is a minefield of expensive germanium junk.

Boy, you got that right! Other things, too. I tend to be suspicious of "Exact Replacement" junk that has an unknown pedigree.

The only thing I have with germanium transistors in it is an old Heathkit GC-1A "Mohican" general coverage receiver. I had several of these at one time, and built one 'pristine' units from them. And I saved all the germanium parts from the ones I junked.
 
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