Heathkit Blues with AA-21 Amp

[P90]

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

 

[drjim]

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!

 

[P90]

Thank you for replying.

LOL - no offense taken. I am a noob at this. I did go over the wiring, but I will go back and triple check. You're likely correct.

Someone on DIY Audio suggested that using a VTVM, (which is what Heathkit says to use - in bold print), can be significantly different than readings taken with a modern multi-meter like my little Fluke 101. Interestingly, the measurements with the Fluke start at or near what would be correct, but then gradually climb over about 30 seconds too much higher levels.

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![/QUOTE

 

[fromnowon]

I acquired a super-rare NIB un-assembled Heathkit AA-21 amplifier

Wow!

 

[P90]

Wow!

I have the matching tuner kit and walnut cabinets as well.

 

[drjim]

That's going to be quite a nice vintage combo! I know there's people who would excoriate you for building these "virgin" Heathkits, and even though you could flip them on ePay or similar sites, I say enjoy them!

Anyway....The meter you have should be fine for making these kinds of measurements, Could you post "Figure 4" from the manual? Until I see what test points they're using, and compare them to the schematic, I can only guess why the readings are that high.

 

[P90]

That's going to be quite a nice vintage combo! I know there's people who would excoriate you for building these "virgin" Heathkits, and even though you could flip them on ePay or similar sites, I say enjoy them!

Anyway....The meter you have should be fine for making these kinds of measurements, Could you post "Figure 4" from the manual? Until I see what test points they're using, and compare them to the schematic, I can only guess why the readings are that high.

I will find and post that image.

- I struggled with the same dilemma. I paid a hefty price for these because I wanted to re-live the excitement of building them. I thought about flipping them for a profit. Ultimately I realized that I would get far more enjoyment by building them than I would from whatever financial gain I might garner. One day I may no longer have the dexterity to enjoy building them, which was the whole point of purchasing these in the first place. Easy decision.

 

[redk9258]

Someone on DIY Audio suggested that using a VTVM, (which is what Heathkit says to use - in bold print), can be significantly different than readings taken with a modern multi-meter like my little Fluke 101.

I think the Vacuum Tube Volt Meter is high impedance. A digital multi meter should be high impedance unlike an old meter like a Simpson 260.

 

[P90]

I think the Vacuum Tube Volt Meter is high impedance. A digital multi meter should be high impedance unlike an old meter like a Simpson 260.

I read the same thing.

 

[P90]

That's going to be quite a nice vintage combo! I know there's people who would excoriate you for building these "virgin" Heathkits, and even though you could flip them on ePay or similar sites, I say enjoy them!

Anyway....The meter you have should be fine for making these kinds of measurements, Could you post "Figure 4" from the manual? Until I see what test points they're using, and compare them to the schematic, I can only guess why the readings are that high.

Revised images, including Fig. 4

]

 

[drjim]

I think the Vacuum Tube Volt Meter is high impedance. A digital multi meter should be high impedance unlike an old meter like a Simpson 260.

You are correct, Sir! It doesn't load the circuit like a 260.

The first "real" kit I built was a Knight-Kit VTVM, I was the envy of my friends!

 

[drjim]

Revised images, including Fig. 4

View attachment 2582102 View attachment 2582103 View attachment 2582105 ]

Are the transistors in the sockets? Are are the controls and switches in the correct position?That whole section is AC Coupled through the Inter-stage Transformer T1R, and is basically "floating" between the positive and negative power supply rails. It pretty much has no ground reference that I can see. Maybe somebody else can spot something I'm missing, but I don't see how they got those numbers referenced to ground.

And.....Oh, Lordy....DO NOT short the speaker outputs to ground! The output transistors would likely pop.

 

[P90]

Are the transistors in the sockets? That whole section is AC Coupled through the Inter-stage Transformer T1R, and is basically "floating" between the positive and negative power supply rails. It pretty much has no ground reference that I can see. Maybe somebody else can spot something I'm missing, but I don't see how they got those numbers referenced to ground.

And.....Oh, Lordy....DO NOT short the speaker outputs to ground! The output transistors would likely pop.

Yes, all of the T03 devices are in sockets.

 

[P90]

Are the transistors in the sockets? Are are the controls and switches in the correct position?That whole section is AC Coupled through the Inter-stage Transformer T1R, and is basically "floating" between the positive and negative power supply rails. It pretty much has no ground reference that I can see. Maybe somebody else can spot something I'm missing, but I don't see how they got those numbers referenced to ground.

And.....Oh, Lordy....DO NOT short the speaker outputs to ground! The output transistors would likely pop.

I can post the pages that describe the theory of operation from the manual if that would be helpful.

 

[P90]

 

[P90]

View attachment 2582127

Yes, all of the T03 devices are in sockets.

 

[fromnowon]

I'm sure you'll figure it out!
Interesting that all of the resistances measurements are off so much and all on the high side, if I'm following. I wonder what could cause that.

 

[chazix]

Someone on DIY Audio suggested that using a VTVM, (which is what Heathkit says to use - in bold print), can be significantly different than readings taken with a modern multi-meter like my little Fluke 101. Interestingly, the measurements with the Fluke start at or near what would be correct, but then gradually climb over about 30 seconds too much higher levels.

I think the diyAudio person may be on to something, but it isn't a question of impedance. (At least not in the usual sense of speaking about the impedance of a multimeter, which would be in the context of measuring voltages.)

But in some cases with solid-state circuits, the Fluke can behave very differently from an old-school multimeter with respect to measuring resistances, because the Fluke applies a very small voltage across the resistance - too small to cause a semiconductor to conduct. An old-school multimeter applies a voltage that is typically high enough to cause conduction - but it will depend on the polarity of the test leads. (If the Heathkit manual says nothing about test lead polarity for measuring resistances, that could be a sign that I'm all wet.)

From what I understand, all the resistance measurements so far have been with the Fluke, and all of them have been higher than expected. In that circumstance, the very first thing I would do is measure a simple fixed resistor, and see if the reading was correct. Maybe the Fluke is just broken.

The next thing I would to is try the Heathkit readings with an old-school mutimeter. For resistance measurements, I don't think it matters if it is a VTVM. A Simpson 260 would be a great choice! (But you might need to use the opposite test lead polarity with a purely-passive multimeter like the Simpson than you would use with an actual VTVM.)

 

[P90]

@chazix,
The manual does in fact say that if the results are unexpected, to try reversing the leads. The fluke is dead-on accurate when measuring isolated resistors. I actually tested each resistor prior to installing it. Only when performing the resistance checks do the readings start at a normal value and then gradually increase to a much higher value and stabilize within 15-30 seconds. The Fluke is powered by 2 aaa batteries, so not much there at all.

 

[P90]

A Simpson 260 would be a great choice! (But you might need to use the opposite test lead polarity with a purely-passive multimeter like the Simpson than you would use with an actual VTVM.)

Great advice. Just bought a 260.