Heathkit AR-15 sound problems

[haskellbob]

Hi all. I have this Heathkit AR-15 here, and it has had this symptom: One channel takes a good fifteen seconds to come fully alive. It moves from zero sound through distorted sound to full-on sound. But then at intervals it gets distorted again.
I think it has finally decided to die, though, because it will not come fully up out of distortion-land anymore.
It has this massive output capacitor, a 4000 mfd giant cylinder. Well, it HAD one; the previous owner (I am a recent Ebay buyer of this much-treasured receiver. At least by me) replaced it with a nice new one, I think Nakamichi? I can look up the brand but I've just put it all back together and don't feel like taking it back apart just now. The point is that it's been replaced so I am inclined to disregard it as the culprit. But I'm in over my head; I'm just an amateur; I do not know how to determine which component (because I'm not sure it's a capacitor, except that this problem tends to be due to a dying capacitor, from what I've heard).
I know it's advisable to do a complete re-cap, or at least it is for some people. For the moment, though, I'd like to isolate the ONE CULPRIT, replace it, and get back to listening. Until the next one dies.
At any rate, I'm posting the schematic for the power amplifier section. I will run a test to make sure the problem is in the power amp (I'll take the pre-amp output and run it through another amplifier and see if it sounds OK).
But for the moment, could somebody take a look at the circuit and tell me, in their opinion, which capacitor (assuming it is a capacitor!) would be the one you'd look at first, and then second and maybe even third?

I'd really appreciate anybody further along in this hobby than me - I'm just a dilettante - throwing me a line to get back to the pleasure of listening without distortion.

Thanks every and anybody!

Bob H.

 

[haskellbob]

Hi Bob, this is Bob (???) with an update: Connected preamp output to alternate amp and sound is great. Of course, now the original amp, connected to smaller speakers, is working fine too. It knows it is under scrutiny, the little bugger, but it will have to give up the ghost eventually. I know that right channel has a weakness in it and I want to root it out once and for all!
I'm guessing that some combination of a) the amplifier's having had a chance to cool off, and b) the fact that it is connected to much more efficient speakers, is the reason the amp is working fine for the moment.
Willing to be shown any weaknesses of my own, in my reasoning, for example!

Thanks again,

Bob

 

[haskellbob]

Sure enough. Left the rig on and went to breakfast. When I got back, pure distortion on the right channel. The old boy just can't keep it up. (is there Viagra for old receivers?). But the missing channel, fed through another amp out of the preamp, sounds fine there. That pretty much proves the problem is in the power amp, I'd say. But logic can only get me so far; it's the lack of experience and application of electronic theory that stops me where others can easily go! I sure would welcome suggestions at this point!

 

[haskellbob]

Hi all. I have this Heathkit AR-15 here, and it has had this symptom: One channel takes a good fifteen seconds to come fully alive. It moves from zero sound through distorted sound to full-on sound. But then at intervals it gets distorted again.
I think it has finally decided to die, though, because it will not come fully up out of distortion-land anymore.
It has this massive output capacitor, a 4000 mfd giant cylinder. Well, it HAD one; the previous owner (I am a recent Ebay buyer of this much-treasured receiver. At least by me) replaced it with a nice new one, I think Nakamichi? I can look up the brand but I've just put it all back together and don't feel like taking it back apart just now. The point is that it's been replaced so I am inclined to disregard it as the culprit. But I'm in over my head; I'm just an amateur; I do not know how to determine which component (because I'm not sure it's a capacitor, except that this problem tends to be due to a dying capacitor, from what I've heard).
I know it's advisable to do a complete re-cap, or at least it is for some people. For the moment, though, I'd like to isolate the culprit, replace it, and get back to listening.
I'm posting the schematic for the power amplifier section. I ran a test to make sure the problem is in the power amp (took the pre-amp output and ran it through another amplifier. It sounds OK).
Could somebody take a look at the circuit and tell me, in their opinion, which capacitor (assuming it is a capacitor) would be the one you'd suspect of having died?
Or what else could the problem be?
I'd really appreciate anybody further along in this hobby than me - I'm just a dilettante.

I've circled the electrolytics in the circuit on the schematic.

Thanks for any help.

Bob H.

Attached Files:

 

[Ylli]

May very well be a capacitor, but do a voltage check first. Measure and compare with the schematic both when it is working well and when it is distorting. You may be getting a DC shift somewhere that is taking the amp out of its linear operating range.

 

[haskellbob]

I will do that. Where should I measure; what are the most important voltage points?
And: supposing it's not a capacitor and there's a DC shift - what would cause that?

 

[DJL33B]

Have you checked power supply voltage(s)? Sometimes you don't need to know the exact voltage, just compare voltage during distortion vs voltage without distortion. It sounds like you may have a power supply problem. Look for capacitors that appear to 'bulge', especially on the top.

 

[Ylli]

There are not that many active devices, would be a good idea to measure them all (E-B-C of all transistors). Why? All things drift/fail with age. Another thing you could try before replacing caps is to try using a freeze spray (or canned air upside down) to see if anything is thermally sensitive.

Just trying to spell out some basic trouble shooting steps before arbitrarily replacing parts.

 

[haskellbob]

I do think something is thermally sensitive. When the temperature goes up in the afternoon all I get is distortion. But right now it's cool outside (and inside) and it's working fine. What tend to be the most thermally sensitive components?

 

[Ylli]

Could really be any of the active devices - could even be one of those capacitors. If you use freeze spray, try to spray only one part at a time, and give it a couple seconds to respond before going on to the next.

 

[haskellbob]

Will do. Thanks.

 

[Harald Kapp]

Could really be any of the active devices

+ could be a cold solder joint. Solder contracting and expanding with temperature results in varying contact resistance which can lead to the same effects as observed.

 

[73's de Edd]

Sir haskellbob . . . . . (aka Bob Haskell)

" I cud'ent reeds yer skeematics MAIN Bee-plussamuss-es-es voltage"
So, this provided schematic shows 83VDC, therefore, a properly operating output section should halve that voltage at my RED mark up square reference.
Also, the output stages are being direct DC coupled, to the circuitry right of my BLUE TRIANGLE line.
Any DC error at the front will progressively be more noticeable at the rear.

HOWEVER . . . .
With the time constant that you mention and the fact that that output stages 4000 motherlyfarad cap had been changed , I would initially check to see if that seller "dewde" had a full knowledge and concept of polarity ?
If such is not the case, then initially take cold readings of the base voltages of Q203 -Q204 and Q205 and then power up and wait for an appreciable change in performance and then take a quick voltage test of those same points and compare .

ASIDE . . . .
If I remembers correctly, and certainly I believe that I do, those 4xxxx series transistors were being RCA's initial pre production numberings for a combined set intended for audio output stage use . This was a decided move up in performance from the previous use of germanium units in such applications .

HEATH AR15 AF OUTPUT STAGE SCHEMATIC SNIPPET . . . . .




73's de Edd . . . . .

I'm absolutely SUPERB at multitasking. I can waste time, be unproductive, and procrastinate . . . . . all at once.


.

 

[Harald Kapp]

I merged two threads of @haskellbob into one. Sorry for the inconvenience of double posts resulting thereof.