Heathkit AR-15 refurb

[tarazed]

I am refurbing my Heathkit AR-15 that my brother and I built 48 years ago, and am trying to clean up the hiss coming from the preamps. I think that the transistors should be replaced, and it uses:

phono preamp: MPS6520, 2N3859A
control preamp: 2N3393, MPS6517

Any advice on which transistors to substitute? Thanks!

 

[fred soop]

Are you sure the hiss is from the preamp? Mine is coming from the power amp. I can even short the power amp inputs and it's still there.

If it is the preamp, I'd consider replacing resistors with low noise metal film before the transistors.

 

[tarazed]

Are you sure the hiss is from the preamp? Mine is coming from the power amp. I can even short the power amp inputs and it's still there.

If it is the preamp, I'd consider replacing resistors with low noise metal film before the transistors.

Actually, I know that the majority of the hiss is coming from the left channel treble/bass circuitry, as it largely disappears when I bypass treble/bass by pulling out on the treble pot knob. Only resistors, caps and transistors involved and I read that hiss is probably transistor and rarely capacitor. Also, the phono preamp has a noticeable amount of hiss, so I figured to replace the transistors there as well. The idea that it might be coming from the resistors is something I hadn't considered!

 

[Osibisa]

Agree with Fred on starting with PS first, then moving along to the preamp.
If you are doing a full refurb I would recommend replacing the carbon resistors and caps. At 49 years old the caps are long past their lifespan.
I have a AA-15 with hiss as well but waiting until I get really really bored before tackling a full on rebuild with modern components. Good winter project though.

 

[fred soop]

So far, the only problems with mine in 48 years (other than the above mentioned hiss) were replacement of the output coupling capacitors and the power switch. Couldn't find a proper power switch or a slide switch where the handle could be transplanted. First fix was to flip the power switch upside down and use the other set of contacts but the center contacts were not too good. Finally swapped it with one of the other switches, either Tape Monitor or Mono (don't remember which). I did redesign/rebuild the multiplex section with an IC based circuit to eliminate all the phase and other front panel adjustments.

I don't consider this a good candidate for a full rebuild because of the now totally obsolete capacitor coupled output. There is a 5 volt turn on thump and electrolytics in the signal path cause a lot of distortion unless they are about 10 times larger than needed for the low frequency response (proven by Douglas Self). There is no center tap on the power transformer, so no way to split the power supply. The later AR-1500 is a much better choice for rebuild and modernization.

 

[tarazed]

As I wrote, I know the hiss is coming from the treble/bass section of the preamp, as it disappears when that circuitry is bypassed. So, back to my original question on the transistor substitutions - any ideas on that? Thanks.

 

[ivandezande]

The 2N3859A could be a KSC2383, the others web search isn't turning up much of anything so you might have to get creative and go through a lot of datasheets. Replace resistors. Fred made some good points as far as the potential of this amp however.

 

[fred soop]

KSA992 (PNP) and KSC1845 (NPN) are good modern small signal transistors for audio. You could even use 2N3904 (NPN) and 2N3906 (PNP).

 

[tarazed]

Thanks for the transistor suggestions - they will be helpful. Just this evening I had a good discovery while tapping resistors in the treble/bass circuit. I decided to also tap on the slide switch attached to the treble pot that disables the tone circuitry, and to my surprise I eliminated most of the hiss/hum by pushing sideways on the switch sliding button. So, I am going to replace the guts of that switch with the innards of an nos switch that I purchased on ebay. I have already fixed the main power switch by doing the same type of replacement. Next I am going to try to clean up the phono preamp hiss a bit - maybe I will get similarly lucky!