Heathkit AA-161 Rebuild

[streetwise]

I will be rebuilding an AA-161; still works but the bass is completely gone. I bought a kit on the bay to save time but immediately found the parts to be incomplete missing electrolytic caps and haphazardly thought out; for instance C13 is part of the PEC and that was offered in the kit without the rest of the caps and resistors necessary to replace the PEC. I plan on replacing Capacitors 1-5,8-10,16,18,19,32,33 and adding a thermistor to the fuse area. What other parts should be replaced or upgraded? I remember Cathode resistors were upgraded in basic kits i've used in the past.

Here is a list of parts on the schematic
Capacitors
C1 30 polarized
C2 20 polarized
C3 20 polarized
C4 20 polarized
C5 10 polarized
C6 .01
C7 .0035
C8 .1
C9 .1
C10 .022
C11 .02
C12 .002
C13 .02 PEC
C14 .002 PEC
C15 200UUF PEC
C16 .1
C17 56UUF
C18 .05 *on the original schematic for AA-161 & EA-3 this is also labeled C16 I modified the schematic because there was no C18
C19 .05
C20
C21 .047
C22 56UUF
C23
C24
C25
C26
C27
C28
C29
C30 33UUF
C31 14UUF
C32 50 polarized
C33 .1

R1 33k
R2 2.2MEG
R3 *47K check value
R4 1200
R5 22K
R6 470K
R7 1MEG
R8 2700
R9 100K
R10 100K
R11 22K
R12 1MEG Volume
R13 1200
R14 100K
R15 100K PEC
R16 1MEG Bass
R17 10K PEC
R18 100K PEC
R19 1MEG Treble
R20 10K
R21 100K
R22 1000
R23 680K
R24 220K
R25 36K
R26 36K
R27 6800
R28 470K
R29 470K
R30 210 7Watt
R31 27K
R32 100
R33 470K
R34 22K
R35 270K
R36 220K
R37 150K
R38 220K
R39 18K

Hopefully this modified schematic and parts list are useful for someone in the future every schematic I found online was split between two pages.

 

[gadget73]

I'd suggest diagnosis before throwing a ton of parts at it. Find out where it dies so you know you have the parts to fix it.

 

[streetwise]

I'd suggest diagnosis before throwing a ton of parts at it. Find out where it dies so you know you have the parts to fix it.

Not sure how to go about doing that the only thing ive noticed is there is no bass anymore all mid and treble. Electrolytics etc.. are original

 

[gadget73]

try jumping out the PEC just to see if its in that section. Looks like pin 1 to 3, across C15. That should bypass the tone controls. If you get more normal response, its probably in the PEC and rebuilding that will fix it. Failing that maybe feed it a 100hz signal and do some signal tracing.

 

[streetwise]

I will give that a try

 

[gadget73]

don't get me wrong it really ought to have all of the caps changed for reliability, but just to make sure you're fishing in the right pond here. I just hate throwing a bunch of parts at something that isn't basically working. If it comes out the other side not working its hard to be certain if a new problem was introduced, or the old problem is still there.

Another thing that can do it is if the cathode bypass cap fails low value or very high ESR. It could partially bypass at high frequencies which will give normal-ish gain there but low gain at the bottom end. It will end up sounding like the bass control is turned all the way down. Open bass pot may do this too.