• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Heathkit SA-2: Need help to track down distortion in one channel.

oldsmobuick

oldsmobuick

Active Member
I have a fully restored (all caps and resistors) Heathkit SA-2 that as of today has very bad distortion in one channel (channel A). The volume is also lower and the high freqs are dead-sounding. Overall channel A is bottoming out, sounding like a blown speaker. Both speakers are good and when I reverse the wiring to the two speakers, the distortion does not follow the speaker. I've tried Aux and Tape inputs and two sources (Sonos ZP-80 and my iPhone).

I haven't used the amp in a few weeks or so. In addition to the above troubleshooting, I switched the output tubes to a different set, and the other 3 pair of tubes i switched with their neighboring tube. Same result, bad distortion in channel A.

The restoration was a while ago and it sounded great after that, work was done by Dan-O and looks of high quality. I'm hoping the group can help- what else can I check and in what order?

Thanks!!
 
Last edited:
Register to hide this ad
You might try cleaning the tube sockets, esp. the driver tubes and output tubes.

You might try cleaning the selector switch, speaker switch and volume control as well.

Clean the rca inputs as well.

Won't cost you a lot.

I have an AA-151. Sweet amps.

David
 
Thanks David, I have done as you suggested. I hit everything with D5 and it's drying now. Will report back. Does D5 ever dry?
 
OK, I cleaned the tube pins and sockets as well as all pots with D5. Let dry a few hours and testing now it's still the same (maybe a hair better?). Volume in channel A is still low and bass bottoms out in that channel like a blown woofer sound.
 
Try swapping the tubes, left channel to right. Move one at a time, starting with the small signal tubes. Test after each swap. When you get to the output tubes, swap them as pairs. If the distortion changes from one channel to the other after a swap, replace that tube.

Good Luck

Shelly_D
 
Thanks Shelly, it seems unrelated to tubes. I switched everything side to side including replacing all 4 outputs.
 
I checked underneath and did not find any open resistors (didn't check for values because I think I need to lift a leg for that). Also did not see any bad solder joints.

One thing seemed odd to me though: all 6 bias pot lugs (3 per pot) short out to each other and to the chassis. All 6 lugs show zero resistance between any one lug and any other, and the chassis is also zero ohms to all the lugs. Is this expected?

I guess it wouldn't explain a problem in one channel if they are both like that, but it seemed worth mentioning.

Hoping for some troubleshooting next-steps from those here in the know. Thanks!
 
You have checked the obvious things. I think it is time to start hunting down an out spec component. I would take the next step of taking DC readings and compare the two channels pin by pin between the two channels. Compare plate to plate, grid to grid, screen to screen (on the outputs and the pentodes) and cathode to cathode. Start at the outputs and move forward through the amp. Post your readings where the 2 channels have a substantial difference, more then 10% of the value of the good channel. On the output tubes, the two tubes of each channel should show similar readings, withen a few percent, on the same pins, cathode to cathode, screen to screen, ... etc.

Shelly_D
 
OK, so I think I got it! After checking for leaky caps and drifting resistor values (none of wither found), I checked the volume pot and read 1M on channel B as it should be, but channel A had 1.3M resistance at full volume. I rewired the pots subbing on for the other and now the low volume and distortion moved from channel A to channel B! I thin kI may have cracked it. Now where the hell do I find a volume pot?!
 
I'm seeing a few dual 1 Meg pots on eBay, would I need linear or log? Taper? Center tap? Any other parameters?

Here's an example of one I see there:

It is 1Meg ohm audio log, with standard 1/4" shaft extenting roughly 3/8" from the front of the threads, or the shaft extends 5/8" from the back of the threads. Includes nut and washers. 1/2 watt, 350 volt, 500 volt peak. Price is $3.99 each.
 
Last edited:
Log taper.

the tap is for loudness. Not sure if yours has it or not. If you get one without the tap and the original had one, then the loudness will not function.

If you can find one thats a little better than $4, go for it. Alps pots are usually a little better than the cheapest thing you can find. They have a nice fancy Blue Velvet pot thats very nice, but they can get pricey, and are physically on the large side. No idea how much room you've got to work with.
 
OK, so I have the pot out and the carbon film is destroyed inside. I'm trying but can't find a replacement online. The markings on the pot are:

D-12-19 P&R 1 MEG AUD 1376022

Pics attached. As you can see, this has an inner and outer rotating shaft to control volume for channels A and B (kind of a combined volume and balance function).

Anyone have a good source for such an old pot?
 
Glad you tracked down the issue! Is it just the three terminals per gang in use or are the taps being used as well? Any dual gang audio 1 meg pot should do if you are only dealing with 3 wires per channel.
 
Wiktor Amps said:
Glad you tracked down the issue! Is it just the three terminals per gang in use or are the taps being used as well? Any dual gang audio 1 meg pot should do if you are only dealing with 3 wires per channel.
Click to expand...

Hey Vic! Thanks for your help the other day! It is just the three lugs per. I'll attach some pics here. The hard part seems to be finding a pot capable of the two knobs for independent channel volume.
-Nick
 

Attachments

  • IMG_0235.jpg
    IMG_0235.jpg
    56.2 KB · Views: 30
  • IMG_0234.jpg
    IMG_0234.jpg
    57.3 KB · Views: 33
  • IMG_0233.jpg
    IMG_0233.jpg
    50.3 KB · Views: 32
  • IMG_0230.jpg
    IMG_0230.jpg
    49.6 KB · Views: 30
Oh yeah, thats certainly a unique pot. According to the codes on the pot its a CTS from the 22nd week of 1960. You could try to find any CTS pot from this vintage with a 1 meg audio taper and then just swap that wafer into your dual gang pot assuming the wiper itself is still intact and ok. I've had varying levels of success doing wafer swaps between pots in the past. FWIW if you find it damn near impossible right now and just want to listen to the unit, throw in a standard dual gang 1 meg audio pot into it, you will have just the one knob to control volume for now, and keep an eye out on a replacement pot in the meantime. I would also try some of the Heathkit specialists on AK to see if someone has a spare.
 
Meh. Nothing unique about that pot. You just need to find a dual concentric 1M volume pot. A regular linked 1M volume pot will work fine as a substitute, having the controls separated was a way of adjusting the channels separately without a separate balance control. It was also a good way of dealing with badly-made pot components.
 
Why don't you just get an Alps, and forget the dual control and see how it works, without a balance control. In my PAS 3s I removed the balance control and went with an Alps. No problem with channel balance.

Be aware that some of the Alps sold now are not Alps, but knock offs. AFAIK the real Alps are not stepped.

The wiring of the terminals may not be the same as the old control, so you will have to determine which wire goes where.

David
 
Thanks guys, I want to keep the amp stock so am looking for the dual concentric solution, I just can't find one. I may look into Mark Oppat but am looking for an economical route.
 
You must log in or register to reply here.
Back
Top Bottom