[baldrick]

I've been trying to revive an Iris preamp that was non-functional. Apart from a blown fuse and a burned tantalum cap on the remote input board I didn't see anything else odd about it.

The current symptom is this:

Channel Right tracks properly with regards to volume.
Channel Left does not. If you nudge the volume pot even a very small amount past minimum the volume in the right channel jumps to 3/4 volume immediately.
Nudging it back just a tiny bit causes the left output to drop back to barely audible. So a very very small increase in the volume pot cause the volume to jump drastically, but only in the left channel.
I verified that the pot is tracking correctly out-of-circuit - linearly from 0 to 50.5K.
I also replaced the usual 4.7 and 2.2 uF Tantalum caps in the audio control circuit that often cause issues.

Can someone point me in the right direction?
I am having trouble deciphering the schematic - it's quite complex with regards to the optical photo resistive volume controls.

Oh, and to be sure it wasn't related to the remote input board I removed it from the preamp. Same issue.

[HAL]

First step with a preamp of that age would be to replace all the electrolytic/tantalum caps with new units.  Did that for a friends NAD1300 preamp and the cap values were all over the place compared to the circuit values.

The CdS cells in optical volume controls or the light bulbs sources can go bad with time if they are that style.  Newer ones are LED/CdS cells that last longer, but need different drive circuits.

From the description the light voltage drive circuit for that channel is not working properly.  I do not have a schematic, but could be the device controlling the voltage/current to the light source.  When I designed one, used a small power MOSFET with gate voltage drive to act like a current source for the LED.

Post a link to the schematic, and might be able to give some ideas for testing.

[baldrick]

Here's the schematic.
Apologies for the low resolution.  It's the best on the inter webs.

https://www.hifiengine.com/manual_library/hafler/iris_pre.shtml

You'll need to create an account to access it, though.

[baldrick]

Progress (and maybe fixed)

I had a closer look at the PCB on both sides around the optical controllers.
Looking at the circuit, the whole volume control is a pretty complex feedback-monitored high impedance analog computer.

I decided to clean up some residual solder flux and noticed that on the component side of the board that it may not have been properly (or completely) soldered in some places.

So I decided to reflow the solder and added a touch around the optical controllers, making sure that there was good connection to the PCB.

And problem goes away!

I believe it may have had something to do with the fact that this PCB is completely bare - no solder mask, no HASL treatment, nothing, so a cold-solder joint or poorly adhering solder joint that failed after years of use due to pcb oxidation is a likely culprit.

Anyway, I still plan to renew the electrolytic capacitors in that circuit, based on reports that many have found they have drifted in value over the years or have dried up.
But for now the preamp seems to be working.