9090db low freq choke in right channel

[Bolan13]

Hi everyone,

I’m running into an issue with the right channel on my Sansui 9090 and am hoping to get some feedback from the Sansui experts here.

The unit was expertly restored a couple of year ago by an AK member here with bypassing the Dolby board. I recently noticed the right channel sounded noticeably thinner.

I did some basic trouble shooting with my multi meter:

Inputs & Sources: Tested via Aux and Tape Monitor inputs using 60Hz and 1,250 Hz test tones from a release on Qobuz, Issue persists identically across both inputs.

Speaker Output Banks & Headphones: Tested on Speaker A and B outputs, as well as through headphones via the front panel jack. The right-channel sounding thinner is present on headphones as well

Speakers & Cabinets: Swapped Left/Right speaker cabinets to rule out driver issues. The low-end loss stays strictly locked to the right channel output of the receiver.

Control Baseline: All testing was conducted with Loudness OFF, Low/High Filters OFF, and Tone controls set flat to establish a direct, un-boosted signal path.

Signal Testing & Voltage Measurements

1. Initial Output Check (At Speaker Terminals):

1,250 Hz Tone: Left = 0.80V AC | Right = 0.89V AC (Channels match closely in upper mids/treble)
60 Hz Tone: Left = 0.70V AC | Right = 0.24V AC (Right channel collapses significantly in the sub-bass)

2. Pre-Out Only (Unloaded / Jumpers Out at 50 Hz):

Left Pre-Out: 0.16V AC
Right Pre-Out: 0.17V AC

Result: Unloaded preamp output voltages match almost perfectly.

3. Main-In Direct Feed (Preamp Bypassed, External Source into Power Amp at 50 Hz):

Left Speaker Terminal: 0.25V AC
Right Speaker Terminal: 0.25V AC

Result: The power amplifier stage operates completely balanced on both channels down to 50 Hz.

4. Normal Operation (Pre-Out Connected to Main-In at 50 Hz):

Left Speaker Terminal: 0.52V AC
Right Speaker Terminal: 0.18V AC

Result: As soon as the preamp stage drives the power amp under normal load, the right-channel low frequency drops by over 60%.

Summary & Question

Because the power amp measures fine directly and the preamp outputs match when unloaded, it appears the right channel preamp is choking under load specifically at low frequencies.

Has anyone encountered this specific load-sensitive low-frequency drop?

Appreciate any insights or advice!

 

[eiraved]

The unit was expertly restored a couple of year ago by an AK member here with bypassing the Dolby board. I recently noticed the right channel sounded noticeably thinner.

This might sound obvious, but is there a reason you're not directing this question to the AK member who expertly restored your receiver? Seems to me he'd be able to narrow the right channel issue down quicker than we would, especially since he worked on it last and knows specifically what he messed with during the restoration.

 

[Scott Muni]

Have you tried swapping the preamp L&R output to the main amp. Use cables instead of the jumpers to narrow down where the problem is.

 

[Bolan13]

Have you tried swapping the preamp L&R output to the main amp. Use cables instead of the jumpers to narrow down where the problem is.

By narrowing down where the problem is, do mean that if the right channel still has less output after swapping left/right, then the problem is in the power amp?

 

[Bolan13]

Well that was fun, i am an idiot, it was my AirPort Express / cable. I was using it for the test tones and running it into different the different inputs rather than switching the actual source. I found a test tone cd and used my cd player, then used my iPod. I got the same readings this time. .45 on the left, .42 on the right. I imagine that difference is minuscule and fine for vintage gear.

The only thing though is why were the voltages matching when I pulled the jumpers?