Yes that's it. Do keep in mind the amp may or may not be the problem. Without more measurements it's hard to tell. If you have a scope, checking for oscillation (while "load" is connected) is something worth doing. Replacing the amp isn't that hard, it's just that the board needs to come out. Then with 2 big blobs of solder on each side (or hot air) it can come out easily.
It would be possible to measure for actual voltage drop when the receiver / tape loop is switched on, I would just need to customize a cable to create test points. Before doing that, though, I thought I would try the Dolby cassette in my D6—connected via the line out to my receiver tape-in. Guess what, the level meter drops a unit when the receiver is on, and jumps back up when receiver / tape loop is off, same as the KA1ESA... The receiver is a Harmon Kardon HK3490.
From calibrations done before on D6/D6Cs the difference between measuring directly (multimeter 10MΩ input) and with 47kΩ load is about 20-30mV RMS. Given the Dolby level is in between -5dB and 0dB marking on this VU-meter, it can indeed drop from 3 LEDs lit to just 2 lit, especially if HK3490 has only 10kΩ impedance on line in. How much are the levels dropping (in mV) on the K1ESA when the receiver is connected ? From what I understand so far, the initial problem was an imbalance, which was not solved by adjusting the levels at the line out (with 47kΩ load), but was solved by adjusting at the Dolby IC. Is my undertstanding correct ?
I haven’t had a chance to test the mV drop on output yet (need to customize a cable first). I had also read that dirty connectors and switches on the receiver can increase impedance, so I planned to pull that and clean it first. (I’ve also been having issues with the receiver briefly dropping signals - coax and HDMI - so another reason to clean connections first). I’ll try to get to that this weekend. But, yes, a channel imbalance was the original issue I was testing for. With the levels set correctly on the Dolby chip and line out on the KA1ESA, there is no audible channel imbalance, but as noted in post #18 above, the VU meters show a left channel imbalance with the deck disconnected from the receiver; but when I hook up the deck to the receiver, that VU imbalance resolves. It’s a head scratcher for sure. I’ll proceed with the steps above and see if it makes any difference. I’ve a feeling I may have to check the main board anyway: at the weekend I played two cassettes (about 2 hrs of music) and I did notice the 2nd cassette felt very warm to the touch when I removed it, so there may indeed be a failed component somewhere here.
Following up on the levels measurement on my KA1ESA. After giving all the inputs and switches a clean on my receiver, I made a testing cable to measure voltage drop on line out from the deck when connected to receiver tape loop (see pics). First of all, checked the playback levels on the deck again and confirmed: Levels on Dolby chip = 388mV Levels on line out (with 47k load, unconnected to receiver) = 432 mV Then, using the testing cable pictured, measured level at line out connected to receiver's tape loop (no load): With receiver off = 430 mV With receiver on = 390 mV So, a voltage drop of 40 mV VU meters still behaving same as in post #18 above...
