The stereo/mono switch which I always thought was odd.
Finding space for another switch may have been difficult, plus it would've added cost to an already pricy unit. Not seeing the meters bounce in mono mode was minor enough to forego.
I pulled the relevant circuits for the meters from the schematic. My description may not be 100%, so I welcome any corrections.
The schematic shows the switch (S601) in stereo mode. In this position, the meters show audio levels. This is in the down position - the middle and lower pins of short together. You can see the little black rectangles representing the slider contacts.
In the up position, the middle and upper pins short together, which sets mono mode and shows the power level on the left meter, and tuning strength on the right meter.
The switch has four sections, but only one of them is involved with toggling mono and stereo mode: S601-1.
On this section, the preamp's right channel output (R-CH IN) ties to the center pin, and the left channel is on the upper pin. Nothing is on the lower pin, so while in the down position for stereo mode, the right and left channels stay disconnected. In the up position, the right and left channels short together.
S601-2 toggles the right meter functionality. The center pin is tied to the meter and meter amp. The lower pin is a resistor to ground, which allows the meter amp to sink current and drive the meter with audio levels. When the switch is in the upper position for mono mode, the meter is connected to the upper pin, which has the tuning strength signal. Without the grounding resistor on the lower pin, the meter amp isn't able to send audio levels to the meter and interfere with the tuning level.
However, if you are recording in mono mode, you get audio levels back! This is accomplished by way of the recording status signal from the preamp. It controls Q405, which, when enabled, reconnects the meter amp output to a resistor to ground - the same value resistor as found on the lower pin. The tuning signal level takes a back seat to the meter amp because it's got a higher resistance (2.7k vs. 1.2k)
S601-4 is the same as section 2 above, except for the left meter. Instead of tuning strength, the upper position ties to main power, to check battery level. However, this is over-ridden by the meter amp if the recording signal is active at Q305, which pulls harder than the battery level resistor (68k vs. 1.2k)
S601-3 has two purposes - toggling the stereo LED, and toggling the audio output from the meter amps. The center pin is ground, which means you're grounding one pin while *not* grounding the other pin.
In the down position for stereo mode, the lower pin - which goes to the base of the meter amp output switches - is grounded. This holds the meter amps open, to allow the audio signals to the meters.
In the up position for mono mode, the upper pin - which goes to the driver circuit on the tuner board for the stereo LED - is grounded. This prevents the stereo LED from shining during mono mode. At the same time, the meter amp output switches (Q304/404) are no longer tied to ground, preventing them from passing audio levels to the meter and interfering with the battery level and tuning strength.
The one thing I'm not sure about is why I don't get audio on the meters in mono mode while playing a cassette. The play signal from the preamp ties to the meter amp's switching transistors Q304/404. In mono mode, their bases aren't grounded, but should be pulling to ground enough through R320 and R420 that it should be allowing meter audio, similar to how the record signal restores audio on the meters during record mode (see S601-2 and S601-4).
Looking elsewhere, the audio coming in from the preamp at CN601P pins 7 and 8 pass through DC blocking caps C322/422 and into the bases of Q303/403. The stronger the audio signal, the harder the output at the collector can pull to ground through the emitter.
VR301/401 adjusts the output (how strongly the signal can flow to ground)
R316/416 sets the gain on the transistors
R317/417 draw the proper amount of power from a 9V regulator circuit (Q602).
Anyway, since it's affecting both channels, I'd check Q602, D602, or FR603 at the +9V regulator circuit. The only other thing it powers is a small muting circuit, which you may not even notice isn't working.
The only other thing that affects both meters simultaneously is the gate for the meter amp switches Q304/404. Maybe they're not getting grounded properly at S601-3? I really doubt you lost both transistors for both channels. I'm betting the meter amps (Q303/403) and meter switches (Q304/404) are okay, and you're getting audio on CN601P pins 7 and 8,
Hope that helps!
