I have an Onkyo TX2500 mk II receiver that I'm hoping to fix up for a friend, which is not working properly on AM or FM. Tuning to AM gives low frequency oscillation, and a cursory inspection shows that for some reason someone has physically snipped off and removed the external AM loopstick antenna. Since the loopstick coil appears to be integral to the AM tuner, this was a pretty dumb thing to do and I'll have to find some kind of replacement to get AM working again.
Tuning to FM with the muting on gives no output; disengaging the muting circuit, with no external antenna, I get static and a few very faint stations. In my area with most receivers I get plenty of strong stations with no external FM antenna.
If I put my finger across the FM terminals as a makeshift antenna I get better reception and some strong stations, but even when they're tuned in strongest they're noisy and badly distorted, and the voltage on the transistor collector controlling the "LOCKED" lamp never falls to engage the lamp (the lamp itself is burnt out.) Stereo decoding never engages either.
On FM the "SIGNAL" meter is completely dead, even when a (distorted) signal is booming in. The "CENTER" meter does deflect in response to signals, but it only seems to deflect in one direction.
All power supply rails test close to the indicated values on the schematic.
I haven't done much work on these old receivers, so before I pull out the scope and multimeter can anyone give me any pointers on what area to suspect? The way the meters are misbehaving leads me to suspect that the FM IF/demodulator IC HA1137 or associated circuitry is faulty. Thanks for any advice.
Here's a schematic of the FM tuner section: