Onkyo TX2500 mk II

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 again and investigate further 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.

Schematic of FM tuner section:

"Bitrex = Fuckwit "

FM receivers normally give no output with no antenna.

Portables and clock radios have ANTENNAS !!!

... Phil

I don't own a proper a FM antenna. I almost never use a receiver to listen to the radio. When I do I get plenty of strong stations with no external antenna, as I mentioned.

I live about eight miles from several 40,000-50,000 watt ERP FM stations

- the other receiver I have here picks them up just fine with great sounding audio without an external antenna. It's magic!

Sorry, Phil... Tuners and receivers often have an output without an antenna attached. This is due to poor shielding of the front end.

Good morning,

The missing AM antenna will be a show stopper for the AM. Unfortunately is is not uncommon to see these ripped off the receiver. People seem to think they make a great handle. Finding a junk unit with a good antenna might be your only bet.

As to the FM, these do require an external antenna. An inexpensive Dipole will likely do if you are in a good reception area. SOME receivers will not do proper MPX if the MPX light is burned out. I don't remember if the TX2500-II is one of those or not.

Personally, its a very nice receiver when working properly.

Regards, Tim Schwartz Bristol Electronics

Thanks for your reply. I did a bit more work on the unit today, using a random wire antenna and checking out the waveforms going into and out of the FM IF IC. The audio output from the IC is weak, noisy, and appears to be half-wave rectified. I can't see any signal at all going into the demodulator IC, down to the limit of my scope (5 mV/div with 1x probes.) Obviously there must be some signal there to produce any output at all, but I would think that with 2 stages of RF and IF amplification before the demodulator IC there should be some signal visible there at the IF frequency. This makes me think that the problem is in the FM front end somewhere.

As for AM, I think I may just get an "aftermarket" loopstick antenna and mount it to the receiver somehow.

Could be... possibly a fried first-stage RF amplifier? Depending on the design, these may be vulnerable to damage due to induced voltage from (e.g.) a nearby lightning strike, or even just bad static electricity on the feedline.

Or it could be in the detector / demodulator. The FM detector usually contains multiple gain stages within itself, with the "limiter" cranking up the gain until the signal is clipping fairly cleanly. A feedback loop (the AGC) controls the amount of added gain in the detector, and the AGC voltage is often used to drive a signal-strength indicator.

So, you could have weak (or no) signal coming into the detector, or you could have a detector whose gain stages or AGC loop are frotzed.

Next step is probably to kluge together some sort of FM signal source which you can use to inject a few millivolts (or microvolts) of RF into the circuit via a probe, work backwards, and find out how far back towards the front end (and how little RF you can inject) to get it working.

Yeah, but it may indeed require alignment after all these years. Onkyo's seem to drift in their adjustments as they age.

Mark Z.