Adding stereo FM modulation to the 8657A - Page 1
Forum section: electronics / repair

author: Zenwizard
author_rank: Regular Contributor
timestamp: June 04, 2021, 11:14:23 pm
content: I need to do an alignment on a Harmon karden tuner for my parents. It is the tuner they have had forever and I am fixing it up for them. I have an HP 8654A in the lab that will work for most of the alignment. I need a function generator that can be set up to provide stereo FM. to the 8657A or provide the modulation directly to the tuner in question. I will admit I am a bit out of my depth here but learning quite quickly.

author: bdunham7
author_rank: Super Contributor
timestamp: June 04, 2021, 11:41:05 pm
content: You can buy inexpensive FM Stereo transmitters that should work well enough for basic testing. I have seen a kit for one that just does the FM Stereo multiplexing, but I don't know if your RF generator actually supports FM modulation at sufficient rate and depth for this. My HP 8656A does not, for example.If you want something more accurate or adjustable, you need specific FM alignment equipment from someone like Sencore or the like. My R&S CMU200 has a basic FM Stereo output module, but it doesn't have the capability of being modulated by an analog input, so it is limited. In conjunction with a cheap FM stereo transmitter, I get by.Synthesizing the signal yourself well enough for calibration, adjustment or just high-quality broadcast is a lot easier said than done. You have to generate a 19kHz pilot tone and then a sideband suppressed amplitude modulated 38kHz subcarrier that is exactly in phase with the 19kHz pilot tone. The main FM carrier is modulated with the L+R (mono, combined) signal and the subcarrier is amplitude-modulated with an L-R (difference) signal. There were custom chips for doing this, like the ROHM BA1320 or you can look up kits for FM stereo encoding or multiplexing. Some of the methods used are cruder than others. This would be a pretty deep dive to just fix one receiver.

author: jh15
author_rank: Frequent Contributor
timestamp: June 04, 2021, 11:51:03 pm
content: I have a Sound Technology 1000a generator bought new and rarely left powered off for long periods. If your near Maine/NH . A veteran of the hifi era, not the audiophools thing.

author: bob91343
author_rank: Super Contributor
timestamp: June 05, 2021, 12:46:02 am
content: I bought a gadget for very little that does an interesting job. It's intended to plug into a car lighter socket and has a stereo input jack for maybe a portable cassette player. It transmits a nice stereo signal at any frequency in the FM band. Looking at its output on my spectrum analyzer it actually looks pretty good. And accurate too.

author: jh15
author_rank: Frequent Contributor
timestamp: June 05, 2021, 01:48:56 am
content: you can use the hp to perfect the front end sensitivity, and since there is very little quality in the commercial realm, tweak it for best sensitivity on weaker community radio or classical stations.You need to set the ratio detector, etc. if a relatively modern rig, after that, it may only have a pll freq adj, and a separation adj. Lift the spkr grounds, tie them together, and find a good classical performance, and adjust it for maximum "spaciousness.

author: bob91343
author_rank: Super Contributor
timestamp: June 05, 2021, 05:14:00 am
content: That's okay for alignment but for troubleshooting one needs to know the pilot amplitude and so on. Let's say the stereo lamp doesn't light. The 19 kHz pilot has to be doubled to 38 kHz, or has to lock a PLL, etc. There should be provision for switching the pilot off or adjusting its amplitude.So the HP can be frequency modulated by an audio oscillator set to 19 kHz to see how well the receiver locks.