H All. I have the 5500 Mk2 tuner that works perfectly and sounds good, but every "locked" station shows a frequency of +0.05MHz. QUESTION, is this just a case of the frequency counter of the display needing a fine adjustment or does more complex adjustment need to be done? I realise it would benefit from a total accurate realignment, but cannot find anyone in the UK that can do it. If it is just the frequency counter how do I adjust this please? .....Thanks for reading.
There is no adjustment in the service manual for this. These tuners actually lock and measure the local oscillator and assume a 10.7 MHz IF. The IF frequency is fixed by the ceramic filters.
I suspect the 4.5 MHz crystal X501, used by the PLL MCU IC501 has drifted off a little with age (1984 model!). Fixing this is quite involved - buying crystals as specials is not easy these days, so if possible just live with it.
I suspect the 4.5 MHz crystal X501, used by the PLL MCU IC501 has drifted off a little with age (1984 model!). Fixing this is quite involved - buying crystals as specials is not easy these days, so if possible just live with it.
Usually it's just the discriminator alignment that needs a tweak in this situation. Very common. Adjust L204 and L202 as described in the service manual on a real station while tuned to its nominal frequency, that should work just about as well as a signal generator. If in doubt, grab another PLL-synthesized FM receiver of conventional, no-frills construction and tune in its LO signal at tuned frequency + 10.7 MHz for a cross-check.
I have tried a number of PLL-synthesized shortwave portables in my possession, and none have disagreed by more than a few hundred Hz on FM, so actual reference oscillator inaccuracy wouldn't be high on my list of possible causes.
I have tried a number of PLL-synthesized shortwave portables in my possession, and none have disagreed by more than a few hundred Hz on FM, so actual reference oscillator inaccuracy wouldn't be high on my list of possible causes.
I guess the answer is to accurately measure the crystal, and the FM detector. It will try to lock to on a 0 DCV from the discriminator. Correcting this and the other coil will also lower distortion. That's FM alignment in general.
I note that there are 2 variants of this tuner; the European uses 50khz steps, the US 100khz steps. could this have anything to do with regards to the discrepancy you're seeing?
The only difference is the US tuner will not display 50 KHz increments, and it will not lock onto any station that has that offset. It's just a switch or jumper. It also causes the AM tuner to step by 9 KHz instead of 10 KHz here.
I had a look at the service manual and there are quite a few differing component values. There is a switch to swap the AM parameters, but not the FM ones.