Mercury DSH 303A VHF Repair: Power Failure, Isostat Issues, Varicap Voltage, WPF Board Swap

I have not written here for a long time and I have not rummaged in our equipment. After a few years, I started to "just retune" the aforementioned radio. Before that, I gave up. Now, after tediously searching for the reason for the silence in the VHF speakers, I have reached a power failure. The reason turned out to be an already established isostat from this range. The head power is supplied through its contacts. Well, but I stopped again.

The noise on the VHF is uniform, it does not change under the influence of turning the tuning potentiometer. The varicap voltage changes ~ 3-30V (I will deal with it later) on the head. Trying to limit some potential causes, I swapped the entire WPF board from another Mercury, but the symptom is exactly the same (the board is also unreliable, comes from Mercury where the voltage from the power supply is 30V instead of 15, so maybe it is damaged too.

Now what should I check? There are approx.

The 15 volt power supply on the T701 and T702 must be functional. Disconnect the wire from pin 715 of the power supply and check its correctness on the dummy load.
A track usually works in the opposite direction to the signal passing through it, that is, from the end. Start at section 13.14 of the U switch. You give a signal to C241, C242 from a signal generator or a finder (a generator for example on NE555, harmonic generator on gates), then C228, then C216, C202 (10.7MHz). If the result is positive everywhere, the defect must have the VHF head.
In the head, you check the operation of the T103 heterodyne.

OK, after "a few days" I got it all. The problem was not very working isostats, sometimes they connected, sometimes not, the effect is the lack of power to the head. And when it was already powered, something else did not connect. In the second copy a similar case. I bought a working plate for olx for PLN 20. After the exchange, it's already downhill. The R320 resistor started to heat up and smoke, but stopped. Now that the tensions have been set up, it's time to tune in. Only this time the AM part does not work ... complete silence, but VHF tuned :)

Section 13-14, even that is marked on the diagram found on the net, so I guess that's the broader problem of these buttons.




By the way, is it a symmetrizator?
Is each symmetrizator two-sided, i.e. you can connect both the receiver and the antenna for 75 and 300?

This is a cable for a collective antenna, as it seems to me - a 75 ohm plug for a wall socket (if you still have an efficient installation of a collective antenna - if at all), and flat plugs for radio antenna sockets - one for VHF and the other for AM.

That's what I did. I have a collective installation in working order, recently made, but so far it has not been better than on the wire itself. I will check where the TV still works.

It probably won't be better, I used to have the Zodiac hooked on. Either the signal was too strong or there was so much noise in the installation. He hummed mercilessly and when you heard something without hum, you could hear other radio broadcasts in the background. Receiving on a 30 cm wire was a "needle", even FaM's radio from Wołomin could be received in stereo.

I agree with the previous speaker - most of the collective installations were of poor quality. With a few friends, I even saw a connected TV antenna instead of a cable - when asked why - "Because the signal is better than from the collective ...".
In my case, when there was still a collective TV antenna installation, he was receiving something, but it was impossible to listen to the radio - the signal was weak, noisy, and sometimes even on "Trójka" the stereo disappeared. It was better on a piece of wire.

No, when I was playing with another applet, I had a very good signal from the summary one, but without any devices in between, also maybe this balun / converter is damaged. And it simply collects the signal from the cable as an antenna, because after unplugging from the installation nothing changes, and everything is the same as on an ordinary wire.
Everything will go wrong if I connect the radio directly to 75?

Drive the hot wire into one of the VHF socket contacts and the cable braid into the ground connection of the AM socket.

Yes, thanks. I just read recently that the transformers at the input are tapped and you can connect this way and that's what it is, the tap is on the ground.

I have an old TV antenna on the wall in the style of http://www.dipol.com.pl/antena_asp-8w_lux_ze_wzmociąg_lna-177_A0024.htm and the second one http://www.dipol.com.pl/antena_telewzyjna_dipol_44-21-60_tri_digit_dvb-t_uhf_A2670.
Is one suitable or suppresses the radio band?

I connected a Condor to the first one while on vacation in the countryside, because the entrance fits.
Was it better? Probably not, because I was not able to pick up a station from Warsaw (approx. 60 km.), Only a broadcast in the background of noise.

There are dedicated antennas for radio reception and they look significantly different from what you showed.

According to the data from the site, none of them will come up because they have a band from the 6th channel upwards.

Well, I'll make a simple dipole, if I have some symmetrical conductor