Repair of a Sommerkamp TS-788DX (Coffee-warmer 3)

Its unbelievable, I got one 🙂
I have a Sommerkamp TS-788DX. The third Coffee-warmer. That was a long dream from me 😀
And I payed not so much for it. But it was sold as part broken.
This is the TS788DX without cc. That means without the 10m repeater function.

This sommerkamp has the problem, that you can’t change the frequency any more, when the radio is warm. If you wait leave it some hours off and you switch it on, it works again.
Something like this is hard to find !

So I open it and see what it can be.

If I take Ice-spray and use it on that marked IC, it works again for some minutes.
THis tree ICs in this area are NAND Gates for the Channel Selector and the up and down buttons.

At first I checked the Channel selector. It have a little bit bad contacts, but it works with contact spray. I checked also the four diodes at that switch.
The problem is still here!

The I checked over 50 solder pins in more then one week work.
The problem is still here!

I use washers, under the board, that the circuit board is not braced
The problem is still here!

Then I changed the other two ICs from that circuit.

I install sockets

And checked the IC with a test board before I plug it in again.
And I change a lot of diodes around this ICs.
I re-solder all contacts at the upper side of this board.
The problem is still here!

I checked the schematic and the datasheet from the NAND gate and I think, I know now why !
The channel selector is wired with some pins of that IC. And they are comes together at the red marked pin. If you change the frequency, this pin is high and has near 7V.
This is a NAND (not and) gate.
So If the yellow marked pin has no volts and the red one has 7V, the green marked pin has always 7V volts and is high. If you change the frequency or not!

We know now that the channel selector the switch the red marked pin high. But The yellow one must have volts too, to get the green one low!
Only then its possible to change the frequency!
I try this out, give the yellow one 7V and the frequency is changed.

I checked every part in this line. But I does not find whats broken. The voltage at this yellow pin is fluctuate from 0.30V to 6V! Alone! That not normally.
If this voltage at the yellow pin is near so high as the red one, it works.

I checked the blue marked IC too and I adjust the voltages (they are in the manual).
That’s important for the step size and the frequency differences at 26.000 and 29.999 MHz.
The problem is still here! But its better. I can work now with for many hours 🙂

After that and 3 weeks of work, I need a break. I clean everything with air.

Everything looks clean now

Looks nice and clean from the other side too.

And build it together.
In that state I can work with for some QSOs.  Later I will continue with this project.

Update 1:
I had try to give the pin with the missing voltage, the power what he need. I made a bridge from IC32 Pin 8 to IC31 Pin 9. Now the radio is working and i can use for change the frequency the UP and DOWN on the mike. But the Scanner is without function then.
Everything fine so far and I can life with. Maybe I have a better Idea later.

I made some tests and a alignment
Two tone test on USB. Not bad so far.

The clearness of SSB.

The USB frequency with 1 KHZ modulation

The LSB frequency with 1 KHZ modulation

The AM modulation. I be not sure what I see here 🙁  But it sounds not bad.

Th FM deviration is about 1.65 KHZ min. and 1.78 KHZ max.
Its clear but not so loud. More is not possible!

The frequency on FM and AM.
The frequency drifts from cold state to warm state over 1 KHZ! After 1 hours it is stable

Update 2:

I have done a modification with the FM-Mod. I replaced the C166 from 0.033uF to 0.1uF and I add to the resistor R287 a second one from 47kOhm.
Now are 2.6KHZ derivation not a problem.
The power is via a 40db attentuator. So 0.69mW are 69Watts on FM 🙂
The FM mod is a little bit better now. The USB Mod is still not so fine, but it works.

Update 3:

After some Weeks of fun with this radio, I leave it yesterday run with receiving a frequency the hole day. I did not transmit yesterday. In the late evening, I see that the red light (SWR protection) is on. I switch the radio off and on again, but the light goes not off. 🙁

So I check the schematic, how this SWR protection is working.
The D89 is a Thyristor. The gate of this become a positive voltage from the reflecting of the SWR bridge. Then the D89 is switching and the red light bulb become minus.
This circuit blocks also the working of the TR16 over the TR17, and no output is come out of the transceiver.  If the transceiver is switched off and on, the Thyristor is a sort time powerless, he blogs the minus of the light bulb, and the red light is off.
So if this problem happend, it can be that the D89 or the TR17 is broken. In my case was it a bad solder pad in the area of TR17.
It cost me over 4 hours, to find this. Now it’s working again.

lets see how long 🙂
See the Videos of this radio:

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