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:

12 thoughts on “Repair of a Sommerkamp TS-788DX (Coffee-warmer 3)

  1. Hello Donald,

    a typical Sommerkamp TS 788dx problem. The frequency cannot (always) be changed. I acquaired a TS 788dx lately with the same problem. All troughholes were soldered by the previous owner. I replaced all electrolytics and tantaal capacitors on the subboard. That did the trick. It’s working flawless. So maybe a recap is in order. The transceiver is over 30 years old. Best 73, Henk

    • Hi Henk .

      Do you have a full list of the number of capacitors (electrolytic and tantatum) on that board and their values . Just so I can buy them before I get started .



  2. hej Henk,,,
    intressant läsning, tack för du skriver,
    jag har en sommerkamp 780dx, er du kunnig på denna radio, jag hade blivit glad om du har något tips om hur man kan få Fm att fungera, fm fungerar ju inte med nya radios som har de nya fm-system… jag skulle också vilja öka power så mycket det går, tack ,,,
    wolfgang / Landskrona- Skåne-Sverige

  3. Hallo Donald ,,,
    interessante Lektüre, vielen Dank für das Schreiben,
    Ich habe ein Sommercamp 780dx, Sie kennen sich mit diesem Radio aus, ich war froh, wenn Sie irgendwelche Tipps haben, wie man Fm zur Arbeit bringt, fm funktioniert nicht mit neuen Radios, die die neuen FM-Systeme haben … ich hätte auch Lust verschrauben Sie die Ausgabe so viel wie möglich, vielen danke ,,,
    Wolfgang / Landskrona – Schweden

    • Hallo Wolfgang,
      FM ist nicht leider zu gebrauchen und auch nicht durch einen einfachen Umbau zu modifizieren. Das Gerät aus einer Zeit als FM noch eine exotische Betriebsart war 🙂 Man kann die Lautstärke etwas erhöhen, durch eine Modifikation, die macht es aber auch nicht klarer, da der Bandpassfilter zu schmal ist, leider. Die 788 bleibt ein Sammlerstück, was auf SSB und AM gut zu gebrauchen ist.
      Wenn Sie aber denken das FM bei der 788dx schlecht wäre, hören sie mal das FM bei der 780dx 🙂 HaHaHa

  4. J ai exactement le meme problème mais je ne trouve pas la solution. Pensez vous que cela peut venir des condensateurs ?
    Jean Yves

  5. Guten Morgen!
    Ich habe 2 TS-780 DX die auf FM sehr gut funktionieren!
    Es gibt FM Platinen zu kaufen in Östereich !
    Einfach mal im Internet suchem!
    Auch für die Sommerkamp TS-788 DX gibt es eine komplette Platine für die Frequenzaufbereitung inklusive FM!
    Der Frequenzdrift gehört dann der Vergangenheit an ebenso das leiern der Modulation und halt auch besseres FM!

  6. Hello Guys ,
    I need the colors from the Mike FM 80
    for the TS788 DX
    Red Up ?
    Green down ?
    White Mich?
    Yellow Loudspeaker ?
    Can anybody help me.

  7. Hello, hny 2020! Old 788dx problems pll quartz 11,300 not in function, other near frequ 11,1 have function but pll is not ok. Need good schema off this side of station, variation on net is too bad for reading – I mean side with pll and frequ. devider and showing decade. Can please somebody help me – thanks! Kurt

  8. I’ve just picked up one of these too , and it has the same frequency change issue . As per previous suggestions I try recapping the radio , as it’s getting on in years .

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