EMG TR0463 function generator - Page 1
Forum section: electronics / repair

author: Gominalo
author_rank: Contributor
timestamp: October 04, 2025, 08:45:38 pm
content: Hello eev community,I am trying to repair an hungarian function generator fully made of discrete components.At first it didn't power on, after two bad caps, a broken potentiometer and a blown fuse it did.Then I connected the oscilloscope and the only portion that works correctly is the modulator (at least i have sine, triangular and square signals)I've been trying to troubleshoot and by the moment i can tell that i doesn't have trigger, and external trigger does nothing (not sure if i am inyecting correctly)Any clues to where have a look into it?Best regards,Gonzalo.

author: Swake
author_rank: Frequent Contributor
timestamp: October 06, 2025, 07:38:28 am
content: Let's start with the trigger input: See schema '7' on Page 55 of the PDF.First: double check the power supplies. It looks like you need + and - 14 V, sorry the schema is not very clear.Second: With the generator in power off and with a multimeter in diode mode you measure the base-collector and base-emitter junctions in the 3 transistors. If these are ok proceed to next step;Third: Set the generator to some very low frequency like 0.3 Hz and check if the signal is found on pin 9 of the 7400 (fuchsia circle). Preferably with a scope. If you do not have a scope the low frequency can help you to see the signal with a multimeter.Forth: The trigger input is a TTL signal (5 volt) (red line) that goes to the NAND ports of a 7400 chip. To check if it works you can feed your signal and follow its path (light blue arrow) one port and one transistor at a time till it reaches the output.Please report what you found.

author: Swake
author_rank: Frequent Contributor
timestamp: October 06, 2025, 08:13:57 am
content: btw: I think the switches might have bad contacts.

author: ondraN
author_rank: Regular Contributor
timestamp: October 06, 2025, 12:39:23 pm
content: These Hungarian EMG products have big problems with poor quality switches, potentiometers and other components. The switches suffer from contact corrosion, resistors and capacitors lose value and transistors suffer from h21e degradation. This is a result of the production of very poor quality components in the former Eastern Bloc countries. If it is a device for a collection, then it is OK, but for some work where reliability, accuracy and stability are needed, I would definitely not recommend it.

author: Gominalo
author_rank: Contributor
timestamp: October 07, 2025, 09:04:56 pm
content: Quote from: Swake on October 06, 2025, 07:38:28 amHello Swake,7400 is working fineFollowing the path of external TTL signal it gets all the way untill pin 3, when it goes through C382 i get solid 5 volts with rising and falling spikes that goes into pin 13, on pin 9 there's no signal, only 4'8v.The transistors TR359, TR360 &TR361 looks fine at least with multimeter (arround 0'7v)Checking with oscilloscope on the base of TR359 i see like a cut triangle signal and it should be like "hj" signal seen in the schematics, in the collector 14v and in the emitter, 0v.I guess i could inject a triangular signal directly to the base and see what happens, but i would like to see where it gets cut off.Best regards,Gonzalo.

author: Swake
author_rank: Frequent Contributor
timestamp: October 08, 2025, 09:06:57 am
content: I somehow forgot to add the attachment in my previous message. Edited that message and have added it.But you understood the idea without the imageYour new picture shows there is a much better quality manual out there: here is the link:https://archive.org/details/emg_EMG_TR0463/mode/2upI can read the actual numbers nowWithout a trigger I believe the 'hj' signal should be near to 0 V. This matches with the measured 40 mV according to the scope image.When there is a trigger 'hj' should be negative something. This because a trigger will activate the 7400 flip-flop (ports with diagonal) that will activate the base of TR356 and that will set the collector of TR356 to a negative voltage during the trigger as it is connected to -14 V over R500.I believe that when the trigger signal on the EXT IN transits from low to high the generator is triggered ones and one trigger equals to one signal period output.

author: Swake
author_rank: Frequent Contributor
timestamp: October 08, 2025, 09:08:07 am
content: I'm wrong, the collector of TR356 is not connected to 'hj' but to the base of TR357.Wait a minute...

author: Swake
author_rank: Frequent Contributor
timestamp: October 08, 2025, 09:41:42 am
content: It is not entirely wrong what I wrote but for another reason.The generator signal comes from that big switch having the capacitors to select the frequency decade. When 'TRIG' button is pressed the TR337 pulls this signal to ground until the trigger happens. When the trigger happens its base is pulled negative and it stops conducting thus letting the signal through.I don't know exactly how the duo TR359/TR360 works, but these probably clean the 'hj' signal and after one cycle and the help of TR361 reset the flipflop (7400 pin9).