Thandar TG501 signal generator

[high_vacuum_house]

This signal generator was bought at the recent BVWS Biggleswade function at the end of the day for a few pounds. They are well made function generators and all of the components are standard off the shelf type if in need of repair. All of the IC's are socketed which is a nice touch. No strange unobtainium semiconductors or FPGA type IC's here!

i thought about trying it out later at home. Upon applying power, I was rewarded with a loud crack and a cloud of smoke out of the top!

Upon inspection, I found that a 10µF tantalum had let go and there were signs of several burnt out resistors in one corner.

I downloaded the service manual and found out that all of the burnt resistors were in the output attenuator stage which the drawing for wasn't in the service manual but luckily the resistor values were on the parts list and duly ordered. Two being 249R which is not a common value. Another was specified as 61.9R which I didn't have but I thought 62R would be adequate as I had some!!

The amount of damage in the attenuator stage meant that quite a bit of power must have accidently been pushed up the output BNC. Fortunately, the output transistors somehow survived their ordeal, though they are quite beefy.

I had bought a new nozzle for my Weller desoldering station as the original one was completely worn out. This made removing all of the dead bits off of the PCB a doddle and all were quickly replaced.

Upon powering up again, there was another almighty crack from within, The replacement tantalum had failed and was now missing its top!! Testing a few from the batch I bought, showed that although they were 10µF, they certainly were not rated for 25V as printed on them and all duly failed not far above 10V. Never rely on new parts!! Another older one I found was checked and seemed happy enough and was fitted.

No more bangs on powering up but a loud hum from the transformer! Fearing another capacitor was going to expire, I quickly switched off and checks around the power supply and supply rails showed no signs of distress. The hum was still there and then I noticed one of the transformer holding nuts merrily rotating round on its screw!! Tightening it soon shut it up!!

The switches were given a clean and the knobs rotated, so the indicator lines were in a more sensible position. The calibration procedure went ok and all the functions and waveforms worked well. The 2 halves of the cabinet were given some foaming cleaner and refitted to finish off.

[G6fylneil]

I managed to burn out the attenuator in my generator, I was using it to check a DVM I'd picked up at a rally. I used a variac to check the higher ac voltage ranges but left the generator connected! It fried the output transistors too!. These days the mains isn't as sinusoidal as it used to be.

[Trigon.]

Originally Posted by high_vacuum_house

I downloaded the service manual and found out that all of the burnt resistors were in the output attenuator stage which the drawing for wasn't in the service manual

Service manual with full schematic here:-

http://www.philipsradios.nl/Schemas/...TI%20TG501.pdf

Cheers

[JohnBHanson]

If you want to calculate odd resistor values try

http://80.5.233.129/cgi-bin/resistor-comb

(Written by me!)

[factory]

The 61.9 & 249 are both 1% E96 series values, they are readily available at the larger component suppliers, for example the Vishay MRS25 series from Farnell.

David