Faulty Wayne Kerr AF Signal Generator

[dperry]

Hello,

I have one of these i am eager to repair, it used to work but was out of range and not creating the desired or 'advertised' frequnecies.

Looking at the manual, it stated that capacitors C1-C12 were resposible for frequency control, so due to their old age and the fact they are wax coated paper capacitors, i assumed they would be leaky...I desoldered them, measured them and they were fine, but upon reattaching them the unit no longer gives a stable output.

In addition to this, there is evidence of burning on the board, around the base of V2 (12BH7 valve), and around R12 and R24.

Here is a schematic, sorry, the pdf is a bit too big to upload here...i can email to people if they wish though

http://pdfcast.org/pdf/af-sig-gen-sche

Can anyone please tell where to test the circuit for fault finding? I am not sure if its oscillating at all anymore

I have access to decent test equipment (DMMs, RLC, Scopes)

Thanks

Duncan

 

[davenn]

HI Duncan,

been having a look at the circuit ... been ages since I did much work on bottled gear used to restore old valve radio receivers, so much fun getting them back to their former glory.

Ok on the caps you noted, obviously you didnt mean that all of them were the old wax paper type as 4 of them are variable caps ... C1,3, 7 and 9. The old wax ones can be replaced by decent hi voltage poly prop. type 250V or greater rating. I wonder what you put in there ?? standard disc ceramics are no good they will have poor stability. That would be improved with the use of NPO type disc ceramics or preferably other more temperature stable type as mentioned earlier.

out of interest you should measure the supply rails and see what they are like and how much they vary from stated values. see how much AC ripple of on the DC rails.
Chech C23 ... 50uF and C24 .... 8uF. this unit is obviously old. I would be replacing those 2 electrolytics anyway as a matter of course.

ok there's a few initial thoughts

Dave

 

[dperry]

Hello,

Thanks for the reply.

Sorry if i didnt explain myself very well, I desoldered the fixed value caps in the group of C1-C12 and measured them....they were all fine and more or less perfectly in their range, which was quite surprising given their age

Before i started messing around with it, the unit was doing what it was supposed to, just out of range.

Now, it doesn't give any sort of stable output whatsoever....regardless of the position of the pots and switches, it jumps around anywhere between 30hz to 80hz.

 

[davenn]

Hello,

Thanks for the reply.

Sorry if i didnt explain myself very well, I desoldered the fixed value caps in the group of C1-C12 and measured them....they were all fine and more or less perfectly in their range, which was quite surprising given their age

Before i started messing around with it, the unit was doing what it was supposed to, just out of range.

Now, it doesn't give any sort of stable output whatsoever....regardless of the position of the pots and switches, it jumps around anywhere between 30hz to 80hz.

ahhh ok ..... you have user error then you must have resoldered something back in, in the wrong place or maybe a solder bridge somewhere take a photo of the area as sharp an image as possible and post on here, lets see if we can spot a mistake

Dave

 

[dperry]

I wish it were that simple....

So one elese has been messing with it, and it no longer works at all

The power valve (V3) sparks with bright lights