Finding a thyristor... refurbishing an old Lexicon delay

[synthiaks]

No load is good, so you can Narrow it down. If the PSU by itself doesn't measure correctly then you have to start there. And you don't risk breaking anything else with wrong voltages.

When the PSU are back to normal you can connect it to the rest and see if it still delivers under load.

Me personally,  Just the looks of those huge PSU filter caps gives me the creeps. That's the first thing i would replace. And any Tantalium in there. If they read good now they probably wont in 5 years anyways, and who knows  what they do when put under DC..

I just get a feeling there is a short to ground somewhere. (Tantals tend to do that when the go bad...)

/s

 

[Studiogearlover]

No load is good, so you can Narrow it down. If the PSU by itself doesn't measure correctly then you have to start there. And you don't risk breaking anything else with wrong voltages.

When the PSU are back to normal you can connect it to the rest and see if it still delivers under load.

Me personally,  Just the looks of those huge PSU filter caps gives me the creeps. That's the first thing i would replace. And any Tantalium in there. If they read good now they probably wont in 5 years anyways, and who knows  what they do when put under DC..

I just get a feeling there is a short to ground somewhere. (Tantals tend to do that when the go bad...)

/s

Thanks. I see your point. I do have another can cap.. but its much higher capacitance one. The large on the pic I've shared I have  27000uF 15V. Can I swap this with a 46000uf?  Interestingly this cap now says leaky on my LCR ... few days ago was OK.

Will swap them all.. 

 

[Monte McGuire]

In general, I have found that the larger PSU filter caps tend not to degrade as easily or quickly as smaller electrolytic caps. Given that mechanical issues can make their replacement very difficult, and the replacements may be just as old as the originals, my policy is to try to measure them somehow to see if they are within some semblance of their faceplate specs,. If so, leave them alone and replace all of the rest of the smaller caps, which are often not still good in older gear.

1974 was still in the era of slightly "iffy" commercial semiconductors, so it is entirely conceivable that some device decided to fail somewhere, and this is precipitating the problem.

Re SCRs: back in the old days, an SCR was used as an output clamp so that if a PSU failed with an excessive output voltage, the driven circuitry would be somewhat protected from this excessive voltage. Expensive and large devices like mixing consoles and expensive digital delays would be good candidates for using an output SCR on each power supply rail. It is potentially possible that these SCRs have failed and need to be replaced. However, I would not just "remove them" since they may be saving the downstream circuitry, as intended, by clamping an over-voltage, possibly caused by a dead regulator or open zener diode! But try to determine if they are dead or not somehow and replace as needed.

I have some of the earlier Delta-T 101 units at home, but I'm traveling now, so I can't offer much specific help. But, I'll try to dig for data!

 

[Studiogearlover]

Thank you guys once again the help. I am still struggling finding the source of the problem. I did go ahead and put new filter caps in there. Still, I am not getting this right. These wet tantalums are going to be replaced this morning as next step.

Few of you mention that you might have the schematic? Please,let me know if you do. I am also happy to pay any cost involved for the photocopies etc... please, if you do, it would help me understand more the PSU section and the points of didgtal vs chassy ground. ( I am getting confused on that where is what)

Please, help me if you can.

Thank you so much

 

[Studiogearlover]

I had a simliar problem on my Delta 102 stereo a couple of years
one of the memory cards draw too much current
and crashed the -5v.

It's a complicated circuit but if a IIRC it was one of the power transistors that was bad
have you checked them?

I have the manual, will look at it when I have time and see if I made any notes.
It also has the board layout which helps a lot.

Hi...  hope you doing fine.. just wondering, if you do have the PSU schematic or manual please do let me know. I am really puzzled what is going on with this Lexicon and why I cannot find this issue. I am sure the manual or the PSU schematic would help me at least to see where are the digital grounds and the actual grounds ( as there are several, and different locations) ..i was hoping to exclude anything I am doing wrong, and to get the correct readings.

I would be really really thankful...please do let me know if you have the manual/ schema.

Thanks sooo much ! 

 

[Studiogearlover]

hey guys..please help..

I am still struggling with this Lexicon unit... the -12 V is what I am not getting. Instead, I am measuring -7 V. I did replace the LM7912.

As there is no schematic, i did a quick (sorry for the quality, can't do it better) of the -12V circuit. I am unsure if this is correct but if it is, i am puzzled why i am not getting the -12v out with this regulator.

Input is 15VAC if I am not mistaken...

Thanks guys !

 

[Walrus]

That diagram is wrong, the input from rectifier/smoothing cap goes into Pin 2 and output comes out Pin 3.

What is the voltage across the main smoothing cap straight after the rectifier?

 

[Studiogearlover]

That diagram is wrong, the input from rectifier/smoothing cap goes into Pin 2 and output comes out Pin 3.

What is the voltage across the main smoothing cap straight after the rectifier?

Thanks.. I have got 19.22 VDC across the smoothing cap.

There is no other way to solder in the regulator as the place of this is factory set and the pcb is the factory made one.. could this regulator be different to an old type of regulator from 1974? 

here is the bottom view and the top view with a BD242 where now the regulator is... ( the BD242 came from me replacing the TIP30 which was also at the same place here, after being advised that this should be a LM7912)

Not sure what to do...