Finding a thyristor... refurbishing an old Lexicon delay
« on: June 10, 2020, 08:27:22 AM »
Hey Guys.. please help me with something very specific and I can not work it out.  Its a DELTA T 102 from 1974.

I have an old Lexicon unit, a wonderful one and I am loving it. Cut it short, sound is distorted. So grabbed the operation manual and went through the troubleshooting. This device sends the following voltages to the delay and other modules:

-12V, +15V, -15V,  +5 and -5V.

Troubleshooting advised how to exclude the issues with the installed cards, which I did and concluded that there is no -5V and no -12V  going out to the cards. +5V I have, and also I have +5 where I am suppose to have -5 V.

I did inspect the main PSU board. At the voltage regulator I had all of the above voltages. But when I am going to the following parts in the circuit, at 2N3055 and two of the TIP33s, there isnt any measurable voltages in fact i am getting some -1.4V instead or something like that.

Schematic is untraceable, only one offered on ebay for 250 USD for a schematic which I found that this person who is selling it is living in a fantasy world.

So, decided to go and replace the two TIP33 the 2N3055 transistor and there one... picture attached....which is responsible for the +5V ( is I am not mistaken) is a thyristor as one of the leg has a letter "G". There is absolutely zero markings on this. Next to this I have a 250uF cap.

The only information I have from the manufacturer that next to this capacitor in the early models, they had a 2N681 thyristor but no mentioning on what was this replaced to later on. The capacitor is to filter out the +5V and reduce noise.

My question is what could be this thyristor type?  Its a bummer that there is no markings on this what it is and wondered if there is any later plastic model for this 2N681? Google is not my best friend this time.

I hope I am on the right path tracing down this issue. There is also a SG3501AJ dual voltage tracking regulator... should I start there instead? It is easily replaceable however rare stuff...

I would greatly appreciate of you could advise me or pointing me to the right direction.

Thanks guys !
« Last Edit: June 11, 2020, 01:41:17 AM by Studiogearlover »


Walrus

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #1 on: June 10, 2020, 09:02:41 AM »
Never heard of a thyristor being used to filter psu noise!
But AMS units, and API 650 PSU's use them as overvoltage crowbars. In the case of the AMS's, the zener used to check the voltage is known to go bad and cause the thyristor to short too soon.
You can tell if the thyristor is short circuit with a resistance check.
Kevin.

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #2 on: June 10, 2020, 09:32:06 AM »
Never heard of a thyristor being used to filter psu noise!
But AMS units, and API 650 PSU's use them as overvoltage crowbars. In the case of the AMS's, the zener used to check the voltage is known to go bad and cause the thyristor to short too soon.
You can tell if the thyristor is short circuit with a resistance check.

hey Kevin, thanks for your note... for the noise filtering is the capacitor I was referring to not the thyristor...apologies if I was not clear enough... lots going on in my head  :)

Resistance between the anode and cathode is 1kOhm. If this is bad, how do I know what thyristor I need as this one has no markings on it at all. Just a plain black plastic.

Cheers!

JohnRoberts

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #3 on: June 10, 2020, 09:45:43 AM »
Hey Guys.. please help me with something very specific and I can not work it out.

I have an old Lexicon unit, a wonderful one and I am loving it. Cut it short, sound is distorted. So grabbed the operation manual and went through the troubleshooting. This device sends the following voltages to the delay and other modules:

-12V, +15V, -15V,  +5 and -5V.

Troubleshooting advised how to exclude the issues with the installed cards, which I did and concluded that there is no -5V and no -12V  going out to the cards. +5V I have, and also I have +5 where I am suppose to have -5 V.

I did inspect the main PSU board. At the voltage regulator I had all of the above voltages.
including -5V and -12V?
Quote
But when I am going to the following parts in the circuit, at 2N3055 and two of the TIP33s, there isnt any measurable voltages in fact i am getting some -1.4V instead or something like that.
If voltages are correct at PS but not in circuit, sounds like perhaps a wiring issue.
Quote
Schematic is untraceable, only one offered on ebay for 250 USD for a schematic which I found that this person who is selling it is living in a fantasy world.

So, decided to go and replace the two TIP33 the 2N3055 transistor and there one... picture attached....which is responsible for the +5V ( is I am not mistaken) is a thyristor as one of the leg has a letter "G". There is absolutely zero markings on this. Next to this I have a 250uF cap.
not sure what a thyristor is doing in a simple PS , perhaps a protection shut down?
Quote
The only information I have from the manufacturer that next to this capacitor in the early models, they had a 2N681 thyristor but no mentioning on what was this replaced to later on. The capacitor is to filter out the +5V and reduce noise.

My question is what could be this thyristor type?  Its a bummer that there is no markings on this what it is and wondered if there is any later plastic model for this 2N681? Google is not my best friend this time.
2n681 appears to be a 25A 25V SCR....   still unsure of what it is doing.https://media.digikey.com/pdf/Data%20Sheets/Central%20Semiconductor/2N681(A)%20thru%202N692(A).pdf
Quote
I hope I am on the right path tracing down this issue. There is also a SG3501AJ dual voltage tracking regulator... should I start there instead? It is easily replaceable however rare stuff...
Am I misunderstanding that you wrote the voltages were correct at the PS?
Quote
I would greatly appreciate of you could advise me or pointing me to the right direction.

Thanks guys !
You can probably trace out the PS circuit better than I can guess...  good luck

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Walrus

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #4 on: June 10, 2020, 10:16:05 AM »
Quote
Resistance between the anode and cathode is 1kOhm.

Normally when they die it will be a short circuit, or a few Ohms.

What is the model number of the Lexicon?  Someone may have some info on it.
Kevin.

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #5 on: June 10, 2020, 10:42:02 AM »
Quote
Resistance between the anode and cathode is 1kOhm.

Normally when they die it will be a short circuit, or a few Ohms.

What is the model number of the Lexicon?  Someone may have some info on it.

Thanks Kevin, its a Lexicon Delta T 102 delay module. Some might say why I am wasting my time on it but even with a distorted sound, does something magical to the sound so it is worth it for me to get this right.  I did try and search on here for this, there are several threads but with different issues or not much responses...  seems to be a very rare unit.  Gotham Delta 102 was the original one... reading the manual this was built in 1974,

Quote from the manual " We welcome to assist you  by thelephone for help...As of this writing (  Sept.1974) , Chris Moore, Chuck Bagnaschi or Frank Ruel at Lexicon"...

I wish to talk to these geniuses...   ;D


In the meantime... this thyristor and the capacitor mentioned earlier is at the crowbar SCR circuit... see attached.

« Last Edit: June 10, 2020, 11:24:17 AM by Studiogearlover »

mjrippe

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #6 on: June 10, 2020, 03:42:40 PM »
Hey Guys.. please help me with something very specific and I can not work it out.

I have an old Lexicon unit, a wonderful one and I am loving it. *snip*

So, what is it?  A Delta-T, a Timeline, PCM-42, some sort of broadcast delay?  Help us help you  :)

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #7 on: June 11, 2020, 01:38:14 AM »
So, what is it?  A Delta-T, a Timeline, PCM-42, some sort of broadcast delay?  Help us help you  :)


hey... its a Delta T 102 from 1974.. I did say that earlier..i think as it was quoted many times, got missed :)  I have updated the first thread with this info :)

« Last Edit: June 11, 2020, 01:41:57 AM by Studiogearlover »

Walrus

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #8 on: June 11, 2020, 07:52:03 AM »
Here is the psu schematic from the Lexicon 224, it may share similarities with the Delta T, and will give you an idea how the thyristor is probably configured.
Kevin.

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #9 on: June 11, 2020, 08:18:05 AM »
Here is the psu schematic from the Lexicon 224, it may share similarities with the Delta T, and will give you an idea how the thyristor is probably configured.

Kevin, thanks a million for this !! On the picture I took maybe there is a 7905 negative voltage regulator and not a thyristor at all... will carefully study and compare this later on...many thanks again!


mjrippe

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #10 on: June 11, 2020, 11:35:12 AM »

hey... its a Delta T 102 from 1974.. I did say that earlier..i think as it was quoted many times, got missed :)  I have updated the first thread with this info :)

Ahhhh, now I see it - indented in a quote somehow  :D  I have a few docs, but no schematic.  There is an application note (about 20pp) and some tech doc on a BPO option? as well as a digitized copy of the demo record that came with the app note.  I've put it on Dropbox as the files are too big for GDIY. 
https://www.dropbox.com/sh/ntswfv7p49d6sgc/AAAgxVCaASke-cEDGE6cYiu7a?dl=0

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #11 on: June 12, 2020, 02:02:39 AM »
Ahhhh, now I see it - indented in a quote somehow  :D  I have a few docs, but no schematic.  There is an application note (about 20pp) and some tech doc on a BPO option? as well as a digitized copy of the demo record that came with the app note.  I've put it on Dropbox as the files are too big for GDIY. 
https://www.dropbox.com/sh/ntswfv7p49d6sgc/AAAgxVCaASke-cEDGE6cYiu7a?dl=0

Thank you soo much! This is fantastic. I do have the user manual but without the application note. :) Very useful stuff. I have noticed that this unit at some point were serviced since 1974. One TIP33 had been replaced to a Radioshack 2N3055. Also, one electrolytic capacitor (again RadioShack) too. and one diode around this part of the circuit ( the 5V's) Maybe (or not) this little voltage regulator which I thought to be a thyristor has been also replaced, does not look factory soldering on the pins. I wish to know really what it is... is it a 7912 or a 7902.  I am getting closer to this..

15v's I have
12Vs I have on the PSU board but not at the connection point to the delay unit.
5V i dont have instead , I have around 1.2 V +/-  more or less

Would you advise t o replace these old RadioShack capacitor? It reads fine... and there is an original one1000uF  Illinois Cap, reads fine.

Thanks!

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #12 on: July 03, 2020, 01:31:38 PM »
Guys please help !  :-[

Again, what I need to achieve here is the following:

1) +15/ -15 V  ---> done. Its there.
2) -5V is more or less , -4.8V there, depends where I am measuring against with the negative probe
3) +5V not there at all.
4) -12V. I am getting now -16

This is a bit of a complex circuit. It all comes down to the following:

The +5 V 7A is achieved form the +15/-15 V regulated source. According to the manual troubleshooting, if these voltages are not correct, or there is a short, will affect the 5V output. The 5V is regulated by an LM723 and a darlington connected series pass transistors for regulation and current foldback  limiting beyond 7A. In addition, an SCR crowbar circuit is sensing over voltages ( over about 6V) and clamp a short on the output for protection.

I can not measure 5V anywhere. First, I thought the LM723 is the faulty part, so I did swap that, having the same issue. No 5V. Everything else seems to be OK altough I did not check the resistors yet, but the diodes I did. Not much else here to check besides the lytic caps and they are OK too.  ( there is a ceramic capacitor and a varistor S10K20(?) and I did not check these)

Started to wonder, the reason why I am not getting this 5V at the output as somewhere I am getting an overvoltage (6V+) and the SCR crowbar circuit shorting it out as above explained in the manual? Could this be a possible reason?

The -5V is also powered from the same transformer windings and regulated by a LM104H. I can measure -4.8V  now, but it depends where my ground point is. 

I am getting really confused and i am lost a bit....  I went through this power circuit and checked/ replaced all of the transistors as most of them were bad. I did upgraded the diodes, not all of them, but checked all of them and they are OK.

The last bit is the -12V which is generated by an IC 12V regulator ( this was a TIP30) and a full wave bridge rectifier set of diodes and separate tranformer windings. I did swapped the TIP30 as looked a bit burnt and did not get the -12V but -11V. Swapped and replaced with a better specs BD 242. Now here I am getting -16V?! How on earth?

I know all of these are very difficult to check without having a schematic, but I decided to throw this in as my last resort.. maybe some of you experienced similar issues and found the way resolving it?

Many thanks for all the suggestions !  :)
« Last Edit: July 03, 2020, 02:09:42 PM by Studiogearlover »

synthiaks

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #13 on: July 03, 2020, 06:11:20 PM »
Is the PSU doing the same without load?

Did you measure the caps for DC leakage?

With a normal LCR they could test OK but put under DC they could be faulty. I would Remove all old caps in there.

With potential seeping DC all over the circuit it's almost like Witchcraft to troubleshoot.

1974 is a looong time ago. The gray 470uf cap on the picture has "Got to go" written all over it :-)

/S
"ex observer in silence"

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #14 on: July 04, 2020, 01:15:55 AM »
Is the PSU doing the same without load?

Did you measure the caps for DC leakage?

With a normal LCR they could test OK but put under DC they could be faulty. I would Remove all old caps in there.

With potential seeping DC all over the circuit it's almost like Witchcraft to troubleshoot.

1974 is a looong time ago. The gray 470uf cap on the picture has "Got to go" written all over it :-)

/S

Hey


Thanks for the suggestions. Since that pic was taken, i did replace that grey cap and there were another 2 similar  bigger within the circuit. There are 4 filter caps as big as my coffee mug which are Mallory computer grades and tested for ESR, 0.14hms and they test OK.


This whole PSU frame is not connected any load. I am measuring the outputs without load on.  Attached  a pic of the PSU unit. I am following the troubleshooting manual for the PSU section. It does not mention I should test the voltages with or without load ...  :(

I am seriously pulling my hair out.  Also not getting why the TIP30 swap to BD242 increased the voltages by 4 volts... unless, I should test these voltages with the load on connected to all the outputs of this PSU?

I will do a new fresh eye testing today. Could not sleep the whole night because of this..  :D

mg73

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #15 on: July 04, 2020, 02:02:10 AM »
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.

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #16 on: July 04, 2020, 02:49:12 AM »
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.

Thank you SO much ! That would help a lot. Yes, I did replace all the transistors...  but if some other parts in the unit is still faulty, could  again kill some transistors...i hope this is not the case tho.. :(

If you have the board layout or schematic of the PSU.. would be very very thankful.  :)

jensenmann

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #17 on: July 04, 2020, 06:25:00 AM »
Like mentioned before it´s a good idea to fully recap all electrolytics and tantalums. I would not bother to troubleshoot such an old piece of gear before it´s recapped completely (unless the fault is very obvious).
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #18 on: July 04, 2020, 06:28:06 AM »
Like mentioned before it´s a good idea to fully recap all electrolytics and tantalums. I would not bother to troubleshoot such an old piece of gear before it´s recapped completely (unless the fault is very obvious).

hi Jens, thank you for your note. Yes, I agree. I did recp tho except thos massive blue can Mallory ones... they do measure spot on. Would you recap these too? I know you mentioned ALL the caps... so this includes the tantalums, the ceramics and everything?


jensenmann

Re: Finding a thyristor... refurbishing an old Lexicon delay
« Reply #19 on: July 04, 2020, 07:32:20 AM »
I meant electrolytics and tantalums only. Other types don´t age as much as these do.
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort


 

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