Lexicon 224xl repair

Hi all,

long time lurker around here but first post.
I purchased a good working Lexicon 224xl reverb some month ago and started to change caps especially on the power supply. Some two month ago I repalced the fan as I could hear the bearing were worn out. I found a Papst 115v fan that was a fit.

Now I do not recall anymore if that is related or if the issue was around before or after but on startup I can hear a loud hum while the unit is doing its diagnostics. As soon as the diagnostics are finished there is a pop, some sorf of fanfare sound which decays after some 2 seconds. After this the unit is working fine, no errors in the diagnostics but an minimal audible hum still just above the noise floor.

I do believe the problem is in the AIN or AOUT board. I would really appreciate if somebody could chime in who has experience with the 224s and may be this is something which does sound familiar to one of you.

Thanks a lot
Tom

Thats sounds distinctly odd, I dont remember any noise during diagnostics. Have you checked your power supplies are all good?
I modify 224's so that there are leds for all the voltage rails on the cards. That way you can see at a glance if the rails are there.
The most common fault on the analog cards is opamp failures. Run the unit in the in/out mode and check that noise and distortion are to spec.

Thanks for answering. It is really a bit weird. I mean that thing fires up with no issues but keeps humming from when I push the on/off button. I have to admit I did not check the voltage first as in my mind that thing would not startup w/o errors if there would be something wrong with the voltages. Anyway I checked the AIN board at the 15v test points and it would give me something like 11,5v. Not sure what to make of it but I will check the rails now.

Anyway, I was trying to wrap my mind about where on the analog boards an issue may arise and there are exactly two locations in the schematics of the service where the signal is split to half of an opamp each before being sent to symmetric driver circuits. I will replace the respective opamps and let you know if that made a difference.

BTW there is some switching noise when going between i.e. Dark Hall and Concert Hall. Does that ring a bell with you?
Thanks a lot
Tom

One question: Do you know a replacement for the LM 398 ICs? Struggling to find information on that type....

Ok, took the time to measure the rails...
Looks like something wrong with the +-15V rail.

With AIN an d AOUT removed the rail measures 14,85v.
With AIN inserted it drops to 13,4.
With AIN + AOUT it drops to 11,8.

As the audio signal is fine except for that modulating hum in the background I am not sure if the opamps are the issue as this would mean there would be issues on both cards.  For me it seems like the power supply is not able to keep up with the demand as voltage drops with each time another card is inserted. Possibly a faulty voltage regulation for the 15v rail?

Check the filter cap before the 15 volt regulator. If that is OK then it could be the regulator. You should have 15v +/- .5 volt.
Where is the LM398 used?

Thanks for the hint. I am currently looking at the service manual and the only caps in front of the regulators are C6 and C10, which happen to be the two 3300uf caps I just replaced with brand new Nichicons. I guess they sould be fine.  Next I will check the voltages at the measurement points as depicted in the schematics and report back.

Anyway, as I am not an expert, can you tell me which of both regulators, 317 or7912, is the reference for the opposite polarity rail? thx
Tom

The LM 398 is just right before the Mux stage on the input board and directly tied to the 15v rails which is the reason why it caught my eye. On top it is the only opamp which is not DIP but has the transistor type looks with long legs. As ist is also surrounded by tantalums they are surely affected by the dopped voltage and so far I havent been successful finding out an adequate replacement in case they blow. Looks like they are unobtanium. If you happen to know a drop in replcement plz let me know

You can use LF398 instead of LM398, plenty available. The 7912 is the negative regulator, in this design it works as a tracking regulator with the 317 as master. However if you have replaced caps, my guess would be you didn't get it quite right. Its possible to ruin the VIAs on those boards taking parts out. Check with a scope for the ripple on the input side of the regulator.
Tants are a real problem on these units. The best way is check all supply lines on all cards for shorts, then work through the tants till you find the shorted one. Pull the cards first of course! Then re-assemble the frame starting from the bottom card.

By the way, this is where LEDS on the rails comes in handy.  Although the supplies have short circuit protection, its not good to keep them shorted too long.

Great input, thanks a lot.

Looked up the LF398 and the LF398H seems to be the equivalent type. Good to know.
Regarding the replaced electrolytics I will have a look at whether I killed a route but doubt it. I use a Denon desoldering gun for all my work and usually that does the job very gentle and clean but I will check to be sure.

Yes, the tantalums are really a pain on this machine. I made that experience on a TC 1210 Chorus that I bought defective. It had 2 blown tantalums which took out the ICs along the route. Replaced the tantalums and the ICs and voila, it came back to life.  For now I only checked the tantalums on the PS, the AIN and AOUT board with a DVM.They seem to be ok but I will check the  tants on the other cards as you suggested.

Would you replace them with new tantalums or with electrolytics as many people do? If so what series from Panasonic or Nichicon would be suitable regarding ESR and high frequency response?

One question still.  317 is the master and the LM301 looks like taking care of tracking the 7912. Looking at the specs I was just wondering why the 7912 is in as it is designed to supply 12v while the 7915 is by design setup for 15v. How comes that the 7912 is used here on a -15v rail?

Thx

You can use electros or tantalums, the tantalums are better now. Or solid aluminium are good. Use higher voltage tantalums.
The design with the 7912 works fine, they offset the gnd pin with the opamp.

jackson5 said:

It had 2 blown tantalums which took out the ICs along the route. Replaced the tantalums and the ICs and voila, it came back to life. 

Would you replace them with new tantalums or with electrolytics as many people do? If so what series from Panasonic or Nichicon would be suitable regarding ESR and high frequency response?

Click to expand...

The first statement, "2 blown tantalums which took out the ICs along the route" should answer your question about aluminum vs tantalum caps.

Hi Andy,
totally agree...I will try to replace each one of them, especially in the PS once I find the time.

@RadarDoug
Kudos! Thanks for answering and all your hinsing me to the solution, really appreciated.

Now wait, the story gets even better:
I checked all the cards once again and there was this big green blob of a 22uf tantalum cap which was right in my face all the time when I ripped out the leaky NiCd rechargeables last week - and it was shorted! Replaced it straight away but for now I did not see any difference. Looking for corrosion thanks to the fallout of the NiCds I found that some previous owner or a "qualidfied technican" obviously changed one of the ICs but left one pin such that it was bend in underneath the chip. You cannot make up that sh....
 
Now that I fixed all of it I could not be happier. This reverb really sounds unbelievable and is worth all the hassle. I do have the 70 and 81 Lexicon, an Eventide but they cannot compete, it is really awesome. Having said that the real kicker is the LARC. NO deep dive menus but simple direct manipulation. Glad I made the deal....

My plan now is to make that unit ready for the 21st century as far as possible in the coming weeks. Therefor I would  need advice replacing the tantalums. My question is again which of the many series of Nichicon and Panasonic have similar properties and therefor could be used w/o any concern? 

Lets put it like this: Most of the time you can get away with just replacing them with aluminum type of similar value but occasionally they were really used intentionally by the designer due to their low ESR. So I am not sure if it is always the best to change to electrolytics w/o any further considerations.

Would be great if somebody could answer....

Mostly you can just put in new electros, the parts today have much lower ESRs than the old ones. Or if you are worried, find some solid aluminium caps.  But new tantalums are better than the old parts. Use a higher voltage rating. Tantalums often fail on the switchon surge, and go to short. A higher voltage part will be less likely to do this. In electro's, stick with the same voltage rating.