Lexicon PCM-80

[W DeMarco]

Our lexicon pcm-80 started behaving nasty. The hammer could'nt fixit , thought Id post up and see if anyone has experienced the same issues.

Symptoms began with the unit not always starting up. Power would sometimes do nothing then either switching it of and on or letting it sit there with the powwer turned on would provoke to actually turn on. This has now become a unit that does not turn on at all. I am thinking it might be the battery soldered to the pcb.

Any ideas?

Will

 

[Geert Heyrman]

Hi ,

Did you try replacing that internal battery. I recognise the problem with my collection of SOny's DPS V77. they do the same thing when the CR2032 battery (I think that's the type) is flat.

Geert

 

[radiance]

Does it have extra memory installed? If so, take it out and see if it powers up. The simms might have gone bad (happened to me once..) and it will not start up because of that.
Another thing to check is the power supply.

 

[SSLtech]

Switched-Mode Power Supply.

You can TRY to repair it if you really like masochism, but I assume you don't, and I don't think that anyone here will be able to assist you... apart from Maybe Svart.

Keith

 

[Svart]

Did I hear my name? :green:

First thing you need to look at with any unit with a switcher is the fuse. then look at the power switch(ohm it to see if it actually contacts inside) then we can look at the powersupply. Switchers usually have a feedback circuit that will shut down the outputs if they draw too much current so you might have a short somewhere, but again we need to determine this through some detective work. Disconnect the PSU from the mainboard if possible and try to power it up again. Check DCV on the outputs. If there isn't any output then we need to test parts on the PSU.

If you determine that the AC is getting to the PSU, then do a DCV test after the bridge on the switcher to make sure the bridge is ok.

With switchers, you usually get a few things going wrong in this order:

1. bulk cap after bridge dries up and goes high esr and possibly shorts.
2. FET before HV transformer dies.
3. PWM controller dies from FET shorting internally and applying HV to the gate driver.
4. Output regulators dead.
5. Output caps dead.

It might be easy to troubleshoot if you take close pictures of the board so that we can look at them and determine where you should take your readings.

 

[SSLtech]

Hey Svart...

Can I send you twenty-seven Switch-Mode Power supplies that I've given up on here...?

Keith

 

[mkoz]

Lexicon's PSUs are dreadful. They are made to work just on the limit. I tried to repair one for my friend's MPX-1 in vain. Then ordered a new one and replaced it. Cost him 65 quid or something back then.

It's not worth trying to fix it, just buy a new one and replace it.

Mach

 

[Svart]

Nah switchers are easy to fix and easy to make better. they cheap out on certain components just like other types of psus.

Hey Keef, what's wrong with them and what are they from? I might be able to do something with them if you want... Or I can at least help you.

:thumb:

 

[W DeMarco]

I checked the Fuse, its good,

I checked the switch, it provides ac to the fuse no problem.

Battery read 3 volts with no load

Im gonna take some pics and post as was suggested.

I also checked to see if additional simms we're ever installed and the two slots are unused.

Thanks guys

Will

 

[tomelectro]

I replaced a PSU in one of those about a year ago. If I recall, it was only about $55, and the replacement was supposedly an improved version. Normally I try to repair them, but it didn't seem worth it in that case.

 

[Svart]

unless the HV tranny was fried, chances are you can fix and improve for less than 10$. I bet it uses a uc3842 IC..

 

[W DeMarco]

I plan to post up some pics of the PSU Later in the week. Thanks for the input guys.

 

[burdij]

Your switching supply may have some bad electrolytics. The main filters (the big caps) usually aren't the problem. Its one or more of the smaller ones that will have a problem.

I have noticed that when switchers start working intermittently (rather than failing suddenly) the problem is often the electrolytics radically changing values. This is due to heating in a confined space and is particularly evident in 1RU switchers. The solution is to carefully remove each electrolytic and measure its value with a capacitance bridge. Replace any that don't read close to the marked value. Try to use 105 deg. C caps rather than 85 deg. C units to prevent future failures.

 

[Svart]

Actually the bulk cap after rectification is a big problem in most switchers. the ESR goes through the roof as the cap degrades from being pounded by smoothing the DC and sourcing huge surge currents to the FET chopper circuit. Charging a transformer winding is no small feat for any cap especially if the FET is mismatched for the application, the leading cause of FET failures in SMPS. A short explaination is that the cap heats up internally due to shorting the RF and from current demands and then dries out leading to value change. Unless the cap is obviously bad the only true test is an ESR/value test through a dedicated testing device.

Output smoothing caps are a general failure and can lead to the unit going into current limiting or shutdown. 'Lytic caps rarely short internally and then unshort in cycles. Usually once shorted they overheat and vent or explode. Again, they almost never change value signifigantly during a normal usage cycle and once "bad" they don't go back to a normal value. Again this is a failure associated with caps that have "dried up". RF noise from the switching and rectification tends to lead to parasitic problems for the regulation which can cause erratic behavior once the caps have gone bad.

I have yet to see a unit that internally reaches the 85 or 105c max for most caps, as that is 185-221F.. The cases would be smoking hot before that temp is reached as well as the temp ratings of the caps being rated for a period of time at that temp.

Your symptoms actually point to a more common solder fatigue problem. I would of course follow my SMPS troubleshooting guide and check all solder joints around areas that see mechanical stresses such as the transformer, or electrical stresses such as the FET etc. Parts in a switcher don't usually go bad then suddenly become OK to work for a while and then go bad again, but solder joints do arc and fuse for short periods of time until they cool down and the joints pull apart again. This generally happens a few times until the gap is too wide, from the arcs eating the edges of the solder away, to make contact and the unit ceases to work altogether. And remember that the gaps I am mentioning can be extremely tiny, it would be benficial to reflow each joint and then try the unit again.

Please post pics, especially of the bottom of the board.

And don't worry we'll get you fixed up.

:thumb:

 

[radiance]

[quote author="Svart"]

And don't worry we'll get you fixed up.

:thumb:[/quote]

This forum rocks! Can't wait for my PCM80 to break down..... :wink:

 

[radiance]

I thought that the life span of a capacitor lenghtens when it's run cooler.
That is: the 105C cap will last longer than the 85C one if they're both run at a temperature of say 40C

 

[Fabriguitar]

Hello boys, I hope that after 15 years someone still has an interest in the circuits of a PCM 80. It still has a very sweet plate ... ;-)
My problem is that after the diagnostics, often the "click" is not heard and the analog section does not start.
When it does not start, if I measure the voltages at the 6 wire output connector from the power supply, the 15 volt contacts are ok, but the 5v ones are close to 0 V. Often when I do this measurement, I hear the "click" and the PCM80 starts working. Obviously, the correct voltages are restored to the 6 wires.
By disconnecting the output connector from the power supply, all 6 wires measure approximately 0 Volts.
When the PCM works (connector plugged in) often the voltages at the 6 wires are correct (to be precise, the 5 V contacts measure about 4.9 V ...).

So each time it may just randomly start or fail, randomly. If I open the cover and disconnect the 6-wire connector, or if I measure the voltages with the tester, it usually restarts.

Reading Svart's answers, I believe I need to change the capacitors of the power supply, starting with the small ones.
Could someone please make a better diagnosis than mine by reading my description of the problem?
Thanks in advance to those who will take an interest in the problem. Ciao
Fabrizio

 

[Rob Flinn]

You may just have a bad connection. Might be worth reflowing the solder on the PSU.

 

[Fabriguitar]

Hello Rob, Thanks for your kind replay.
You're right, of all my assumptions I've never thought of a bad solder.
Soon I'll try to do reflowing!
Ciao
Fabrizio

 

[radardoug]

Molex type connectors on power supplies are often over rated. Also, is your connector pushed in wires? If so, solder them. This is a very bad method of connection. Soldering in the top is messy but possible. Much burnt plastic smell!