Echoplex EP1 power problem

[norbert]

Hello,

I was lucky to come across a very early 1st run CMI Echoplex EP1 s/n 169 from a friend who had it in his basement for the past 3 decades. It is in really great condition but when powered up the first time snapping sounds occurred so I found a surplus GE multisection 40/40/30/20 350v. cap (original was Mallory FP) and installed it. It ran and I was able to grab the contents of the 30yr old loop on my recorder and had it powered for about 5mins and turned it off. Later when I tried turning it back on the motor didn't turn at first, so I pulled it back out of the box and it then did start to run but then the 100ohm 1/2 watt resistor (r29) smoked that is going from the diodes to the 40uf (c18a) section of the GE cap, I replaced it and the next one heated also.

I looked at the published schematics but this EP1 is not wired as the series 5000 which is posted online. I think this is a close schematic, http://www.loopers-delight.com/tools...lex_schem.html. I still need to trace it all to confirm. If anyone has tips on what I can check for voltages etc. and how to confine any further testing from damaging tubes or other parts and to confirm that possibly the NOS surplus (I was told it was reformed by the surplus company and supposed to be good as a new one) cap is also bad I would appreciate it. My LCR meter shows the cap values correct. Any opinions on the historic value of the unit vs. upgrading it with newer caps would also be welcome, I'd like to keep it as stock as possible. Any comments on good or bad experiences with cutting the top off the original Mallory cap and using a few new electrolytics inside to keep the vintage look also welcome. If someone knows where to get a fresh c18 cap that would be good news if that seems likely to be the problem.

Thanks,

Norbert

[52 Bill]

Welcome.

the 100ohm 1/2 watt resistor (r29) smoked that is going from the diodes to the 40uf (c18a) section of the GE cap, I replaced it and the next one heated also.

Something is drawing too much current causing r29 to overheat.

Start by pulling out the tubes to see if they are the cause. If the resistor still overheats, unhook r28 and or r31 an see if removing either circuit from the power supply reduces the current draw. If the resistor still burns, the new cap should be checked.

[Regis]

NOS caps are a bad idea. You can get a new one built to spec at Ted Weber's site for twenty bucks.

[norbert]

tracing power problem

Thanks for the tips Bill and Regis,

I've been to your Echoplex site a number of times Regis, thanks for putting that up. Schematic Heaven is where I grabbed the manual with various model schematics. This model is not consistent with the above link schematic beyond the power transformer to the rectifier so am in the process of drawing an "as built".

So I took the tubes out, still smoking R5 and noted quickly warming transformer, measured only 40vdc at the diode side of R5 and 66vac at the transformer side of each diode. The above link EP1 schematic show cap side of R5 should be at 260v. The EP-1 model 5000 schematic shows the diode side of R5 at 140v as does EP2 schematic. The AC side of the diodes is reading 66vac to ground.

Started backing out towards the power transformer. Lifted cap side of R5 and measured slight increase on R5 diode side at 49vdc but still hot transformer same 66vac. Disconnected different cap sections same result. Removed the cap entirely, same.

Disconnected DC side of diodes and now get 127vac on the transformer sides to ground 240v across hots. One diode is getting 53vdc to ground, the other zero. When measured across open diode DC ends voltage is 100vdc. Testing the diodes with ohmmeter shows that the one with zero volts is zero ohms in both directions with the meter. The one with voltage is showing 38ohms and infinity in the other direction.

I'm assuming this means that a rectifier diode needs replacement. The markings look like they are Semitron 1401 and Google shows many references to obsolete etc. The manual p.36 shows that the EP1-2 are the same rectifier diodes part# 919-043244-001. I've seen other model rectifier references to 100v/1a but since some are full wave and driving only transistors, I'm not sure that is a safe assumption.

I could use some help to verify that it seems the problem is the diode and what is a suitable replacement or source for NOS, assuming no shelf life issues. The NOS GE cap does test OK but won't be going back in and the Ted Weber page is not working so replacement cap options are welcome.

I'm also still looking for opinions on the value in restoring to working condition with new caps etc. vs. trying to keep it vintage and if need be cutting the mallory FP top open and installing new guts, assuming 4 suitable caps will fit.

[52 Bill]

The manual p.36 shows that the EP1-2 are the same rectifier diodes part# 919-043244-001. I've seen other model rectifier references to 100v/1a but since some are full wave and driving only transistors, I'm not sure that is a safe assumption.

Have you tested the diodes with your meter? In that circuit a 100 volt diode will not work. 1 amp would be more than enough current, but I would suggest at least a 400 volt rating. A standard 1N4007 would work fine.

I'm also still looking for opinions on the value in restoring to working condition with new caps etc. vs. trying to keep it vintage and if need be cutting the mallory FP top open and installing new guts, assuming 4 suitable caps will fit.

Restored and working will always be worth more to me than vintage broken. Whether you replace the cap can with a new one or rebuild the old one for looks is up to you. I've rebuilt can caps and they do look good. I suggest opening up the can from the bottom by prying up or grinding off the sealing edge of the can.

[norbert]

Hi Bill,

The resistance on one diode as zero in both directions, the other with 38 and infinity. I grabbed a pair of 3a 400v 1n5404s from Radio Shack figuring higher rating would be safe.

I like the suggestion to open the can from the bottom, it sounds cleaner than slicing it and trying to pair the top and base later. Need to get some caps to fit it next.

Since it is all open, there are a few Good All .1 400v (paper?) caps as well as about 4 10uf 50v. caps in it. First thought is to leave them to keep it sounding /looking original if it works. Most articles suggest replacing all non-mica caps. I'm not up on good substitutions so suggestions regarding replacements to reduce noise and chance of failure etc. vs. leaving it alone are welcome.

Thanks again,

Norbert

[52 Bill]

Those diodes should be fine.

So as to not confuse things too much, get the unit up and working before you start to change anything. I may be necessary to replace a few caps, but make it work first.

[norbert]

Good advice! I agree with taking it a step at a time but... since I need to order caps I'm trying to minimize many separate orders. I'd rather spend a little $ on stuff I might need than on repeated shipping and always waiting for orders. There are a lot of "tone" related expensive caps and resistors being marketed that seem suspect to excessive hype so first-hand experiences with replacements would be very helpful.

I was mistaken earlier regarding later model EPs using full waves vs. the EP1 and see now that this is a full wave rectifier with the center tap transformer setup.

The diodes are in and I now get a reading of +116vdc to ground but am a little concerned that the schematics all show 140v at that point. Not sure how that would be the case since there is only 128V coming out of the transformer's secondary legs to ground. ?

[52 Bill]

I'd rather spend a little $ on stuff I might need than on repeated shipping and always waiting for orders. There are a lot of "tone" related expensive caps and resistors being marketed that seem suspect to excessive hype so first-hand experiences with replacements would be very helpful.

That's a good way to order things. So use the existing cap until you make it work and then order everything at once.

As for high tech replacement parts, there may be some benefit from using some of them, but remember that you'll always be limited by the frequency response of the analog tape.

The diodes are in and I now get a reading of +116vdc to ground but am a little concerned that the schematics all show 140v at that point. Not sure how that would be the case since there is only 128V coming out of the transformer's secondary legs to ground. ?

Is the output of the two diodes connected to a voltage divider with a 33K going to ground?

The standard formula is to multiply the ac voltage by 1.4 for the approximate dc output voltage. So 128 X 1.4=179.2v

[norbert]

EP1 back from the dead

Hi Bill,

I had originally written off the surplus cap as the problem but thanks to your advice to work with what I have and get it working I reconsidered what had happened.

When it was first plugged in the orig. Mallory snapped and something smelled smoked. In addition to the top four screws, the EP1 has 3 screws on the bottom, two to the chassis and one to the drive motor mount. The chassis depth is direct to the box but the drive motor mount is not and needs a spacer. The original screw and spacer were removed before I got it so I used what looked like a good depth plastic spacer and J-clip to secure the motor. I think it was not long enough and when tightened might have caused the mount to be pulled down and bind the motor. That would explain why it didn't turn when first put back in the box and did when I removed it, unfortunately this is when the R5 smoked. If anyone has an EP1 and can show a pic of how the stock drive mount is supported to the box and dimensions of any (rubber motor mount type?) spacer that would be great.

I'm guessing that either the bound drive motor (only a few seconds powered on) or the original cap that was shorted could have damaged the rectifier diode and in turn the short was overloading R5 causing it to smoke when connected and also causing the transformer to heat quickly even with no load to R5. I checked for shorts and resistance on the NOS cap and it looks OK so I ended up putting the NOS cap back in and it seems all working well again. The damage total was minimal and included only the original cap, 1 diode and a resistor.

Next step is to finish a good schematic and get voltages etc. and start looking at it's performance and sound with new tape etc. I'm not sure where the voltage doubler circuit is. The new diodes fixed the rectifier but the voltage still looks low according to schematics so I'm going to replace the pair of 3A with the 1A diodes for comparison and to restore it more to original specs.

The cap replacement has been an issue from the start and there are a few options I've found for consideration, I may be wrong about the following and welcome feedback in case the NOS cap smokes again.

The most stock option is the surplus NOS cap but most comments on old caps are to avoid them. The other option is to stuff micro sized electrolytics into the original can. I found an interesting post by a radio restorer that advises against this as the smaller caps likely will not last long with the high ripple current from the rectifier.

It looks like to get low ESR and higher voltage rated micro caps would increase their ability to handle the ripple current but then the size issue (each cap diameter larger than 16mm) of being stuffed into the 1.5" mallory could be a problem. There is also a Vishay guide that advises at least 10mm between caps to allow heat dissipation. Trying to get a handle on all the replacement options is a chore, if anyone has done stuffing on a Mallory 40/40/30/20 350v and can provide cap details and how long it has been used since it would be good to know. The smaller caps under the chassis with the original clipped and left mounted seems probably be the cheapest and least destructive to the originality of the EP.

Larger caps such as the Sprague Atoms would seem to be better as individual replacements but they are huge and would take up too much space and look totally non-vintage as would newer CDE polypropylene ones. The poly ones are about the same size as Atoms would never dry out as an advantage over electrolytics and is seems interesting if concerned with making upgrade mods.

Thanks again,

Norbert

[52 Bill]

I'm guessing that either the bound drive motor (only a few seconds powered on) or the original cap that was shorted could have damaged the rectifier diode and in turn the short was overloading R5 causing it to smoke when connected and also causing the transformer to heat quickly even with no load to R5.

The motor is ac driven from the ac line, so there would be no reason that a binding motor would cause a dc supply problem. It has been a long time since I've worked on a EP-1 and I own an EP=2, so I don't know the answer to your spacer mounting question. For what it's worth, I don't ever remember having to use a spacer in any of these things.

I'm not sure where the voltage doubler circuit is. The new diodes fixed the rectifier but the voltage still looks low according to schematics so I'm going to replace the pair of 3A with the 1A diodes for comparison and to restore it more to original specs.

As far as I know, there is no voltage doubler in these things. If you meant the voltage divider that I had asked about, then look for a 33K resistor to ground from the end of R5 where the diodes are connected.

The cap replacement has been an issue from the start and there are a few options I've found for consideration,

You've done a lot of research into the whole cap replacement question. Opinions about this subject are quite varied and often subject to pure conjecture rather than fact. There has to be some middle ground regarding vintage originality and safe working operation.