Echoplex EP-3--No Output

[wsscott]

A buddy recently got an Echoplex EP-3 that had been "in a barn" for years, and although it runs, it has no Output.  I told him I'd take a look and maybe it would be something easy, but I'm running into a dead end.  The filter caps all test fine.  I ran 1 KHZ test tone into the unit, and hooked up my baby Oscilloscope in AC mode to see if I could trace the signal.  I'm attaching a copy of the schematic.  I was able to trace the signal to the 220K resistor coming off the Echo/Sustain pot.  I've got a signal going into the 220K resistor, but nothing after that point.  The wiring from that point, over to the Volume pot, and then onto the Output Jack is fine.

Any thoughts/suggestions?  Thanks.

[Keppy]

Sounds like either:

• the 220k is open, as in many infinite Ω instead of 220k, or
• the dead end of the 220k is shorted to ground.

Have you metered for either of those conditions?

[wsscott]

I checked the resistor, in circuit which I thought I could do without false readings, and it read 139k ohms. So I replaced it with one I tested first and which read 220k on the money. No change. I then tested it in circuit and it read 135k ohms! So I checked the one I removed and it was reading 225k. So both are fine. So am I not able to read this resistor in circuit, or is this the sign of the problem? One weird thing is when I took the chassis out of the cabinet I found a 2” black wire that looked liked it had been soldered somewhere at sometime, but I can’t find any spots where it was attached. And I heard what turned out to be the wire loose in the cabinet before I opened things up. So I didn’t break it loose.

[Willabe]

Might have to check the whole amps wiring to see if you can find where that 2" piece of wire came from?

Here's Doug's double checking methods link;   

https://el34world.com/Forum/index.php?topic=17701.0

[wsscott]

I just put my meter on the "dead" end of the 220K and checked it for continuity with chassis ground.  Sure enough when I touch the lead to the chassis it shows continuity, so its leaking to GND.  Which I take it that it shouldn't be.  I looked at the traces on the board casually and didn't see any issues.  Any thoughts on where to go from here?

[Willabe]

I just put my meter on the "dead" end of the 220K and checked it for continuity with chassis ground.  Sure enough when I touch the lead to the chassis it shows continuity, so its leaking to GND.

You can't test it that way. You have to test it for resistance to ground.

[wsscott]

Obviously my skill set is limited.  So tell me if this is the correct way to do it:  I set the DMM to Ohms, connected the red lead to the dead side of the resistor, and connected the black lead to chassis ground.  It read about 20 ohms or so, numbers were changing a bit.

[sluckey]

Obviously my skill set is limited.  So tell me if this is the correct way to do it:  I set the DMM to Ohms, connected the red lead to the dead side of the resistor, and connected the black lead to chassis ground.  It read about 20 ohms or so, numbers were changing a bit.

That's quite normal if the ECHO VOL is turned down. Does that resistance reading change as you turn up the ECHO VOL?

[mat janssen]

Could it be that the heads are dirty?? Clean them, maybe it helps.

[wsscott]

Sluckey, yes it increases from about 20 ohms to about 120,000 ohms as I increase the volume pot setting.

[sluckey]

Sluckey, yes it increases from about 20 ohms to about 120,000 ohms as I increase the volume pot setting.

Well then you're in the wrong rabbit hole. Keep looking.

[wsscott]

Yea, I guess so.  I put the meter on the output jack and ground to chassis, and turned the Volume knob and the resistance changed just like it did at the resistor.

[wsscott]

So does it make sense that I can see a change in resistance (consistent with it when measured at the "dead" side of the 220K resistor) at the Output jack when I turn the Volume pot, but no signal comes out of that resistor and on to the Volume pot even though I can trace a signal going into the 220K resistor?

[sluckey]

I assume you are putting a test signal in at the INPUT jack and you have no signal at the OUTPUT jack? The dry signal path is from input jack through Q5 and a couple 100K resistors to the output jack. That 220K ain't even in the dry path. The 220K takes the high frequency ECHO head signal, strips the hi-freq signal with that 470pF/100K filter, leaving only the echo signal to be mixed with the dry signal.

So tell me, if you put a 500mV_pp 1KHz signal into the input jack can you get an output signal at the output jack with the ECHO VOL pot set to midrange?

[wsscott]

Yes, I was injecting a 1Khz signal at the Input jack and have no signal at the Output jack.  I don't remember how strong it was, so I will do it again at 500mVpp and let you know the results.  I did adjust the Echo Volume in the process with no response.  Thanks.

[wsscott]

Sluckey-Nothing at the Output jack.  I do get signal at both of the Foot Switch jacks.

[sluckey]

Trace the dry path. Got signal at point 4? Got signal at point 23? Got signal at bottom of ECHO VOL pot (as viewed from schematic)?

[Keppy]

So does it make sense that I can see a change in resistance (consistent with it when measured at the "dead" side of the 220K resistor) at the Output jack when I turn the Volume pot, but no signal comes out of that resistor and on to the Volume pot even though I can trace a signal going into the 220K resistor?

No, that does not make sense. If you have signal on one side of the resistor, you should have signal on the other side unless there's low resistance to ground (or to another low impedance node, like a transistor output). Have you double-checked to make sure the scope reading was done with the volume pot set appropriately? And was the scope attached directly to the leg of the resistor, not some other component/wire?

[wsscott]

I traced the signal from point 4 then going into Q5, but nothing coming out on either leg of the transistor.  Nothing at 23 either.

[sluckey]

OK. Now check the dc voltages on Q5. Should be about +14V on drain and about 1.1V on Source. If voltages are wrong also check voltage at top of that 22K resistor. I don't know exactly what it will be but it should be between 14V and 22V. Also check voltage at points 12 and 5 (should be 22V). Report. And meanwhile start searching for a replacement for Q5.

[wsscott]

The 3 caps are reading 33, 25 and 14 VDC respectively.  I'm reading 14 VDC at points 5, 12 and 23 and at the bottom of the Echo Vol pot, as well as at the top of the 22K resistor, and on BOTH the Drain and the Source.

[sluckey]

The 3 caps are reading 33, 25 and 14 VDC respectively.  I'm reading 14 VDC at points 5, 12 and 23 and at the bottom of the Echo Vol pot, as well as at the top of the 22K resistor, and on BOTH the Drain and the Source.

Those are some good clues. Time to hone up your troubleshooting tools.

[wsscott]

Sluckey, as you know, I"m a novice.  I don't know if I did the Q5 readings properly.  I connected the red lead of the DMM at point 5, and the black lead to the Source and then to the Drain, and I got these readings of 14VDC on each of them.  I later thought maybe I should have connected the red lead to the Source and then to the Drain, and the black lead to the chassis.  When I did that, I got only about 1.4 mv on each of them.  So was either of these connections done properly by me?  Presumably not.  As I think about it, it seems that if 14VDC is getting to Q5 as it should be, then either the Source or the Drain will be hot, and so connecting the black lead to the chassis would read the voltage.  I screwed up one way or the other.

[sluckey]

I can't do this. I'm dropping off this thread.

[wsscott]

I understand.  Sorry.

I'll close it out.

[wsscott]

I found that the 35uf cap coming off the 22K resistor was shorted internally, and I replaced that with a 50uf, all that I had, and now have sound coming from the Output.  Its not being processed by the Echo, but I'll see what I can find there.

[wsscott]

Well I've finally got it working.  I'm getting Echo and SOS.  Sounds a bit rough, but hopefully cleaning and demagnetizing the heads will clean that up.

[Willabe]

Good job! 

Clean the pinch wheel and capstan too.