Dynacord EC504 - No Duration (Feedback)

[smilan]

Hi,
I have on the bench a Dynacord EC504 with almost no "DURATION" (feedback)
What is currently going on is that the signal at the "RETURN" (B1) is much stronger compared to the signal at "DURATION" (B2). I have no Idea if this should be like that.
When I'm injecting a 1kHz at 40mV sine wave directly to B1 I'm getting 600mV at the output of I001.
When I'm injecting a 1kHz at 40mV sine wave directly to B2 I'm getting 600mV at the output of I002.
So I can tell both op-amps are working as they should.
If I'm getting it right, the signals at those points (B1 and B2) come from the playback heads to the head amplifiers and from there to the "heads switching board". From the "head switching board" the signal goes to B1 and B2.
Since the dry signal goes to the output and I'm getting the echo (without the duration / feedback) and since the VU meter and the OL LED are working I can tell that the amplifier feeding the heads (board 82051) is working well and the bias voltage is correct (I've tested it with a scope).
I've noticed that the signal level drops from 600mV to 40mV at point B3 (at the other side of R035 the signal is strong and the resistance is 21.86K R so it's not the resistor that drops the volume.
What can cause this drop on the side of R036, C024?
Anything else that might be worth to test?

 

[smilan]

I made a video showing the symptoms of this unit. Any help with troubleshooting this issue will be highly appreciated.

 

[smilan]

I finally found the problem, one end of L301 was disconnected and only high frequencies could pass to the record head via C306 and there was not enough signal to make feedback. With the L301 fixed there is very loud feed back. I've also added a switch at the back of the machine that can remove the dry signal from the output so it can be used with a mixing console. Thank to all of you for your comments. I made a new video with this machine after the repair.

 

[Voyager10]

Is the "81 130" block on the schematic a separate PCB which can be unplugged?

If so you could inject a signal on the output (pin 1) of the module and measure the B2 signal (when the appropriate switch is on). From the schematic there will be about an 11:1 attenuation (set by the R020 / R021 ratio) so if pin 1 is 300mV it will be about 27mV at B2.

There will be attenuation of the signal between the R034 wiper and the R035/R036 junction, because R036 is being driven from a low-impedance source (Q003 emitter) so R035/R036 form a voltage divider. So if you're seeing 40mV at B3 when there's 600mV at the I002 output, that's not wildly wrong.


_Edit:_ Ah - ignore that - I see you found the problem!