Melcor AE-20 - EQ Does Not Work

[smilan]

I have a Melcor AE-20, it's a preamp with EQ (looks like a GME-20).
The problem with this unit is that the EQ does not work at all.
Since I can't find a schematic I'm having trouble to follow the audio path and troubleshoot the problem.
There's three discrete op amps on the board, two of them are labeled "Melcor 1731" and the third one is open and contains a 2N2102 transistor and another transistor which I can't see it's label.
The unlabeled op amp amplifies the mic input, the three gain switches works and after the switches the signal goes thru a 470uF cap and a 10 Oham resistor to the output (label "B" on the multi connector).
The other two op amps does not reviving any signal on both inputs.
All op amps receiving +15V and -15V power.
When I connected a tone generator to both 1731 I got amplified signal on the output.
Do you have an idea what could went wrong here?

 

[gridcurrent]

the un-potted vertical op-amp is unusual.
AE-20 modules normally have three 1731 amps in a row, all horizontal.
the preamplifier section has its own in/out connections.


A equaliser circuit common

B preamplifier unbalanced output via blocking capacitor and 10 ohm resistor

C equaliser unbalanced output via blocking capacitor and 10 ohm resistor

D preamplifier circuit common

E -15 VDC

F chassis

H microphone preamplifier input

J microphone preamplifier input CT

K equaliser circuit common

L microphone preamplifier input

M +15 VDC

N preamplifier circuit common

P equaliser circuit common

R equaliser unbalanced input

 

[smilan]

Thank you Gridcurrent
I seems like someone has messed with this unit before my time.
The connector in this unit wires as follows:
A to F connected the way you mentioned.
H connected to pin 2 of the mic input jack.
J is connected with a white wire to the red wire of the input transformer.
On the connector that coming out from the circuit J has disconnected. (is it OK?)
Between H to J there's 20 ohms of dc resistance. (is it OK?)
K    connected to the equalizer circuit common (brown wire) and disconnected on the connector that coming out from the circuit. (is it OK?)
L connected to the microphone preamplifier input.
M  connected to the +15 VDC.
N connected to the inverting input of the vertical op-amp via a 270 Ohms resistor. it seems like someone have added this resistor and disconnected the gray wire coming from the N point of the multi connector from where it was connected when this unit has made be Melcor.
Where should the gray wire connects originally in the circuit?
On this modded unit the side of the N pin on the connector that coming out from the circuit connected to the wiper of a 5K pot that connected to chassis ground on the other side and used as an external "Gain" control.
P connected to the equalizer circuit common.
R connected to the equalizer unbalanced input via a 3.6k resistor.
There's also a 1K resistor that seems to been added to the circuit between the "COM" pin of the middle op amp to the non inverting input of the vertical op amp.

 

[mjrippe]

So, just to be clear, have you tried connecting to only the eq, using points P&R for input and A&C for output?  If the eq is not working on it's own, the most likely problem is a bad opamp. You can use any 2520 type that is designed for +/-16V as a replacement.  I have several Melcor pres and EQs (including one AE-20) and I have found the GAR1731 to be an excellent choice. 

I can provide un-modded photos if needed.  I would also be interested in learning more about that vertical opamp.  I always keep original Melcors in the mic pre position because I think they have more impact on the sound there than they do in the eq.

 

[smilan]

Thanks, I found the problem, one of the PCB traces was broken, now after I fixed it the unit works and sounds great.
The disconnect wire from K point has made some occasional crackles, after connecting this wire to chassis ground the crackles stopped.
The unit racked by another engineer in a way that there's two input jacks (mic / EQ) and two output jacks  (mic / EQ) .
You can connect the mic out to the EQ input and use the whole circuit.
The vertical op amp PCB is glued to the chassis and I don't want to disconnect it.
I've tried to look with a dentist mirror to read the model of this op amp but there's no space to insert the mirror.
By the texture of this op amp PCB it looks very similar to the original Melcor main PCB.