Guyatone AE-5 Analog Echo

[Yazoo]

I am trying to repair a Guyatone AE-5. It seems complete apart from one component. It is close to the power input from a transformer rated at 0-12 0-12 wired in parallel. The two wires go to a diode bridge. The positive output from the diode bridge then goes to the middle pin of the missing component (3 holes). On the back of the pcb it looks like one pad of this has lifted. I would guess this would be a voltage regulator(?) but a 7912 would expect negative voltage.

If you have any photos of a working unit or a schematic, I would be grateful. When I checked the BBD, it is a Coolaudio V3205, which can't be the original as this unit is quite old. When I did an online search, the only info I could find was that the BBD was an MN3005 but the voltage supply for this would be negative, not positive. Again, if you can confirm the correct chip, I would be grateful.

[Mark Hammer]

A 3205 can be made to work in the circuit, because it is functionally identical to a 3005.  But it is NOT, I repeat NOT, a pin-for-pin plug in replacement.  The power connections are different, and you will have to make some small adjustments to the board.  I'm sure there is someone here whose ability to point you to the appropriate thread is greater than mine.

[Yazoo]

I've started to trace the circuit out and the power connections are correct for a 3205 rather than a 3005. I checked the ground and power lines and there is no evidence of any alterations to the pcb. There are other chips on the board, a 4013, a 4016 and 2 LB1415s and the power and ground lnes synch up. I've tried powering the unit up with a DC bench power supply and it comes up and passes signal but no echo. So far, so good.

[Mark Hammer]

Fingers crossed.

[Yazoo]

OK, first things first - I got it wrong.

I tried jumpering a V3205 on a breadboard but it would not play. I then looked at the pcb again. I think it is actually an MN3005, but I would be grateful if this could be confirmed for me. The VDD pin on the 3005 is connected to ground. The Ground pin on the 3005 is connected to the positive voltage, I'm guessing 12V (the mains transformer came unconnected). If I understand this correctly, this would mean that VDD is in effect -12v as far as the chip is concerned, which is what it needs. Is that correct?

[Yazoo]

It does work with an MN3005 in the way described. I bought an Xvive MN3005 and after setting the DC bias, it is now working, much to my relief. Some BBDs are expensive, though not as bad as a Reticon.