Telefunken echomixer troubleshooting

[eternal slang]

Dear all,

I'm having a hard time trying to fix an issue with a Telefunken echomixer spring reverb.

All the voltage readings are accurate except on the summing amplification stage where its reading 4.01 vdc at transistor emitter, 4.02 vdc at base and 3.98 at collector. Also, there is no more AC signal just before C12 and therefore, no output signal. (see schematics with notes in attachment)

Here is what I did with the unit:
I replaced all the capacitors, checked the transistors and swapped them, checked the continuity between components and between all the ground points, remade most of the soldering points, checked for a short to ground and for an eventual out of tolerance resistor. I didn't find anything wrong, the components and the overall circuit seems healthy.

Does anyone have a clue about the next thing to look for?

Many thanks for your help, it would be much apreciated!

 

[PRR]

At a glance, the echo tank output coil looks "open". They run the transistor bias string through it. If open, the Base goes way high, roughly limited by 6.8K and 5.6k, roughly the observed voltage. In this condition the transistor input is a near-short to signal. (It will always be a way-low impedance due to the 120k NFB.)

 

[eternal slang]

Right that make sense, many thanks for your reply and help! I wont be able to test it until next week.
If that is the case, is there a way of knowing the reverb tank output coil impedance besides finding the specs sheet?
Is rewinding an affordable option or I should just find a replacement tank?

 

[JohnRoberts]

Dear all,

I'm having a hard time trying to fix an issue with a Telefunken echomixer spring reverb.

All the voltage readings are accurate except on the summing amplification stage where its reading 4.01 vdc at transistor emitter, 4.02 vdc at base and 3.98 at collector. Also, there is no more AC signal just before C12 and therefore, no output signal. (see schematics with notes in attachment)

near identical voltages at base,emitter, and collector is usually evidence of a failed transistor, or solder bridges on pcb. There may be other stuff wrong too.

JR

Here is what I did with the unit:
I replaced all the capacitors, checked the transistors and swapped them, checked the continuity between components and between all the ground points, remade most of the soldering points, checked for a short to ground and for an eventual out of tolerance resistor. I didn't find anything wrong, the components and the overall circuit seems healthy.

Does anyone have a clue about the next thing to look for?

Many thanks for your help, it would be much apreciated!

 

[Audio1Man]

1. Short the BASE to ground, if the COLLECTOR goes high (to the power supply voltage) and the EMITTER goes to zero the transistor is good.
2. Short the bottom end of 1K9, and if the BASE goes to about 1.57 volts the spring coil is open or a bad connection as PRR said.

 

[eternal slang]

After conducting the tests above it seems like the output coil from the tank is open.
Does anyone have info about this tank (see attached picture)? The following numbers are written at the back: 1122-6307.
I'm unable to find impedance values online.

Thanks for your help!

 

[PRR]

It's a plain old Hammond tank. There's a stamped number somewhere which indicates what coils were installed.

https://www.amplifiedparts.com/tech-corner/spring-reverb-tanks-explained-and-compared

 

[eternal slang]

Thanks everyone for helping! After replacing the tank the unit is working perfectly and sounds gorgeous!

 

[andow]

Thanks everyone for helping! After replacing the tank the unit is working perfectly and sounds gorgeous!

May I ask which specific tank you used?

 

[eternal slang]

I couldn't find any stamp on the actual tank except an H wich wasn't referenced on amplifiedparts.com.
So I had a C tank type IV laying around and just decided to try and it works great.

 

[salomonander]

May i bump this dinosaur? I had the same issue. hardly any output signal and wrong voltages at the final transistor. the output coil of the tank missed its connection to ground. I reconnected it and now i hear lots of reverb and the voltages are correct.
Channel 1 (the one without reverb) is a bit louder than the other channels. Is this normal?