[SOLVED] Thompsonic radio - doesn't work

[Lukisek]

Hi everyone,

I found an old radio tower in my basement. But after I delivered to it voltage there is no signs of life. Even the POWER led doesn't light. Only speaker hums and bangs after I touch a potentiometer of volume. I don't need FM (I know that I need to do retuning), I don't need even tapes (it also doesn't works, I checked it). The only thing I care about is AUX. It also doesn't work. I figured out, that there is no voltage in the PCB with controls, but from back PCB (next to transformer) voltage is delivered (16 V). What should I do to reacitvate AUX? None of capacitors and resistors has voltage on this PCB. I checked transistors and diodes and they looks like they working properly. Thanks in advance for any tips. Photos in attachment.

Regards,
Lukisek.

 

[BradtheRad]

from back PCB (next to transformer) voltage is delivered (16 V).

It's important to determine if it's DC or AC?

1.
If DC then it sounds as though the power supply board works. So if voltage doesn't reach the other boards, then you must find the faulty connection. The On-Off switch is one place to check. Soldered wires can break at the solder joint if flexed a few times.

This assumes your 16V is smooth DC, and is sufficient to run the circuitry. (Some amplifiers have both positive and negative supply rails. If one polarity is faulty then results are unpredictable.)

2.
If it's AC then there's a chance the transformer puts out AC but the power supply board no longer converts AC to DC. One or more faulty components may need to be replaced. Frequently to test a component you need to first clip its leads to take it out of circuit.

Of course if you have a spare DC power supply, then you might hook that up to the unit in hopes it can aid diagnosis. It's a strategy that requires know-how and caution for obvious reasons.

 

[vfone]

Only speaker hums and bangs after I touch a potentiometer of volume.

In old and unused for years electronic devices, first that start to brake are the electrolytic capacitors placed on the DC power supply. Hum on speakers tells that may need to check and replace some of them.

 

[Lukisek]

@BradtheRad thanks for your reply. Yes, 16 VDC, not AC. So you propose to track e.g. one path with voltage? I'll try.

@vfone thanks for your reply. How can I determine which capacitors need to be replaced?

 

[BradtheRad]

So you propose to track e.g. one path with voltage?

Two wires are needed to carry power. Traditionally ground is black and supply is red.

 

[Lukisek]

@BradtheRad yes, but I mean that I have to check every supply path with ground. I will try. Thanks.

 

[Lukisek]

Ok. Today I had some free time and I checked paths. Supply is going to a switch with three states. I set it on CD/AUX state and tried to find where supply is going forward, but I didn't find it. I think that switch should transfer supply or GND forward on another pin of switch, shouldn't it? When switch is in AUX position, then on the second pin from the left on bottom it has supply. But it isn't connected to anything... So my question is how such a switch should work?

On the first picture I selected GND and supply delivered to this switch. On the second picture I selected second pin with supply and GND when switch is in AUX state.

Thanks in advance for any tips how to solve problem.

 

[Lukisek]

I don't know how to edit my last post, so I have to create next one. Problem solved now. AUX is working, but everything else like FM or tapes still doesn't work and it's ok for me. I figured out that on the back board (next to transformer) there is a cable with 3 wires and its captioned "AUDIO INPUT". So I decided to solder three wires into AUX plugs and the second terminal of these three wires i plugged into "AUDIO INPUT" plug. I took photos. It's working but I can't even increase or decrease a volume (only by my mobile phone). In attachement two photos, with AUX plugs and "AUDIO INPUT" plug with board in background. Thank you guys for your replies and good tips.

Regards,
Lukisek.