Montgomery Ward Model 450 with dead signal strength meter

[ecluser]

just lurking on this one ahead of doing my HK. Can you just briefly explain how changing ohms at either of these adjust offset? Thanks!

When I suggested to change R405 and R409, the purpose was to increase the voltage drop in R413. Obviously, the emitter and collector current of Q405 will increase. The collector current of Q405 is the tail current for the differential input stage. It is important to realize that if the tail current increases, it is the current in Q403 that will increase. The current in Q401 will not change because it is set by R411 and the Vb-e of Q407. Now, if the collector current in Q403 increases, so does its base current and there will be more voltage drop in R421 and R427. The offset at the output will go positive (if it was zero initially). In order to restore the output offset to zero Volt you need to reduce the current in Q403 by increasing the current in Q401. You can do that by reducing the resistance R411. That said, in this circuit, if you have a positive offset at the output, you must reduce R411. If you have a negative offset at the output, you need to increase R411. Same thing for the other channel, trimming R412.

 

[JJCalvillo]

It looks like VR101 is the adjustment and C151 needs to be functioning for the signal strength meter to operate properly. Just a first glance guess, though.

When I test VR101, a 10K ohm VR, it goes from a little over 10K ohms down to 953 ohms (.953K ohms). Signal strength indicator peaks at the 953 ohms (fully clockwise on the VR). I'm pretty sure the signal strength is good, as even weaker stations are lighting the Stereo Lamp, but the meter isn't reflecting that.
Also, the meter deflects as it should on AM, so I don't think it's a meter fault.

Two Questions:
• Can I conclude I need to install an new trimmer that will adjust closer toward 0 ohms to push the SS indicator past 1.5?
• Could I use a 5K ohm VR istead of a 10K since the adjustment needs to be somewhere lower than 1K ohms?

 

[ecluser]

Go for a 5K VR. Make sure that D105 is a germanium diode. I've seen some folks saying that the 1N60 is a silicon diode, which is not true.

 

[JJCalvillo]

Put in the new VR and no difference. Already replaced C151. Don't have the gear to do an alignment, besides I don't think weak signal is the problem.

Any other thoughts on what I should look at?

Photo of D105, is it a germanium? Tests at 3.26 forward voltage.

 

[ecluser]

It looks like your diode is defective. The 1N60P is a germanium diode. Cross reference to the NTE110A. I've just measured a few Ge diodes that I have and all of them have a Vf of 0.35-0.38V on my small MTester. The elbow is not as sharp as silicon diodes, so the mesured Vf will depend on the forward current. I doubt that Vf can be as large as 0.5V however in actual applications.

 

[JJCalvillo]

So I’ll look for a new diode. Thank you @ecluser .

 

[Warren-Oates]

Are there any silicon diodes that will work? One with a lower vf? Schotky, if I recall? Or is a 1060p the only thing that will work here.

 

[ecluser]

A silicon diode should possibly work here. But germanium diodes are not hard to find. The 1N1060p is not the only one that will work here. Any germanium diode will work.

 

[JJCalvillo]

Apparently not the diode, as I installed a new NTE110A and the peak on the SS meter is still 4 of 10 (2 of 5). If that's correct it gets has incredibly good sound from that weak a signal. No clue what the next step would be.

Edit: If there isn't something else that could affect the meter, maybe someone has done a po' dudes alignment on the 450?

 

[JJCalvillo]

Figured I'd see what the voltage was going into the meter and found 4.5mV detuned, and 42mV tuned to a strong station. Put the 5K VR back in and was able to get it up a little past 2 on the meter.

Went to check the collector of Q105 and it had black leg disease. Also, it imploded when I attached the probe and measured OL with the unit powered up to FM and there was no Stereo. I BROKE IT ALL THE WAY!

Good news is I had KSC945 to replace the 2SC460 I killed. Now, it tests at 90mV tuned, Stereo is back and the SS meter show strong signal!




Thanks for your help @ecluser .

 

[ecluser]

With such a low voltage at the meter, I doubt that a silicon diode would work in that application. Happy to learn that all is good now.

 

[JJCalvillo]

Someone here 'splained to me that my 40 wpc HK730 couldn't sound like more than 40 per side I thought it was. When I tested this to determine the output, I got past 50 wpc before the sine waves started to fuzz on the ends. So HK was pretty conservative when it came to their specs.

One question I have is it looks like the second set of Phono inputs is for a ceramic cartridge as it has "CER" next to it. Does that mean those inputs don't work with a regular cartridge?

 

[ecluser]

One question I have is it looks like the second set of Phono inputs is for a ceramic cartridge as it has "CER" next to it. Does that mean those inputs don't work with a regular cartridge?

In all the amplifiers and receivers that I've seen with a magnetic and a ceramic phono inputs, there was a resistive-capatitive network, wright after the input jacks, to reduce the level of the ceramic cartridge to a level comparable to a magnetic cartridge. You can not connect a magnetic cartridge to the ceramic input. Rotel made a lot of amplifiers with those double phono inputs.