KLH Model 21 found at Goodwill today!

[jr_tech]

Since the IF board is shielded, I can't see if the active devices are transistors or ics.

But first things first, regardless of the type of device used for IF amplification, IMHO, the hum problem needs to be resolved before further problems can be addressed.

jr

[Captainclock]

Originally Posted by jr_tech

Since the IF board is shielded, I can't see if the active devices are transistors or ics.

But first things first, regardless of the type of device used for IF amplification, IMHO, the hum problem needs to be resolved before further problems can be addressed.

jr

Well if the filter capacitor wasn't the cause of the hum (it still hummed even after the filter capacitor was replaced) what else could cause the radio to have a 60 Hz hum?

[Ed in Tx]

Post 9 jr tech mentioned the rectifiers. My thought too. You checked both of those?

[Captainclock]

Originally Posted by Ed in Tx

Post 9 jr tech mentioned the rectifiers. My thought too. You checked both of those?

No I didn't check the diodes yet, that was the next thing I was going to check, would I check them with the diode function on my DMM? Also when I go to check the diodes do I need to remove them from the circuit to check them or can they be in circuit yet to check them? Also would a couple 1N4007 diodes work in place of the original diodes if they do turn out to be bad?

Edit: I checked the diodes and here are my results:

Diode 1:
171
118 in reverse

Diode 2:
174
119 in reverse

I measured the diodes with the radio unplugged and turned off, I'm not sure what the numbers mean but maybe some of you guys on here might be able to interpret the results for me as to whether or not my measurements mean the diodes are good or bad, or whether or not I need to have the radio plugged in and powered on to test more accurately, also the diodes were tested using the diode function on my DMM.

[jr_tech]

Likely you will need to unsolder one end of each diode to get a good test... should read "OL" in one direction and something around 300 to 700 mv in the other direction (depending on the type of diode).

jr

[Captainclock]

Originally Posted by jr_tech

Likely you will need to unsolder one end of each diode to get a good test... should read "OL" in one direction and something around 300 to 700 mv in the other direction (depending on the type of diode).

jr

They are of the "top hat" variety of diodes it says on the diode case 1N2859 on both. They're soldered in rather precariously so I may just cut one of the leads and if they test fine I can just solder them back into place.

[Captainclock]

Originally Posted by jr_tech

Likely you will need to unsolder one end of each diode to get a good test... should read "OL" in one direction and something around 300 to 700 mv in the other direction (depending on the type of diode).

jr

Well I cut one side of each diode out of the circuit and they measured around 600mV for both of them one way and measured "OL" going the other way which means they are working as they should be apparently which means that there must be something else causeing the humming and unresponsive tuner issue besides the diodes.

[zeno]

On IC's it may not have used them. They went disco in the
80's IIRC. The way the IF is built I would NOT go
there unless needed.

If the main filter is not the hum check the outputs next
& any transistors direct coupled to them. They will probably get HOT fast.
Its probably the transistors mounted on the chassis in round
holders.

73 Zeno

[jr_tech]

My first guess was on post 9 (bad power supply diodes), but there are additional filter caps on the audio board to check/ replace. Shorted output transistors could also place a hogh load on the power supply and cause hum.
Does the volume control change the loudness of the hum or is it always there?

jr