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Variable inductor for audio adjustment

S

SFM-Ptgal

I've been fine-tuning my vintage Harris cart recorder. It's a bit have a bit over trebled. The manual says that the hf adjustment is made by 2 (L+R) variable inductors. Adjusting the ferrite core doesn't make the wanted frequency change (reduce recorded high frenquencies). Where should I look at besides the inductors?... Thanks!
 
First make sure you've properly set the bias, and for the type of tape being used.
 
Consider that the effect of tuning such inductors may be dependent on an expected capacitance "load"
within that section of amplifier. Changes in coupling caps or bypass caps, whether because changed value in replacement or by age can affect the result.
Most likely is an electrolytic coupling capacitor that has lost capacity and now greatly "favors" higher frequencies.

Is this variable inductor really in the playback circuit? Just seems an odd and expensive way to vary "treble" in playback mode.
 
I don't recall any Harris recorder which used variable inductors to adjust HF response.
They did use variable inductors in some of their record electronics to keep the record bias from back-feeding into the audio record electronics.
The high frequency response is almost always controlled by a variable resistor or sometimes a variable capacitor (in the record electronics).

Your best bet is to adjust the record bias a bit higher. This will bring the high frequency response down.
 
Bias on the Bias Oscilator Control card is adjusted with varicaps and inductors, on the record amp card there are two indutors (squared cans) to adjust the HF... While tweaking the BOC card inductors I've damaged the ferrite adjustement core screw, it was stuck, forced it too much and the ferrite desintegrated...oh boy. The caps issue camed to my mind too... no mention regarding caps on AF adjustment on the manual...
 
frankberry said:
I don't recall any Harris recorder which used variable inductors to adjust HF response.
They did use variable inductors in some of their record electronics to keep the record bias from back-feeding into the audio record electronics.
The high frequency response is almost always controlled by a variable resistor or sometimes a variable capacitor (in the record electronics).

Your best bet is to adjust the record bias a bit higher. This will bring the high frequency response down.
Click to expand...

The variable inductor is the Bias Trap. I swapped the L and R and got the result of a dead inductor. Nothing to loose, removed and opened the can: cracked spool due to age, broken wire. Now I got a vintage piece with one dead record channel. I can't find a replacement anywhere. The value is 81k to 20K uH (as writen on the manual). The can is about 1cmx1cmx1,5cm... :'(
 
Perhaps someone in the U.S. has a spare card for you.
Many of the ITC machines had similar bias traps. Perhaps an inductor from an ITC machine will work.
You may need to increase or decrease the value of the capacitor which is associated with the inductor.
Good luck.
 
Old topic brought to life :)
I got a card containing the 2 variable inductors I need (Thanks to Ebay and... Thanks "Micrental"!!!!)

Now I need to make an extender card. I need the socket to insert the pcb. Due to my ignorance I can't find on the web what I need, wich is a socket to connect to a coppered prototype board and the insert the BIAS single sided pcb with 12 contacts. What words should I google?...
 
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