Inductance meter and tape head issue - Page 1
Forum section: electronics / beginners

author: Quantumplate
author_rank: Regular Contributor
timestamp: September 01, 2022, 08:27:56 pm
content: I'm just checking the operation of a new vintage digital meter, and it seems ok at measuring relay coils, speakers, ferrite rod coils and transformers, but when it comes to stereo cassette heads I can only get a reading if I measure both channels in parallel. I'm getting between 50 and 62mH on a couple of heads I have lying around, which is about right for a parallel configuration.What kind of weirdness might be going on...?

author: David Hess
author_rank: Super Contributor
timestamp: September 01, 2022, 09:04:49 pm
content: Tape heads are easy to saturate. Could the inductance meter be saturating them when measured alone but not when placed in parallel?

author: Quantumplate
author_rank: Regular Contributor
timestamp: September 01, 2022, 09:17:22 pm
content: I think you've hit the nail..Just looking at the calibration section of the manual - when using known test coils it states that they must not saturate with the 3.5mA peak current coming off the meter's relaxation oscillator.Damn! I got this mainly for measuring tape heads. Oh well, learnt something new.

author: floobydust
author_rank: Super Contributor
timestamp: September 02, 2022, 01:37:27 am
content: What is the test frequency? Is the head magnetized? I've seen some LCR meters unable to read certain inductors but never figured out why.Tape heads have a tiny air gap but not enough to help with saturatione.g. 16-track 2" AMC head:Playback 95mH, 136Ω, 510T #47 AWG, gap 5µin (0.127 µm) and usually smaller.Record 70mH, 114ΩErase 1mH, 11Ω 50Vrms/60mA rms

author: Quantumplate
author_rank: Regular Contributor
timestamp: September 02, 2022, 10:44:15 am
content: Quote from: floobydust on September 02, 2022, 01:37:27 amNot sure. The service manual is coy, or it could be me, and I don't have the user manual. Some old marketing blurb I found says that for large Ls it uses a freq of between 25 & 250Hz, and for small Ls it uses 1MHz. Not sure whether there are frequencies inbetween.The head is not magnetized.

author: floobydust
author_rank: Super Contributor
timestamp: September 03, 2022, 02:14:26 am
content: I meant the test frequency of the L-meter you are using, not too high at say 100kHz or lowish at say 100Hz. At high frequencies the winding capacitance causes oddball effects.Why you only get a reading for a pair verses one winding is odd. I tried a junkbox cassette playback head, about 120mH 320Ω each, and half that in parallel, at 1kHz.The actual inductance is important for the erase head especially as this is resonant tuned.I find it's common for tape heads to fail open-circuit due to the very fine wire and thermal stress from the epoxy, or flux corrosion inside these are my theories.If you take an ohmmeter reading (or likely an inductance reading) consider the head magnetized, so I will degauss afterwards.

author: Quantumplate
author_rank: Regular Contributor
timestamp: September 03, 2022, 01:42:50 pm
content: Quote from: floobydust on September 03, 2022, 02:14:26 amYes, I know you did, the meter uses different frequencies depending on the size of the inductor it senses...clever for 1978. Well probably, what do I know?You mentioned previously about the air gap not being enough to help with saturation - could you explain a little more about that please? Feeling lost. Why would a large gap help??