RE-301 Erase Head Issue (Solved!)

MagnetoSound said:

What happens if you disconnect the head?

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both measurements where done with disconnected heads on the broken and working unit (erase and record is one connector). same measurement with connected heads gives the following (disconnected voltages in brackets).

it gives -13,26 at R415 and after R414 -13,02 (same without head connected on both units)

at collector and at coil in it gives 11Vpp instead of 24Vpp at the working unit with head connected (14 Vpp without head connected on broken unit instead of approx. 24Vpp compared with a working unit)

88Vpp at coil out and at the erase head pin instead of 180Vpp at working unit both with head connected (112Vpp without head connected on broken unit and 180Vpp on the working one)

28Vpp at record head instead of 64Vpp with heads connected (52Vpp without head connected on broken unit and working unit as well)

does that make any sense to you?

Since the AC voltages are at approximately half of where they should be when the heads are disconnected, it suggests that one leg of the primary circuit may be bad.

There are not too many possibilities. Is the waveform identical at each end of the primary? If you take the transistors out of circuit do you get sensible readings across each junction? Is C312 leaky?

MagnetoSound said:

Since the AC voltages are at approximately half of where they should be when the heads are disconnected, it suggests that one leg of the primary circuit may be bad.

There are not too many possibilities. Is the waveform identical at each end of the primary? If you take the transistors out of circuit do you get sensible readings across each junction? Is C312 leaky?

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as i am not an native english speaker i am not sure if i got you right. if you mean "at each end of the primary winding of the coil" with "at each end of the primary" as i understand it, then yes it is almost. approx. 17us for one period at the same voltage-pp.

what do you meen by sensible reading when tranistors taken out? how to see if C312 is leaky? Sorry for the questions ( :

and i took out the transistors and they seem to be fine (diode test). so it can only be the caps right? cause there is nothing else there, and the coil seems to work as well as voltage increases afterwards and in circuit check was good, right?

So transistors seem to be good, if they pass the diode test.

Transformer primary winding seems to be good - if the amplitude is even across both halves then the probability of a short is low - so, either the oscillator circuit is putting out too feeble a signal or there is an abnormal load downstream.

There still exists the possibility of a short in the secondary winding, but the next logical step would be to take C309, C310 and C312 out and check them for capacitance value and leakage. It's a good idea to have replacement caps ready. If you cannot test them fully you can still replace each cap with new, each time stopping briefly to power up and see what develops.

hey i will get the caps tomorrow and replace them all as i do not have a meter that can test the capacity. i will take these from the store nearby.

http://www.conrad.de/ce/de/product/455350/KONDENS-MKS2-0022F-63VDC-20/SHOP_AREA_17435&promotionareaSearchDetail=005

http://www.conrad.de/ce/de/product/455024/KONDENS-FKS2-1500PF-100VDC-20/SHOP_AREA_17435&promotionareaSearchDetail=005

http://www.conrad.de/ce/de/product/450587/KONDENS-FKP2-2200PF-630VDC-25/SHOP_AREA_17435&promotionareaSearchDetail=005

if there is the broken secondary wiring within the coil, than it should show no connection passage when tested right? maybe i should take it out for that and measure the resistance again, but incircuit said it has a connection and was the same as with the working unit. or what would you say?

ps. thanks alot so far!

inputhone said:

if there is the broken secondary wiring within the coil, than it should show no connection passage when tested right? maybe i should take it out for that and measure the resistance again, but incircuit said it has a connection and was the same as with the working unit. or what would you say?

ps. thanks alot so far!

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There could still be a shorted turn (a partial short) in the secondary, which would put a strain on the oscillator, perhaps enough to reduce the available voltage by the amount you are experiencing. DCR measurement might not show this up easily. As a last resort, you could swap the transformers and see if the fault follows ...

Caps fail more easily with age, though.

okay now i got what you meant. thanks alot. yeah, a short within the wiring could cause that. i'll change the caps tomorrow and then if nothing happens i'll swap the transformers just for a try - careful not to ruin the good one.

but in case it really is the oscillation-coil there would be nothing i could do about it, as for this part i could not get a replacement? and i don't think it's possible to open it and fix the short..

Hey, good news. I swapped the caps today and the voltages came back to normal. It really was a leaky cap within the oscillation ciruit. I was quite suprised, cause i never have had a broken foil cap in one of these units but only dried up electrolytics. But in the end the coil is fine, and i was able to proceed with the calibration. All good now, carefully calibrated, sounds fine and back to a good state again.

Thanks a lot folks, finished and done! 

Glad you fixed it.