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Old 19th Oct 2021, 9:13 pm   #1
1955APREN
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Default Grundig PT1421 / PT1420

About 3 yrs ago I acquired this 14 ins MTV PT1421 when I received it very poor Frame Lin was repaired by replacing a 2200 uf. a few months ago it let out a loud crack then was dead . So it was placed on pile for things to look at
recently it was time to take a look . First power supply was ok with crt heater
ok at (12v) Looking for the service manual I noticed that on Pauls DVD listed a Grundig 1420 , looked like a similarly set ? downloaded.

Then noted the following difference's
1 Tuning control 1421 as band switches 1420 as touch pads
this not problem as my fault is not in the section

2 Line Driver stage 1421 DOES NOT HAVE A DRIVER transformer
1420 as transformer with two transistor
see scan of 1420 and my trace of 1421 line driver stage .

The line output transformer is top of my list as faulty . this looks to be the same transformer part No for the one in my set (1421) 9270;052
these transformer do not have a direct connection between the eht winding
and the primary winding but have a drive winding connected from tapings
on the primary and the moulded eht winding is over this.At the present time I do not have a working scope , but using my digital meter on frequency range I have 15.3Kcs on the base of the line output transistor and 12v on the collector.
If any one a the full manual for the 1421 could you please confirm if the transform's are the same in both sets.
Thanks in advance for any help
Derrick
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Old 15th Nov 2021, 9:26 pm   #2
1955APREN
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Default Re: Grundig PT1421 / PT1420

Further test on line output transformer proving it is faulty

1 Transformer was removed form pc

2 A 47ohm resistor was connected from the collector to the 12v line
( this was to test if line output transistor was not going base / collector oc under load)
This dropped 6 volts But still no frequency on collector even with scan coil
unplugged
3 The resistor was removed and replaced with a coil of 0.68mh
I now had 15kc/s on the collector when the scan coils were reconnected
this frequency then doubled to 30kc/s ?
Please see my request in parts required sections
Derrick
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Old 22nd Dec 2021, 11:10 pm   #3
1955APREN
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Default Re: Grundig PT1421 / PT1420

further update regarding the above mtv . I removed transformer from set
then removed over winding (EHT) . When transformer was refitted to set
without winding, secondary voltage all came on 15Kcs on collector even
with scan coils connected. So it looks like this little set will have to taken
11 miles to nearest recycling yard.

Derrick
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Old 23rd Dec 2021, 7:59 pm   #4
Maarten
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Default Re: Grundig PT1421 / PT1420

That's a shame for such a nice little set. Maybe someone else wants to give it a try? While getting more scarce, a defective LOPT is not a death sentence.
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