Magavox PTV705 blows HOT instantly

[David]

Bad flyback, cracked blue crt (very common), bad hv divider, etc.
Just use standard tv set high voltage circuit troubleshooting techniques.
David

 

[Guest]

Hi. Magnavox PTV705, blows Q802 HOT instantly. Have checked, good H drive to
base from proc chip, with HOT removed and HV cable detached to avoid crt
problems. Anyway, done up with HOT removed and HV isolated, I can fire up
for 1 second. During this time I have good base signal to HOT base
connector. If I put in new transistor, makes a not pretty noise, then is
instantly shorted. Have found no obvious bad parts near the area. Any ideas?

Thanks in advance,

Danny

 

[Bill Jr]

That Q802 is the Horizontal Output Transistor.
The High voltage is done with Q906, the HVOT.
These sets suffered from lack of sealant for the anode connectors to the
CRT's causing your exact symptoms.

Good Luck,
Bill Jr

 

[Guest]

Hi. Thanks for the ideas. It does blow the Q802 with HV cable removed from
splitter and isolated also. Reckon this set doesn't like having no load on
HV output? First time I've run into this, so a bit of a learning curve. I
gotta change the tube oil anyway, it is grungy, so I'll redo the RTV at HV
connectors while I'm at it. Just trying to sort out the running problem
first before the cosmetics.

Cheers, Danny

 

[David Heller]

Hi. Thanks for the ideas. It does blow the Q802 with HV cable removed from
splitter and isolated also. Reckon this set doesn't like having no load on
HV output? First time I've run into this, so a bit of a learning curve. I
gotta change the tube oil anyway, it is grungy, so I'll redo the RTV at HV
connectors while I'm at it. Just trying to sort out the running problem
first before the cosmetics.

Cheers, Danny

Danny

I would reflow the horiz drive transformer. You need to do the four pins
where the tiny leads from the windings are connected to. and resolder
the connections where the transformer is soldered to the board. Other
possibilites are a bad yoke or horiz drive problems Also the damper
diode could be bad if not internal to the output transistor.

Dave

Good Luck

 

[Alan Harriman]

Danny

I would reflow the horiz drive transformer. You need to do the four pins
where the tiny leads from the windings are connected to. and resolder
the connections where the transformer is soldered to the board. Other
possibilites are a bad yoke or horiz drive problems Also the damper
diode could be bad if not internal to the output transistor.

Also check for open safety (tuning) caps off the horizontal output. I recall a
Philips PTV620 few years ago the would blow the HOT instantly. In that case, the
cause was C817 (0.0091uf/1600v)

Alan Harriman