Found this on the side of I-840 today, Magnavox console tv.

[mr_fixer]

PM sent,
new transistor, no change in action. I have 30 volts on the hot which i think is the start up voltage, i have 320 in a different section, but don't know enough about this weird power supply.

Originally Posted by Findm-Keepm

I've got the service lit - most of it. I have the model specific data and the supplement with schematic, parts list, breakdown list and interchangeability data. I also have the Switchmode power Supply manual (theory, general operation, a must for the "bitch mode" supplies these have...) from NAP, all yours for postage. PM me your address, and I'll get it out to you.

In the meantime, do you have 130V across the diode (D404) on the yoke? Negative lead to the anode (hot ground...)That was always our starting point - D404 is a high-failure item in these chassis, and my earlier attachment provides a list of stuff to check as well - lots of bad solder jobs on the early runs of these chassis - the 25C55X series didn't seem to have the solder issues, just the earlier 25C53X and 25C54X stuff.

January 1986....when the Challenger tragedy struck.....

[zeno]

Watch for disc caps & box caps off the C of HOT. Common
for them to burn, melt or short. Also burned PCB that
may be hidden under frame plastic. Noise is probably from
SMPS running off freq to keep the overload from wrecking everything.
Assuming HOT is not shorted see if it gets hot in 30 sec.
If so probably bad FBT.

73 Zeno
LFOD !

[DavGoodlin]

I too cannot let such a good example like that just sit. I always find them a home. Hats off to you!

I worked for a dealer then and remember that as the mid-level set. A one knob-tuned with UHF presets was the entry level.

[Jon A.]

Seems almost everyone starts hating the cold and snow upon growing up. I'm one of VERY few adults (well, chronologically anyway) who like winter. We've hardly had any snow since winter began but we're finally getting it today, and I'm thankful for it. The night I pushed the Heathkit console home it was 10 below, making the activity that much more comfortable.

[davet753]

I remember the C5 chassis very well. It seems like they only had two types of service issues: either they were a quick and easy fix, or they sat on the bench for days while every tech in the shop pulled his hair out trying to troubleshoot it.

[mr_fixer]

Hi all, I got the service manual from Findm keepm today and spent most of the day checking everything i can, to no avail. the 2 main symptoms are low voltage in the horizontal driver circuit, expected 33 volts got 6 and the 130 supply is around 21 volts. this is one of those switching power supplies that has 2 types of protection circuits over voltage shutdown and current limiting. I've taken a pic of the waveform on the switch mode regulator what i have vs what the book says it should look like, any hint on what i should try next?
Logan

[zeno]

Unhook the C of the HOT & see if the voltage comes up. Also
do the heat test of the HOT.
Dont forget you are working with 3 grounds on this set.
Hot gnd for the primary of the PS
Cold ground the chassis ground / PS secondary.
I dont think this one does but some sets also had another ground for the
hoz osc/out.
CRT ground. Most NAPs do NOT ground the CRT dag as normal sets.
We used to just grab the CRT ground but that dont work on these.

73 Zeno
LFOD !

[zeno]

OOPS !
Just pulled the training manual & the hoz output is
also on the hot ground side !

[mr_fixer]

Ok, i disconnected the hot and flyback from the 130v supply, voltage went up to 138 v, I connected a small 40w lightbulb from 130v to ground for a load, volts dropped to 27v. Horizontal driver supply voltage is unchanged at 6-7 volts where 33 is expected.
hmmmm. resistors that drop the 33 supply from 312 volts are good and there is a diode that somehow is supposed to bridge the 2x20k resistors and 7 volts worth of zener diodes to ground. I'm not sure how a regular diode can act like a resistor, any thoughts?
Logan

Originally Posted by zeno

Unhook the C of the HOT & see if the voltage comes up. Also
do the heat test of the HOT.
Dont forget you are working with 3 grounds on this set.
Hot gnd for the primary of the PS
Cold ground the chassis ground / PS secondary.
I dont think this one does but some sets also had another ground for the
hoz osc/out.
CRT ground. Most NAPs do NOT ground the CRT dag as normal sets.
We used to just grab the CRT ground but that dont work on these.

73 Zeno
LFOD !

[RCAZenith]

I-840 - Nashville is an interesting place to say the least. My father was in a country band in the late 90's about 2 steps from making it big and it didn't pan out for them. I was up there for training a couple of of years ago. I don't think I met anybody the whole time I was there that was local!

I've yet to see any tvs on I-285. I bet they would be smashed to bits long before any of us were to come along. That and I try and stay out of atlanta anyways...


Sent from my iPhone using Tapatalk

[zeno]

What you need is horz drive from IC340 pin 11
That goes to the optic isolator IC469.
VCC for IC340 pin 10 comes from T453 / D461
Hoz drive is a must have to run the PS.

Since unloaded it seems to be doing what it should
I would suspect over voltage shutdown. Maybe test the small
transistors by lifting 2 leads for accuracy.
BTW one of our distributors made up kits for these to fix
them after lightning. Worked 100%. IIRC had an isolator,
4 small transistors & a diode.

The similar C3 chassis had a lot of bad FBT's Symptoms
were dead set, HOT gets too warm, R460/461 get warm.

73 Zeno
LFOD !

[davet753]

Be sure to check for cracked solder joints on the horizontal driver transformer. C5 was about the time Philips started using the driver transformers that had a metal pin on each corner of the frame (the windings soldered onto the top of the pin and the bottom of the pin came through the board)....it's a good idea to re-solder all 8 points.

A bad connection on that transformer will most always zap the horiz. output or more.