Hoffman 610 Lost B+
 

I think I'm in over my head on this one. I got this from a fellow member, and it was unrestored. I was told it would get a partial raster, and that's all. I was hoping a simple recap would do the job.

I started recapping the electrolytics and paper caps, and I got the raster almost full, but no signal yet. When I ordered the last electrolytics, I also decided to replace the 2 candohms R178 (1650) and R 174A (1500).

When I tried a power up after this, I had no raster and no B+. I have checked and rechecked all my work and even disconnected everything from the filter choke, then connected it up one by one.

Here are my findings: When I disconnect R206 (560ohm) from pin 4 of V105, the B+ goes to about 30V. With it connected, I have about 8. Disconnecting either side of R178 givers me the 260V I need. I even hooked up the old candohm of R178 to make sure I had the right part in. Putting the old one back in didn't help.

Any Ideas on where to go now?

Correction: I misread the schematic. The 560ohm resistor that is pulling the voltage down on V105 is R113. It goes to pin 8 in parallel with a 20uf electrolytic.

With R113 connected, and R178 disconnected, I have about 80V.
With both R113 and R178 connected, I have about 25V B+.
With both R113 and R178 disconnected, I have the required 260V B+.

(With the set off) check resistance from centertap of power xfmr. to ground. What reading do you get? It should vary a bit with the focus control setting.

Well, from what you gave us, either you are not getting the current needed to drive the set....
So check out the 3 windings on the B+ that feeds the 5U4, check out the 5U4, are the
lights on..? Like O-C said, check the resistance on the transformer with the set off and
unplugged. CT also.

Other option, you got a short, or over driving that audio output tube..... is the 470K
resistor ok r112. Is that tube looking a little hot....? Audio output transformer a little hot...?

I'm thinking O-C is got a good idea in looking at the transformer, at least go there
first, look at the 5U4 too...

.

Are you certain you have the polarity of the Caps correct? The + of 152c and the - of 153b are tied together at ground, a simple mistake would be to assume both - connections go to ground, but without seeing how the chassis is laid out I don't know if that could happen.

Again (if you haven't already done so), check continuity from the centertap of the Power xfmr's HV secondary to ground.

It should show continuity, and read several hundred ohms (variable a bit with the focus control setting).

What reading do you get?

(EDIT) I just noticed sumpthin' in the schematic concerning the two 80uf filter caps C152B and C152C. The polarity markings are correct, however the symbol for C152C is backward, i.e., The radiused (curved) line is on the wrong side.
Be sure C152C is installed correctly (negative side to point 'C' and positive side to ground).

There is a fair chance that you made a connection error in replacing the components. The old saw, about one never seeing their own mistakes, is true here. Ironically one of the two most difficult sets to repair and the cause of my most wasted time back in the 1950's when I was learning TV servicing, was a Hoffman 7JP4 set where they omitted a CandOhm bleeder resistor on both the schematic and the parts list. This component proved to be both open and shorted to ground. To keep heat away from sensitive parts, it was located on the opposite side of the chassis to the circuit to which it connected and I had missed it in my circuit checking. Ironically, neither Sams nor Rider had my schematic, but there was one factory attached inside the cabinet and it was incorrect!

I don't know where you found your schematic, but the drawing of the power supply is unnecessarily complex.

So you can look at the circuit with fresh eyes, I'd suggest that you go to the ETF website and download the Rider schematic for the 610 Hoffman, and print out pages 15 and 16.

http://www.earlytelevision.org/tv_sc..._diagrams.html

(The Sams schematic on ETF shows a more robust power supply using two 5U4's and is also marked "preliminary", so I'd avoid that one.)

While you have the full Riders, verify that you are looking at the correct model and chassis number.

The first thing to check is the connections to the resistors in the focus network, as these resistors and focus coil setup the different negatives in the power supply. Also do a visual inspection for solder bridges, especially terminal to case on the candohm, and cold soldered connections in the power supply. On both the focus coil and power transformer, sometimes just wiggling the wires can accidentally open the connection where the solder flux has weakened the copper.

Don't worry if some electrolytics are a slightly different value or a different physical mounting, just check the polarities.

Hope this helps.

James

Edit: In addition to Old Coot88 & UserName1's good suggestions, I left out test 101 !

When powering up, look at the plates of the 5U4 in reduced lighting. If they start to turn a dull red, the set is drawing too much current. If the 5U4 plates stay their original color, there is probably a high resistance or open connection somewhere.

If the plates indicate too much current, it will do no harm to yank the Audio Output V105 to see if it corrects the B+ problem, in the remote chance that tube has shorted while you were rewiring the set. Otherwise, it probably is an inverted electrolytic or an incorrect connection.

First, thanks for all the input. I started checking all of your ideas and finally figured out PART of my problem.
R174 is a big candohm in the focus circuit. The old one measures 3000 ohms which is what I ordered in a 50W power resistor. I failed to note that it has a GROUNDED CENTERTAP. That's why hooking the old one back up with jumper wires on the ends did not solve the problem. Hooking it back up with the center tap grounded gives me the correct B+ to some of the circuits. However, now the other candohm R178 starts frying. I had used a 10W 1650 ohm resistor for it. Maybe I used too small a wattage.

Before I order correct power resistors, what would be a good wattage to replace the old candohms?

The schematic I have IS the one on the ETF site. However it's confusing compared to my set.

Ohms law will give you the wattage. There's 260 on one side and 140 on the other of R178 for a difference of 120 volts. P = (V * V) / R or about 8.7 watts. A good rule of thumb is to double the resistor so I'd go with a 20W resistor.

Aha. So the center connection of R174A/ R174B, being left ungrounded, was breaking continuity-to-ground for the power xfmr's centertap, thus killing the B+ supply and the negative voltage supply.

From what I can gather, Yes. The only info on this set is the Riders, which I downloaded from ETF. The schematic is very confusing. There are differences in it, and my set, and between the schematic and the pictorial of the parts diagram.

I don't know why Sams didn't cover this set other than a reprint of the original schematic.

I've ordered two 20W 1500ohm resistors to put in series with the center connection being grounded.

Here is where I am today. Taking Bandersen's suggestion, I found I did have a 1620, 20W resistor on hand to replace the 10W that was frying for R178.

I've got 2 new 1500 ohm 20W resistors ordered to replace R174 candohm which I will put in series tying the center point to ground. For right now, I have put the old R174 back in with the center tap grounded.

That corrects my 260 B+ voltage. However, the 140 volt line is only reading about 54 volts. 260V should go into R178,and drop it down to 140, but it's pulling it down to 54.

I guess I will unhook everything tied to the 140V line and start checking

Voltages are now correct on V105 audio output 6K6 tube. Touching the meter probe to in 4, I can hear some static in the speaker.

I cant see the schematic, too small & blurry. Also it is
laid out messy. Give a point to Sams for drawing all
schematics the same & with sense.

If something is loading down a source & the dropping
resistor is the right watts it should get real hot fast.
Measure ohms to GND from the source. If it reads say
5K look for anything that is fed with about 5K resistor.
Example if a 4.7K feeds the G-3 of a tube there is usually
a decoupling cap to ground that may short. Pulling
any tube using the source is another test.
Another place I found a few times was the tuner B+
feed-through shorted.
Good luck

73 Zeno:smoke:

I'm back in business, sort of. Before I acquired this set, the member I got it from warned me ahead of time that a mouse had been inside the cabinet on top of the chassis. The mouse had peed all over the IF board leaving that part of the chassis rusty and corroded.

I had tried cleaning that part of the chassis with various rust cleaners and steel wool. Either the mouse pee had corroded some of the tube sockets or maybe a combo of the steel wool and cleaners shorted something.

With everything connected, I would get an arch in certain tube sockets, and several 1K resistors fried. After replacing 4 tube sockets, my voltages are back to normal. However, I've lost vertical as my raster has come back to just a line. I'll start troubleshooting the vertical now then continue loking into why no signal is coming thru.

Steel wool? :eek: Yeeikes!!

Ah, yes, steel wool. I learned my lesson back in the late 1950s when I cleaned my Lionel train tracks with steel wool and some of the fibers remained behind. A very interesting light show for a 7-year-old.

Well after finding a replacement filter cap in the vertical section in which I had pushed it where the leads were touching (why I had no vertical), and another short; I am finally ready to post my 1str results on the screen.

I can't tune the signal in. The tuning slug doesn't get it any plainer than this. Tuner tubes and IF tubes have been subbed with no change. Sound is just a weak buzz.

Video IF voltages check ok.. I'll start checking other voltages next

The horizontal frequency is off, try turning the slug in the horizontal coil, it should be somewhere near the oscillator tube.

Edit: L111 on the schematic.

Actually, when I replaced one of the tube sockets, I forgot to put a resistor back. That was my problem. NOW, I have a pretty good pic, BUT no audio al all. The audio section from the 6AL5 forward is working because wiggling that tube produces cackling in the speaker. Voltages check normal on the audio tubes

I've gotten it t where I have a god pic and weak sound. I don't seem to be getting any gain thru the 1st Sound IF(T100). It's got two internal caps, but the schematic doesn't say what they are. I'm going to try and find a replacement IF. Hoffman 5304. I crossed it to a Miller 6190.

my IF transformer came in. After much tweeking, I have a good pic, but the audio is still week. With the volume at max, the sound is adequate but not overly loud. When adjusting the tuner slug the seems to be only one brief place where audio comes thru, but the video comes thru at a much wider range. The video doesn't seem to be at it's best where he audio is. It's on the bench and has been playing for over an hour.