Hello to Antique Radios forum. I am new to the electrical/mechanical aspects of antique radios. Up to now, I have been a learning spectator of the forum. I want to thank all of you for what I have had the opportunity to learn from this forum. Quite honestly, so many of the threads are intimidating to me at this point. The knowledge and experience in this forum is incredible.

Perseverence and research is just not getting me anywhere it seems with this second set I am working on. Please forgive any incorrect nomenclature. I have done alot of studying but have so much more to learn.

This is a Howard model 68 I am working on. The electrolytic can of two 8mfd electrolytics were previously replaced and new ones were installed to replace those. I have replaced all of the caps and a few resists. The previously installed elect caps were installed incorrectly, as best I could tell, which caused the dc voltages to be far under spec. They were re-routed according to the schematic and the voltages came into spec. The tubes were tested and were all good prior to this event. The set played very well until the tranny, rect tube and field coil overheated. I could be wrong, but I think the pilot lights that were out of sight and not installed at the time, were shorting against the chassis. After cooling down, tested tubes and all ok, left the rect. tube out, plugged into a light bulb in series and no shorts indicated, worried that the tranny and/or field coil was lost, checked the voltage going into the rect tube heaters and it is 5.7vac, voltage going into the other two rect tube electrodes is 120vac, the other two wires from the tranny shows 3.2 vac. The other side of the 1200 ohm field coil shows 5.7vac, detached the field coil leads and the coil shows 560 ohms.
Replaced both of the new 10mfd electrolytes with new ones again even though they tested ok, installed the rect tube while connected in series with the lightbulb and the light bulb shows a short. I have replaced the rect tube with two other of the same rect tubes that test good and still showing short.
I can't seem to find how to do further tests on the tranny or field coil. The tests that I see on google results, say what to do but not what the results indicate. Honestly, maybe I just don't understand.
I thought maybe the tube socket is grounding against the chassis but I don't want to take the socket out etc if my thoughts are out of line.
I am stuck now, with an unknown short situation in the tranny, rect and field coil system.
I am hoping that someone can help me to understand how to repair this circuit.
Thank you in advance for any help.
Sudsy

There are the odd ones, where the fault is obsure. In saying that the fault is to be found in proceeding in a logical order. A shorted 80 normally blows itself appart. Double check the wiring. and make sure the electrolytics are not reversed

Get rid of the cap on the mains. I do not use dim bulb. The filter caps should be 500V or better. If they are under rated and there is an open circuit they risk failure as will surge.

You start as you seem to have with the rectifier out. to check the transformer. Beware of the filament winding. I have had one of them cut through under a metal boxed cap. As soon as there was B+ it shorted. Filament winding carries B+. Do look for pieces of solder & wire, that can short and a miswire. Do not 'dilly dally' as thing can get hot if there is a short. if there is no B+ voltage in say five to ten secs. with 80 shut it off.

You could have a heater cathode short but initially I would pull all of the tubes. Do you have a variac? If you pull the speaker plug (after checking that attached to & preceeding it) You can build the voltage up. Monitor B+ with an analogue meter (1000Vdc range initially) (digitals stop & flatten bateries) Computer type "clip leads" are good. If the voltage fails to fall and it will go very high 450V or more: All is well. Still with tubes out you can then try the same with the speaker plug in.

The heater winding should be simple. No volts = short. Bayonet sockets can short with the globes out.

Judging where you are, I doubt you have an insulation tester, however some tube testers have them. This can be used to see if there is any leakage to the frame. Do check the wiring on it for shorts from bad wire and that incudes looking in the plug. You can also disconnect the outfeed from the field (socket) to prove it if there is a continuing problem.

Marc

In case you're not clear, he's talking about C16. Snip it out and see if you get a different result. It's not crucial to operation. And by all means, do use the dim bulb. There are some strong opinions here pro and con about them, but diagnosing a short is what they are fantastic at. Not only do they indicate with brightness whether or not there is a problem, but they protect the radio by taking the brunt of the short so that further damage to the radio is unlikely.
http://www.nostalgiaair.org/Resources/v ... 009342.pdf

First, pull out all the tubes and try the dim-bulb test.

If the light is bright, you have a shorted transformer or a short on the wires going out from the transformer. Unsolder the wires going out ONE AT A TIME and try the dim-bulb test each time.

If the bulb is dim with all the tubes out, then plug in the rectifier tube. If the bulb gets bright, you have a short to ground somewhere after the rectifier tube. Use your ohmmeter and check from rectifier tube heater to chassis-- it should be several thousand ohms. If it's much less than that, start unsoldering things from the B+ bus until you narrow down which item has the short to chassis.

Relatively simple, if you stay methodical, and change just ONE THING at a time.

Whilst in the 5V instance the insulation cut through was fabric type sleeving, some of the three grades of rubber that tended to be used were well known for turning to jelly, getting attacked biologically, or just plain falling off.

I will never power a radio until I have visually inspected the mains & transformer. Some times if its wiring is bad I will not bother turning the set on at all: I know it is unlikely to work, with old paper caps. &electrolytics. C16 is one I have seen examples of in the last month and were no longer there when I powered up.

Inspect every wire that goes through a hole, or can be crushed. There is a "Breville" radio featured in Australias "Silicon Chip" mag. this month. That failed inspection as the rubber wire feeding the dial lights was bare at the chassis and was one of several where the insulation was conspicuous by its absence. New work on IF & RF coils going through a chassis can see solder droop onto the chassis, and fibrils of multi strand wire cause a bridge.

The transformer was continuity & insulation tested, and the set refurbished before power up. "Touch wood" I have not replaced a radio transformer for decades (faiure rare), so that was a reasonable commercial risk.

Do not rush, take one step at a time. Lets get the PSU working first, in logical steps.

Marc

After returning home, read your much appreciated replies. Did dim light test as prescribed, still shorted. Used the after rect logic and the wires through chassis logic and thought back to what areas that were worked on after the set played well. The tuning eye resistor replacement was the last step done since the eye was not functioning correctly. Pulled the eye socket apart and found two wires inside the bottom piece of the socket assy that the casing was frayed with bare wire exposed. Repaired these, dim light test showed no short condition, dim light with tubes reinstalled and no short condition. Put set under normal power and SOUNDS GREAT!!!
You guys are great! I would still have been chasing my tail without your help. THANK YOU.

I would like to ask your advice once again please. The set has a squealing sound when attempting to tune in weaker stations but on strong stations sound is great. I did not have any metal tubes for the 6k7 or 6q7 so I changed to glass tubes. When I touch the glass of the tubes while tuning the weaker stations, the squeal is significantly reduced but does not tune the weaker stations in to an audible state. I am only using a 2 ft section of speaker wire held in my fingers and I know that is not a sufficient antenna method but do you think the squealing is from glass tubes rather than the metal tubes? or maybe it is from something completely different?

Thank you all again for getting me this far.

BTW the rect to chassis did show only 600 ohms

Sudsy

Yes, using glass rather than metal tubes will make set prone to squealing. Glass tubes with metal shields over them would be the other option.

Ok! pleased to see that that the original issue is dead & buried. I would agree that glass tubes where the originals in the RF section were metals, can result in feedback. 6D6 and 6U7 being two shockers. Even the type of shield on a 6D6 can be an issue and de-stabilise it.

Investigate the shielding first. Pin 1 on an octal metal is normally the shield / body; Pin 1 should in that instance only have a grounding wire to chassis and nothing else. Problems have occured when the shield pin is used as a tag of convenience and a metal tube is installed.

Marc

Hmm, 600 ohms is a bit low-- that would imply 166 milliamps of current draw per hundred volts of B+, waaay too much current draw. Probe the rest of the B+ feed circuit with the ohmmeter-- some point farther down the line may have a dead short to chassis. That's the most likely thing.

If that doesn't narrow it down, although it should, start unsoldering things from the B+ and see what it takes to get the resistance up into the many thousands.

Very happy that the tranny or coil was not lost too!!!

Found metal tubes and they took away the squeal. The strong stations come in very well, but on lower frequencies in std bcst, police and shortwave, there is a low hum. Was able to tune in a station on swave but inbetween is the low toned hum.
All caps were replaced and all but three resists. I saw that there is a .01 mfd cap spec'd in the schem but one is not installed in the set.

You know, I didn't think to recheck the ohms after repairing the eye wires. Will do that. Thank you.

In my house, I get a low hum caused by a wall wart power supply for a cordless phone. When I unplug the power supply, the hum is gone from the radios.
Don

I wrote this, went to another page and forgot to click submit, now doing it over.... AAARGH!

After fixing the short from the eye tube, chassis to rect tube heater is 11,000 ohms with all tubes out. Tubes in the voltage is 380 at rect heater and 300 after field coil.

Original schem says target 385v and the revised schem says 320v. I don't know if this means anything... I checked all non el caps to schem and all have the correct value with two exceptions, and seem to be in the correct connections (as best my limited talent can tell). The exceptions are: that C8 was in the chassis at .02mfd and it has been changed to .1mfd per schem. the other exception is that C15 with value of .05mfd is showing on the orig schem but is not included on the revised schem. There is no C15 currently installed in the set. Should C15 be installed?

The sound that is present must be the motorboating referenced in the books. The sound can be heard for a moment when first turning the set on and then again below about 950 on std broadcast and all the time on police and swave.

Is my next step to go through all of the solder joints?

Thank you for your continued help.

Normally If the voltage is extremely high some thing is not conducting due to a faulty tube, faulty resistor etc. Low you know about.

If the voltage is normally quoted plus or minus 20% to allow for mains fluctuations. The caps will not draw current unless faulty. Check that one of the blades on the oscillator gang rotor, does not short to the stator in the position where it oscillates.

Moving parts around when replacing can cause instability. There are a few instances (& Midwest'ss are one that I have had problems with) especially around the det audio in a few sets where I have needed to add shileded wire around the path to the volume control. Wires too far away from the chassis often do not help matters.

If you get a non metal probe like a "chopstick" you can do a bit of prodding to see if a slight movement of a wire etc. causes the frequency to change. Caps from mains to chassis on many transformer sets, to me can bleed AC onto the chassis & you if it lets go. A practice I consider dangerous in many cases, as I have recently had examples of this on metal cased instruments, with the case connected to the chassis. Totally non compliant with regulations here that basically say ... That if a metal part can become live & touched, it has to be connected to the mains earthing / grounding. ie three wire cable. A N E. If the mains cap shorts the chassis can be live.

Marc

Good morning Marc! Thank you for the direction. I will proceed.

After cleaning up all the solder joints and inspecting for debris etc... set still has the motorboating. Took out the eye tube and still mbg. I just re-checked all of the tubes and they test out good. I will proceed next with your direction.
Thank you!

Amen....

When I put my finger on or near the grid cap of the 6k7, the mtg stops. The reception overall weakens a bit also.
Does that mean anything for narrowing the search down?

Thank you.

Just clipped it off. Still mbg...

Does this mean anything?
The double elect cap can was replaced with two 10mfd el caps. One connects to #1 of rect tube, tranny wire and field coil lead, opposite end is to chassis. #1 rect tube is wired to speaker large electrode (lower).

The other el cap is connected to the other large speaker electode (upper) and to the other side of the field coil, opposite end is to chassis.

Maybe any causes with this configuration?

Thank you.

Putting your hand near the tube suggest thet that is where the radiation is. If it is a glass tube it may need shielding if it is metal make sure pin one of the socket is actually tied to the chassis.

There may also be a need to sheild the grid cap wire if it is long

Marc