The AM Forum

Hello all,  my name is Doug and a ratty HQ-145a followed me home.no Case and ultra smoke infested, a real Beauty!  heh   anyway  I cleaned it extensively with simple green and hosed it off so the chassis looks pretty good ( front panel had some rust so its not that great)

anyway after drying it it sort of worked for a hour or so then no sound, the audio xformer had been hacked up and encased in silicone anyway. I think it shorted out internally and tried to take the main power xformer with it   ( the main pwr xformer was arcing a little I saw as I killed the power)   anyway I replaced the 2 rectifier diodes (one was shorted) and now it will make 250 volt dc and not blow a 2 amp fuse. I have 250ish vdc going into the audio transformer but not coming out.   
My question,  would a transformer from a old tube cb radio that had 6bq5 audio tubes work in the Hammarlund that takes a  6aq5  ?    Im trying to get a better handle on how the stock one is wired up, but its pretty hacked up, looks like it has 5 or 6 leads (have to dble check that)

How critical is the swap, or should I give up and wait till I find an original transformer.

Sorry for the long post

thanks
Doug
KD8HLV

She had originally a dual-secondary OPT:  3.2 and 500 Ohms, respectively.

If you can't find an original, anything designed for a 6V6 or 6AQ5 will work, but you may have to rewire where the phones get their feed from if you use a single secondary type.

73DG

The primary impedance isnt very critical and most single tube transformers are fine. Antique Electronic Supply also carries new ones.

Carl

A Radio Shack 110/6.3V filament transformer works FB as a replacement for anything having a 6AQ5 output.  When I did this in a NC303 the fil transfomer even sounded better than the original.

Thanks for the advise , Im glad I didnt throw those junk cb set away now. they dont have the 500 ohm tap  but I dont need one anyway for now.

W3JN,  is this transformer like the one you used on the 303?  for 10 bucks locally, it might be worth a try.
http://www.radioshack.com/product/index.jsp?productId=2102700

Ill report back after I swap it out.

Doug

Yep.  Use the center tap and one side.  Or try both sides of the secondary, pick whichever sounds better.

a quick update,   good news and bad news
the good news is I pulled a audio transformer from a pearce simpson guardian 23 and the radio worked for a half hour or 45 min then the main xformer started smoking so I shut it off.  Im pretty sure it was hurt from the audio trans shorting out.     The transformers out of the old cb donars look slightly smaller, I dont know what they are rated at yet but I suspect they might be borderline current wise.  Maybe tomorrow Ill dig in further , and Daytons next week so I can find something there.

Thanks for the help   ( I still want to try the filament xfmr on the audio out to see how it works just for kicks)

Doug
KD8HLV

Doug,

Before you go any further I would replace the main filter capacitors AND the cathode bypass capacitor for the 6AQ5 output stage.  You also need to make sure there is no positive DC voltage on the control grid of the 6AQ5 (from a shorted capacitor in the couplate Z2).  Either of the latter two problems would cause failure of the audio output transformer along with overheating of the power supply.

When restoring a tube type receiver, I ALWAYS replace the coupling/DC blocking capacitor to the output tube and the cathode bypass capacitor because failure of either of these causes expensive collateral damage.

So you are saying that the main transformer might not have been ruined, and replacing the usual suspects could clear up the possible overcurrent problem. I would agree it's worth trying.

For sure, if the power transformer does not smoke when the rectifiers are unplugged, then it is probably OK and those likely other problems are the cause.

Another thing to check, after making sure B+ is good, is to check the audio output's grid coupling capacitor. This cap, when leaking,has destroyed many output transformers in many kinds of equipment over the decades. With B+ restored, pull the audio tube and check the grid terminal voltage with the volume turned down. It should be zero or much less than 1 volt in any case. Any positive voltage is due to leakage through the capacitor although a small voltage might be from dirt on the socket.

The main transformer is toast,   I pulled the b+ line from the new and tested pair of diodes and turned it on for about 15 min and the transformer was almost too hot to hang on to .
   I have not found a suitable replacement in my just box so Ill be on the lookout in Dayton.     today I replaced all the main filter caps and checked the coupling caps for shorts ect,  so when I get a new hunk of iron  Ill be ready to rock.

Thanks for all the great help,  Ill report back when I get the tranny
 ;D
Doug

Doug,

When I work on such gear the first thing I do before seeing if it works is to pull the secondary leads off the rectifiers or if it still has the tube rectifiers just unplug them.  Then let it run for awhile and you will know if the transformer is bad or not in short order.  If that is good then proceed from that point.  I have seen to many cases where something downstream has toasted the transformer.   

A lot of times these multivoltage transformers develop a short on the rectifier filament winding due to inadequate insulation.  With a directly heated tube the filament winding will be at the HV potential.  Removing the rectifier filament wires from the tube socket and putting SS diodes in is a good way to keep that failure from happening.  In particular, the HT-37's transformers are well known for that problem.  Tape the rectifier filament winding leads off and be done with them!   

I have a very pristine HQ-145 receiver. if your interested you can send me an email. 

Fun stuff.....

Joe, W3GMS