The AM Forum

Well, I spent a good many hours this weekend recapping my National HRO 50 T receiver. I never turned it on until I recapped the entire receiver. The electrolytics, all the .1, .01 bypass/coupling caps. There is nothing more gratifying than turning on a receiver to hear it come to life. The receiver works well. After spending time working out the scratchy pots and control, I started to become more critical of the audio. It sounds really good although there is an annoying low level buzz. After some schematic contemplation and control switching and signal tracing, I notice this odd condition. The phono jack is not active when the mode switch is in phono. In the phono position the audio is muted/grounded.  ??? The phono jack seems to be active when the mode switch is in AM & CW.  ??? wtf ! The phono jack appears to have a jumper running to the select-o-ject jack on the back panel.  ??? Something is not making sense. The schematic shows the phono jack completed isolated. The schematic shows the audio from the receiver being routed through only the select-o-ject jack. The audio cabling looks factory original but doesnt make sense. I get the impression this buzz is due to ground loop pickup.

Anyone have any insight on this ?

The buzz is getting in before the audio pot as when I turn down the pot, the buzz also does.

Steph, check out the rectifier tube in the power supply. If you have a scope see if you can trace the AC riding on the B+. I spent days rebuilding a -50 that I bought at an auction in NJ and it is one of my primary listening boxes. Gud luck.

I did test it on my EICO tester and it seemed fine. I will pop a new one in there just to see what happens.

I pulled out my schematic on the HRO-50.  The correct audio path is from the pin 5 of V-10 which goes to to pin 1 of the select-o jet socket.  From there its jumpered with a jumper plugged that in turn connect it to pin pin 5 of the same socket.  From that point it goes to the AM-CW-NFM and phono selector swith.  The common wiper on that switch goes down to the high side of the AF volume control.  The phono input is never grounded and gets routed to the high side of the volume control only when the phono postion is selected.  It looks like someone may have moved a wire on that switch if yours is otherwise.  All of the audio lines are shielded to prevent hum pickup.  I would put it back as it was designed if yours is different.  The Select-O-Jet was an audio filter that National sold as an accessory to offer audio level filtering to reject hetrodynes and thing as such. 
Hope that helps...
Joe, W3GMS   

That is a big mistake IMO. I start with the PS and audio stage caps and then Variac it up. Once a benchmark has been established then I replace 2- 3 caps at a time and any resistors at those tiepoints that are out of tolerance. Then its turned on and the next benchmark is recorded. That way you never get too deep and can easily backtrack.

Nationals are known for bad resistors and it doesnt matter what model. My HRO-50 and 60 required around 15 each.

The NC-98 Im doing for a customer has 4 different resistor vendors, 2 of which are garbage; a 100K screen dropper was actually open but looked new and others were way high. The only signal I could get when first turned on was hum from touching the volume control. Even the sensitivity pot was open yet the receiver is in fantastic cosmetic condition with a perfect copper plated chassis.

The buzz could be heater cathode leakage (especially the 6H6), a miswire of a cap, a prior mod. Since the NBFM and Selectoject sockets were often used for other purposes Id also check those areas.

Carl
KM1H

One of the dangers of working on these things is you can easily misplace a part or a wire which will cause the buzz.  As Carl pointed out, do the power supply first and get it playing.  Then replace components in one small area at a time being careful of parts and wire placement.  Hopefully all grounds are good and that should help prevent the problem or maybe others.

Then there are modifications and/or repairs made before you found the rig.  Finding those can be a real hair puller.

Well, everyone is entitled to their opinion. I inspected the components and did the work with the utmost care. I really consider myself a top notch tech when it comes to doing electronic repair work. I will share my findings about the buzz when I resolve the issue. The obvious rework of the audio wiring on the rear jack is a telltale sign to me, especially since the audio wiring is hovering directly over the 115/230 switch.  ;D  Oh, and I did take extensive photos before I touched the first part. I do remember one tube having some leakage not exceeding 1M....Hmmm...wonder if that was the 6H6 ?

Thanks for the confirmation. This was helpful..thanks Joe !

Yep, but when I offered mine I didnt realize you were a female ;D. I was simply offering comments based on 47 years experience.

Carl
KM1H
National Radio 1963-69
Service Tech, Service Manager

I like to verify that there is nothing severely wrong with the radio as soon as possible during the process. typically I'll change all the filters and any other large value lytics  like bias filters or large value (>.33) bypasses and give it the juice. About 70% of the time the radio comes up playing then and there. play it for a short time, maybe a minute, then you know it's a player then and there and worthy of whatever more work is required to make it 100%+.

I developed this by going to deep into BCL sets that turned out to have open osc coils and such that would never play unless you rewind the coil or have an original replacement.

Check the set as fast as possible into the process for playing, even if its distorted, weak, etc. - you know it's easy from there on out.

I always just plugged things in and tried it.
If it did not work right, go from there.
When I was playing with the stuff, I never had to re cap anything but the power supply, but stuff is much older now.

If I was recapping a radio, I think it might be a good idea to do it in stages and test it.
A good idea, but I would likely just replace them all one shot!
I was never big on making things easy for myself... 

Brett

Your welcome Stephanie.  Good luck with the receiver.
Joe, W3GMS

No problem Carl....I appreciate the advice  :)

Understood. All good points that a number of folks have mentioned. I guess I decided to take a gamble.

As it turns out, someone did in fact modify the audio to the phono jack. I removed the jumper they installed and also re-bonded the shielded audio cables to the screw terminal where a ground lug had broken off. There was a significant improvement. There still is a buzz with the volume control wide open (Above 6). It is pre-volume control. With the RF Gain up around 7-10 and the volume around 3 or 4, there is plenty of great audio and not even hear the buzz. Anyone notice a buzz with the volume control wide open with the RF gain all the way down on their 50T ?

Those P-P 6V6's should blow the windows out of the house. I think mine always had nice audio.

Stephanie,

My 50T is in the barn shack but my HRO-60 does have a little hum with the RF gain down and volumefully up.  

Any chance some of the buzz is due to local interference?  I temporarily had one of those ceramic disc heaters in the basement radio room and in the reduced heat setting it created so much hash on the AC line that it was transferred to the low level audio lines.  I spent quite a bit of time troubleshooting the Gonset communicator on my bench that had worked perfectly earlier in the day but suddenly developed a horrible audio hum.  I should have realized earlier that it was my new heater.  

Are you listening on headphones when you pick up this buzz?   A bit of hum and buzz is common when using more modern headphones with most vintage receivers.  I find using an Autek QF-1A provides worthwhile audio filtering and provides better operation with headphones for most vintage receivers.

My opinion is nothing beats a good scope for tracking down hum, intermittent crackles, etc.  If you don't have one on your bench once you start using one you will never want to be without it.

As was posted earlier, the HRO-50/60 have a great audio section.  If you have a decent AM broadcast station in the area it almost makes the expensive E and F coils worthwhile to utilize the receiver's capability.

Rodger WQ9E

and they do when cranked !   ;D

I checked to make sure the heater I have plugged in was off when I checked for the buzz. The buzz is still there

I just checked my 50 and 60 and both have a very slight hum with the RF gain off and audio on full. Its no different with the other radios here and I suspect that the only cure is DC filaments.

Check all your external wiring, cables, etc for ground loops. Ive also found most noise problems by shutting off the AC panel breakers one at a time and then devising a fix for the offenders.

Carl
KM1H

I tried out my 50T and 50T-1 this morning and like Carl's there is only a very slight hum at full volume and I had to get the nearby cat to quit purring to hear that amount of hum.  My HRO-60 has a bit more but I have never gone through it so it has the original filter caps and perhaps a tube with some heater to cathode leakage.  But the 60 cycle hum is covered by the RF/Mixer noise even with no antenna connected.

What happens when you switch yours to the phono position?  Is it the same?  If so there may still be some problems with the previous owner's wiring changes.  If the hum goes away maybe take a look at the detector and noise limiter stages.

I went through an HRO-50T-1 that I bought and had to remove extensive audio system changes to get it to work normally.  This one had 6L6 tubes in the output stage along with a very large output transformer and some sort of solid state regulator.  Although it had hum that was the least of its problems.

Rodger WQ9E

As a side note, what is the distinct difference between the -50T and the -50T-1?

The -1 has double tuned IF transformers and an additional IF stage so the skirt selectivity is a bit better.  I believe a few of the tube types also changed.  

It works a little better than the straight 50 under crowded AM conditions but still doesn't compare favorably to one of the Halli or Hammarlund selectable sideband receivers under battle conditions.

Rodger WQ9E

I appreciate the feedback here from everyone. I discern between a hum and a buzz. A hum is different than a buzz. I have a couple more things to check like a leaky 6H6. I dont think the buzz comes from an external AC source. I actually have a very good brute force filter in the shack.