HRO 60 audio problem

[Peter/SWL]

My AF gain is set to 10 so I can hear, but the audio is very low.


Check voltages...ok
Check tubes...ok
Changed resistors, electrolytic cap and bought new audio tubes for audio section.
Two new electrolytic 40uf 500v power supply filters
New rectifier
Check ballast tube...ok
Check for bad grounds...so far ok
Traced schematic...can't find anything wired wrong.

I think it might be the audio transformer, but don't know how to check it.
Anybody have any glues as to what it could be?  
 

Peter

[WQ9E]

Peter,

Insert and remove a headphone plug into the headphone jack a couple of times.  Often these develop poor contact from sitting around and a bit of use returns it to normal operation.  Double check that you do have an 8 ohm speaker connected from the 8 ohm jack to common (ground).

You can also try plugging a regular audio cable (RCA plugs) into the phono input and set the receiver to phono.  Touch your finger to the RCA plug center contact and advance the volume, you should get a healthy level of hum if the audio stages and speaker are working properly.

[Peter/SWL]

WQ9E

Thanks for your reply

I took the headphone jack out already and cleaned it, it's ok. I also checked the wires going to the headphone jack too see
if the wires are good, they are. The audio came up a little bit after cleaning it.

Speaker is hooked up correct and I have the original speaker. I also hooked up another speaker, it works but low
audio still. And there is no phono cable in this receiver.

I use to get great audio with AF gain at 3.
 

Peter

EDIT POST: failed to mention that this problem started 4 weeks ago when I turned it on and very low audio.

[WQ9E]

Peter,

There should be a round phono jack (usually white ceramic unless the original was broken and replaced) on the rear panel just above the antenna terminal strip.  Set the front panel mode switch to Phono and either attach a standard audio cable and touch the center conductor with your finger or use a small blade screwdriver to touch the jack center connection and touch the blade with your finger, with the gain advanced either will result in hum.
This test will help determine if the low volume is the audio section or in the detector or prior radio stages which will help narrow your troubleshooting area.

Is the S meter still indicating normally (i.e. shows relatively high reading on strong stations)?

[Peter/SWL]

Rodger

Yes I do have a round phono jack on the rear panel and tried the test. NO hum.
Second, I found the original phono cable and installed it back where it's suppose to be. Repeat the test still NO hum.

Yes the S meter is acting normal on strong stations. But I noticed In AM MODE with the phono jack in,
the signal on the meter goes down, when you pull it out the signal goes back up. And if it matters, in
AM MODE with the Limiter "off" the voices are raspy sounding with Limiter "on" it clears the voices up.


Peter

[WQ9E]

Peter,

What you described almost certainly narrows the problem down to the audio section.  This is the point where having a scope would be very nice to trace where the signal is disappearing.  Alternatively and audio signal generator can be used to inject a signal to narrow down the trouble point.

What you are finding with the phono cable is a little odd but I am going to assume it is just an interaction with your receiver antenna and it is likely changing the RF ground characteristic of the receiver.  The phono input is disconnected except in the phono position so it normally has no effect on the receiver.  What you are finding with the limiter is the reverse of what would be expected, normally it will create a raspy characteristic when on instead of off. 

You can try the "hum test" again at the center terminal on the AF gain control to rule out a problem with the mode switch but it isn't likely the problem is there.  Carefully recheck all of the voltage readings for V10B, V13, V14, and V15.  Measure the resistors and suspect any old capacitors in this circuit that were not replaced.

[nq5t]

Checked or not, since something odd is happening in the NL stage, it would be worth subbing the NL (V-12) with another 6H6 if you have a spare.

[Peter/SWL]

Rodger

Correction on my post above about the "phono cable".

Years ago I took it out because I didn't use that feature. But I remembered where it was soldered in. And all this time it should of have been soldered to the mode switch wheel. It wasn't! Yesterday I found the cable I soldered it according to the placement on the schematic and did the test again. It HUMS loud now.

And again, yes the S meter is acting normal on strong stations. Voltages on the tubes you mentioned check ok. Sangamo red paper tubular caps  are still installed in the receiver but I didn't switch out any caps, but tested them instead. I found C71 0.5 200V and C110 .25 200v is waaay off value. The other ones checked within 20%. Electrolytic caps are new. Also tested the resistors and found alot off value and replaced those. I've been workin on the problem for about a month now.

And no the limiter is not working the way it should.


Nq5t
I have a spare and already replaced V-12 6H6 and tested most of the tubes 3 times to make sure I'm using good ones.  

Peter

[WQ9E]

Peter,

Based upon the "hum" test you need to take a closer look at the detector and noise limiter section, it sounds like your audio section is working as it should.

[AB2EZ]

Plug P1 should be providing a jumper between pin 1 and pin 5 of the socket that it plugs into ("SELECT-O-JECT" socket K3).

It provides the connection between the noise limiter output and the 1st audio amplifier input in AM mode... via the mode switch. It plays no role in phono mode.

You might want to check this path with an ohm meter to make sure there is essentially zero resistance... between the top of potentiometer R40 at the input to the audio amp and the right side of the 0.01uF coupling capacitor (C69) and the 220k ohm series resistor (R-41) connected to the plate (pin 5) of the noise limiter.

Stu

[Peter/SWL]

Ok, I went right for the noise limiter V-12 first. I installed new resistors on and around that tube and one was installed wrong BY ME! There's suppose to be a 220k resistor R-34 on pin3 with a .01 ceramic C-66. I had it installed on pin 4. I corrected the resistor. Got audio back and limiter works!

Phew! do you know how many times I looked in that area and missed it!?

Originally pin 1 to 5 (black wire) is installed on the select-o-ject socket. I never touch that.

I'll never know now what made the audio go out in the first place. Must of hit it by replacing ALL the resistors and buying new audio tubes.

Saved a trip to Mouser and spending 70 bucks on a new audio transformer!

Thank you!
Peter
   

[WQ9E]

Congratulations Peter!