I have one that was working pretty good until yesterday when the volume suddenly increased over a two second period.

I first thought it could be the audio section, but connecting an input to the AUX jack confirmed that the amp hasn't increased in sensitivity.

Based on that I seem to have an AVC issue.

I had to trace the wiring as I have no schematic. AVC voltage gets to where I assume it should be based on what all connects to the AVC buss.

Console was given a full restoration (nearly 2-3 years ago and was not used that much as I needed a speaker) to include replacing all resistors along with all paper and electrolytic caps. One IF can had to be replaced as well.

All tubes are NOS and check good.

Another dead giveaway I have an AVC issue is when receiving my two tube broadcaster the closer the antenna wire (alligator clip lead as chassis is on my bench) is to the broadcaster's antenna the louder the sound gets. With properly functioning AVC that shouldn't happen. Also audio gets more distorted the closer the antenna is brought to the broadcasters antenna.

Any other areas I should check?

It does act like a tube. I'll try switching around the 6BA6 tubes one by one and see if operation returns to normal.

I assume it is possible for a tube to still work while not responding properly to a negative grid voltage?

The chassis uses a 6C4 in the phase inverter circuit, but the odd thing is the chassis is picky as to what 6C4 I use. I used a NOS 6C4 and got hum. I reinstalled the Zenith branded 6C4 and no more hum.

We might be able to give better help if we had a model number.

I posted a thread in the antique radio discussion forum in hopes someone knows the model number. I took a picture of what I believe is the model number, but it is too smudged for me to make it out.

Model number appears to be HFT-1080

An online search even on radio museum shows nothing with that model number.

When I first turned on the radio set to my two tube broadcaster the AVC was about -13 Vdc and the radio was working decently. It then jumped up to -30 and has distorted audio at all volume control settings.

Seems like something in the front end is pulling down the AVC voltage, but it is nearly impossible to find what it is without the schematic.

I was able to get proper AVC operation for about a minute last night after turning the radio on, but then the volume ramped up over the space of one second again.

There is a 2.2 meg resistor that feeds the AVC voltage to the front end. What's odd is as I bring the antenna wire closer to my two tube broadcaster antenna the main AVC voltage goes more negative, but the AVC voltage after the 2.2 meg resistor goes less negative . I swapped tubes earlier with no change.

Anyone?

We, unfamiliar with the set, really have to see the circuit to see how it develops AGC and if it's simple, or delayed with one diode audio & the other AGC, or a triode doing the lot? Going from -13 to -30 is not the voltage being pulled down. It is actually not being pulled down.

Often the AGC is taken from a voltage divider. In that circumstance the positive end of the divider could be going open. The increase in negative on the AGC would shut tubes attached to it, down.

It is then a dart board; RF circuit could also be going into supersonic oscillation, due to poor screening, induction, neutralisation, or a missing decoupling cap.?

Marc

Okay, you have the classic symptoms of a short to ground on the AVC line, somewhere away from the AVC detector.

Most AVC circuits are NOT all that complex-- just start at the pin of the IF can opposite the detector diode, there should be lots, like you say, -30 volts there. Then follow the resistors- there should be NO voltage drop in any of the series resistors. Well, a bit, if your meter has a 10 or 20 meg resistance, but not more than 10% or so. When you get to some series resistor that has more than a 10% drop across it, then there's a short or leak to ground on the low side of the resistor. Turn off the radio and use your ohmmeter-- 95% of radios have NO resistors from AVC line to ground, so you should measure many, many megohms from AVC line to ground. If your measurements go lower as you go down the AVC chain, there is a leak to ground there. Usually caused by a leaky or shorted capacitor, less often, by a gassy tube or a tub with a speck of cathode material between cathode and grid. You can tell it's gas by tube substitution, or by noting that that one tube has a much lower or even positive first grid, compared to the other signal path tubes. The last one of these I saw, there was a very gassy 12SA7 driving the AVC line to +3 volts, even as the detector was trying to push about -20 volts into it.

An even more sophisticated debugging technique-- put the minus lead of your meter on the place that has the highest negative AVC voltage, usually the very last IF coil. Then probe down the AVC chain from there, following each series resistor. Since there (often) is zero voltage drop in the chain, your mter will have ZERO volts across its input resistance, so it CANT draw any current and make the AVC voltage droop, its just acting like a null-detector. On most radios you will just measure no voltage drop down the chain, well maybe a tenth of a volt due to the tube grid currents. If there are leaks or shorts to ground you will measure many volts across some resistor, and that will be all true leakage, with no meter effect in play.

When AVC increases from - 13 to -30 volts it may not be getting to grid of one or more RF or IF tubes. AVC voltage is generated by the detector stage. From there it goes through a high value resistor and cap filter. In your radio it gets that far.

From there AVC voltage will go through IF Transformers or other components to grid #1 of tubes. Your -30 AVC isn't getting to tube grids or as Marc mentions the circuit is in oscillation. Is this a Zenith made to use "G" style tubes? If so do not use metal tubes unless pin #1 is grounded to chassis.

Be sure tube shields are in place where required.

Oh, I forgot, like Norm said, the prob can also be caused by an OPEN series resistor. These are usually like 100 to 470K, an open one is easy to find with your ohmmeter.

You can also get -30 volts if a stage goes into oscillation, but you usually notice that due to a whole lot of hissing or squealing going on.

Look for a .05 mfd (or so) cap somewhere after the 2.2 meg resistor and going to ground.

It serves two purposes: (1) bypasses to ground any remaining I.F. or audio signal in the AVC line, and (2) acts as a time-delay buffer (fraction of a second) to the DC AVC voltage.

If it is defective (leaky) or missing it could cause AVC voltage to behave erratically.

Unfortunately I have been unable to find a schematic. At first I thought there were two diodes, but there is only one used which provides detection and the AVC voltage.

What I don't understand is if something in the front end is loading the AVC buss down then why does the AVC voltage go less negative after the 2 meg resistor while the rest of the AVC voltage is going more negative.

Norm, I will have to check on that. Without a schematic I have no real clue what is supposed to see AVC voltage and what is not.

No provision for tube shields.

The tube lineup is this:

6V6
6V6
6C4
6AT6
6J5
5U4
6BA6
6BA6
6BA6
6BE6

I wonder if it may be an alignment issue? If so I cannot align it without the service manual as it uses several adjustible coils and trimmer caps in the front end.

Will be very time consuming, but I could draw up a schematic if I have to.

Ancient hacker, all resistors are new and was one of the first things I checked along with the new capacitors. Seems to be no unwanted oscillation as I hear no squealing and if the antenna ia far enough away from my two tube broadcaster the audio sounds very clean. Keep in mind the radio worked good for maybe an hour or so before the problem developed.

Oldbear, there are a couple capacitors one right after the 2.2 meg and one after another resistor that is fed by the 2.2 meg resistor. Both seem to pass an ohm meter check and when I removed them it seemed to have no effect on the problem except gain was reduced somewhat with the AVC still not right. For the capacitor right after the 2.2 meg resistor I substituted in another .047 uF cap with no change in operation. I did not yet substitute for the second capacitor, but IIRC when it was disconnected AVC was still not right.

I do know that the mica caps are still original to the chassis. Not sure if any of them connect to the AVC buss, but if so could one of them be leaky?

Listing the tubes does help. Look at pin #1 on 6BA6's and pin #7 on 6BE6. These are input grids and most likely have AVC voltage.

We think of lower voltage as being loaded down but in case of AVC going more negative it's the opposite. When AVC is loaded down it doesn't go negative enough. Going more negative shows it isn't doing the job by not getting to grid #1 on one or more tubes.

Not an alignment problem. Alignment is very good when AVC is that negative.

I'll check that out.

I am still puzzled by the fact that as the AVC voltage before the 2.2 meg resistor goes more negative the AVC voltage after the 2.2 meg resistor goes less negative. Something is loading the AVC line down after the 2.2 meg resistor and I for the life of me cannot find it.

Naturally I would have this kind of problem on a radio I cannot seem to find the schematic for.

I've googled the chassis number and also what has been thought as the model number and only hits I get are to my postings on this site.

Possible a gassy/leaky tube in RF or IF stage has a positive grid #1? If AVC goes through enough high value resistors the side of one of them may be positive? Only work with 6BA6 and 6BE6 tubes. Others listed are audio or other circuits.

I thought maybe a tube, but I swapped the 6BA6 tubes around with no change. Also all tubes were checked Saturday and are good.

When I turned on the radio Saturday night to troubleshoot it worked ok for maybe a minute or less then I had the AVC problem again.

Swaping tubes around may not make a difference. All 6BA6's may be going to the same AVC line? See if pin #1 on any of them is positive. This will detune the radio but still none should be positive.

I'll check them again to be sure none are positive. I have no clue if they all go to AVC or not.

I can post the chassis number here if that will help locate a schematic.

If I do find a schematic I will then have something to contribute to radio museum since they don't have this listed at all

Once I get the AVC problem solved I will check the IF to see if anything needs to be adjusted. Pretty sure I need to readjust as when I replaced one IF transformer I only used a DMM to adjust the IF as I didn't have a VTVM at the time.

Any chance the replaced IF Transformer is feeding positive back to AVC? AVC and plate voltage are on each transformer.

I'll ohm the transformer to see if there is any resistance between the two windings. If the transformer was indeed feeding some positive voltage wouldn't that make the main AVC voltage less negative and affect the whole AVC buss?

It depends. Usually the entire AVC line would be positive but this radio may have different AVC lines? With that -30 volts AVC is trying to cut back volume. For some reason it can't.