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Old 29th Jul 2019, 2:02 pm   #1
donutty
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Default Zenith G730 (7C05 chassis)

I was on business in Canada last week and did a bit of antique shopping and managed to bag a Zenith G730 for less than £20.

It was sold as not working, but no further details and I thought it was a nice enough set so I would give it a shot. It managed to survive the baggage handlers and I got it powered up after changing the wax capacitors and the selenium rectifier, which looked like it had an 'exit wound' - I didn't power it up before doing a basic overhaul because I didn't want a selenium air freshener in my face. The sound is stunning (a very well balanced 6" main and an electrostatic tweeter) and I really like the set and its construction but it is having problems with the AFC circuit:

After about 20 minutes (maybe first 5 minutes on lamp limiter, then the rest with full juice) on FM (AFC setting), the sound starts warbling; a constant 'strobing' like the output it being modulated. There is no sound overlaid, like motorboating or mains hum - the programme its self is 'wobbling'. Switching from AFC to 'normal' FM and it is back to being good again. Note again it is fine for the first 20 minutes on AFC.

I have monitored the HT and it is about 6% high probably due to the silicon diode I used to replace the selenium and the line voltage from my transformer will be 120v rather than the 117 in the spec.

This set uses a varicap diode in the AFC circuit - has anybody ever known these to go bad? I have a stash of BA102s that I could try as replacement, but I'd like to know more about this symptom first.

Anybody have any ideas?


And by the way, when all is well I will stick a compact step down transformer in the back and stick on a properly polarised UK plug.

It should be noted that this radio had been reasonably bodged by it's last owner - see pictures of wire used to bypass a coupling capacitor on the phono input, a huge witness mark of something having gone 'pop' discovered under a slapped in replacement multi-segment electrolytic, and the resistor feeding the HT was 150R when it should have been 22R.... Everything else looks original with the exception of only one original Zenith valve remaining, the others being a mixture of other makes. I removed the tacked-on electrolytic which had been paralleled onto the original one and this seems to be performing OK and keeping it's cool. I will likely put some brand new replacements (40 = 47 and 80 = 82 microfarad) in there when I seal it all back up to give it a fighting chance for the future.

Also see what I think looks like a 'magic smoke' outlet on the selenium rectifier and how I stuffed the insides with paper to protect the 'toobs' during the flight.
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Old 29th Jul 2019, 3:20 pm   #2
Nuvistor
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Default Re: Zenith G730 (7C05 chassis)

Is the AFC voltage modulated, presume it uses the voltage from the discriminator for AFC, if so the electrolytic cap across the ratio det is worth checking.

Is there a circuit available?
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Old 29th Jul 2019, 4:48 pm   #3
donutty
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Default Re: Zenith G730 (7C05 chassis)

Service info here:

https://app.box.com/s/wkl2emoj97ylmq0kck4q
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Old 29th Jul 2019, 5:10 pm   #4
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Default Re: Zenith G730 (7C05 chassis)

It’s a Foster Seely discriminator not a ratio detector so no large electrolytic cap. However there is some smoothing on the AFC line which is also the audio output of the discriminator so I would check those, they are R&C’s so easy to obtain if faulty.
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Old 30th Jul 2019, 5:25 pm   #5
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Default Re: Zenith G730 (7C05 chassis)

Zenith made that radio in many forms since the early 50's. I have several of models of the set using both the wood and the plastic cabinets, I have at least one like yours with the walnut cabinet. Yours does show a high degree of hackwork.
It's a good thing you looked it over well, as that jumper on the isolating cap made it a shock hazard even on our 120 volt mains.
Dave, a non-hackery repairer and restorer.
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Old 31st Jul 2019, 9:42 am   #6
donutty
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Default Re: Zenith G730 (7C05 chassis)

Yes, whilst on my travels I also saw a couple of radios using the same chassis in the plastic case, which also look nice.

I don't know why somebody would bypass the phono input decoupling cap ... but it was the first hack I removed!!

AFC wise... I checked out a few resistors on that circuit. The 3.9M was over at 4.3M so I replaced it with the nearest I had at 3.7M. The reception on AFC lasted longer and did not start to warble after the 20 minute warm-up but after about an hour it promptly went silent without warning. Turning to non-AFC FM restored sound, going back to AFC lost it again. Hmmmm....
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Old 31st Jul 2019, 4:47 pm   #7
usradcoll1
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Default Re: Zenith G730 (7C05 chassis)

I never had problems with the AFC circuit in one of those.
Also, most of them had mostly original valves, except for the 35C5 and the 19T8, the 35C5 on the start and the 19T8 on the end of the heater chain.
If you like that set, you'll love the larger eight valve model that was offered at the same time.
Dave, USradcoll1
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Old 2nd Aug 2019, 11:18 am   #8
SteveCG
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Default Re: Zenith G730 (7C05 chassis)

When you have sorted out the AFC issue, just bear in mind that North American FM receivers use a 75 uSec deemphasis time constant, whereas European receivers use 50 uSec. The result is that the sound from your tuner as it stands will sound duller than it need be.

I have not looked at the circuit diagram but suspect that just changing the value of one capacitor is all that you'll need to do to get the expected performance.
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Old 5th Aug 2019, 11:00 am   #9
donutty
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Default Re: Zenith G730 (7C05 chassis)

Thanks for that tip Steve.. I'll pore over the circuit and see what it could be to tweak the time constant.

I can also report I appear to have fixed the AFC issue - it was one of the smoothing electrolytics in the original 2-cap can that the previous bodger had tacked a replacement multi-segment can onto. I presumed this was done to fix a hum caused by an expired main smoothing cap but upon removing it found no such issue and left it off.

The set has now been running for about 6 hours without a hitch!


Cheers all !!
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Old 5th Aug 2019, 11:21 am   #10
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Default Re: Zenith G730 (7C05 chassis)

Although the de-emphasis will be slightly out, you probably won't be able to hear much difference with a radio of this type. Even some hifi tuners used a 'compromise' de-emphasis value so that there didn't need to be different models for different regions.
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Old 7th Aug 2019, 1:57 am   #11
usradcoll1
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Default Re: Zenith G730 (7C05 chassis)

Quote:
Originally Posted by donutty View Post
Thanks for that tip Steve.. I'll pore over the circuit and see what it could be to tweak the time constant.

I can also report I appear to have fixed the AFC issue - it was one of the smoothing electrolytics in the original 2-cap can that the previous bodger had tacked a replacement multi-segment can onto. I presumed this was done to fix a hum caused by an expired main smoothing cap but upon removing it found no such issue and left it off.

The set has now been running for about 6 hours without a hitch!


Cheers all !!
I'll definitely keep that one in mind!
Cheers to you as well! Dave, USradcoll1.
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Old 8th Aug 2019, 2:31 am   #12
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Default Re: Zenith G730 (7C05 chassis)

Is there a simple way to decode the Zenith radio chassis numbers?

The first digit does seem to be the valve count, as is in:

5 for AA5 AM-only receivers, such as 5F04, 5J04.

6 for AM-only receivers with RF amplifiers, such as 6E05, 6G05, 6H03.
Also for late FM-only receivers with single valve (12DT8) front ends, such as 6H06.

7 appears to cover three sub-types, namely:

FM-only receivers, such as 7F03
FM-AM receivers without AM RF amplifier, such as 7C05, 7E02
FM-AM receivers with AM RF amplifier, such as 7F01, 7K01

The second letter might be chronological, in that say a 7E chassis would be later than a 7C. But that said, the 7C05, the subject of this thread, had the later front end valve combination, 6BJ6 plus 12AT7, whereas the 7E01 and 7E02 had the early 12BA6 plus 12BE6.

The final two digits might address detail variations. Based upon a limited dataset, within the 7-series, 01 appears to cover FM-AM with AM RF amplifier, 02, 04 and 05 FM-AM without AM RF amplifier, and 03 FM-only.

Some other observations are that until the arrival of the single-valve unit FM front end (late 1950s in the US case, allegedly inspired by German imports), Zenith used combined AM-FM front ends, 12BA6 RF amplifier with 12BE6 autodyne mixer in early post-WWII models, then 12BA6 RF amplifier with 12AT7 mixer-oscillator, the 6BJ6 RF amplifier with 12AT7 mixer-oscillator. And the parallel availability of models with and without AM RF amplifiers appeared to stay until the end of the valve era.


Cheers,
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