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Old 17th Jan 2019, 1:39 pm   #1
SteveCG
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Default TEAC T-H300 AM/FM Tuner

I've just got this Tuner - it had been languishing, neglected on the floor, in a Second Hand place for at least a couple of months. As has been mentioned recently on the forum, these AM/FM tuners have little value these days and attract little interest.

I've got some information on it from the hifiengine web-site, but I've a couple of simple questions about it:

1) When was it likely to have been made?

2) I remember TEAC having a good name for its products. Is it likely that the performance of this tuner is a cut above the average?

Thanks
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Old 17th Jan 2019, 2:00 pm   #2
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Default Re: TEAC T-H300 AM/FM Tuner

I don't know when your tuner was made, but, as to TEAC's good name, if it's any guide, I've owned a TEAC Turntable for many years, and a TEAC Dual Cassette deck for maybe 10 7ears now, and never had any trouble with either.
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Old 17th Jan 2019, 2:46 pm   #3
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Default Re: TEAC T-H300 AM/FM Tuner

It appears to be part of a late 90s mini separates system. Lots of the big Japanese hifi names made similar systems, and while some people saw this as 'slumming it' and moving downmarket, they generally performed very well. The internals of 'real' hifi components had become very small by then anyway, and the big boxes contained mostly air.

I don't doubt that this will perform very well. It's just a pity you don't have the matching amp and CD player.
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Old 18th Jan 2019, 12:11 pm   #4
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Default Re: TEAC T-H300 AM/FM Tuner

An update:

I've not taken the cover of it yet but there a couple of things I've discovered:

1) The dot matrix type display is a bit dimmer on the left hand side, with the horizontal lines of dots being of a different intensity than the vertical lines of dots;

2) Whilst the stations can be stored in memory, on powering off the information is lost. No doubt a back-up battery has died.

The tuner I think is a mk 1 version and it has working RDS, as well as AM. hifiengine has the service type info for the mk 3 version. This is more of a block diagram listing than a detailed circuit and set-up instructions.
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Old 18th Jan 2019, 12:21 pm   #5
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Default Re: TEAC T-H300 AM/FM Tuner

Not all of these retain preset settings when completely disconnected from the mains. The assumption is they will be plugged in all the time.
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Old 18th Jan 2019, 12:26 pm   #6
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Default Re: TEAC T-H300 AM/FM Tuner

Paul, the instruction manual (in typical manual "English") says that the tuner can be unplugged from the mains for upto 15 days before loosing the stored information.

Clearly it needs to have been powered up for a bit before getting towards that information storage longevity, but I think I had it on for long enough. I'm going to take the cover off and see what's what.
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Old 18th Jan 2019, 3:30 pm   #7
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Default Re: TEAC T-H300 AM/FM Tuner

Further update:

After removing the cover - which was a bit of a struggle for no apparent reason, sharp steel so watch your fingers - I found a well laid out pcb. For some reason the Japanese don't do mounting components vertically or horizontally, rather they were mounted at all sorts of angles. Anyway I straight away saw a 3 cell, 3.6v, 80 mAh, green shrink wrapped NiCd battery around whose +ve end the pcb was darker than elsewhere. I reckon the battery is just starting to ooze. To remove the battery I need to remove the main pcb, which looks to be time consuming rather than difficult.

I've not decided whether to replace the battery, but remove it I definitely shall. This now adds to the list of the bits of electronics I have recently dealt with where failed NiCd batteries pose a threat to the rest of the circuitry. I would recommend anybody with this Tuner or its near dated cousins to check the physical state of the battery.

Apart from that, the display is on a separate pcb and so the display brightness issue is for the future.

One of the things I wanted to do with tuner (and part of the reason for buying it) was to see what filter circuitry was between the FM IF output and the stereo decoder, and also what filter circuitry was on the output of the stereo decoder. I may not have the circuit diagram but I reckon I can trace the circuit, so I'm pleased about that.
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Old 19th Jan 2019, 11:17 am   #8
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Default Re: TEAC T-H300 AM/FM Tuner

A correction to post no. 7

On further reading with a stronger light and magnifying glass:

The battery in question is a GP brand GP40BVH X3, 3.6V 40mAh.
The pcb writing says "3.6V 30 mAh" which I had misread as 80 mAh

I checked on CPC, and Amazon for that matter, and the battery is not available, presumably because of the NiCd technology. There is a GP NiMh 3.6 V 80mAh available from CPC (GP70BVH3A3H, CPC order code BT01325) but whether that battery would substitute, being a NiMh instead of NiCd, is a question whose answer I'm not sure about given the charging regime of the Tuner.

BTW, dealing with a single sided pcb, that has separate finger-sized components with colour codes, is a treat after multi-layer pcbs and smc components!
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Old 19th Jan 2019, 12:04 pm   #9
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Default Re: TEAC T-H300 AM/FM Tuner

I doubt if the battery type is very critical. You could probably get away with a lithium 2032 in a holder, or even 3 Poundland NiMH AAAs if there's room.
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Old 19th Jan 2019, 12:21 pm   #10
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Default Re: TEAC T-H300 AM/FM Tuner

Interesting suggestions Paul.

I had already thought of the AAA NiMH route - but your (non-rechargeable) CR2032 in a holder suggestion may be an easier route for me as I have both a holder and cell already. To check on the feasibility I'll try to establish what the current flows are around this part of the circuitry when 1) in operation, 2) standby and 3) mains disconnected. I can do that by removing the present faulty battery and substituting an outboard 3 cell NiMH battery with a current meter in series with the battery.
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Old 19th Jan 2019, 1:03 pm   #11
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Default Re: TEAC T-H300 AM/FM Tuner

I would expect the battery to be slowly trickle charged by a resistor, and the current may not be enough to damage a 2032. Of course, if you can identify the charging circuit you can just disconnect it.
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Old 20th Jan 2019, 12:23 pm   #12
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Default Re: TEAC T-H300 AM/FM Tuner

NiMH technology is said to be less tolerant of long-term trickle charging regimes than NiCd, so fitting one with a larger mAh rating will make it less of a problem given the same trickle rate. I'm pretty certain that a green-wrapped backup battery will have been an NiMH type anyway, so the circuit it's in may well have been designed to suit from the outset. (When the insidiously poisonous cadmium component was displaced by environmental considerations and legislation, the battery makers were keen to identify their new type with, guess what, a green wrapper to distinguish them from the older "bogey" types which often had blue heatshrink, but also grey, red, black..... sometimes). The change was at least 2 decades ago, again making it more likely to have been NiMH original fitment.

I've got the version of this "shoe box" format tuner with DAB (Richer Sounds clearance), inside it's a real bitsa with about a dozen separate little PCBs tacked and stacked on each other with lots of multi-way pin arrays. The DAB bit is the standard Frontier Silicon module fitted in a zillion early-generation tuners and radios from most of the Far Eastern usual suspects plus Pure and other odd-ball outfits. From new, the back-up time on mine was more like 2 days than the claimed 15, but there are no less than 3 "supercap" backups scattered around various of the little PCBs, life's too short to investigate further....
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Old 21st Jan 2019, 11:31 am   #13
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Default Re: TEAC T-H300 AM/FM Tuner

Well, I was able to remove the main pcb quite straightforwardly with no soldering required. I removed the battery and cleaned up the crud around the pins. The brown pcb around the pins was a noticeably darker hue of brown - rather like earlier pcbs that had had valve bases directly mounted on them where the heat from the valves had cooked the pcb. Except in this case there was no obvious source of heat and the rest of the pcb, including all of the underside with the copper tracks, was in immaculate condition.

The battery itself was not obviously encrusted, however this morning I found that a piece of newspaper I had put it on whilst doing some further tests on it had a damp patch on it, so the battery has gone to the battery recycle bin! - I'll just retrieve it to confirm whether it is a NiHM instead of what I thought it was - a NiCd.

I thought further about the CR2032. I concluded that if a 30mAh battery was required to keep back-up state for 15 days then a CR2032 would be flattened in no time at all.

I also considered again replacing it with a NiMH, but decided that since I was likely only to use the Tuner occasionally then all I would do is flatten the battery with all the "reverse charging" damage to it that would happen when the terminal voltage sank below 3.6V.

So I think I'll simply omit the battery and put up with having to reset it every power-up.

Turretslug. Right, so the DAB version is a right cat's cradle. In comparison this unit is pleasure itself to deal with (mind you, I've not tried removing the separate display pcb...) and I hope to do some circuit tracing in due course - warmer weather permitting!
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Old 21st Jan 2019, 11:48 am   #14
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Default Re: TEAC T-H300 AM/FM Tuner

The battery is likely to have been a standard part and the capacity rating will be irrelevant. The duration of the no-power backup will be down to the self discharge characteristics of NiCd technology.

It would be easy enough to lash up a 3.6V source and measure the current being drawn to maintain the memory. I bet it's absolutely tiny.
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Old 21st Jan 2019, 12:06 pm   #15
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Default Re: TEAC T-H300 AM/FM Tuner

Alright Paul - I'll try that.
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Old 22nd Jan 2019, 1:30 pm   #16
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Default Re: TEAC T-H300 AM/FM Tuner

Paul,

Using a CR2032 with a 120 Ohm resistor in series (just as a safety feature whilst experimenting) with my DMM on its uA range, I measure 10 uA.

So if a Cr2032 is about 200 mAh then I'd have 20,000 hrs back-up, approx 2 years.

A quick question for turretslug and his DAB model: Is the clock part of the display dimmer than the rest?
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Old 22nd Jan 2019, 9:23 pm   #17
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Default Re: TEAC T-H300 AM/FM Tuner

I've just had a peek at search images for the AM/FM version and the display looks to be a somewhat different format to the T-H300DAB- whilst still VFD, the -DAB has a 16 character by 2 line dot-matrix setup.

What I did notice after perhaps 10 years of ownership was that the display looked somewhat mottled when running through the presets, i.e. some dots far dimmer than others. Acting on a hunch, and a bit straw-clutchingly, I thought that I tended to listen mainly to BBC stations, i.e. Radio (x) on the display and wondered if some dots had lost light emission through under-, rather than over-use. Changing through stations and leaving the display on less-used names, I was amazed at just how quickly the dim dots came back to similar brightness as the others- literally a couple of minutes or so- it was almost as if the lesser-used anodic dots had suffered something like a cathode interface-like loss of effectiveness. (I had found a similar effect years earlier with a decidedly swanky Sony tuner (STD-777) rescued from near the bottom of a well-loaded skip after considerable effort and some risk! That too had a scattering of dim dots that recovered after an overnight stint on "Demonstration Scroll"). Before that, dimmed dots/segments on elderly VCRs/clocks invariably seemed an irreversible thing.

Does the clock continue to display on standby, maybe those phosphors have become more tired than the switched remainder of the display?

Not relevant to the thread, but after purchasing my T-H300DAB, I found that there was a MkII version (that's why clearance offers happen, innit....). Prompted by this thread, good ol' Electrotanya came up with the MkII service manual, it looks far more integrated and refined inside than my MkI. Notably, it uses a single 3.6V NiMH back-up, rather than the scattered "supercaps" of mine- maybe they weren't so super after all?

Last edited by turretslug; 22nd Jan 2019 at 9:29 pm.
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Old 23rd Jan 2019, 12:31 pm   #18
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Default Re: TEAC T-H300 AM/FM Tuner

Thank you for that turretslug. A couple of bits of information:

1) I now think the failed 3.6V back-up battery was a NiMH and not a NiCd. The battery does not actually say what it was but the picture in the CPC catalogue of the potential NiMH replacement I mentioned in post no. 8 looks to be identical in style.

2) The clock does continue to display if the tuner is powered-up but in standby. I noticed that some 'sections' are brighter than others, I'll have to have a closer study to see if I can infer usage as you did.

My next moves are to look up the information on 4 ICs in the tuner, these are:

1) LA1266;
2) LM7001;
3) LA3410;
4) MC4558S

What I'm hoping to do is to get a feel, from the associated circuitry, of the response of the Tuner to a crowded VHF/FM band where 100 kc/s spaced stations abound. I may even find that an IC manufacturer's recommended circuit design has been followed...
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Old 28th Jan 2019, 12:05 pm   #19
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Default Re: TEAC T-H300 AM/FM Tuner

Following on from post no. 7:

I have traced the circuit around the LA3410 Stereo decoder. It has a filter on the input to the decoder. It turns out that this filter was not against the RDS subcarrier stuff (as I had wondered given this tuner is RDS equipped) centred 57 kc/s, rather it is a low pass filter that is flat out to about 62 kc/s and then dives to a zero at about 114 kc/s (3 x the stereo subcarrier frequency). I measured these results by injecting a sine-wave before the filter and then observing with an oscilloscope the input to the decoder.

As a further expt I set the tuner up to be receiving a stereo signal and then with the sine-wave still being injected, I altered the injection frequency to see what would cause audible effects. The answer was around 38 kc/s and around 114 kc/s. The upper frequency limit I could inject was 150 kc/s.

Last edited by SteveCG; 28th Jan 2019 at 12:05 pm. Reason: punctuation
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Old 28th Jan 2019, 1:36 pm   #20
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Default Re: TEAC T-H300 AM/FM Tuner

A clarification to post no. 19:

For the second (audible birdies) expt I injected the signal from my oscillator after the low pass filter, ie straight into the stereo decoder.
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