In the "old days" of TV servicing often I'd come across a set in which the IF transformers had been twiddled, some local boffin had messed up the set and I'd called in to fix the set. On a house call this little signal generator was used to get the IF alignment back to the correct frequencies. Simple procedure, retune the sound IFs first, then locate the sound trap in the vision IF amplifier, for example in most post 1955 receivers the standard BREMA sound IF is 38.15Mhz. Peak the vision IFs to 36Mhz, that will roughly set the vision passband slope for the 34.65Mhz carrier. Some sets have 33.15Mhz adjacent channel sound trap, the same trap can set the vision response curve limits. You'll recall the topic about the Ekco T370 and this is the alignment procedure is used for this set. Before supplying any power to this signal generator I'd better determine the type of battery employed, 6 or 9 volts? There is an AF114 type of transistor in the signal generator, whiskers problem?
Till Eulenspiegel.
A nice little sig gen.
You were lucky that cores weren't broken or coils ripped off their formers, or did you have to contend with as well?
For some reason the customers we had didn't tend to have the back off their sets so customer damage was a rare occurrence.
Frank
Back in the sixties I'd often have to order replacement IF transformers. Damaged formers were common. Often the reason why the IFs were twiddled was the aerial signal was too strong causing vision on sound and vice-versa. Seems the bodgers would try to solve the fault condition by wrecking the IFs. An attenuator would have solves the problem.
Till Eulenspiegel.
Within a few days time I'll be in a position to carry out tests on the signal circuits in an HMV 1804 TV receiver. This is the receiver that had received extensive modifications to sound and vision circuits. Let's find out if the Nombrex signal generator is any good. First task is to fit a PP3 battery connector.
The attachments show the original EMI and the modified sound IF amplifier and demodulator circuits. Start off with the easy to fix parts of the set and later move on to more complicated RF and vision IF circuits. The little Nombrex will be ideal for this kind of work.
Till Eulenspiegel.
The modified circuit "appears" straightforward, the one diode of the double diode triode providing AGC to the sound IF amp.
The double diode providing audio demod and noise limiter.
I am not too sure about the EMI circuit, one diode is demod, the other I assume AGC but C76 seems that it would also work has a shunt limiter for impulse interference, not sure also what L20 is providing, is it a choke or part of a tuned circuit?
Lots of questions David, I have been following on the UKVRRR, just not said very much.
Frank
Hi Frank,
In the original EMI 1804 sound circuit the D63/6H6 double-diode functioned as the demodulator and noise pulse canceller. The was no AVC applied to the KTW61 vari-mu beam RF tetrode IF amplifer. The later circuit is as you say is more conventional in having a series noise limiter and AVC. When found the KTW61 had been replaced with a Mazda SP61, I wasn't willing to countenance a Mazda valve in a set made by EMI. It has subsequently been replaced by a W61 vari-mu pentode.
Getting back to the signal generator, it doesn't work. It's possible the RF transistor has the "whisker" problem that besets that type of transistor.
Till Eulenspiegel.
Till Eulenspiegel said
Back in the sixties I'd often have to order replacement IF transformers.
I saw a lot of twiddled IFs but never had to replace an IFT, thankfully.
The old slotted cores weren't too bad because the twiddler rarely realised that there were two cores in the IFT so, if they chewed up the slot in the upper core, you could remove the lower core and get to it from the other end to remove it.
The real problem came with the introduction of cores with hexagonal holes through them. one touch with a screwdriver rammed in the hole was usually enough to crack the core and jam it in the former!
I found that worked was careful use with a range of twist drills, increasing the size gradually until the cracked core was so thin that judicious use of a small screwdriver with a sharpened blade could break it up into small enough bits to pass through the former or even be poured out of it if the IF strip was a separate small chassis.
I recall one story I heard about an engineer who went to a set and told the customer that someone had been interfering with it to which the woman sprang to the defensive saying that no-one had touched it. It was a set with the back and bottom made in one 'L' shaped piece and, when the engineer removed it completely, a number of dust iron cores fell out onto the carpet!
The woman turned red and suggested that her husband 'might' have had a quick look inside to see if it was something simple!
When all else fails, read the instructions
