R106 MK11 Reconversion

[ms660]

Hello my next receiver project is the reconversion of this receiver from B7G front end back to octal, the original osc/mixer circuit was totally redesigned to take B7G.
My question is as follows:

In the original circuit was there a particular capacitor in the Osc. that had a specific -ve temp coeff. if so which one, none of the circuitry in the coil packs was modified, it is just the receiver chassis that I want to put back to spec.
I have the book of words.

Lawrence.

[trh01uk]

Lawrence,

I think the answer is no. At least there is no direct evidence in the EMER parts list that the one fixed capacitor that appears in certain coil packs was a negative tempco type.

The fact that on certain bands there are no fixed capacitors at all (e.g. band H) suggest that no attempt at temperature compensation was made. Anyway, you say your coil packs are as original, so if you certain coil packs have tempco caps fitted, you are home and dry.

When the remainder of the circuit (outside the coil pack) is examined there are only two contenders. There is the variable tuning capacitor, C58, but I am not aware that variable caps can be made temp compensating, and presumably yours is original anyway. Then there is C30, the grid coupling capacitor, which is just listed as 100pF +-5%. And in that position, it would not be able to do any temp compensation anyway.

Richard

[ms660]

I have more or less come to the same conclusion, I have a selection of both ceramic and mica to reinstate the grid coupling capacitor with and I also have an original ceramic octal base to replace the minature valve base with.
My other HRO is solid as a rock after 20 mins or so, I will compare any drift to that when I have done the re conversion and take it from there.
At some point the person who originally converted the front end must have had problems with Osc. drift as a VR stabiser valve had been fitted ad hoc to the chassis to feed the Osc HT, the rest of the conversion was, from a layout point of view, done very well. however the screen feed to the Osc valve was/is single ended ie: not fed from a divider network, I suspect that this might be contributing to the Osc drift which is still present after warm up in the receivers unrevoked modified state. Also in it's present set up the Osc. injection is to the mixers G1 and not G2. Heater voltages are stable. All four front end valves are minature types as it stands.

Lawrence.

[trh01uk]

Lawrence,

most of these "improvements" were done by amateurs following a recipe given in some mag article, though I don't have a reference to hand in this particular case.

The fitting of a stabiliser valve to the oscillator HT makes sense, since all oscillators will drift if the supply wanders up and down. Remembering that the HRO was a 1930s design, I don't think such concerns had really surfaced at such an early date.

The switch of oscillator feed from G2 to G1 is curious. Its possible it was done to improve the intermodulation performance of the receiver. This parameter was barely recognised even by the WWII era, but might have been an issue by the time amateurs got their hands on these receivers by the 50s and 60s, particularly on the 40m amateur band where high power broadcast signals abound.

The R106 parts list is particularly vague about the precise type of components fitted, with the "Type" column against the capacitors either blank or unhelpful - e.g. "1002 type" whatever that was! However you can't go far wrong with ceramic or mica caps in RF circuits.

Sounds like you are making a good job of putting this particular example back to its original state. If you need the original docs for the R106 (specifically) then you should be able to find copies on the vmars archive website - or failing that PM me and I will send you copies.


Richard

[ms660]

I am ok for the correct manual etc Richard, but just to throw a spanner in the works on the subject of drift I have couple of receivers of American origin but dual mains voltage, I run them on a Variac set to 230V AC and at night when I am done I wind the variac down with the receiver still on and I am always suprised at how low you can run them before the Local Osc/BFO starts to drift out of the capture of an SSB signal say on 40 or 80 meters, the overall gain (Inc. Audio) is always the first to take a dive.
The G1 & G2 mixer thing is also interesting I have some sets where the Osc is injected at G1 and in both cases the highest band range (10 meters) will only track when the Osc. is running below Fs and not above, a fellow forum member commented on this and pointed me in the direction of the Radiotron Designers Handbook, it made very interesting reading, these early valve designers really knew there stuff.
Many of these early sets were modified when they became surplus and available both by Ham operators and SWL's (including myself) but that was by necessity and was of its time.
I remember when I was kid around 11 years old Mum and Dad had an old Bush DAC10 (I think it was) on the shelf, it was push button job, one Saturday night when they had gone down the Pub I decided to take it appart and put it back together again before they got back home...I failed...my Dad went ape, my Mum said don't worry George are Lawrence will fix it back ok, I did, it took me about two weeks, I guess that ripping it appart taught me the beginings of radio engineering. However at this point in time I am of the opinion that this old stuff should be saved and preserved (pref. working) as best as we are able to.

Lawrence