Armstrong Jubilee Mark 2 tuner/amp - general advice

[Barnjet]

I am not at all familiar with Armstrong tuner/amps. It has been installed into a home-made cabinet, with no obvious means of access to the chassis or components, other than what can be got at through the back. It looks like I would have to damage the cabinet to get the chassis out. It also weighs a ton! I've never come across a unit so heavy before.

A previous owner has marked one of the components "No M/L wave". And I can't see how the current cabinet installation would provide adequate ventilation? At this early stage, I am wondering if it is worth restoring?

Thanks

[Simon Gittins]

https://www.service-data.com/product.php/338/639/m10338
In your second photo, two screws can be seen at either side fixing the chassis to the wood base. It may be that all you need to do is remove these and the knobs (may have grub screws). The service data says there are four fixing holes, if someone managed to get screws into the front two then extracting could be a challenge!
I think the message means that Medium and Long wave aren't working for some reason. Plugs with sleeved pins were introduced in 1984 so it may have been in use fairly recently.

[Barnjet]

Originally Posted by Simon Gittins

https://www.service-data.com/product.php/338/639/m10338
In your second photo, two screws can be seen at either side fixing the chassis to the wood base. It may be that all you need to do is remove these and the knobs (may have grub screws). The service data says there are four fixing holes, if someone managed to get screws into the front two then extracting could be a challenge!
I think the message means that Medium and Long wave aren't working for some reason. Plugs with sleeved pins were introduced in 1984 so it may have been in use fairly recently.

Thanks Simon, good points

Max

[Mr 1936]

Hi Barnjet

Some information on this here:

http://ukhhsoc.torrens.org/makers/Ar...Mk2/index.html

It looks quite an attractive unit, and well worth restoring. A plus point is that it covers the full FM band up to 108 MHz, so no modification needed for present day stations. Nothing too unconventional about the circuit, so should be possible to get AM working again.

I agree about the inadequate ventilation. Thie minimalist approach would be to cut a rounded square hole in the top of the case and fit some expanded metal mesh over it from the inside. Might be a good idea to connect this to the chassis, and then fit a 3 core mains lead with earth to chassis.

[Edward Huggins]

It most certainly is worth restoring! This well built unit has everything on it for great Mono radio, record player and tape/iPhone sound. The 10 watt amplifier has Baxendall type tone controls and I well remember hearing these when they were new. It will form the basis of the great system. But do get it out of that heat hogging cabinet ASAP! I fear both the transformers will have absorbed much heat from this multi-valved chassis in that confined area.

[Barnjet]

Originally Posted by Mr 1936

Hi Barnjet

Some information on this here:

http://ukhhsoc.torrens.org/makers/Ar...Mk2/index.html

It looks quite an attractive unit, and well worth restoring. A plus point is that it covers the full FM band up to 108 MHz, so no modification needed for present day stations. Nothing too unconventional about the circuit, so should be possible to get AM working again.

I agree about the inadequate ventilation. Thie minimalist approach would be to cut a rounded square hole in the top of the case and fit some expanded metal mesh over it from the inside. Might be a good idea to connect this to the chassis, and then fit a 3 core mains lead with earth to chassis.

Very helpful, thanks

Max

[Barnjet]

A fellow forum member has recently done me a very good turn, and so I have gifted him this set in gratitude. He is keenly looking forward to restoring it.

[cathoderay57]

Hi folks, I am the lucky recipient of the Armstrong Jubilee Mk2. I have extracted the chassis and begun to take a look. I also have a copy of the manufacturer's manual including circuit. Interestingly (to me, anyway) there are some significant differences from the circuit, all of which are without doubt original. The AM detector diode in the circuit also provides AGC whereas in my example there is a third OA81 diode providing AGC. The AM detector diode in mine is also followed by different value components. I'll sketch what I find and post it later. Maybe I've got a Jubilee Mk 2 and a half..... The push-pull ECL82 output valves have their cathodes strapped to chassis and are provided with fixed negative grid bias from an OA81 diode via a 6.8k/100k potential divider across one of the 205v ac HT windings, so sourcing 13v ac. The diode is open circuit and I was thinking of replacing it with a 1N4148. The grid current should be very low. Any issues with that? A few Wima humbug caps to deal with. Cheers, Jerry

[cathoderay57]

Here's the sketch of the AM detector/AGC that differs from the manufacturer's circuit. Jerry

[cathoderay57]

Originally Posted by cathoderay57

The OA81 diode is open circuit and I was thinking of replacing it with a 1N4148. The grid current should be very low. Any issues with that?

I found an answer elsewhere on the Forum https://www.vintage-radio.net/forum/...ad.php?t=49806. In fact all three OA81s are o/c. The loss of 0.5v or so needs to be considered for all 3:
Negative grid bias for push-pull ECL82s: Probably OK?
AGC: OK.
AM Detector: Not sure if this is going to affect audio level noticeably. There appears to be bags of gain with half an ECC85 pre-amp prior to the phase splitter, and an 8W output, so I suspect not noticeable.
Any comments?
Jerry

[cathoderay57]

The FM discriminator circuit is slightly different on mine from that shown in the circuit diagram. The EB91 diode output load resistors are each 10x the value shown, now 27k, and, instead of the single 4uF electrolytic, mine has two 25uF capacitors connected in series with the centre point earthed. See sketch. Jerry

[cathoderay57]

Hi folks, I've worked my way through cap replacement and tomorrow will try to reform the 2 main electrolytics. Meanwhile, my knowledge is rather sketchy with the detailed functioning of FM circuits. If they work reasonably I tend to leave well alone. This circuit employs two ECC85s. The first one is the FM front end which appears to be fairly standard using V1a as RF amplifier and V1b as mixer/oscillator. However, can somebody please explain what V6a is doing? V6b (not shown in the extract) is clearly the AF pre-amp. Cheers, Jerry

[PJL]

RF amplifier? bit difficult to tell without seeing the rest of the circuit.

[cathoderay57]

Unfortunately, posting the whole circuit breaks the rules. If it helps, the 2 wires going to the grid of V6a via R9 & R8 comprise:
1. DC negative grid bias from the fixed bias rail.
2. A feedback output from the FM detector diodes.
Clearly the output at the anode of V6a is fed via C14, L6 and L5 to the grid of V1b.
Makes me wonder whether it is some form of amplified AGC for FM?

[cathoderay57]

Further, I've found a couple of errors in the circuit, also in the above clip. The resistor top right should be labelled as R11 (label missing). It provides a directly-coupled feed from the FM detector. As drawn this signal wouldn't do anything because the end of R11 is shorted to chassis. The actual circuit has got a 0.1uF decoupling cap between the end of R11 and chassis, instead of the s/c. Jerry

[Trevor]

AFC control

[Edward Huggins]

Also unusual to see an ECC85 in a Pre-Amp/Stage. Its biasing will need some attention.

[G6Tanuki]

Yes the 'odd' triode is being used as a reactance valve across the local oscillator tuned circuit to provide AFC.

[cathoderay57]

Aha, AFC, that makes sense. Thank you very much! I should have been tuned into that idea (forgive the pun) since there is a similar principle used in my other current project, the McMichael 802 motor-tuned set, to correct mechanical mis-tuning arising from imprecise rotor disk position. Re: biasing the second half (AF) ECC85, the set provides fixed bias derived from a diode circuit. The manual states -0.6v on the grid, measured with an Avo 8 on 10v dc range so at least I'll be able to check it, allowing for a bit more negative volts using a DVM. Cheers, Jerry

[cathoderay57]

This afternoon I tested the HT smoothers. These are two aluminium can 50+50uF 350v caps. All reformed acceptably (leakage current <2.5mA) except the second smoother which started to reform, leakage went progressively down to 2.9mA then it started rising. When it got to 4mA and was still going up I abandoned ship and have disconnected it and soldered in a radial replacement beneath the chassis. Will power up tomorrow and see if the set makes any noise. Cheers, Jerry