Wonderful Bush PB65 work in progress

[jmedley]

This radio was from a family that had bought it new just pre-war and had been in the garage for the past 20 years. The photo of the front of the cabinet shows the plastic finger guards still in place behind the knobs and the cabinet in fine condition. Luckily I was able to download the Bush manual from this site and discovered that this model had a 3 position selectable IF response with adjustment available using the far right knob on the front.
With the back removed, horror, the 3 main electrolitics in boxes on the top of the chassis had leaked out their contents causing much surface rust.
The valves are 2 x VP4B, TH4B, TDD4, 354V, ACO44, 43IU, TV4.
All tested out OK, except the frequency changer TH4B, which had failed in both halves. I was very pleased that the output valve, a Mullard ACO44 was good. (Bush quote a Cossor 4XP to be an alternative ! ) they are rare and very expensive.
After cleaning and de-rusting I removed both cap. boxes and made up a new box to hold 1x 16uF and 2x 8uF electrolytics, changed the coupling caps to the 354V and the output valve AC044, the old caps were sort of OK but read high in value, which was more than double, as marked on them. These caps are cardboard tubes with the wires emerging not from the flat end pieces but from the circumference of the tube, near the ends. Underneath the chassis there is a pair of electrolytics of 50uF, both in a single cardboard tube, these are for cathode bypass and both had failed, so replaced.
I prefer not to paint a chassis but it was needed in this set, I had some grey undercoat/primer which went on well.
Then I switched on, it lit up OK without any humming or buzzing bad smells or smoke. Then tickling the Gram input I had a healthy buzz from the speaker. Sice I had changed the frequency changer I checked the the IF performance by injecting 460kHz to the top cap (grid) of the freq. changer.
The tone from the speaker was low so I attempted re-adjustment of the ferrite cores of IF1 and IF2, (4 in all) all was going well until I could not move the lower core of IF1 . Yes a damaged ferrite core, I have been there before with a DAC90. After carefully noting where the wires go I removed the complete can and stripped it down. The cores fit inside a tube made from something like carboard or paper mashie, and the lower portion was cracked and some time in the past the core had been adjusted with force and the threaded brass rod that is moulded into the ferrite was adrift.
The ferrites were 10mm diameter and luckily I found a twist drill of about 10.5mm (13/32" ) I carefully hand twisted the drill into the tube, leaving it in place while the PVA glue dried. Then the brass rod was Araldited back onto the ferrite core and all was left for 24 hours to dry and set.
All re-assembled and now the IFs tuned up beautifully. This only took me 2 days!! How in my previous life I repaired most things in half an hour I have no idea!! Hope more to follow after I have rested up a bit
Jim

[vinrads]

Excellent work Jim, the chassis is looking splendid , looking forward to the next episode. Mick.

[HamishBoxer]

Another nice set badly stored but the chassis looks great now.

[McMurdo]

nice to see a few pre-war bushes, makes a change from all the usual suspects!

[electrogram]

I have always fancied an exmple of that model. Does it suffer from the pre-war Bush condenser paradox? About thirty years ago a chap brought me three bush sets of about the same year, it was interesting to note that all the decoupling caps were beyond reproach as far as leakage goes, these were 'BI' make, and covered in some brown goo, 'stickies' rather than waxies, yet many of the puffers were miles out, and had to be changed, these were LEM or LEMCO.

[stevehertz]

Amazing transformation!

[jmedley]

Thanks for the your kind words and your replies.
The caps certainly do not follow the usual trend. The brown "stickies" are the ones I mentioned in the first post, strangely I have found the ones tested so far, are too high in value, around the IF area there are a few 0.003uF which measured up to 0.05uF, I changed all of them to get the IF working decently
The small pFs are silver mica types but are not encapsulated, the rivets are visible, some are dodgy and due to be relaplaced, again the ones in the IF cans I have already changed. Usually I have not had problems with the proper silver encapsulated mica caps. No doubt the problem here will be with the rivets.

I am now concentrating under the chassis with the valves in place, I have bolted 2 bits of plywood to the chassis to give me a stability. Its too easy however careful I am, to tip the chassis over and damage precious bits like the valves or scale glass, and yes i have done it before and I have beaten myself up about it. I really enjoy Bush sets they are so well made, physically and electrically. I dont blame them for small component failure after 75 years. I just purr over their well designed little brackets and small parts all held together with 6BA and 4BA brass nuts and bolts.
The Bush manufacturers manual downloaded from this site is very comprehensive, it provides dozens of test voltages with readings from both sides of the component in question.
Some more pics attached

Jim

[jmedley]

Problem uploading pics will try later

[jmedley]

I have found my error with picture uploading, I was trying too many pictures at one upload.
Now for :- WHAT A LOAD of NONSENSE with ONE CAP !!!
After taking a break from this project I went back to it, after changing many of the "brown capacitors" . Some are very well hidden and a bit challenging to get to and remove, since they were fitted behind the tag board during manufacture, before all the complete chassis parts were finally assembled.
I confirmed that the output stage was OK by inputting audio into the Gram socket from an iPhone, it nearly lifted the roof off, lots of power and good quality. The radio stations were all down and only about half volume of the gram input. Usually this is because I have'nt connected an aerial but yes I had my long wire plugged in. In a previous post I mentioned that I had set up the I.Fs with the siggy gen. so I didn't suspect a problem in that area and after much messing about I decided it was time to get serious, so out with the scope. I rarely get too serious when repairng radios and I don't document every thing I do as I go along, but maybe I should now, being in my 78th year.
The I.F wave form looked at a glance quite OK, after the 1st IF however the amplitude after the second I.F was not lifted as high as I would have expected (I remember many years ago concerning computers some one gave me very good advice being" RTFS" ) you can work that one out.
I began to read the waveform amplitudes and sure enough the 2nd amplifier was not doing much. The I.F cans are very easy to remove, they are just a bayonet type fitting, and I suspected one of the tuning caps being 150pf on one winding and 160pf across the other, they were both spot on!!!
After being disappointed that my guess was wrong I carried on checking with my headache getting worse. The primary section of the winding would not tune in at all when the ferrite core was adjusted from top to bottom of the winding with no peak of signal at all. The primary winding has an HT supply via a resistor and is decoupled in the normal manner with a 0.05uF to ground, well I couldn't find this cap anywhere, on or near the tagboard.
It was one of those caps, as previously mentioned, that was originally fitted out of plain sight under the tagboard. Somehow idiotically, I had cut it out and then not replaced it. I then triple checked where it should go and soldered in a new 0.047uF cap.
The transformation was dramatic, the ferrite core tuned in very well with massive signal improvement. Re-checked tuning of all 4 alignment cores and the reception was absolutely sparkling on all bands.
My problem was overlooking a simple problem and getting obsessed with the finer details, this radio is the only model I have worked on that has a 3 position switch that provides selection of the I.F response, making the circuit around both I.Fs more complicated with the addition of 4 screened cables a 3 position switch and extra components. So next time I may be more careful but then again it all adds to the fun.
The HT feed resistor is shown in pic 3 . It is the one immediately to the right of the screening shield plate 10k ohms, the decoupler cap was underneath
Pictures attached
Jim