Bush A.C.91

[DonaldStott]

Originally Posted by DonaldStott

Thanks guys - over the weekend I'll revisit the Anode readings on V5 and get to the bottom of this.

I'm sure it's the valve socket/valve pin connection that is causing the trouble as I did get a reading of 236V at one point on the rectifier Anode.

Apologies for the very long pause but I've had other family priorities and matters to attend to. I'll also mention in passing my Velleman K7000 "adventures" only recently resolved with the extraordinary help, patience and perseverance of David G4EBT - thanks again David for your assistance, notwithstanding the supply of many components including two assembled probes for the Velleman!

So where were we - dodgy readings on the Anode for V5 - the rectifier valve ...

I was getting fluctuating readings when I wiggled the valve about but this stopped when I used my long nosed pliers to crimp the connection tightly to the pin - now getting a consistent 236V. Only slight issue is that the pin connector is now loose in the valve base - some epoxy will fix that.

See attached drawing for V5 readings.

Moving on to the IF coils I have the following measurements (assuming my probes were all in the right places):-

L7 8.5 Ohms
L8 5.5 Ohms

L15 9.0 Ohms
L16 6.6 Ohms

These should all be 5 Ohms so L8 and L16 are passable - on the assumption that L7 and L15 are the upper coils in the IF cans does this provide more circumstantial evidence that the IF cores are broken?? The good news, as David G4EBT has advised, is that none of these are open circuit.

Moving on, next on my list were the trimmer coils which measured as follows: -

L1 0.3
L2 0.4
L3 0.7
L4 4.3
L5 35.2
L6 17.0
L9 0.3
L10 0.4
L11 0.8
L12 3.5
L13 1.6
L14 4.6

These seem reasonable to me?

Now that I'm almost Velleman-enabled (just to fit all the components in the box) I can start to use it in earnest to help pin point the ongoing issues I have with this "dead set"!

Lastly (for now) I mentioned in an earlier Post that I've also installed two new HT line smoothing capacitors C27 and C29 but still got that hum??

[ms660]

Broken or missing slugs shouldn't affect the DC resistance of the inductors.

Lawrence.

[DonaldStott]

Thanks for the clarification Lawrence - what are possible causes??

[ms660]

Wrong info in the manual.

Poor meter/probes/contact.

Greenspot...possibly.

If they are indeed high I wouldn't worry to much, a few ohms extra in series with the inductance won't make a lot of difference.

The key thing is voltage and signal continuity.

Lawrence.

[Voxophone]

Originally Posted by DonaldStott

Lastly (for now) I mentioned in an earlier Post that I've also installed two new HT line smoothing capacitors C27 and C29 but still got that hum??

Could be that the volume control body isn't connected properly to chassis - although I think you mentioned the connection was there a few pages back.

Alternatively it could be due to a faulty audio valve (V3 or V4) superimposing AC from the heater onto the audio signal. Not sure if the CL33 is particulary prone to this but its extremely common with the UL41 in the DAC90A.

Liam

[DonaldStott]

Could be that the volume control body isn't connected properly to chassis - although I think you mentioned the connection was there a few pages back.

Thanks Liam - you are correct that I mentioned the metal volume control body in an earlier post but the short connection to an earth tag on the chassis doesn't seem to make a difference to the hum?

I also replaced and relocated the V3 Cathode Decoupling capacitor (C21) to under the chassis using another suitable earth tag - could that be the culprit?

[ms660]

Just a thought, I would sort the IF transformers out first before worrying about any hum.

Lawrence.

[DonaldStott]

Of course the IF transformers are top of my list - just doing some multi-tasking...!

[Voxophone]

Originally Posted by DonaldStott

Thanks Liam - you are correct that I mentioned the metal volume control body in an earlier post but the short connection to an earth tag on the chassis doesn't seem to make a difference to the hum?

If the metal body isn't connected to chassis it can cause hum, but since you've confirmed the connection is there this is unlikely to be an issue (unless the chassis tag isn't screwed on tight enough or there's a dry joint).

One way to narrow down the source of the hum could be to try shorting the control grid of V4 to chassis. If this removes the hum, it proves it isn’t coming from the h.t. line, hence the smoothing electrolytic capacitors are probably ok. It can only be coming from V4 itself (internal leak) or a preceding stage.

Follow this up by removing one end of C26 (V4 control grid coupling capacitor). If the hum persists, it must be coming from inside V4 itself.

[DonaldStott]

Thanks to DavidG4EBT the Velleman K7000 signal injector/tracer is finished at last - see picture below.

And thanks to all those who contributed ABS hole cutting suggestions.

Now, hopefully, we can get this Thread back on track??

[David G4EBT]

Neat job Alistair - good look with your signal injecting/tracing!

It will be interesting to discover whether the IFTs are in fact duff.

[DonaldStott]

Sorry Moderators, I originally Posted this in my Valve Base Wanted Thread ...

Thanks to Ed Dinning a replacement valve base arrived in the post today - thanks Ed!

Although this is the valve base for the Bush AC91 rectifier with only three connections required it seems like the Bush people were enthusiastic users of spare pins as connection points.

What's the thinking on this practice and its impact, if any, on the rectifier valve?

[Boater Sam]

It was a cost saving measure used by most manufacturers. The effects on a rectifier valve would be nil as the valve will not have any connections to the pins not listed on the spec sheet. Unused pins were often used as tag points for components.
The same is not true of other valves, there may be differences between valve makers too.
The Ul41 is a case in point, pin 4 may or may not have a connection, use of this pin as a tag point can cause problems.

[DonaldStott]

Hi guys - I've been busy over the last couple of months focusing on family priorities and then before you know it, Christmas and New Year.

In between I've rebuilt and restored an HMV 101 portable wind-up gramophone helped in no small way by Audio1950 (Barry) who cleaned and re-greased the main spring for me. Working well now and spinning at the correct speed - sound quality is fine and much improved with a replacement No. 4 Soundbox.

So last weekend I decided to get back in the saddle (so to speak) with the Bush AC91 to see if i could finally crack this enigma - if only out of respect for the number of people who have given me help, guidance and spare parts.

I went back in and re-familiarised myself with where I was and rechecked all the transformer values, the valve voltages, heater voltages, IF coils, trimmer coils, resistors and continuity where I could - all was where I left them and working according to to the Service Manual. But I still had a dead set ...!

I had a problem with a broken valve base on V5 but this has now been replaced (thanks to Ed Dinning) - that really was a difficult job to un-solder and then re-solder the various pin connections. In so doing I disturbed the two connections on the top of VR1 - oops! So I rewired all the connections around VR1 including C27 and C29 (the HT Line Smoothing Capacitors) which have also been replaced.

Tonight, as usual, I plugged in a simple wire to act as an aerial and draped that around the room, connected the set to the mains using an RCD and fiddled about with the tuning and switching wavebands. Nothing I could hear until suddenly a faint station Medium Wave, could it be, YES some music on Clyde 2 ... !

I was so excited I made a short video and posted it on YouTube - hope this allowed on the Forum?

https://youtu.be/5VV4DlRdyR0

Not a great sound and you cant really pick up the loud hum that still persists but I can try and sort that out later.

Meantime, turned the set upside down and immediately the music stopped as we heard the familiar clunk - turned it back round and the volume increased dramatically for a second and then the music went back to where it was. Tried again giving the set a slight shake and it's certainly sounding more alive than its ever done!

Before I delve in and have a go at the IF slugs which now appear to be the prime suspect, is there anything else that could cause the behaviour I have described or other things to check first??

It's good to be back - but you may not agree?

[Boater Sam]

Its the slug disease alright. Its very difficult to glue them back onto the brass thread exactly in line unless you have a lathe to line them up on.

I've thought of 2 remedies, not yet tried.
Working on the assumption that the alignment is correct with the slug touching the thread, all that need to be done is hold it there.

Idea 1
Remove the adjuster and slug, preferably the loose one though how you can establish that I have yet to work out.
Put a thin very weak BRASS coil spring in the former between the slugs to hold then against the adjusters.

Idea 2
Same really but use a sliver of foam rubber as a spring.

As its a Bush, these IF transformers were used on lots of models, if you could find replacements that is probably the best way.
I have found to my cost that trying to realign these often breaks the slugs off due to too much wax being used to lock the threads, it has dribbled down and stuck the slug in the former, one twist and it breaks off.

[DonaldStott]

Thanks Sam for your quick Reply and suggested Ideas.

In the interests of my own education and extending my skills I will endeavour to have a look inside the IFTs and see what falls out.

I will also, however, Post in the Wanted section for replacement IFTs as these should be fairly common as you suggest.

Meantime, how do I get inside these beasts without causing too much damage in the process: -

https://www.vintage-radio.net/forum/...pictureid=3852

[DonaldStott]

Last night I "liberated" the IFTs from the chassis and am now pondering the means by which I get inside these fragile looking components without extensive surgery - see attached pictures.

As a fall back I'll also advertise in the Wanted section for replacement IFTs - as Sam advised these are Bush IFTs so should be fairly common.

Both the IFTs have that tell-tale click when turned upside down and tactile feedback hints at lots of small pieces inside - aargh!

[DonaldStott]

Back again with some major progress and good news – at last!

Thanks to David G4EBT who put me in touch with 60 oldjohn I managed to acquire a pair of replacement IFTs for my Bush AC91 – no clicking noise when you turn them upside down so the ferrite slugs are intact – hurrah!

Using some of that excellent silicone cable from SWB 18 I got the IFTs cleaned up, wired up and installed – see attached picture for the pre-installation phase.

And the great news is that the radio is now working with tuneable stations on Short, Medium and Long Wave - amazing! You will appreciate how happy and relieved I am at long last to have got to this stage!!!!

Still got quite a bit of a hum that doesn't vary and quite a low volume even when turned up full? I still suspect that there are some dodgy connections from the previous phantom around VR1, C27 and C29 (the HT Line Smoothing capacitors) and thr mains supply.

So I feel that we are now on the home straight with this set with only the low volume and hum to sort out (!)

Focusing on the hum first what I did was to use my Velleman K7000 to inject a signal at the volume control slider and got a really nice loud beep. Did the same thing at the junction of C26/R16 into V4 and got an even louder beep - so sounding good so far?

Measured voltages on V4 and V5 again - Voltages in brackets are from the Valve Data table in the Service Instructions: -

V4 CL33 - Output Pentode
Anode - Pin 3 : 254V (245V)
Screen - Pin 4 : 147V (115V)
Cathode - Pin 8 : 5.1V (4V)

V5 CY31 - Half Wave Rectiifeier
Anode - Pin 5 : 236V A.C. (220V A.C.)
Cathode - Pin 8 : 274V (260V)

Tried shorting the control grid of V4 to chassis (Pin 5 to which R16 is connected) - the music that was playing stopped and I was left with just the hum??

I also removed one end of C26 (V4 control grid coupling capacitor) but a bit concerned that this was associated with the faulty Output Transformer. The hum remained - from information I've gleaned from the Forum that would appear to indicate that the hum must be coming from inside V4 itself. Reconnecting C26 I get music and the hum!

So the question for you Forum experts is where else could the hum be coming from – is it from the HT line due to inadequate smoothing (C27 and C29) or mains by-pass (C30 – X2 type) bearing in mind that all these have been replaced.

Lastly, for now, I'm still intrigued (perplexed?) by some undocumented wires and connections in and around V5 - the common chassis tag for C27 and C29 has a wire running to Pin 3 on V5 and from there another short wire leads to another chassis tag. I've no idea what these are for or what they are doing as Pin 3 in unused in V5??

I’ve had lots of additional advice, guidance and mentoring from David G4EBT along with helpful suggestions and support from 60 oldjohn and tim.norris – can’t thank them enough.

[ms660]

""Still got quite a bit of a hum that doesn't vary and quite a low volume even when turned up full?""

I would investigate the low volume first.

""Focusing on the hum first what I did was to use my Velleman K7000 to inject a signal at the volume control slider and got a really nice loud beep. Did the same thing at the junction of C26/R16 into V4 and got an even louder beep - so sounding good so far?""

With the volume control at maximum, a signal injected at the volume controls wiper should sound louder than the same level of signal injected at the pentodes g1, so you need to confirm/verify that first then take it from there.

Lawrence.

[DonaldStott]

Originally Posted by ms660

I would investigate the low volume first.

With the volume control at maximum, a signal injected at the volume controls wiper should sound louder than the same level of signal injected at the pentodes g1, so you need to confirm/verify that first then take it from there.

Thanks Lawrence - will confirm/verify tonight and report back.