Bush A.C.91

[ms660]

I've never taken one apart but looking at various pictures I would say removing whatever secures the tapped adjuster plate(s) would be the key.

I'm not sure what the slugs are made of.

EDIT: Looking at the photo you've posted I would say removing the two slotted screws shown would release the adjuster plate.

Lawrence.

[Station X]

From what I remember, I screwed the adjusting screw right in and extracted it and the broken pieces of core from the bottom of the can. What I don't remember though is what stopped everything falling out.

[David G4EBT]

Has it been established beyond doubt that the slugs are broken?

Personally, I'd leave the IFTs well alone till everything from the volume control onwards is working - that is the audio pre-amp V3 (which also takes care of detection and AVC). As far as I can gather, there is no voltage at the 'hot' end of R12 - the anode feed from the HT to V3 and if that is still the case, there isn't much point in delving into the IFTs until that fault is resolved. Once the audio stages are working, which - in the absence of a signal injector should provide a very loud sound if the centre tag of the VC is scratched - then fine, move to the earlier stages - the first and second IF & the frequency changer (which a signal tracer on the VC centre tag may actually prove to be working, so faults should not be assumed).

Without a signal generator I'd be wary about disturbing the IFTs till I was satisfied that the IF stages were proved to be malfunctioning, and true, you can roughly align IFTs on a working set without a signal generator if the cores are removed and repaired if found to be faulty. (It would be interesting to know if there are signs that the IFT cores have been got at by the 'phantom' before the set fell into Alistair's hands).

Just my personal thoughts - others might see things differently, which is fine.

It's certainly been a valuable learning exercise and full marks to Alistair for his enthusiasm, commitment and willingness to seek and take advice.

His next set should be a walk in the park!

[DonaldStott]

Thanks David - just to confirm that I won't be going anywhere near the IF cans until the associated problems with V3's stage are resolved.

While Googling I came across this book "Valve Radio and Audio Repair Handbook" by Chas Miller - on page 116 I noted this: -



IFT slugs - when I turn the set upside down I am clearly hearing/feeling two distinctive clicks or clunks - the implications from Tim's post and others was that this indicated broken slugs??

David - I may have to rearrange the words in your last sentence though and instead of tackling another set I may just go for a walk in the park!

[David G4EBT]

From post 192 your HT line is 135V - a bit higher than the 110V on the circuit, but no big deal.

The circuit states that if all's well, there should be 45V on V3 anode, but when you've got 135V on the 'hot' side of R12, the anode voltage may be a little higher. The cathode voltage is stated at 0.7V, but that was with an AVO7 on the 10V range, and given that the AVO7 is only 1,000 Ohms per volt, that's the equivalent of putting a 10K resistor in parallel with the 1,000 Ohm cathode resistor (R11), pulling the voltage down below what it would be if the Avo wasn't in circuit.

The AVO will have measured the voltage accurately, but that voltage would have been lowered due to the loading of the AVO. Hence, if measured with a modern DVM, which won't load R11, the voltage will be higher - maybe in excess of 1V. I don't want to 'muddy the waters' - just saying don't worry if cathode voltages on this or any other set are a little higher than stated on service data when the original tests were done on an AVO7.

You'll get the better of this set before too long!

And yes, as Chas says in his book, if you touch your finger on the top cap when it's the grid, you should hear a loud buzz from the speaker if all is well.

Someone on the forum quipped that whether the top cap is the anode or the grid, you'll get a buzz either way!

[julie_m]

Don't put your fingers anywhere near this set! It is powered by an autotransformer, not a "proper" transformer, so the chassis is directly connected to the mains.

[DonaldStott]

Following more continuity and HT testing that included re-soldering a couple of "older" joints that looked dry and crumbly I'm now reacquinted with a persistent background hum!

But on V3 I've now got:-

Anode 56.5 v

Cathode. 0.8 v

These now look o.k. to me?

I also went back and tried David's tip and scratched the centre tag of the volume control with a screwdriver and heard scratching noises from the speaker!

And a reassurance to Julie that I have been aware of the chassis being attached directly to the mains right from the start of this Project - but thanks for the reminder! What I did to was to tap the top cap of V3 with an insulated screwdriver and got the expected crackling noises!

So some progress tonight ...

EDIT: The volume control on/off switch reads "Radiospares Pat. No. 439028" - can anyone confirm that this is a 500k VR - thanks.

[Station X]

V3 is looking much better.

If you want to check the resistance of the volume control, just measure it.

[DonaldStott]

If you want to check the resistance of the volume control, just measure it.

Yeah, I didn't phrase that very well! What I was trying to ascertain was whether or not this had been replaced?

[David G4EBT]

I very much doubt it. Looking at earlier pics that you posted, it has a metal shaft, a single pole switch, is large in diameter, looks undisturbed and is 500K. For decades now, potentiometers have had double pole switches, plastic shafts and 500K hasn't been a 'preferred value' - the nearest being 470K. It's rather hemmed in by several components, so to change it would be a bit of a faff, albeit there would be some benefits, in that a replacement would have a double pole switch and plastic shaft.

I think you've already got quite enough on your plate to cope with, so if the pot is working, maybe best to leave it in situ? The most likely faults that the pot would develop would be an open circuit switch or scratchy track, both of which are often capable of being cured, leastways, for a time.

It sounds like you've got the audio pre-amp stage working now, so time to move on to the second IF stage maybe?

The residual 50Hz hum you can hear from the half-wave rectifier stage is most likely C27 & C29 (reservoir and smoothing capacitors) on their way out, but that can wait till later.
(At least it's another sign that the audio stage seems to be working!).

[ms660]

Radiospares supplied replacement components to the repair trade, I don't think they supplied to the manufactures:

https://www.google.co.uk/search?q=ra...=1498647888490

Lawrence.

[DonaldStott]

Originally Posted by David G4EBT

It sounds like you've got the audio pre-amp stage working now, so time to move on to the second IF stage maybe?

Agreed - the set now feels "alive" for the first time...!

I was just interested in VR1 as part of my ongoing education but will seek to replace C27 and C29 at some point as they were installed by the previous "phantom repairer".

There is, however, a voice at the back of my head repeating the words "IFT slugs" ...!

Meantime, let's move on to the second IF stage - any tips as to what I should be looking for apart from the usual checks?? Although this is quite a complicated stage I understand from reading other parts of the Forum that the RF and IF circuits are generally fairly reliable?

[ms660]

The IF amplifier stage is not that complicated, It's a standard pentode amplifier, the IF amplifier valve is a variable u type as is the mixer valve (hexode section of the triode/hexode) in effect the stage gain is controlled by the AGC voltage applied to the valves control grid, more -ve = less gain, less -ve = more gain, the AGC voltage is developed from the AGC detector, that's the diode that's fed via C24, C24 is connected to a tap on the IFT's primary, the tap being the point where the signal is taken from to feed the AGC detector, strong signal = more -ve AGC voltage, weaker signal = less -ve AGC voltage.

Checking the valve heaters are lit and measuring the valve voltages is usually the first job.

Note that the voltages given in most manuals are measured under no signal conditions with the receiver usually switched to MW, this is relevant to AGC controlled stages in particular....

A quick once over about the variable u valve:

http://www.r-type.org/addtext/add100.htm

Lawrence.

[DonaldStott]

Originally Posted by ms660

The IF amplifier stage is not that complicated ...

It was for me until your eloquent explanation - now it seems quite straightforward and I now understand (I think) how AGC works?

Originally Posted by ms660

Checking the valve heaters are lit and measuring the valve voltages is usually the first job.

I've checked heater continuity for all valves but are you able to see that the valve heaters are lit for valves with metallised coating?

Originally Posted by ms660

Note that the voltages given in most manuals are measured under no signal conditions with the receiver usually switched to MW, this is relevant to AGC controlled stages in particular....

Noted - thanks.

[Boater Sam]

Originally Posted by Boater Sam

I've checked heater continuity for all valves but are you able to see that the valve heaters are lit for valves with metallised coating?

Not always, but after a while they get warm.
Sam.

[DonaldStott]

Not always, but after a while they get warm.

Doh!

I suppose that's why they call them heaters - thanks Sam.

[Station X]

Given that that valve heaters in this set are wired in series, I'd think it highly unlikely that any aren't lighting up.

The service sheet has voltage readings for V2 and V1, so it'd be well worth checking them in that order.

[David G4EBT]

Originally Posted by DonaldStott

I've checked heater continuity for all valves but are you able to see that the valve heaters are lit for valves with metallised coating?

The valve heaters are all is series, so if any one of them was open circuit, none would be 'lit up'. As the audio stage is working, whether or not you can see any valves lit up, they must all be drawing 200mA current or none would work. In a series heater chain, all the valves have the same current rating, but can, and usually are, of different voltages. In your set they are:

V1: 7.0V
V2: 6.3V
V3: 6.3V
V4: 33V
V5: 20V

Total heater chain is 72.6V (AC).

All the heater pins are 2 & 7. (Make sure your meter is set to AC Volts of course).

The dial lamp is also in series (sort of, but on its own tapping), with the heater chain but if it fails, the heaters won't go out as they're across tags H & L on the auto transformer, but if you were to measure the whole heater chain you'd need to make sure that you measure from pin 1 of V1 to pin 7 of V5, because if you have one prod on say pin 2 of V1 and the other on the chassis rather than pin 7 of V5, you'd have an extra 3.5V for the dial bulb.

Hope that doesn't confuse!

As to the IF stage, (IFT 1/IFT2), ideally, a signal generator would show if the stage is working and the IFTs correctly aligned, but as you don't have one, all you can do at this point is check that the voltages on V2 are more or less as the spec - 65V anode, 50V on G2 and 1V cathode. You can also check the IFT windings - L7 &8, and L15 & 16 to see that they're not open circuit. (They should all read 5 Ohms). Do the cores look as though they've been tampered with?

As to the frequency changer, to check if the oscillator is working, you can use another (working) set if you have one, which has the same I.F. frequency - 465 KHz. If you tune both sets to the same place on the dial - say 1,500 Metres LW, place the sets close together, then tune your DAC91 either side of that frequency, if the oscillator of your AC91 is working, you should hear heterodyne 'whistles' on the working set.

Just a point to mention Alistair, which I hope doesn't sound patronising or stating the blindingly obvious, but your set is unlikely to bring in any signals on any band without an aerial of some sort. Just say 15ft of wire draped around the room would suffice. On a short aerial such as that, plug it into the 'sensitivity' socket rather than the 'selectivity' one to give best signal strength.

Assuming that the voltages on V1 are more or less correct per the data, The most likely faults on the front end are 'switchery' - oxidised switch contacts, and misalignment due to tampering of the coil cores or trimmers. That's usually obvious as the coils are generally sealed with wax, and trimmers with a dab of paint. Also, check the continuity of coil windings for any open circuits. (The Ohmic values of all of the coils is stated on the data sheet).

Onwards and upwards!

[ms660]

Don't forget that valve heaters can go short circuit or partial short, if that happens in a series circuit the other valve heaters will still be lit up but the valve concerned won't be doing a lot.....it happens, it also happens in tellies and CRT's.

Lawrence.

[DonaldStott]

Back from holiday and seeking to pick up where I left off - oh dear, just realised that you've all had to endure 11 pages and #219 Posts - apologies to everyone but appreciate your patience, perseverance and ongoing support!

I've now replaced all the wiring associated with the power socket/on-off switch and installed two new HT line smoothing capacitors C27 and C29. Still got that hum but not even a hint of a radio station on any waveband - I have a long length of cable plugged into the aerial max sensitivity socket.

Some readings for you to ponder and advise:-

V3 - Anode (55.8V) Cathode (1.11V)

V2 - Anode (97.1V) Cathode (1.22V)

V1 - Anode (131.7) Cathode (0.63V)

All seem a bit high so I'll check all the associated resistors but not sure where to go next??