Bush DAC90A can't get tuning knob off

[stevehertz]

Apply matching dots of paint to the two pin mains 'plug' and socket to ensure safe application of mains each time you reconnect it.

[Steve1952]

OK many thanks for all the advice. Not sure if this needs a new thread as still a beginner here, but. Have replaced most of the caps,(see pictures) and the dial bulbs (missing) which was enjoyable, however the radio performance is poor. loud hum, high pitched whistle when tuning which gets lower and just disappears when the station tuned, , only two stations dimly received , Radio 4 I think and an Irish station. I haven't any test equipment yet. Should I start testing everything in sight or can anyone diagnose the problem. The whistle is quite annoying, I assume this is a problem rather than normal. many thanks

[stevehertz]

Hum can be main smoothing caps need replacing. Otherwise either test or replace valves one by one with known good ones.

[Sideband]

Firstly go over all that you have done and make sure there are no errors or wrong value caps fitted. If you are sure that everything is OK, well the problem is most likely I.F instability....typically that shows itself as a tuneable whistle on stations. It's because the I.F amplifier (UF41) is oscillating (it shouldn't)!! That in itself is usually caused by poor IF decoupling (so as above, make sure correct value caps have been fitted). As you also have a hum, it can be caused by poor smoothing (if the smoothing cap is completely open circuit) which also aids decoupling. If this should be the case, you can either try restuffing the old can with new capacitors or just disconnecting it and leaving it in place and just neatly fitting two separate electrolytics under the chassis.

[Radio tirana]

Hi and welcome.

What value capacitor have you fitted for C17 (IF bypass /tone corrector) ? the yellow one just to the left of the volume control from the tagboard to the chassis via a solder tag. Just wondered as these are usually silver mica 390pF capacitors and don't normally need replacing. Not sure I have seen those yellow polypropylene caps in that value before.

Kind regards RT

[HamishBoxer]

BVWS sell excellent replacement caps. I personally have only changed one in all the DAC90's I have restored.

[Radio tirana]

Hi
Sorry I have just realised that I usually replace this capacitor with 390pF to improve the treble response of the radio. The original capacitor would be 0.003uF or 3000pf hope this has not caused confusion
Kind
regards RT

[Steve1952]

Dear all, many thanks for the advice. I got a kit of the main caps for this radio from Ebay, and have checked again and they are all correct as installed. See below. The radial capacitor had been replaced already so I didn't do that one, perhaps I should.
C17 I replaced with a 0.001 in the kit for a better tone response as you suggest RadioTirana, not that any tone is noticeable through the whistle and hum...

The resistor is to go across the dial bulbs which were missing. This was the main success- the tuning dial is now bright at least.

I don't have any valves to replace one by one (or anything else for that matter) so will try and restuff the smoothing cap. Just to show more ignorance, which one is that??

Best Regards Steve

[Radio tirana]

Hi Steve
The smoothing cap is the round silver can about 35mm in diameter fitted through the chassis, it is actually Two capacitors C19 smoothing and C21 Reservoir in a single housing It is unlikely to be faulty.
Do you have the service data for the set? (Available from the link at the top right of your forum page) it would be worth checking the resistors to see that none have drifted out of tolerance. you will need to desolder or snip one end out of circuit and also be aware that your fingers on the voltmeter probes will affect the readings.
I see from your photos that some pins (3 & 4) on V4 UL41 output valve socket have been used as anchor points for part of the circuit, this is how Bush wired it originally but is far from ideal and could well be the source of some of the hum you are experiencing. if you search threads on this forum you will find reference to a modification to the wiring to avoid this. I will search and see if I can find a drawing or photograph to illustrate this modification.
Don't expect too much from the radio, there are not many MW/LW stations operating now and those that remain are often drowned in a sea of background noise,
Please be aware as others have said that this is a Live chassis make absolutely sure that the set is unplugged before you work on it.

Kind regards RT

[Station X]

Originally Posted by Radio tirana

you will need to desolder or snip one end out of circuit

Completely unnecessary I'd say and likely to introduce further faults. In VALVE radios resistors are unlikely to be shunted DC wise and can be tested in circuit.

[ms660]

It depends on whether any relevant capacitors have been replaced or not.

Lawrence.

[Station X]

A DMM is unlikely to be affected by electrically leaky capacitors unless they're very very leaky.

If a reading is grossly wrong then disconnecting the resistor and retesting both it and the shunt capacitor is the way to go.

Of course leaky capacitors and off value resistors will also show up when taking voltage readings.

[Radio tirana]

Point taken Graham. I should have worded it better, I was trying to warn him about measuring resistors particularly ones in the high mega ohm range and touching both meter probes at the same time. Caps have all been replaced according to op, I was not sure if he has the Equipment and or necessary skill set to take voltage readings safely.

Kind regards RT

[ms660]

If the reading is less than expected in a high resistance circuit such as an AGC circuit with no obvious DC shunt path then often it's due to a leaking capacitor, leaking wax capacitors used to show up on my old Avo multimeter many times, the meters battery voltage being generally higher than the AGC voltage that the capacitors were subjected to in circuit.

Lawrence.

[Radio tirana]

HI Steve
here are some pictures of the modification I mentioned on order to isolate pin 4 of the output valve. Second picture shows original wiring.
Third picture is the smoothing Cap.
Kind regards RT