Bush DAC53 oscillator problem

[242B23FT]

Hi All,

I'm an American radio collector who recently got his hands on a Bush DAC 53. British sets like this are very rare here in the states, so this was an exciting find! However it has an issue that is really giving me a tough time.

The radio has been electrically gone through. The filter capacitors, paper capacitors, and out of tolerance resistors have all been replaced. The audio output transformer had an open primary and was replaced with a similar one from a 1937 Philco console. The radio turns on and will receive static but does not tune anything in. After some diagnosis, I found the oscillator in the front end/converter stage is not running. If I couple in a 1895khz signal from an external oscillator into the triode grid of the TH22C, the radio will receive 1430khz on the MW band. This confirms the rest of the radio is working correctly.

This set thankfully came with the Bush Service manual. I spent considerable time tracing through the oscillator circuit and have confirmed everything is good and matches the schematic. This includes the coils, band switch contacts, resistors, Mica capacitors, and voltages. At this point I assumed my issue was with the triode section of the TH22C.

After an unsuccessful search for a TH22C, I discovered a Mullard substitution book that listed the Mazda TH2321 as a suitable replacement. I was able to buy a "NOS" Mazda TH2321 from UK ebay. 2 weeks later the valve arrived and still did not fix my oscillator problem. I again turned to ebay and purchased a Mullard TH21C(recommended by an acquaintance in the UK) which also did not yield me a working oscillator.

At this point I am beginning to question my sanity. I spent another few hours going over the circuit the again(testing every component and connection) and I still cannot find anything wrong. I have been working on old radios most of my life and have never been stumped like this.

Is it possible I have 3 bad valves here? All with bad triodes? With no access to a British 7 pin valve tester, I made an adapter to test them in my American tester as a 6J8 (with the heater voltage set to 20-25v). I found all 3 valves show zero emission on the triode section. Not weak but completely dead. The Heptode section shows plenty of emission on all 3.

Is this a common problem with these 7 pin Triode/Heptode valves? If so can someone point me to where I can purchase a good (preferably tested with proof of some sort) TH22C, TH30C, or any equivalent valve?

Any insight here is appreciated.

Thank you

[cathoderay57]

According to the Avo VCM163 manual the TH22C triode should be tested with Va=150v, Vg=0v which should give Ia=5mA and gm=6mA/v. A TH2321 wants Va=100v, Vg=0v to give Ia=4mA and gm=5.2mA/v. TH21C wants Va=150v, Vg=-2v to give Ia=6mA and gm=1.2mA/v. Not sure what grid bias you were using for your test? Jerry

[vintage_8bit]

I have a DAC 51, which I think is the same minus the magic eye. I also had trouble with the L/O. It was found I had a faulty mica cap/s. I can look latter & see which one it was. Lets hope you haven't got 3 faulty valves. Colin

[Mr 1936]

Hi

Just an odd couple of thoughts.

If all other components check out OK, it's not impossible for the moisture content of the wax coating and even the paxolin (resin bonded paper) insulating former of the oscillator coil to absorb moisture over the years. This increases losses (drops the Q) maybe to the point where the stage no longer oscillates.

If this does turn out to be the problem, you could gently heat the coil to melt out the wax, and replace it with something more modern.

Another thing which can drop the Q is if it is wound with Litz wire and one or more of the strands have gone open circuit, usually right at the start or end of the winding. This will show up as increased resistance on an ohmmeter test, assuming you are lucky enough to have accurate data on what the resistance should be.

[Cathovisor]

There is always the remote possibility that someone previously has wired the MW (BC in your part of the world) oscillator coil backwards, thus introducing a phase error. Does it work okay on the other wavebands?

I do have a "scrapper" I could remove the MW oscillator coil from, should this prove to be the issue.

[Cathovisor]

Originally Posted by vintage_8bit

I have a DAC 51, which I think is the same minus the magic eye.

It is - the Bush service manual for the DAC51 says that it is to be used in conjunction with the DAC53 manual.

[vintage_8bit]

On My set I found the L/O caps : C7, C8, & C9. Were all faulty. I have had this style of mica becoming intermittent in the past & only failing in circuit. Colin.

[242B23FT]

Thank you for the replies!

For testing the valves, my tester only does emission so the control elements are all tied to the anode and the valve is tested as a diode. I was testing with 75V on the anode.

A problem with the coil could be possible. The Oscillator coil is Litz wound on a cardboard tube. I received this radio in parts and had to reassemble it before working on it. It came from the estate of a long time radio collector who had started a restoration and unfortunately was never able to finish it. The socket for the TH22C had been removed to replace the rubber grommets, along with the tuning capacitor. The service manual gives very detailed information about the pin out and DC resistance of all the coils so verifying they were ok and wired correctly was straight forward.

The Mica caps, specifically C6, C7, C8, C9 were testing good on both capacitance and leakage out of the circuit, I assumed these capacitors were good. vintage_8bit hit the nail on the head, they were the issue!!!

I replaced all of these caps with new Micas and the radio now plays beautifully on every band! The micas I usually see fail result in noise and instability in the radio so this is a new one for me.

I aligned the set following the instructions in the service manual and it works fantastic!

The final step is to mount a 110>240V step up transformer inside so this set can be used with standard US mains.

Thank you all for your help, this radio now will live on!

[vintage_8bit]

Thats good news, well done. Although mine has a failing Litz wound I.F. it still performs well, so you should have a good set there. Colin.

[Cathovisor]

Originally Posted by 242B23FT

The Mica caps, specifically C6, C7, C8, C9 were testing good on both capacitance and leakage out of the circuit, I assumed these capacitors were good. vintage_8bit hit the nail on the head, they were the issue!!!

I replaced all of these caps with new Micas and the radio now plays beautifully on every band! The micas I usually see fail result in noise and instability in the radio so this is a new one for me.

I aligned the set following the instructions in the service manual and it works fantastic!

The final step is to mount a 110>240V step up transformer inside so this set can be used with standard US mains.

Thank you all for your help, this radio now will live on!

Absolutely delighted to hear another Bush lives on Stateside