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Old 11th Jun 2020, 11:01 am   #1
Malcolm G6ANZ
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Default CR 300 now working

I have already written about some of the repairs I have carried out on this receiver, so to continue now that its back in working order. When under load the HT dropped to around 140v. The measured current on the HT line was measured and found to be around 40mA which is roughly the stated figure in the manual. To be certain that the receiver wasn't loading the supply and dropping voltage a 10k resistor was substituted for the receiver. Sure enough when connected the volts dropped. The valve was replaced with a pair of 1N4007 diodes and the same test carried out. This time the volts stayed high. Before reconnecting the receiver to the PSU a 300ohm surge limit resistor was fitted in series with the supply.
There are a lot of decoupling capacitors in the CR300. The majority are housed as three 0.1uF in a single can. These cans were probably the first item fitted to the chassis and as such proved impossible to remove and restuff. They needed changing as most were leaking a tar like goo from the base. So I cut off the wires and fitted external 0.1 uF at 600 v to the valve base pins leaving the cans in place. While doing this the resistors were checked and most found to be out of spec these were swapped out for 1W types.
The anode load to each valve is split between the high value resistor and a 470 ohm. The 470 ohm is mounted on a separate tag strip so the volt drop across it can be measured and so the current drawn by each stage calculated. These 470 ohm resistors were all up in value at around 680 ohm.
After changing a set of R's and C's I made sure that the receiver was still working as best it could. It is quite easy to make a mistake as the point to point wiring is a bit of a rats nest and the unused pins on the valve bases are often used a anchor points for components unrelated to the valve.
After one set of changes I switched it on and it was dead, no dial lamps or heaters. Some of the decoupling caps are in the heater chain so I thought that one of these had gone down. Measurement of the heater line showed that there was a direct short to chassis. Removing each valve in turn trying to find the suspect component proved inconclusive as there was always a short to chassis. Eventually I disconnected the heater from the valve chain and there was still a short. This eliminated anything in the chain. The heater supply comes in on a multi-way plug and goes to the on/off switch and then to the heater chain. The wire from the plug to the switch was in a sleeve along with some uninsulated screened wires. The heater wire was single core insulated with varnished systoflex. The rest of the sleeving in the receiver was in good condition but where this sleeving went through the yellow sleeve it had reacted in some way and become very brittle and broken up. This allowed the neater wire to short to the screened wires and cause the fault. The HT wire also went through this sleeve but was unaffected. The faulty wires were replaced with normal multi-strand wire. Other lengths of the yellow sleeving were cut open to check for the same fault but were found to be in good condition.
The set now came on correctly and worked. Using a 20' length of wire it was quite sensitive picking up a lot a stations in the far reaches of Europe.
Injecting an AM signal into the aerial socket from my Marconi 2022 sig gen the measured sensitivity was near enough to the stated spec in the manual, the best being around 5MHz with a sensitivity of approx 1uV EMF. This judged by reducing the output until the modulation just disappeared into the noise. Not very accurate but reasonable I feel.
The front panel had, at some point, be painted gloss black and the controls labelled with green Dymo tape. So the panel was cleaned with paint stripper and sanded back. It was then sprayed with grey paint. The lettering was originally stamped into the panel and the indents were still visible so my son carefully repainted these legends with a very fine brush and a very steady hand.
The receiver now works as it should. The BFO allows reception of amateur ssb although slightly difficult to tune in. The crystal calibrator is also at the correct frequency.
Thanks to all who aided in the recovery of this boat anchor of a radio.
Malcolm
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Old 12th Jun 2020, 12:19 pm   #2
HamishBoxer
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Default Re: CR 300 now working

Very nice job.
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Old 12th Jun 2020, 7:00 pm   #3
vinrads
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Default Re: CR 300 now working

Hi Malcolm I had one of these I think it was my first short wave set , well done getting it working again ,and re painting the front ,I could do with someone with a steady hand to do the lettering on my Pye radio I had to paint the front of .Mick.
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Old 12th Jun 2020, 9:31 pm   #4
Phil G4SPZ
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Default Re: CR 300 now working

Well done on that heroic restoration, Malcolm.
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