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 Circuit diagram wanted for a Democrat radiogram
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 Return to top of page · Post #: 1 · Written at 1:01:06 PM on 31 August 2016.
Scotty's Gravatar
 Scotty
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 387

Hi Folks,

I'm looking for a circuit diagram for a 1950's Democrat radiogram.
I can't find a model number on it, but the valve lineup is as follows:

6V4, 6M5, 6BD7, 6BH5 & 6AN7

It's a fairly basic chassis. The unit has been re capped but the MICA caps are still there and I'm not sure of the values.

The unit works but the overall volume is not great nor is the signal clean. Valves all check out as good.

Thanks in advance.

Cheers
Scott
 
 Return to top of page · Post #: 2 · Written at 3:59:00 PM on 31 August 2016.
Robbbert's avatar
 Robbbert
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 2014

Mica caps around the 6BD7 will be 100pF. If the voltages there check out ok and there's no crackling noises then the caps will most likely be ok.
 
 Return to top of page · Post #: 3 · Written at 5:16:20 PM on 31 August 2016.
Scotty's Gravatar
 Scotty
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 387

Hi Robbert,

Thanks! Generally the overal volume is low and distorted. I will change the mica as you suggest, would like to know the function of the 6BD7 and what voltages I should be seeing.

Cheers
Scott
 
 Return to top of page · Post #: 4 · Written at 5:43:42 PM on 31 August 2016.
Johnny's avatar
 Johnny
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 544

I haven't got a circuit but the 6BD7 would be the detector/audio preamp.
I would expect around 50 to 100 volts on plate, i.e. pin1.
Change cap off pin 1 as it will be the coupling cap, probably .047 or similar.
On the 6M5 output stage check plate pin 7, and screen pin1, should expect 250 and 200 odd volts.
Change cap off pin 7, a .0022 or .0047 which affects the tone, but quite often go leaky or short.
I've seen some bushrangers just cut these out.
JJ
 
 Return to top of page · Post #: 5 · Written at 2:44:27 PM on 3 September 2016.
Scotty's Gravatar
 Scotty
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 387

Thanks, checking the voltages, 6m5 seems ok but pin 1 of 6bd7 is only reading 30v which would explain the low output. The cap of it is a .022μF and is new. Could it be the valve itself?
 
 Return to top of page · Post #: 6 · Written at 3:36:28 PM on 3 September 2016.
Johnny's avatar
 Johnny
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 544

More likely to be the resistor feeding it.(common problem).
The 6BD7 could be gassy or weak, but check the feed first.
JJ
 
 Return to top of page · Post #: 7 · Written at 3:51:20 PM on 3 September 2016.
Scotty's Gravatar
 Scotty
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 387

Swapped put the 6BD7 and volume came back up as did the voltage to 76v. Should I swap the .022 to a .047?
 
 Return to top of page · Post #: 8 · Written at 4:20:14 PM on 3 September 2016.
Johnny's avatar
 Johnny
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 544

Really need a circuit, but .047 won't make much difference.
Maybe a bit more volume and bass, but then it could also load down the detector a bit.
Stick at this stage to the 0.022 and hope the previous repairer wasn't a bushranger.
Although I do know from experience and factory observation that sometimes the accounts section tell the engineers that they have
100,000 certain caps in stock and they have to use them up. Smile.
JJ
 
 Return to top of page · Post #: 9 · Written at 7:21:49 PM on 3 September 2016.
Scotty's Gravatar
 Scotty
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 387

Thanks JJ,

It's working fine and all back together now.

Cheers
Scott
 
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