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 Post subject: Colonial 650 no volume control
PostPosted: Jun Fri 18, 2021 1:13 am 
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Joined: Jun Fri 18, 2021 12:07 am
Posts: 7
Hi,
I'm working on my first radio restoration. It's a Colonial model 650. Schematics:
Attachment:
File comment: Schematic page 1
colonial_radio_corp_650_pg5-1.png
colonial_radio_corp_650_pg5-1.png [ 473.38 KiB | Viewed 721 times ]

Attachment:
File comment: Schematic page 2
colonial_radio_corp_650_pg5-2.png
colonial_radio_corp_650_pg5-2.png [ 574.6 KiB | Viewed 721 times ]


I've replaced all the caps (except for the micas) and any out of tolerance resistors. It now works with the exception of the volume control. Turning the pot has no effect on the volume. I've checked the resistance of the wiper as the knob is turned and it goes from 0-500 Kohm as laid out in the schematic.
I've gone through the whole chassis and looked for anything that doesn't match the schematic, but haven't found anything off. I'm beginning to suspect that the 75 tube isn't working, but don't have a tube tester. Is there any other way to check it? With the tube removed the radio plays, but at a much lower volume, is that the normal behavior?
Any help is be appreciated!
Thanks,
Asa


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 Post subject: Re: Colonial 650 no volume control
PostPosted: Jun Fri 18, 2021 10:02 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 38
Hello,

Did you check the tone control pot and clean the contacts on the wave selector switch?


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 Post subject: Re: Colonial 650 no volume control
PostPosted: Jun Fri 18, 2021 1:00 pm 
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Joined: Jun Fri 18, 2021 12:07 am
Posts: 7
I had not thought to clean the sector switch contacts, but doing so didn't seem to change anything.
With leads connected the tone control pot the resistant measurement is very erratic. With them disconnected it reads 660 Kohm and the wiper is linear from 0-660. This is fairly high compared to the 500 Kohm the schematic says it should be, might this be my problem?


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 Post subject: Re: Colonial 650 no volume control
PostPosted: Jun Sat 19, 2021 6:11 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 8011
Location: Near Brandon, Iowa
Verify that the volume control pot's ground connection is intact. If this connecting wire is broken it will not be possible to control the radio volume.

It's also possible for the volume control to break internally and so lose its connection to ground. If this has happened, your only recourse is to replace the pot. If you find the ground wire to be intact, I'd suspect this next.

A third possibility, although not very likely, is that you have an internally shorted detector/1st audio amp tube such that the demodulated signal is leaking onto the grid of the amp triode section, bypassing the volume control. Since you don't have a tube tester, you might want to purchase a tested-good tube (eBay or one of the used-tube sellers) that you can sub in.


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 Post subject: Re: Colonial 650 no volume control
PostPosted: Jun Sat 19, 2021 1:12 pm 
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Joined: Jun Fri 18, 2021 12:07 am
Posts: 7
Thanks for the suggestions!
The ground connection was intact. At it's minimum position the wiper to ground connection had about 400 ohms of resistance. The minimum resistance between the wiper and the third leg on the other hand was only about 20 ohms, so I shot it with some contact cleaner. Big mistake! The pot now reads as an open connection. I took it apart and it looks like some of the resistive film flaked off near the ground terminal.
I'll update whether the new pot fixes I when I get one ordered and installed.
I may go ahead and order a new old stock 75 tube in case the pot wasn't the problem. I couldn't seem to find a used tested one anywhere.
Thanks again
Asa


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 Post subject: Re: Colonial 650 no volume control
PostPosted: Jun Sat 19, 2021 6:16 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 8011
Location: Near Brandon, Iowa
At its minimum-output setting, the volume pot should have measured just a few ohms from the wiper lug to chassis ground, not 400 ohms. This tells me that you had a bad pot. So, don't worry about having "damaged" it further by cleaning it... it was already dead.


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 Post subject: Re: Colonial 650 no volume control
PostPosted: Jun Fri 25, 2021 2:58 pm 
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Joined: Jun Fri 18, 2021 12:07 am
Posts: 7
Thanks Lorenz200w!

I replaced both the volume and tone control pots, but it didn't change the symptoms. I still have no volume control. I've order a tested 75 replacement tube in case that is the issue. I've noticed a couple of things that I'm not sure about. First is that the voltages on all the plates are significantly higher than the specifications.
Attachment:
20210625_080413.jpg
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The schematic said that an 8 or 14 MFD first filter capacitor could be used, so I replaced both the electrolytics with 12 MFD. Should I have stuck with 8 for the second? Could that be causing the high voltage?

Another odd thing I that the sound ocasionally randomly drops to a whisper (still no volume control when this happens) and then comes back on it's own after a minute or two. If measure between certain spots (e.g. the plate of the 6A7 tube) and the chassis with a multimeter when the sound is low it will immediately come back to the higher volume.
Trying to measure the voltage between the diode plates of the 75 tube and chassis cuts the sound off completely.
I'm too new to this to know if this is a normal behavior or not and appreciate any insight.


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 Post subject: Re: Colonial 650 no volume control
PostPosted: Jun Fri 25, 2021 3:45 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 8011
Location: Near Brandon, Iowa
As a data-gathering exercise, tack-solder a short wire to the WIPER of the volume control pot, then tack the other end to chassis ground. Wire it to a chassis ground lug, not to the adjacent pot pin that is ostensibly connected to ground. Fire up the radio and report what the radio is doing:

If you still have sound output, somehow the demodulated signal (audio) is bypassing the volume control pot and getting onto the grid of the 75 tube's triode. Could be a shorted/leaky 75 tube as discussed earlier. May also be a fault in the 75 tube's socket or an adjacent-node short due to a solder splash or stray wire strand.

If there is no (or very little) sound output... something is still messed up with the volume control circuit, allowing audio to leak onto the wiper (which, being grounded out, will not permit signal to get to the 75 triode).

Cut the ground jumper wire with the set operating and verify that sound returns.

HIGH VOLTAGES: this is probably a combination of three things: (1) meter loading; (2) worn tubes; and (3) high AC line voltage. Meter loading is the resistance a meter presents to a circuit being measured. Most modern meters are very high impedance so the amount that they load a circuit is almost nil. Meters of 85 years ago were relatively low impedance so they tended to read low when applied to a low-current source. If you haven't tested your tubes, I'll bet that they are pretty well depleted (especially the high-power audio output tubes). When tubes get weak, they can't pull the amount of current as a "good" tube. Since the radio's B+ supply is not regulated, low current demand translates to higher than normal voltages. Finally, most AC line voltages these days are well over 120 V- and the set was probably measured while operating on 110V. Higher line voltage means higher voltages throughout the radio.


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 Post subject: Re: Colonial 650 no volume control
PostPosted: Jun Fri 25, 2021 9:03 pm 
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Joined: Jun Fri 18, 2021 12:07 am
Posts: 7
With the volume pot wiper shorted to ground the sound was at the same level as before. I can't see anywhere the that the lead from the 75 triode grid could be be shorted to anything. Does that suggest the 75 tube is faulty?
Thanks,
Asa


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 Post subject: Re: Colonial 650 no volume control
PostPosted: Jun Fri 25, 2021 9:20 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 8011
Location: Near Brandon, Iowa
asaroy wrote:
With the volume pot wiper shorted to ground the sound was at the same level as before. I can't see anywhere the that the lead from the 75 triode grid could be be shorted to anything. Does that suggest the 75 tube is faulty?
Thanks,
Asa

Yes; or there is a "sneak path" inside the tube socket that allows the demodulated audio from the diode section to "leak over" to the triode grid. Worry about that if the new tube doesn't solve the problem.


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 Post subject: Re: Colonial 650 no volume control
PostPosted: Jun Tue 29, 2021 1:30 am 
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Joined: Jun Fri 18, 2021 12:07 am
Posts: 7
I replaced the 75 tube and the volume control works now!
However, it doesn't take much turning (~10% of the full range) for the volume to become unbearably loud. The volume pot I took out was linear taper, would audio taper have been a better choice?


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 Post subject: Re: Colonial 650 no volume control
PostPosted: Jun Tue 29, 2021 1:41 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 8011
Location: Near Brandon, Iowa
asaroy wrote:
I replaced the 75 tube and the volume control works now!
However, it doesn't take much turning (~10% of the full range) for the volume to become unbearably loud. The volume pot I took out was linear taper, would audio taper have been a better choice?

Yes.


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 Post subject: Re: Colonial 650 no volume control
PostPosted: Jun Tue 29, 2021 2:31 am 
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Joined: Jun Fri 18, 2021 12:07 am
Posts: 7
Ok I'll swap it out then.
Thanks for all your help lorenz200w


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