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38-93 low volume - Printable Version

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38-93 low volume - NostalgiaRadioTime - 04-03-2017

Working on what I thought would be a fairly simple one band, 5 tube radio. It's been not so simple so far...

After checking all the tubes, doing a complete recap, and changing the resistors that were way out of tolerance, I'm left with a radio with very low volume with the pot wide open. I even replaced the volume control with a new 500K as per the schematic to no avail. My voltages are rather LOW (unfortunately the schematic doesn't list many voltages other than for plates) and I'm wondering if someone here could take a look at the numbers and perhaps point me towards what the culprit of the low volume may be? I'm working off of the 37-93 schematic, which most all sources online seem to indicate is the same as the 38-93. Just to be sure I'm ordering a set of 38-93's from Chuck. When I touch the grid caps of the tubes with the soldering iron I get a low buzz. It should be a whole lot louder. Ditto for the center tap of the volume control...very low volume buzz. The audio output transformer checks out at 455 ohms. I'm only getting 2 ohms on the 6R7 however (well, 2.2) but it should be 25 volts.

   

   

RE: 38-93 low volume - Radioroslyn - 04-03-2017

Did you put #6 in backward???? + goes to the chassis - goes to #23.

RE: 38-93 low volume - NostalgiaRadioTime - 04-03-2017

That was my first thought but no, I have it in as you described. I knew it was just the opposite of the two 8 mfd's.

RE: 38-93 low volume - NostalgiaRadioTime - 04-03-2017

Now there's a discrepancy between the parts list and the schematic with regards to #16 and #18. On the schematic it says 1.5 MEG ohm but on the parts list it just says 1.5 ohm. The two resistors that were in each measured 2.3 MEG ohm, so I replaced them with two 1.5 MEG's.

RE: 38-93 low volume - morzh - 04-03-2017

Your plate voltages are way low.

1. Is the speaker FC 1140 OHM?
2. Are 13 and 23 resistors good?
4. Is 6F6 running hot? Anything else is too hot?

Your 5Y4 out seems ok but then the plates are low, might mean excessive consumption.

RE: 38-93 low volume - Arran - 04-04-2017

The plate resistor on the 6R7 says .19 meg on the schematic, which is 190,000 Ohms, sort of a weird value, if it was swapped with something close to 19 meg that would explain the low plate voltage on that tube.
Regards
Arran

RE: 38-93 low volume - John Bartley - 04-04-2017

Just a thought .... All the B+ readings are low, except at the rectifier .... in other words, on the rectifier side of the field coil the voltages are ok and on the radio side of the field coil they are all uniformly low. My first suspicion would be a defective field coil. Maybe try a quickie test of paralleling the field coil with a 1K resistor, just for a second to see if the volume improves. Alternately, you could stick a DC ma meter in series with the B+ coming from the field coil to see what sort of current the set is drawing. If it's too high you will end up with a voltage drop such as you have.

RE: 38-93 low volume - Mondial - 04-04-2017

Yes, I agree with Arran that there is a problem in the 6R7 circuit. Double check that resistor 20 is around 190K, and there is full B+ voltage on one side of it.

The 6R7 tube could also be bad. This tube operates with bias derived from the AVC circuit, so the voltage on the plate should change as you tune in a strong station and you vary the volume control. The plate voltage should rise with the strength of the received signal and increasing volume control setting.

RE: 38-93 low volume - Jim Koehler - 04-04-2017

Agree with John on this....
Follow the B+ from the FC to the plate of the 6F6. You have to check the FC for resistance as well as the primary of the OPT. These are static tests. The OPT should measure 400 ohms +/- 10%. The FC 1140 ohms +/- 10%. Another check would to make sure the 6F6 isn't shorting. These tubes run HOT to begin with but a short will drag down the voltage considerably. If you can, recheck the status of your e-caps. More than once I've installed new e-caps that weren't up to par and leaky.
Is the transformer getting too hot? A good way to spot this trouble is to monitor the AC primary with a 0-2 A ammeter. Down and dirty, divide the name plate wattage by the incoming AC voltage and it'll put you in the ballpark on how much amperage you're supposed to be drawing.

RE: 38-93 low volume - NostalgiaRadioTime - 04-04-2017

THANK YOU all for your great suggestions and ideas. I'm happy to report the radio is now singing as it should. The culprit? There was a short between the outer two terminals of the #22 bakelite block. I was getting continuity across the .015 mfd capacitor. After re-doing the block it came alive. Voltages are much closer to where they should be. 210 volts on the plate of the 6F6 and getting 19 volts across the 6R7.... which is a whole lot better than the 2.2 I was getting Icon_thumbup

Next up replace the speaker wires, do an alignment and this chassis should be done.