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marconi 157 - Printable Version

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RE: marconi 157 - Arran - 11-20-2016

 I think that I may own one of these, either that or mine is a Marconi Model 159. In any event the set is from about 1940 or 41, and is definitely Canadian built. One thing to be aware of is that there are some rubber/gutta percha covered leads coming out of the power transformer, which is why I haven't tried powering up my set yet. These sets are a bit unique in that they have a three gang tuning condenser but no RF amplifier stage, and they use a 6K8 triode-hexode converter tube rather then a pentagrid convertor like a 6A8/6SA7 like most sets from then would. As to why it motorboats it could be a number of things, like if you put a 6Q7G in place of a 6Q7 without adding a shield, or an open ground on a bypass capacitor, or a bypass capacitor missing or connected incorrectly.
 You already noted the bias cell, make sure that the polarity is correct, modern watch batteries have the case as the positive so you would need to swap the connections around to use one in place of the Mallory carbon zinc cell. A trick to get rid of the cell is to place a resistor between the cathode of the tube/s it's connected to and ground to make it more positive then the grid, just copy the value from a set that uses that sort of setup, and it will need a bypass cap in parallel with it of about .047 mf.
Regards
Arran

RE: marconi 157 - the picker - 11-20-2016

I replaced the bias cell with a 1.5 v aa battery and I do not have a 6q7g tube and do I check for a open ground on a bypass cap thanks

RE: marconi 157 - the picker - 11-20-2016

(11-19-2016, 09:56 PM)Phlogiston Wrote:  Well, they look canned.

So never mind that, but check the voltages on the tubes starting with the 6F6.

Just curious too, does the eye tube do anything (signal indication)?

Do you have a signal tracer?
tuning eye does not change at all and the tube voltage are tube 6f6 pin 3-271dc pin4-271dc pin7-6.3ac pin8-13dc tube5y4 pin3-322ac pin5-320ac pin7-340dc pin8-340dc tube 6q7 pin3-30dc pin7-6.3ac tube 6k7 pin3-279dc pin4-74dc pin7-6.3ac tube 6k8 pin3-276dc pin4-74dc pin5-.25 pin6-0volts question mark and pin7-6.5ac

RE: marconi 157 - the picker - 11-20-2016

(11-20-2016, 06:08 AM)Arran Wrote:   I think that I may own one of these, either that or mine is a Marconi Model 159. In any event the set is from about 1940 or 41, and is definitely Canadian built. One thing to be aware of is that there are some rubber/gutta percha covered leads coming out of the power transformer, which is why I haven't tried powering up my set yet. These sets are a bit unique in that they have a three gang tuning condenser but no RF amplifier stage, and they use a 6K8 triode-hexode converter tube rather then a pentagrid convertor like a 6A8/6SA7 like most sets from then would. As to why it motorboats it could be a number of things, like if you put a 6Q7G in place of a 6Q7 without adding a shield, or an open ground on a bypass capacitor, or a bypass capacitor missing or connected incorrectly.
 You already noted the bias cell, make sure that the polarity is correct, modern watch batteries have the case as the positive so you would need to swap the connections around to use one in place of the Mallory carbon zinc cell. A trick to get rid of the cell is to place a resistor between the cathode of the tube/s it's connected to and ground to make it more positive then the grid, just copy the value from a set that uses that sort of setup, and it will need a bypass cap in parallel with it of about .047 mf.
Regards
Arran
which side of bias cell connects to pin 6 of tube 6k7 thanks

RE: marconi 157 - morzh - 11-20-2016

It says it right there in the sch. The battery is marked, minus goes to grid.

RE: marconi 157 - the picker - 11-20-2016

I had it hooked up right so I still have the motor boating problem

RE: marconi 157 - the picker - 11-20-2016

also I question the 0 dc v on pin 6 of tube 6k8

RE: marconi 157 - morzh - 11-20-2016

Section two of the switch brings the voltage to the pin6 (anode of triode section). See if the switch us ok.
Then R13, then the RF transformers connected to that section, the primary windings.

RE: marconi 157 - the picker - 11-20-2016

I assume you are talking about the band selector if so I noticed that it has five columns with wire wound around them and one has the wire broken on it so I will take pictures tomorrow sorry for my lack of knowledge I am just learning how to repair radios

RE: marconi 157 - morzh - 11-20-2016

Schematic reading is the key.

RE: marconi 157 - the picker - 11-20-2016

I totally agree but I am finding it hard with no electrical background

RE: marconi 157 - morzh - 11-20-2016

It's easy. Just learn the symbols. And the lines are wires.
Like reading maps. You don't have to be a pro land surveyor or a geography teacher.

RE: marconi 157 - the picker - 11-21-2016

(11-20-2016, 09:14 PM)morzh Wrote:  It's easy. Just learn the symbols. And the lines are wires.
Like reading maps. You don't have to be a pro land surveyor or a geography teacher.

this pic has the broken wire

RE: marconi 157 - the picker - 11-21-2016

(11-20-2016, 09:14 PM)morzh Wrote:  It's easy. Just learn the symbols. And the lines are wires.
Like reading maps. You don't have to be a pro land surveyor or a geography teacher.

here is another pic

RE: marconi 157 - the picker - 11-21-2016

(11-20-2016, 08:48 PM)morzh Wrote:  Schematic reading is the key.

another picture