[Tiddat33]

I was given a Philco 86 Lowboy and as a former electronics technician in the US Army(MOS 33Q20), decided I was up for a challenge.  I have already replaced most of the capacitors and checked resistors….. I can tune to a faint local signal, so it seems everything works.  What I have found is that all of my DC voltages behind the 80 tube are high…. Bad 80 tube?

[MrFixr55]

Hi Tiddat,

Welcome to te Phorum!

The 80 is likely OK. The voltages are high due to a low current draw. These power supplies always run higher when there is no "load" connected to it. Therefore, the issue is that there are circuits that are not drawing what they should.

http://www.nostalgiaair.org/PagesByModel...029653.pdf

According to the above schematic, there should be about 160V on the plates of the 71A Output tubes. If that value is high, the 71As may be bad, and may be the reason for the low volume. I fixed a Gloritone several years ago for a friend with the same symptoms. Audio output was a single '45 tube. When replaced, issue was solved. Radio played loud and proud and B+ voltages returned to normal.

This radio needs a rather long antenna. This is a "TRF" set, very similar to Atwater Kent and RCA TRF sets of the period, except that this et has a Push Pull output which gives superior, lower distortion sound. Unfortunately, the grid leak detector used in this set gives higher distortion than a diode detector. This set may also employ the "Neutrodyne" circuit, and unlike the Radiola 16, 17, 18 and the A-K 5, 40, 35c, the first RF stage is tuned.

This set does have a built in "local" antenna (Capacitive coupling to AC Line. However, ensure that the capacitor 33 is new and of a "Safety" type.

[Tiddat33]

Thanks for the info!  In doing some more measurements, I found that I missed something obvious…. No plate voltage on the 1st A.F. 26 tube.  Could have sworn I checked all my transformers but I missed an open primary on the push-pull input transformer.  Would this have that much of an effect on the rest of the voltages?

Next question…..  Can I use the same workaround I saw used on an Philco 87 driving a pair of 45’s on Ron’s Radio website?  

   

Thanks!!!

[MrFixr55]

The 1st AF does not draw much current, so the open primary shouldn't cause a great increase in voltage.

Did you recap this radio and use higher capacity caps? What is the B+ that you are getting vs te specs? One other issue is that a lot of these radios were designed for 110V, and the average outlet voltage nowadays is 120V., so about 8% or so high.

If the secondary of the interstage transformer is good, Ron's "bridging technique" should work great. I have a Philco 20 with a good transformer, but I am going to attempt to RC couple that one.

[RodB]

Should that coupling cap be .047mfd?

[Ron Ramirez]

No.

0.47 uF is correct. You might get by with 0.22 uF with some loss of fidelity.

Tiddat33 - Thank you, I'm glad someone is actually reading my blog site! And, yes, you may use the audio transformer workaround if you need to.

[RodB]

Thanks for clarification, Ron. I had read the post some time ago but the value seemed high.

[Tiddat33]

All capacitors I have replaced have been with the same capacitances.

So while I’m waiting for more parts, decided to document the existing voltages…

All of my AC voltages (A, B & C) off the power transformer are spot on….. My DC voltages are a whole different story…..

D & G = 112
E = 48
F = 244

Plate voltage on both 71a tubes = 240
Plate voltage on all 3 A.F. 26 tubes = 110

All the tubes light up and I can faintly tune a local station….

[Radioroslyn]

The difficult thing to know on the early Philco sets (the ones that use directly heated tubes) where to connect the - meter lead to match reading on the listing. Most would think chassis ground. But not so much. It could be the hv ct or one of the 80 tube's plates.

I ran up with this in a model 20 in the output stage. If I measured the plate voltage it was high and the grid was low from chassis ground. Measuring from the filament gave a much better picture of what was going on in terms of voltage on the plate and grid. They were well within range according to the rca tube manual

As for the low volume it might be because of the resistance coupling of the two audio stages. Your not getting the benefit of the step up voltage offered by using transformers. I've used these to replace those types  https://www.tubesandmore.com/products/tr...tage-10-ma  Also you might try connecting your ant to each of the grid of the 26's one by one. You may find one of the amp isn't working or a bad ant coil (easy to fix).

Have fun chasing electrons.

[MrFixr55]

Hi Tiddat,

The faint volume, is that with the open interstage transformer primary or have you "bridged" it using the Ramirez Technique? If low because the xformer is open, then replacing it or "bridging it" will resolve.

+1 On Terrys (RadioRoslyn's) suggestion of touching the grid of the detector and RF Amps one at a time to see if volume increases. Primaries have been known to open on early Philco RF transformers.

one other thing and it is a biggie! R35 is a "voltage divider" with multiple taps, and gets its power from the field coil (point G). It is very likely that the resistance section between connections 3 and 4 is open. That would lower field coil current and raise all of the other voltages.

Phorum Experts may have the value of the resistor. All sections should be replaced with wirewounds of at least 5 W.

Good luck, you're getting warm That resistor section was also warm before it got cold.

[Ron Ramirez]

I have to say that my Philco 87 with "bridged" audio transformers (i.e. converted to resistance-capacitance coupling) has plenty of volume, no problems. Yes, conventional transformer coupling will give you a bit more volume, but in an otherwise properly restored radio, you won't miss the extra "oomph" of an interstage transformer with resistance-capacitance coupling.

[RodB]

Especially when it's feeding push-pull power out tubes.

[Arran]

Then there is the "sand" option, to be found in a back copy of the AWA Old timer's bulletin, I would have to dig it out to find it, but that will work if both windings are bad, usually it's the primary that's short, but not always.

[Tiddat33]

I’m hoping my voltage issues are actually related to my referencing everything to chassis….. we’ll see once I put everything back together again. I currently have things in pieces again as I tackle hiding all the new caps inside the cans…. I originally installed them underneath and bypassed the cans.

The faint sound is with the transformer primary open…. Still waiting on the parts for the RC coupling.

All the R35 section measurements are spot on per my schematics….

I really appreciate all the input and help!!!

[Radioroslyn]

Oh I thought you had added the r/c coupling parts already. Yeah you'll have really low audio w/bad transformers!