Recapping a Philco 39-25

Why Recap at All?

The Philco 39-25 left the factory in 1939 with a full complement of wax paper and electrolytic capacitors. At the time, these were perfectly adequate components. Eighty-five years later, they are not. Wax paper capacitors absorb moisture and become leaky. Electrolytics dry out and lose capacitance. A set that appears to play may be operating well below its design specification — or may be on the verge of taking something more expensive with it.

Recapping is not optional on a set of this age. It is the first thing you do, before alignment, before any other diagnosis, before you spend an afternoon chasing a fault that a ten-pence capacitor would have cured.

A recap is not a repair. It is maintenance. You would not drive a car on sixty-year-old brake fluid. Do not play a sixty-year-old radio on sixty-year-old capacitors.

— Practical Wireless Restoration, 3rd edition

Before You Begin

Safety First

The 39-25 is an AC/DC set, which means the chassis is directly connected to the mains with no isolating transformer. This is not a set for beginners to work on live. Always discharge the main filter capacitors before touching anything, and always use an isolation transformer when powering the set on the bench.

What You Will Need

• Isolation transformer, minimum VA rating 100W
• Digital multimeter with capacitance range
• Temperature-controlled soldering iron, 350°C tip
• Solder sucker or desoldering braid
• Long-nose pliers and flush cutters
• Capacitor kit — see parts list below
• ESR meter for testing old electrolytics in-circuit

Sourcing Parts

The 39-25 uses a mix of wax paper signal capacitors and electrolytic power supply capacitors. Original wax paper types are worth saving not worth attempting to reform — replace them all. For electrolytics, measure the original value and voltage rating, and fit a modern equivalent rated at at least 20% above the original working voltage.

The Parts List

The following table lists every capacitor in the 39-25 by circuit reference, original value, original voltage rating, and recommended modern replacement. Values marked critical must be matched exactly — substitutions here affect alignment.

The Procedure

Step 1 — Document Everything

Before removing a single component, photograph the chassis from every angle. Pay particular attention to lead dress — the routing of component leads — as this affects both safety and RF performance. A photograph taken now will save considerable head-scratching later.

Step 2 — Remove the Valves

Pull all valves before applying heat anywhere near the chassis. Valve bases are phenolic and will crack if overheated. Label each valve with its socket position using masking tape. The 39-25 uses the following complement:

• V1 — 6A8 converter
• V2 — 6K7 IF amplifier
• V3 — 6Q7 detector and AVC
• V4 — 25L6 output
• V5 — 25Z6 rectifier

Step 3 — Replace Capacitors

Work systematically from one end of the chassis to the other. Replace one capacitor at a time — remove the old component, clean the pads, fit the new one. Never remove more than one component at a time until you are confident you can identify every connection from your photographs and the service data.

For wax paper types, note the orientation of the original component. One end of a wax paper capacitor is marked with a band indicating the outer foil — this end should connect to the lower-impedance point in the circuit, typically chassis or the anode side of a valve. Modern film capacitors are not polarised, but matching the original orientation is good practice.

Step 4 — First Power-On

Before powering on, check your work against the parts list. Then:

1. Connect the chassis to the isolation transformer — do not connect to the mains directly
2. Set the transformer to 50% output voltage
3. Power on and monitor current draw for 60 seconds
4. If current is stable, bring voltage to 100% over two minutes
5. Check all HT rail voltages against service data
6. Re-fit valves one at a time, checking for shorts after each

Common Faults After Recapping

No HT Voltage

Check C3 and C4 orientation — electrolytics fitted backwards will fail immediately and may vent. Check the rectifier valve V5 is seated correctly.

Hum on Audio

Residual hum after recap usually indicates one of the main filter capacitors (C3 or C4) is open circuit. Measure capacitance out of circuit — a healthy 10 µF electrolytic should read within 20% of marked value.

Weak or Distorted Audio

Check C7, the cathode bypass on the output valve. A failed cathode bypass capacitor causes significant loss of gain and increased distortion. This is the single most common fault on 39-25 sets that have been recapped before by someone who missed it.

Intermittent Reception

If reception is intermittent or sensitive to vibration, suspect the IF transformers rather than the capacitors. The 39-25 IF cans are known for developing internal dry joints with age. This is a separate job — see our guide to IF transformer repair.

A Final Note

Recapping is satisfying work. There is something deeply pleasing about returning a piece of engineering from the 1930s to reliable service. Take your time, work methodically, and do not cut corners on the high-voltage components.

If you get stuck, our workshop is open for enquiries. We are always happy to talk through a difficult chassis.