⚠️ IMMEDIATE SAFETY WARNING: Like all Revox machines of this era, the PR99 contains RIFA paper capacitors that WILL fail explosively. Never apply mains power to an unrestored PR99. Also contains Philips "blue" electrolytics that leak and corrode PCB traces.
Overview
The Revox PR99 is the successor to the A700 and the professional version of the B77. Produced from 1978 to 1985, it features balanced XLR inputs/outputs, +4dBm professional levels, and improved transport mechanics. While sharing many parts with the B77, the PR99 has unique electronic and mechanical considerations that require specific attention during restoration.
📌 Key Differences from B77: Balanced audio (Siemens ICs), +4dBm operation, different oscillator board, XLR connectors, and professional level metering.
Current Market Value (2024)
PR99 values have increased significantly as professional studios seek reliable ¼" machines:
PR99 Mk I (1978-81)
Original version, VU meters, Siemens ICs
$1,200-2,000
PR99 Mk II (1981-83)
LED metering, improved electronics
$1,500-2,500
PR99 Mk III (1983-85)
Final version, most refined
$1,800-3,000+
Fully Restored
Professional recap, calibration, mechanics
$2,500-4,000+
Parts/Project Units: $400-900 (assume full restoration needed)
Rare Variants: PR99 with vari-speed, rack handles, or HS (high speed) command premium prices.
Common Failure Modes
#1 RIFA Capacitors (Mains/Suppression)
Symptom: Smoke, burning smell, cracking, blown fuses, tripped breakers.
Cause: Paper dielectric absorbs moisture, cracks, shorts internally.
Repair: Replace ALL RIFA capacitors on sight. Located on relay board, motor boards, and across mains input.
#2 Philips "Blue" Electrolytic Capacitors
Symptom: Leaking brown goo, corroded traces, lifted pads, audio distortion.
Cause: Philips capacitors from this era have electrolyte formulation issues and leak destructively.
Repair: Replace ALL Philips blue capacitors. Inspect PCB for corrosion damage underneath.
#3 Siemens ICs (Audio Path)
Symptom: Distortion, noise, dead channels, intermittent audio.
Cause: Siemens TBA 931, TCA 580, and other audio ICs develop internal noise or fail completely.
Repair: Replace with modern op-amp adapters (SOIC-8 to DIP) using OPA2134 or similar.
#4 Motor Run Capacitors
Symptom: Motors run hot, won't start, slow speed, capstan instability.
Cause: 3.5µF (capstan) and 6.5+1.5µF (reel motor) capacitors degrade over time.
Repair: Replace with new AC-rated motor run capacitors.
#5 Brake Band Wear
Symptom: Tape spillage, uneven wind, machine won't stop properly.
Cause: Felt brake bands wear or become contaminated with oil.
Repair: Replace brake bands (still available from Revox parts suppliers, part 1.011.646).
#6 Tantalum Capacitors (Red Devils)
Symptom: Short circuits, logic glitches, destroyed ICs.
Cause: Tantalum capacitors fail shorted, taking out expensive ICs.
Repair: Replace all tantalums on audio and logic boards with 35V rated parts or film substitutes.
#7 Reel Motor Bearings
Symptom: Grinding noise, rough rotation, excessive wow/flutter.
Cause: Dried lubricant, worn bearings.
Repair: Replace bearings (SKF 608-2Z or equivalent) or re-oil with PDP65.
#8 Mode Switch Contamination
Symptom: Machine enters wrong mode, erratic transport behavior.
Cause: Gold contacts develop tracking due to contamination.
Repair: Remove, clean with IPA, re-lubricate lightly.
Mk I, Mk II, Mk III Differences
| Feature |
Mk I (1978-81) |
Mk II (1981-83) |
Mk III (1983-85) |
| Metering |
Analog VU meters |
LED bar graph |
LED bar graph (refined) |
| Audio ICs |
Siemens TBA931 |
Hybrid modules |
Hybrid modules |
| Oscillator Board |
Discrete |
Integrated |
Integrated (improved) |
| Capacitor Types |
Philips blue + RIFA |
Philips blue + RIFA |
Fewer problem caps |
| Head Block |
Standard |
Standard |
Improved shielding |
Note: Mk II and Mk III share most electronic assemblies. Mk I has unique audio boards requiring different restoration approach.
Systematic Restoration Process
🔧 RESTORATION PHILOSOPHY: The PR99 is modular—work in stages. Remove audio chassis as a unit. Test after each stage. Document connector positions with photos.
Restoration Checklist:
✅ Stage 0 - Documentation (photos of all connectors)
✅ Stage 1 - Replace ALL RIFA capacitors (safety critical)
✅ Stage 2 - Replace Philips blue electrolytics in PSU
✅ Stage 3 - Test power supply voltages
✅ Stage 4 - Replace motor run capacitors
✅ Stage 5 - Capstan board recap and motor lubrication
✅ Stage 6 - Logic board tantalum replacement
✅ Stage 7 - Mechanical: brakes, bearings, tension arms
✅ Stage 8 - Audio boards: full recap, IC upgrades
✅ Stage 9 - Full calibration
Recommended Restoration Order:
- Stage 0 - Documentation: Photograph all connectors, switches, and board positions before disassembly.
- Stage 1 - Safety First: Replace ALL RIFA capacitors on relay board, motor boards, and mains input BEFORE applying power.
- Stage 2 - Power Supply: Replace all Philips blue electrolytics, check rectifiers, test voltages (24V, 21V, 12V, 5V).
- Stage 3 - Transport Test: Verify basic transport functions with audio boards disconnected.
- Stage 4 - Capstan Board: Replace capacitors, check motor control ICs, lubricate capstan motor.
- Stage 5 - Logic Board: Replace tantalums, clean mode switch.
- Stage 6 - Mechanical: Clean/lube reel motors, replace brake bands if worn, service tension arms.
- Stage 7 - Audio Boards: Replace all electrolytics, tantalums, consider Siemens IC replacement.
- Stage 8 - Calibration: Full electronic alignment (levels, bias, EQ).
Complete Capacitor Replacement Guide
⚠️ CRITICAL - REPLACE BEFORE POWER-ON
| Component |
Location |
Value |
Replacement Notes |
| RIFA Suppression Caps |
Relay Board (1.079.xxx) |
0.1µF 250VAC |
2-3 units, replace with X2 class |
| RIFA Motor Board Caps |
Motor Control Boards |
0.1µF, 0.47µF 250VAC |
Replace all with X2/Y2 types |
| Mains Filter RIFA |
Behind power switch |
100nF X2 |
Replace with modern X2 |
🔋 MOTOR RUN CAPACITORS (AC RATED)
| Component |
Value |
Replacement Part |
| Capstan Motor Capacitor |
3.5µF 250VAC |
Motor run capacitor (Kemet, Ducati) |
| Reel Motor Dual Capacitor |
6.5µF + 1.5µF 250VAC |
Special 3-terminal dual cap (Revox #1.022.410) |
🔵 PHILIPS "BLUE" CAPACITORS - REPLACE ALL
| Common Values |
Location |
Failure Mode |
Modern Replacement |
| 47µF, 100µF, 220µF, 470µF |
Audio boards, PSU |
Leakage, corrosion, lifted pads |
Nichicon FG, Panasonic FR |
| 1000µF, 2200µF |
Power supply |
Dried out, high ESR |
Panasonic FR, Nichicon PW |
| 10µF, 22µF |
Audio coupling |
Distortion, noise |
Nichicon FG, Elna Silmic II |
🔴 TANTALUM CAPACITORS
| Board |
Values |
Solution |
| Audio Boards (1.080.xxx) |
1µF-10µF 16V/25V |
Replace with 35V tantalums or film (WIMA MKS2) |
| Logic Board |
4.7µF, 10µF |
Replace with 35V tantalums |
🎛️ AUDIO PATH ELECTROLYTICS
| Type |
Recommendation |
| Coupling Caps (>10µF) |
Nichicon FG, Elna Silmic II (audio grade) |
| Small Values (<10µF) |
WIMA MKS2 film capacitors (no polarity concerns) |
| Power Supply |
Panasonic FR, Nichicon PW (low ESR) |
Siemens IC Replacement Guide
Mk I PR99 uses Siemens ICs that are prone to failure and noise. Modern replacements dramatically improve performance.
| Original IC |
Function |
Modern Replacement |
Adapter Required |
| TBA 931 |
Audio preamp/line amp |
OPA2134, NE5532, LME49720 |
SOIC-8 to DIP-8 |
| TCA 580 |
Meter driver |
Difficult - check if working |
N/A - keep if good |
| uA739 |
Dual op-amp |
NE5532, OPA2134 |
Direct pin-compatible |
💡 IC Upgrade Tip: For Mk I audio boards, consider the "Sound-Design" replacement kit that includes adapters and high-quality op-amps. This transforms audio quality and reliability.
Critical ICs to Protect:
- TCA 561 - Capstan motor control (expensive/rare)
- SC 10429 - Logic IC (unobtainable)
- LM 317/337 - Voltage regulators (replace if faulty)
✅ Protection Strategy: Replace all tantalum capacitors and check voltages BEFORE powering up to avoid destroying these expensive ICs.
Mechanical Maintenance
Brake Band Replacement Part #1.011.646
- Remove reel motors to access brake bands
- Replace with Revox part 1.011.646 (felt band kit)
- Adjust for proper tape tension per service manual
Capstan Motor Lubrication
- Use genuine PDP65 oil or equivalent
- Remove C-clip, drop shaft, apply to felt rings
- Critical for speed stability and servo lock
Reel Motor Bearing Service
- If noisy: replace bearings (SKF 608-2Z or equivalent)
- Clean old grease, repack with light lithium grease
Tension Arm Damping
- If arms flop loose: disassemble, clean, replace silicon fluid
- Use 300,000 cSt silicon oil for proper damping
Pinch Roller Rebuilding
- If hardened or glazed: replace or send out for rebuilding
- Terry's Rubber Rollers offers service
Mode Switch Cleaning
- Remove switch from transport PCB
- Carefully pry open metal housing
- Clean gold contacts with IPA, reassemble
Audio Board Restoration
Mk I Audio Boards (1.080.xxx)
- Replace all Philips blue electrolytics
- Replace all tantalums
- Consider Siemens IC replacement with adapters
- Check for corroded traces under capacitors
- Replace trimmers (Bias, Level, EQ) with multi-turn Bourns
Mk II / Mk III Audio Boards
- Replace all electrolytics (fewer Philips caps than Mk I)
- Replace tantalums
- Hybrid modules are generally reliable but can be recapped
- Check for cracked solder joints on XLR connectors
Record/Playback Head Care
- Inspect for wear (grooves, reduced output)
- Demagnetize with head demagnetizer
- Clean with isopropyl alcohol
- If worn: relapping or replacement (JRF Magnetics)
Calibration Essentials
After recap, full calibration is required for professional performance:
Required Equipment:
- Test tape (IEC 1: MRL, Revox, or ATR Services)
- Oscilloscope (for bias adjustment)
- Signal generator
- AC millivoltmeter
- Distortion analyzer (optional)
Calibration Sequence (per service manual):
- Power supply voltages
- Repro level and EQ
- Record level and EQ
- Bias adjustment
- Meter calibration
- Check frequency response
⚠️ IMPORTANT: Never attempt calibration without proper test tapes. Improper bias can damage heads and degrade performance.
Restoration Kits & Resources
Comprehensive Restoration Kits:
Basic Cap Kit
RIFA replacements, motor caps, main PSU caps
Full Recap Kit
All electrolytics, tantalums, film caps
Mk I IC Upgrade
Siemens replacements with adapters
Mechanical Kit
Brake bands, pinch roller, belts, PDP65 oil
Parts Sources:
| Supplier |
Specialty |
| Revox (via authorized dealers) |
Original parts, heads, mechanicals |
| Mouser / DigiKey |
Capacitors, ICs, connectors |
| JRF Magnetics |
Head relapping, replacements |
| Terry's Rubber Rollers |
Pinch roller rebuilding |
| ATR Services |
Test tapes, calibration |
| Sound-Design (Germany) |
IC upgrade kits, capacitor sets |
| Studio Sound Electronics |
Hard-to-find Revox parts |
Online Resources:
- Tapeheads.net - Revox subforum, active community
- Revox Forum (German) - Technical deep dives
- HiFi Engine - Service manuals, user guides
- YouTube: "Revox PR99 restoration" - Numerous video guides
- The Revox List (Groups.io) - Email list for enthusiasts
Common Troubleshooting Guide
| Symptom |
Likely Cause |
Solution |
| No power, blown fuse |
RIFA capacitor shorted |
Replace RIFAs, check bridge rectifier |
| Capstan won't lock |
Dry motor bearings, bad capstan cap |
Lubricate motor, replace 3.5µF cap |
| Distorted audio |
Bad electrolytics, Siemens ICs |
Recap audio board, replace op-amps |
| Meter not working |
Bad tantalum, meter driver IC |
Replace tantalums, check TCA580 |
| Tape spills in fast wind |
Worn brake bands |
Replace brake bands (1.011.646), adjust |
| Intermittent transport |
Mode switch contamination |
Clean mode switch contacts |
| Hum in audio |
Bad PSU caps, ground issue |
Recap PSU, check ground straps |
| Low output, dull sound |
Worn heads, bad EQ caps |
Check head wear, recap EQ section |
| Motor runs hot |
Motor run capacitor failing |
Replace motor run caps |
| Noise when adjusting pots |
Dirty controls |
Clean with DeOxit |
Popular Modifications & Upgrades
Audio Upgrades:
- Op-amp upgrade - Replace Siemens ICs with OPA2134, LME49720, or Sparkos discrete op-amps
- Capacitor upgrades - Audio-grade electrolytics, film caps in signal path
- Balanced output transformers - Lundahl, Cinemag for studio use
- Recapping hybrid modules - Replace electrolytics inside Mk II/III modules
Mechanical Upgrades:
- Pinch roller upgrade - New rubber compound, improved grip
- Reel motor bearings - Precision replacements for smoother operation
- Tension arm dampers - Fresh silicon fluid (300,000 cSt)
- Brake band material - Upgraded felt compound
Electronic Upgrades:
- Capstan servo upgrade - Modern replacement for TCA561 (Mister Q module)
- LED lamp conversion - Replace incandescent bulbs with warm-white LEDs
- Vari-speed modification - External speed control with pot or voltage
- Meter LED backlight - For Mk I VU meters
⚡ Performance Note: A fully restored PR99 with modern op-amps rivals machines costing 5x as much. The transport is rock-solid when properly maintained. Many studio engineers prefer a well-maintained PR99 over modern digital emulations.
Model-Specific Considerations
PR99 Mk I
- VU meters: Bulbs may need replacement (warm LED conversion popular)
- Siemens ICs: Plan for replacement - they will fail eventually
- Oscillator board: Discrete components, recap fully
- Serial numbers: Below 10,000 may have early issues
PR99 Mk II
- LED meters: Check driver ICs, power supply to meters
- Hybrid modules: Can be recapped but require careful disassembly
- Improved shielding: Better RF rejection than Mk I
- More reliable transport: Refined over Mk I
PR99 Mk III
- Final revision: Most refined electronically
- Fewer problem caps: Some Philips eliminated
- Best head shielding: Improved head block
- Most valuable: Commands highest prices
Essential Parts Reference
| Part |
Revox Part # |
Modern Equivalent |
| Capstan Motor Capacitor |
1.022.380 |
3.5µF 250VAC motor run |
| Reel Motor Dual Capacitor |
1.022.410 |
6.5+1.5µF 250VAC special |
| Brake Band Set |
1.011.646 |
Revox original only |
| Pinch Roller |
1.011.450 |
Terry's Rubber Rollers rebuild |
| Capstan Motor Oil |
PDP65 |
PDP65 or equivalent |
| Reel Motor Bearings |
1.011.120 |
SKF 608-2Z |
| TBA 931 IC |
1.080.550 |
OPA2134 + adapter |
