Modifying the AVR2 for lowband playback

photo of circuitboard photo of circuitboard
Boards 15 & 16 (switcher/equaliser and FM demodulator)

The Ampex AVR2 can't play lowband, usually.  There are alternative demod and equaliser boards that can be swapped in, but we didn't have any.  We did find a website selling custom modified boards that could be switched between lowband and highband playback, but the cost was rather incredible, and couldn't see that being cost effective, for us.

Since we do have the manuals that detail both the highband and lowband boards, we went down the route of modifying some spare highband boards, and would live with swapping boards over, for the few lowband tapes we have to deal with.  There's very little difference between the highband and lowband boards, namely:

Whilst the low-pass filters can be left in their original highband configuration, they won't attenuate noise that may appear in the picture as much as they probably should do.  And probably don't equalise the video signal as effectively, either.

We couldn't find core formers that would fit on the board, and faced with special ordering something to do the job, disassembling the original cores and rewinding the coils, or finding something else to wind the inductors on, we took the latter route.  We used plastic sewing-machine cotton bobbins to wind new coils on.  While they're not easily tuneable, we don't have access to a reference playback tape to align them with, nor a spectrum analyser, to do a proper alignment, anyway, but they do the job respectably well.

Components to change on the equaliser board 15

10.5/14.5 MHz low pass filter
Component Low band High band
C78 160 pF 1% 115 pF 1%
C79 321 pF 1% 232 pF 1%
C80 2 pF ±0.5 pF 8.7 pF ±0.5 pF
C81 33 pF 1% 38 pF ±0.5 pF
C82 372 pF 1% 270 pF 1%
C83 58.2 pF 1% 42 pF ±0.5 pF (blurry)
C84 41 pF 1% 30 pF ±0.5 pF
C85 134 pF 1% 96.2 pF 1%
C86 910 pF 1% 656 pF 1%
C87 71 pF 1% 51 pF 1%
C88 810 pF 1% 587 pF 1%
C89 71 pF 1% 51 pF 1%
C90 736 pF 1% 532 pF 1%
L11 1.87 µH 1.37 µH
L12 1.56 µH 1.13 µH
L13 1.25 µH 0.91 µH
L14 0.201 µH 0.146 µH (blurry)
L15 0.192 µH 0.139 µH
L16 0.347 µH 0.251 µH
T1 4.7 µH 3.41 µH
T2 3.55 µH 2.57 µH
T3 4.03 µH 2.92 µH

T1, T2 & T3 are centre-tapped inductors.  We made them by trial and error—winding the first half of the coil separately from the second half, testing the halves and the whole (the inductances of the two halves don't add up to the whole inductor value).  All the inductors are supposed to be variable, via an adjustable ferrite slug, but we just created fixed values (we don't have a calibrated test tape to properly adjust them, anyway).  The manual lists them as ±5% tolerance.

Components to change on the demodulator board 16

4.5/6 MHz low pass filter
Component Low band High band
C9 186 pF 1% 140 pF 1%
C21 217.5 pF 1% 163 pF 1%
C40 10 pF 6.8 pF
C42 78.2 pF 1% 582 pF 1%
C43 510 pF 1% 375 pF 1%
C55 500 pF 1% 365 pF 1%
C56 15 pF 10 pF
C63 460 pF 1% 318 pF 1%
C64 15 pF 10 pF
Video out demod de-emphasis
Component Low band High band
C79 2400 pF 2700 pF
C80 51 pF 56 pF
C81 75 pF 62 pF
C83 53 pF 1% 745.6 pF ±0.25% (blurry)
R143 374 Ω 1% 280 Ω 1%
R144 20.5 ㏀ 1% 16.5 ㏀ 1%
R149 14.3 ㏀ 1% 11.5 ㏀ 1%
R150 47.5 Ω 1% 47.5 Ω 1%
R151 536 Ω 1% 544 Ω ±0.25%
R152 3480 Ω 1% 798.15 Ω ±0.25% (blurry)
R175 4.75 ㏀ 1% wasn't present
Sync demod de-emphasis
Component Low band High band
C47 51 pF 1% 700 pF 1%
R74 332 Ω 1% 348 Ω 1%
R76 536 Ω 1% 549 Ω 1%
R77 3480 Ω 1% 825 Ω 1%
R174 4.22 ㏀ 1% wasn't present
Video out level adjust
Component Low band High band
R2 2 ㏀ 1% (change) 3480 Ω 1%
R9 55 Ω 1% (blurry) 100 Ω 1% (different on some boards)

Some of the de-emphasis components affect the frequency response and the output DC and signal levels.  They are the main differences between low-band and high-band demodulation, the low-pass filters are more for extraneous noise-reduction (on both of the boards).

We had to change R2 to get a useful video output level, choosing a value somewhere around 1 to 1.5 ㏀, so that the R1 multiturn trimpot for the video out level was around half-way through its range.  The output of the demod board is DC coupled to the next board, and the demod board's output signal level affects the overall video gain, and the black level.

R174 and R175 may not exist on some boards.  R174 goes from the junction of R74 and Q11 emitter to a negative 12 volt supply rail (e.g. the other end of R77 from where it joins with R76).  R175 goes from the junction of where R143, R144, R149 & R150 all join the emitter of Q22, to a negative 12 volt supply rail (e.g. the other end of R152 from where it joins with R151).

General notes

Be very careful while handling the boards, they are very old, and three of the transistors on our equaliser board broke their legs, just a millimeter away from their casing, while the board were being worked on.  It wasn't an easy job to resolder the legs back onto the transistors (we had no spares to replace them with).

We're a PAL country, and I haven't checked whether different values would be needed for NTSC.  However, the manual has separate pages for some of the high band modules for different television systems, and one low band colour schematic page each (for the equaliser and demod boards) that doesn't list any particular television standards.

Many of the component values are unusual, requiring you to make up the values by combining series and/or parallel components.  Some of them are critical, and using the nearest common value will not do the job.

Rather than try to calculate how many turns to put on each bobbin for the inductors, we just used an LC meter to read the value of the coils as we made them.  The centre-tapped ones were a complete bastard to make up.  Lower value coils ended up with something like twenty turns, higher values may have had sixty turns (well within the ability of hand-winding them).  The original coils were slightly adjustable, but only fractionally around the value required for the highband board, the lowband board required values well beyond their adjustment range.

While you have the boards out, check for bad electrolytic or tantulum capacitors.  We had one machine with many shorted tantulum capacitors, that disabled one or more of the main power supplies, putting the entire machine out of action.  And electrolytics are notorious for drying out.

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