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Old 29th March 2010, 08:55 PM   #1
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Default Improving a "Disco mixer" to mid-fi performance

I bought this Herald RA-88a "Disco Mixer' used at the flea market with 2 turntable inputs, an aux input, and a mike input, to replace my Dynakit PAS-2 preamp with the bad volume control, and the Sony TC230 Tape recorder I was using as a magnetic phono-CD player power amp. The Sony got weak on one phone channel, noisy and inttermittant on the input switch, and has over 100 electrolytic caps that are 40 years old and need replacing. The "disco mixer" was $15. I'm involuntarily retired. I liked that I could keep the magnetic phone input and CD player input on at the same time without switching. I didn't like the hiss of the mixer, maybe 40 db down. So I took alook inside and found 4 JRC 4558D op amps for the 4 inputs. A thread on this website recommended many op amps as better, but the ST Micro 33078's dip op amps at $.35 each fit my budget and were in stock at Newark. I installed Tyco 1825093-2 phosphor bronze Dip sockets at $.45 each as I went, so I could keep experimenting if necessary to go on to Burr Brown or Analogue Devices parts. The 33078's sound great, hiss is gone, now I hear a faint hum. I changed the volume pot this March on the PAS 2 with one I bought from Stereo Cost Cutters in 1982 out of the Dynaco bankruptcy surplus. The surplus pot is also defective, being sonicly quiet, but requiring vice grips on the stem to turn. The disco mixer is now quieter than the Dynakit PAS2, and sounds just as good with a ST120 with the DJoffe mod and two Peavey "T-300 High Frequency Projector" speakers. The PAS2 was updated with a 1982 purchased Rubicon filter can cap and cathode heater axial caps, 1982 RCA rectifier tube, selenium rectifier replaced with 1N4003's, and 1961-1968 12AX7 amp tubes. (I tested them at Sterling Electronics in 1968 and replaced two weak tubes). The Dynakit hisses a little bit, no hum particularly. The disco mixer hums a little, the least hum when the non-polarized 120 VAC plug is plugged in with the stripey side on the hot. (Amateur construction probably). The scary thing about the PAS2 I found out as I was trying to balance the phono cartridge inputs, (one tube is weaker than the other) the PAS2 shorts the inputs not selected with the selector switch . This could kill a transistor radio output or CD player output plugged into the PAS2 and run without selecting, or at least run the battery down. I bought another volume control for the PAS2 from a famous tube vendor in St. Charles, IL. It came in mono instead of stereo like the picture and nomenclature. I feel like the maintenance manager of the N&W Roanoke shops, when he ordered a feedwater pump for a steam locomotive in 1959, and had to pay ten times the usual price, because it was "out of production", and it came in without the latest revisions, because the pump company didn't update the prints as it was going bankrupt. PAS2's are like steam locomotives, interesting, slightly incompatible in some ways (tube radios don't mind if the output transformer is shorted) and poorly understood. Oh, the schematic for the PAS3 I downloaded from the link on the tube suppier's website, locked up my computer without successfully displaying anything but the cover-twice.
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Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 29th March 2010 at 09:02 PM. Reason: cathode heater cap, not bias
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Old 1st April 2010, 07:37 AM   #2
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Old 16th July 2010, 01:17 AM   #3
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Default on towards hi-fi

okay, the Herald after the op amp replacement had this annoying power frequency hum about 40-50 db down on the magnetic phono input only. Went away when the magnetic phono pot was down, reduced when my hand or the headshell was over the turntable platter, or my hand was over the mixer control panel. I tried grounding the case the 3rd pin safety ground (had a 2 prong power plug, North american) but that helped nothing. I didn't think that was the op-amp's fault, thought it had something to do with them grounding all the rings of the RCA jacks to the case, also the turntable shell. Bought some new 47 uf caps, the only ones i could read the numbers on, and some RCA jacks that I could mount in a new ungrounded way like the PAS.\
Well, completed the PAS2 upgrade project with stellar results, so dived into the RA-88a. First, replaced 4 47uf caps coupling the + and - inputs from the RCA magnetic phono jacks to the RIAA filter networks leading to the op-amps. Stopped at 4 caps so I didn't have too many places to look if something screwed up (lots of crackling on the PAS2 from dirt, wax, rosin, flux. Clean, clean, wash, wash). Problem solved. These caps were radials, jammed under the "cue" switches and probably one of them was touching the switch case. New caps were smaller, laid them on their sides to make sure the case didn't touch. Cool, really sounding good. Both the RA-88a with one slider 1/3 way up, and the PAS2 with the normal volume setting, are so quiet I can't hear them over the airconditioner in the next room. I'll have to put the o-scope to them to see which is quieter. Until then playing vinyl records and loving it.
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Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300
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Old 14th December 2010, 03:03 PM   #4
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The hum came back, and you can't edit posts here after about four hours. so here are the further adventures of the hum patrol. The mixer would hum both power on and power off. See the drawing for the original and modified power supplies.
At the top of the drawing you see the original power supply. 17 VAC CT transformer, 4 diodes, 2 electrolytic caps, headphone power and 330 ohm resistors isolating the op amp rails. I didn't measure the original op amp voltages with 4558 IC's, but on the ST33058 op amps the rails were +-2.5V. Something about the ST33078 takes 5 ma and the 4558 takes 3. Wasn't a problem distortion wise on mag phono or CD player, but when I tried to bring in an FM radio instead of the mike input (pulled the microphone op amp) it would clip. So see modified drawing at the bottom. First I put in the turn-off pop capacitor around the power switch, and the CL90 to eliminate turn on pop. Still hummed. 80 mv on rails. Then I built the auxillary board with its own filter caps, 22 ohm resistors on the transformer to protect it if the new caps failed, and toroid chokes out to the old pcb. The toroid chokes did not help the hum, but they did keep the hash from the lamp dimmer in the next room from getting through. (21 turns on PC switcher toroids). It still hummed, power on or power off. Next I broke the negative side of the 120 VAC line with the other side of the power switch. Cut the power off hum a lot, but increased the power on hum, with 100 mv hum ahead of the 330 ohm resistors on board. Put 1n5344 8.2V zeners between the offboard capacitors and the on board, it didn't help anything. Jumpered the 330 ohm resistors, made the hum worse power on. Added 1000 uf capacitors up near the IC's, fiddled with the 330 ohm resistors, ended up with 47 ohm for op amp rails of +- 7.4V. This got the hum on the rails down to 3 mv, and stopped the radio from clipping, but it still hummed a lot power on and also the output oscillated at 1 Mhz. The power amp fan was running full blast. Added .1 uf ceramic power supply caps near 2 ends of the PCB (4 op amps now) and added 22 pf ceramics around the feedback resistors on the output IC. Oscillation stopped. Still had power supply hum power on.
Finally found an 18VDC race car set wall transformer in the attic. Removed the power cord and disconnected the CT internal transformer and the 4 rectifiers from the prefilter caps and the toroids. Brought in the 18VDC from outside, attached to the 22 hom series resistors. No Center tap to ground, the 8.2V Zeners establish the signal ground on board the PCB. Still hums a tiny bit, but the gain is high enough I can reduce the gain pot on the CS800S power amp to 2 clicks above off, and the hum becomes inaudible. Still enough volume to drive me out of the room, I'm able to reduce the mag phono and aux pots below maximum on most records or CD's. Finally!
I also diconnected the bridging on the rings of the 12 RCA jacks, and ran the returns separately to the 2 mag phono IC's, the aux input IC, and the radio going to the mike resistor after the removed mike op amp. These wer through the cable shields already installed by Herald Electronics. I also put 1/4 o-rings around the rca jacks so they wouldn't touch the case. This makes the power amp hum badly when the mixer case is not grounded to the power strip safety pin, but go dead quiet when both power strip ground and turntable headshell ground (separate wire) are together under the screw on the mixer case.
$15 box plus $10 in parts, sounds as good as the PAS2, uses 1/100 of the power and doesn't need vice grip pliers to adjust the volume. Also, all sources can be left on at the same time, and don't have to fiddle with the volume pot most of the time when switching from record to CD to radio or back. Fun!
Attached Files
File Type: pdf schematicRa88aPSModified.pdf (88.1 KB, 158 views)
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Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 14th December 2010 at 03:09 PM.
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Old 25th February 2011, 05:35 PM   #5
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Mag phono in drawing RA88a
I learned how to use the Sun drawing program under Ubuntu, so here is my schematic of the mag phono input on the RA88a. It is one of two stereo pairs. KevinKR says I can improve noise by decreasing the feedback and input resistors by 1/10 and increasing the caps on the feedback and connected to the plus input by 10 times. So I will try that. He also says LM4562 is better than 33078, and I might try that. Stay tuned.
Attached Files
File Type: pdf RA-88a-magphono-in.pdf (85.0 KB, 87 views)
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