Revox B780

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Revox was the consumer brand of Studer, who made some of , if not the, best analog tape machines used in the worlds great recording studios. Back in the day I worked on the A80 (24 track and A800 (24 track) machines, sometimes slaved together to create a 48 track machine. These all used 2inch tape (my favorite was Ampex 456) and were run at either 15ips or 30ips. My all time favorite was using the Studer 1/2″ tape machine running at 30ips (inches per second) to master onto as this captured all the information that the mixing console (SSL/Neve/Harrison etc) was capable of providing. Over the years I had owned a number of Revox tape machines, the A77 and B77 which were also excellent examples of electro-mechanical  construction.

So when my friend asked me to “recap” his B780 FM receiver, of which he had two- one purchased as a spare-I leapt at the opportunity. Revox HiFi gear has always been appreciated and was at the more expensive end of the spectrum when new, though as you were paying a premium, you got a top quality product!

The Revox, in line with many Swiss and German engineered products has a very specific method of construction, which when understood makes working on these units very straight forward- or appearing to be so. There are a number of circuit boards connected by a wiring harness, each board can be removed from the chassis and worked on separately.However there area couple of complexities……

Power supply board: This board supplies the low voltage rails to all parts of the receiver, except the main power amps. At the time Revox, and a number of other European brands, for example Tandberg, used “Frako” electrolytic capacitors, which have not stood the test of time well at all. If you have a unit with these installed, then it is a very good idea to get rid of them. It was also very popular at the time to use Tantalum capacitors both for their small size and low leakage, and without wishing to add to the long running debate on their applicability for audio, I come down on the side of always removing them from the signal path or any audio related functions (including power supplies), however I do tend to leave them in place in RF circuits where they are not involved with the audio chain.

So the power supply board is now fitted with new Vishay Electrolytic capacitors, as the mounting is axial rather than radial, and where applicable I tend toward the Vishay units over the Nichicon VX series. Additionally Nichicon KL low leakage types replaced some tantalums as did WIMA film for the lower value types. Finally Nichicon low impedance types were also employed. The three bridge rectifiers were also upgraded.

Power amp boards: These have separate power supplies for each channel to provide sufficient independent power capability so that a heavy load on one channel will not drain these resources for the other channel. Although not quite as isolated as two independent transformers, this deals with most of the issues created by using a single supply for both channels, or worse a single supply for all the amplifier/receiver functions. I have read that there is a cogent argument for sharing a single transformer, though my personal preference is to have two completely separate power supplies. The photo shows new and old power amp boards side by side, with a polystyrene cap in place of one of the ceramics.

Now the fun began. The power supply boards mount the reservoir capacitors and bridge rectifier on the track side of the board- which makes their removal and replacement somewhat difficult. The capacitors have small stand offs and four connectors, of which only two are active, though fortunately a company ( provides a replacement kit of 4 10,000uf 63v capacitors that replace the original evo 4700uf 63v units. Without these it would have been extremely difficult to undertake this work. Having mastered the challenge of removing these, involving the use of a jewelers saw to get underneath the caps to remove them, the rest of the upgrades went relatively smoothly.

There is another limitation  to these units, which is that some of the PCB’s have tracks that lift with even the smallest amount of heat (applied with a Hakko vacuum desolder system), which resulted in having to track down minute and invisible breaks in the PCB or use the leads of the new components to replace lifted track. On one of the units there had been previous repairs undertaken, and I was relieved to see from this unit and read from others experience that I was not the only one with this challenge.

On both units the output transistors had been replaced, in one case by a good choice and in another by an less good choice, so I replaced the now unavailable 2N6031/2N5631 with MJ15004G/MJ15003G respectively on the unit with the less good choice. The original bridge rectifier was also upgraded in support of the larger value power supply capacitors.

All the electrolytic capacitors were replaced with either Audio or low impedance types, and the low value tantalum by a WIMA film type. Two ceramic capacitors were also replaced by WIMA film types (or on one unit by polystyrene).

Finally the input stage of the power amp employs an LF357 op amp, which has a high slew rate and relatively low noise, however op amps, like most all electronic components have progressed over the last decades and I decided to try some alternatives, however the only drop in replacement is the very expensive OPA627, which at $30 each  certainly impacted the budget. On reflection it would have been a good idea to add some further decoupling to the IC power lines (there is none originally fitted), however with the experience with the PCB tracks and lack of space, I opted to forgo this. Given the cost of the IC’s and one unit being a spare, we left the original IC’s in one unit and installed the OPA627 in the other.

There was a difference in sound, the OPA unit having a more transparent and dynamic overall sound compared to the “warmer” LF357 unit, though I was really impressed by the overall balance of sound delivered by both of these units and would be happy with either configuration.

The rest of the unit received the same treatment, use of WIMA film types where appropriate and possible, especially in the replacement of the tantalum units, Nichicon Audio (FG/KZ/KA) types and low impedance types (PW/HE), ELNA SIMLIC II and some MLCC CGO ceramics as appropriate. In all cases the original capacitance values were replaced like for like, though usually with a higher voltage rating. The RF sections only had their electrolytics changed and the CPU board was left alone.

The result is two fine sounding receivers, which deserve their premium reputation, though working on them certainly provided some challenges.



Photo show the second unit prior to upgrades, with the rear heatsink removed and feet with appropriate footwear

Harmon Kardon 730

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I have worked on a couple of HK330 and wanted to know what the bigger brother would sound like. The Marantz 2230/2235 and Pioneer SX737 are the equivalent contemporary peers of this receiver and share a number of common design philosophies, however the HK*30 series has a dual power supply as the most significant difference. It has always been my belief that two independent power supplies are superior to a single power supply in power amplifiers, so when the opportunity to upgrade a HK730 came along I could not resist.

There are a number of areas which can be effectively upgraded in this receiver and each of these is outlined below.

Power supply: The original power supply used 4,700uf 50V reservoir capacitors which have been upgraded to 10,000uf 80v (the voltage only because of the physical dimensions) and the bridge rectifiers  have also been upgraded in capacity (amperage) and voltage. The other electrolytic capacitors have been replaced with low impedance types (Nichicon) of the same values. The lower voltage rail that supplies the lamps and indicators has a separate power supply board, keeping this supply away from the audio chain. Again all the electrolytics have been replaced with low impedance types.

Power amp boards: The electrolytics have been replaced with either Audio quality (SIMLIC II) or WIMA film types for the audio path or low impedance Nichicon types for power supply decoupling. Some of the original ceramics have either been replaced with WIMA film or CGO MLCC ceramic capacitors.

Preamp: All the capacitors have been replaced by either low impedance or audio electrolytics for the higher values or WIMA film types for the lower values with some original ceramics having also being replaced by MLCC CGO Ceramics.

Equalizer(Phono): There are a number of modifications that improve the RIAA equalization curve that have been published and tested and these have been implemented,with larger capacity electrolytics, both Audio and low impedance types with film WIMA used in lower values and replacement of some ceramics as described above. The overall result is an improvement in the accuracy and bass response of the phono section, with a number of people preferring this to the Pioneer and Marantz equivalents.

RF section: Here there was a minimal intervention approach, replacement of the electrolytics with low impedance and audio types and replacement of of those tantulums in the audio chain with WIMA film types.



Finally the lamps were replaced with LEDs giving that great green glow.

Overall the receiver sounds excellent. Subjectively there is more dynamics and transparency  with a lower noise floor and certainly less distortion than the original. There is a “warmth” to the overall sound that is quite appealing and with suitable speakers, say JBL or other relatively high efficiency types, the overall sound will be certain to impress.  In comparison to the  Marantz or Pioneer- and this is all from memory, so highly subjective, the HK has more “oomph” and a relaxed sense of “power”.

So yes my hypothesis that two power supplies is better than one is borne out in the power amp section, and the phono stage is really very good indeed, which makes these HK series very appealing….and then there is that great green glow…



The picture to the power supply shows the power supply under rebuild with the new capacitors installed on the left.The picture below shows the rebuilt equalizer board, which in this example also includes new low noise transistors, which have been matched for Hfe. There are also a couple of resistors that have been replaced with modern 1% tolerance types.

The picture below shows the rebuilt power amplifier board


Onkyo A7 Upgrade

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I recently completed an upgrade of an Onkyo A7, which is the most comprehensive one for these amplifiers I have done.

Starting with the power supply section, my customer asked that the 25,000uf 55v main capacitors be replaced, so after an analysis of the available options two Nippon Chemicon 27,000uf 100v units were installed, the original units were also Nippon Chemicon. These units are about 20mm taller, but the same diameter, and sit inside the case with no problems. As these had been done I decided to replace the main rectifier diodes, so Fairchild 1N5402were installed. The power supply capacitors were replaced with Nichicon PW series, except for C706 which is Bipolar, and here a Nichicon BP type was used.



Those new power supply capacitors looking serious


Completed unit

The main output transistors were fine, however I replaced all the driver and low level transistors on the main amp board. The differential amp was comprised of 2SC1775/2SA872 and these are now KSC1845/KSA992. The bias stabilization transistors were 2SC945 and these are now KSC2383. The driver amp pair was 2SA912/2SC1885 and these are now KSA1220AYS/KSC2690AYS. The complimentary pair were 2SD358/2SB528 and these are now KSC2690AYS/KSA1220AYS respectively.

One thing that appears to regularly blow on these units are the 56 ohm resistors R653/R655 and R654/R656- so I always check them and generally replace them as well. All the electrolytic capacitors have been replaced, CV605/C606 4.7uf 50v with WIMA film 4.7uf 50v and the others (C619/C620 and C633~C636) with Nichicon PW units. The emitter resistors R659~R662 were replaced with Dale 1% 5W resistors.

The tone control board caps (C409/C410) were replaced with Nichicon KL series low leakage units.

On the pre-amp and equalizer board theimg_0649differential amp (Q301~304) 2SC726 were replaced with KSA992 and the tone amp (Q305/Q306) were replaced with KSA1845. All the electrolytic capacitors were replaced, C201/C202/C303/C304 with WIMA 2.2uf 50v film as were C225/C226/C317/C318 with 1uf 50v WIMA film types. C211/C212 were replaced with Nichicon KL types as were C319/C320. C315/C316 were replaced with Nichicon PW types.

All the switches and controls were deoxed , and the unit aligned as per the service manual.

The result is a very fine sounding amplifier!

May-2017- I have just completed another A-7 upgrade, and measure the distortion at around 0.02% THD+N- which considering the spec is 0.1% is quite an improvement. This unit has all new transistors, including the outputs, except for the phono stage, as well as the mods described above.

Onkyo A-10 upgrades

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the Onkyo A-10 is the big brother of the A-7 and A-5. These were certainly over engineered for their time and have both a heavy aluminum front plate (about 1/8″ thick) and very good quality components and construction.

I have only come across one A-10, and it impressed me a great deal, and effortless sound quality, great detail and solid bass..all with great transparency. I mentioned in an Audio Karma post that I proffered the upgraded version to a Krell KAV300i I have and what were the upgrades etc I undertook.

All the electrolytic caps except the large power supply units (2 x 13kuf 63V) were replaced. Mostly by Nichicon PW low impedance types, however the original circuit had a number of low leakage caps fitted, and these were replaced by Nichicon KL types- except the 2.2uf and lower values, which were replaced by WIMA film types. C275, C276 were replaced with ELNA Simlic II. C253,254 were replaced with Nichicon KZ, as were C311,312. C337,339 were replaced with Nichicon FG.

The 2SA726 were replaced with KSA992, as were the 2SC720. The 2SC1681 were replaced with KSA1845FBU

overall the amp sounded very good, and shows I think a very good underlying circuit and quality components

I recap and upgrade Onkyo A-7s whenever I can find them

NAD 2200 mods and upgrades

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I have been recapping and modding NAD 2200s for a while now- and wanted to give some details back to the community:

I have selected a number of mods from AK and DIY Audio and others listings, as well as my own tests, and these seem to make the majority of difference:

C215-218- These support the 18v rails that provide the power to the initial VAS stage of the power amplifier broads, and as such larger values provide a greater local reservoir. Replacing the original 2.2uf 50v with 470uf 50v makes quite a difference, “more bass” and better dynamics, I have settled on Nichicon HE (ultra low impedance)
C111-114- I have replaced these with COG Ceramics and have added 47uf 50v in parallel with each of these (across the 18v supply lines to the IC) and this again seemed to provide better dynamics, these are Nichicon PW
I tried Pilots (posted in AK) swapping of C109, C110 to 2.2uf 50v Wima- but ended up going back to Nichicon FG 10uf 50v- which gave more solid square wave response at frequencies below 100Hz
C107,C108- these were 820pf ceramics (circuit states 1000pf film)- so replaced these with WIMA 820pf 50v film
C203, C204- I tried a few versions (2.2uf Film, 10uf Film- both WIMA, Nichicon KL, Nichicon FG) and finally settled on ELNA SIMLIC II 10uf 50v- for me this kept the detail and was smoothest- also kept the LF square wave tidy.

I have also replaced a couple of ceramics with WIMA film on the main power amp boards

I replaced the 2043DD IC’s with LM4562NA- I had tried the OPA2134 (which is an IC I like), but again for me the LM4562NA is better suited to the amp-perhaps as it is BJT as is the 2043DD

All the other electrolytic capacitors have been replaced as follows:

Power supply- Nichicon PW same values
Power amp- all PW except c223, C224, C311, C312, C313,C314- all FG and C203,C204- ELNA Simlic II as mentioned
input amp- C109,C110 FG, C115,C116 Nichicon KA
plus the 47uf mentioned above across the 18v supply lines
10000uf 80v with Nichicon or equivalent
the 10000uf 120v are unobtainable, and the 160V versions are both expensive and dont fit- so I now have settled on Nippon Chemicon 10,000uf 100v (125v surge) replacements, which given that the high power rails are 95v, work very well indeed.  These have been operational for over 12 months with no problems.

for me this is the best NAD mod yet- more bass, better dynamics and measures well, though there is some low frequency phase shift and roll off from about 50hz downwards-the LAB inputs less so as expected.

one thought is to bypass the 10k ufs with a film cap (1uf?)- but again I have not tried this as yet.

Pioneer SX 850

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I have just started working on a defunct Pioneer SX 850. When it arrived there was no audio and no relay “click”. The insides, besides being dusty looked OK.

So I started with the power supply and removed and replaced all the electrolytic capacitors with a mix of Nichicon low impedance and audio types (all 105c). There are two transistors which should also always be replaced as they are often prone to failure. Having done this and with unit connected to a variable power supply with a fats trip and current meter, I turned it on, and still now relay click. So I tested all the voltages across the Power supply, ans everything is now as it should be.

So the next board to work on is the protection circuit, again there are two transistors, that should be upgraded, and all the electrolytics replaced. There are two small electrolytics, Sanyo, that I replace with WIMA film caps. Again turned it on, and hey presto the pleasant sound of the relay kicking in, so I then ran some tones through the aux input and all is good. Next is the phono equalizer, where all the transistors are replaced with low noise types and the small value electrolytics by WIMA film capacitors and the other electrolytics with Nichicon Audio types.


so fa so good- now on to the main amp boards- photos to follow


NAD 2100 upgrades

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so I have two NAD 2100s in for upgrade, one had a cracked PCB and is now up and working, all with new electrolytic capacitors throughout- The Panasonic 10k uf 80v from Digikey drop right in perfectly.

However the second NAD is being somewhat recalcitrant and I have pulled out nearly all the driver and and output transistors to try and find the problem. The issue is with the High Voltage rails,

and now I have it coming out of protection, which led me to look into transistor equivalents and replacements. Although the main outputs (Sanken) are also avialable from Digikey, some others, such as the 2SB922

appear to be unobtainable, and have very few appropriate substitutes.